050826-1313 EST USA

We have a surface finish problem on a VF-3, 1998. This shows up worse at 7500 rpm.

Rather than have HAAS replace one component at a time to find the cause I want to find a means thru measurements on the machine to try to pin point the bad component.

So far we have spent about $1000 on HAAS service and talked to the factory without any clear direction.

What we observe ---

The surface finish problem is visual and is probably in the range of 10 to 100 millionths of an inch ( in the micron range ) in surface variation.

This is somewhat erratic but appears to have some component on the order of 5 revolutions of the spindle. In other words a visual pitch of .02" at 30 inch/minute feed and 7500 rpm gives us, 7500 rpm = 125 rps or 125 Hz for the once per revolution frequency, 30 in/min is 0.5 in/sec and .02" corresponds to about 25 Hz.

If we double the feed rate to 60 in/min then the .02" pitch becomes about .04". This implies that the problem is spindle related rather than X or Y axis. At 60 in/min ( 1 in/sec ) I can see an .008 to .010 pitch also. This is a frequency of 100 to 125 Hz or on the order of once per revolution. In other words we see a once per revolution component, and another at 4 to 5 revolutions.

Both end and side milling shows a surface finish problem. Changing cutters from two to three flute, or changing cutter length does not eliminate the problem. Can not say at this time whether the surface finish changed. Running the same cutters, material (6061), and program on a different HAAS ( VF2 of 1993 or VF0 of 2000 ) produces roughly a mirror surface on the side milling.

The side milling surface is identical whether X or Y axis is viewed.

HAAS tests on drawbar force were just slightly under low limit. Their ballbar test was better than some new machines (well below spec limits). Their spectrum analysis from accelerometer measuring perpendicular to bearing housing produced nothing that stood out. And their testing for end play did not indicate anything. Note, the surface finish problem is under very light load conditions.

We are clearly looking at a variation that results from once per revolution and lower. Thus frequencies below 125 Hz.

The spindle speed on the problem VF3 appears to be very stable.

The VF3 with a problem has a HAAS vector drive. The VF-2 and other VF3 are variable frequency and the VF0 is a HAAS vector drive.

Runout (TIR) on all the machines is on the order of 0.000,5". Also from my tests I am fairly sure there is at least one ball bearing cage velocity of about 1/2.3 of the spindle velocity (rpm). HAAS has been unwilling to tell me what cage velocities I might expect. My 2.3 comes from runout frequency analysis. Also this correlates approximately with an open ball bearing (arbitrary) that I measure about 2 1/3 for this value.

I have now run many tests on vibration (perpendicular on spindle housing), power input to the vector drive, and runout on the different machines. In fact the cage runout component on the VF2 is worse than on the VF3 but the VF2 produces better surface finish.

My next test is force vs displacement on the spindles.

Runout by itself does not appear to be the problem.

The problem VF3 spindle feels very free when rotated by hand.

I am not looking for an individual ball problem because this would produce a higher frequency than once per revolution.

We believe the problem may have developed over a year or two period.

I am following the instrumentation path because it is useful to have a way to pinpoint a cause rather than simply use substitution.

Is there anyone else that has had this problem and what was the cause?

.

Does it have a gear box?
I don't know what the gear ratios are in a haas but I could see a bump on one gear could cause every rev and a bump on the other could cause every 4th rev.
Just a thought.

JP

This is a bit of a shot in the dark. Has this machine ever been shipped without the spindle immobilized? If you have brinnelling on the races due to shipping vibration this could give you an effect in synchrony with the cage rotation. The inner and outer race brinnel marks would come into opposition many times every revolution but there would only be a ball in the precisely correct position every cage revolution. (I think)

Personally, I'm surprised that spindles using interchangeable tooling are as good as they are. There aren't any other kinds I guess :D, but if I was looking for perfection, I'd be really looking closely at the fit of the tapers. If the spindle taper is a wee bit off angle (or off in any other way), you'll never notice outright looseness, but still, the tool will be able to precess and vibrate inside the spindle taper.

I suppose you could test for this with some kind of a leverage test, with a fairly long, one piece toolholder, so that you know you are not introducing another flexible connection. Perhaps an extended shell mill holder would work. Hold a chunk of bronze in the vise, and bore a hole in it, for a running fit on the pilot end of the extended toolholder. Then, for the test, lower the pilot into the bushing and apply a few thousandths of deflection by moving the X or Y axis a little bit off center. Perhaps you could place your vibration transducer somewhere on the bushing, to see what it picks up. By measuring the change in vibration from dead centered, to however far off center you dare go, you might be able to detect whether the shank of the toolholder is hammering inside the taper, ever so slightly.

I am with HU..
Get a new tool holder. DY kem it then without a pull stud press it into the spindle bore, and see where the wear is. ( clean well first).. If this has lines showing up it it.. Make one out a Polyethelene on a lathe )If you need one I can sell you one of mine i made).. Then lap it back in. I did this to my bostomatic, and it made a big diffrence in the surface finish and the runout of my test pin went from .00043 to .00016. This is 3" from the collet face. :)...

050827-1957 EST USA

Thanks all. I have read all the messages and they are all important.

I have had some strange results today and will report more later.

A major result of your comments is that now I am running a feed of 120 in/min for some of the tests and the once per rev runout makes a nice visible time marker.

.

From a logical standpoint. I would start looking away from the spindle and more at the tooling, pull-studs, and draw force.
Reasons:
You stated force was below spec. I would fix that just to take it out of the picture.
The problem came over 1-2 years it seems. IMHO a bearing problem would have gotten "ugly" by now I suspect. Just an assumption however. Anything is possible of course.

Questions I have:
Are you doing all this testing with same tool holder? I would try a cross section of holders and see what the results are.
Is there a threashold speed? Meaing do you not see this problem below spindle speed X?
Is there any change with regards to result in testing machine right after power up and after full operating temp?

Best regards,
Sean

050830-1208 EST USA

Hi all:

I have many comments to your various suggestions and no solution yet.

JPM --- yes this machine has a gear box. Our primary problem is in the range of 6000 to 7500 rpm, thus we are in high gear.

The HAAS manual provides little information on specifics. However, one can draw some conclusions. Some of the spindle drawings are about 1/2.88. Gear box drawings are much smaller.

HAAS specifies the spindle pulley as 1.875 and on the drawing it is about 1.5 or 1.5 x 2.88 = 4.32 but this does not correspond with 2 x 1.875. On the other hand if this is the id of the pulley it does not scale correctly either. I can not correlate pulley size on gear box either. What is the length of the belt? Also I do not know that.

On the gear box drawing there is an idler gear between the motor and the gear box output shaft. This is about .18" for the output and .225" for the motor, or a step-up of .225/.18 = 1.25 from motor to output in high gear. I will use this as an assumption.

I can run a test at 7500 rpm with a 1/2" two flute cutter and 120 in/min feed rate and get a good once per revolution pattern on the cut. This corresponds to 125 Hz at the cutter. Ball park runout is 0.000,5" (TIR).

Same at 6000 rpm and got a pattern that correlates with 100 Hz.

We are not concerned with this once per revolution surface finish problem. Rather the problem is a similiar pattern that occurs at 30 in/min and has a period that is about 5 times that of the once per revolution. Thus, at 7500 rpm and 30 in/min thiis corresponds to 25 Hz.

A test at 6900 rpm and 30 in/min has a pattern of about 22 lines for 25 lines on a 7500 rpm sample. This ratio is 25/22 = 1.136 and the ratio of 7500/6900 is 1.086. The fact that the pattern was not unchanged says that there may be a speed related correlation of the surface pattern with spindle speed.

At this point I can not say or find something in the motor , gear box, and belt path that correlates with a factor of 5.

As I have previously indicated there should be no reason to consider a once per ball revolution or other things of a higher than an output shaft once per revolution frequency. Their effect should be of less importance than that of the shaft once per revolution.

Geof --- primarily this would be a high frequency effect. If there were a once per cage rev effect it would not produce an output sinusoidal result. Also as best that I can determine cage velocity is about 1/2.3 of spindle. This does not correlate with a 5 to one ratio.

Hu --- toolholder loosness is not likely, very hard to remove after clamping. Looseness would be unlikely to provide a stable pattern at the 1/5 frequency.

I had previously setup a 3/4'" Oilite bearing in the vise and with our test 3/4 rod in this and varied the side load. Could not produce the 5 factor effect, but did get the expected 2.3 effect in runout. And of course the once per revolution factor.

Kmed --- our spindles on all our machines are quite good for HAAS, .000,1 to .000,2" (TIR). I do not currently have a way to get my LVDT to measure this, and therefore only used a dial indicator. Our toolholders mate very well with the spindles, but do have runout that exceeds the HAAS spindle.

On my runout measurements from the 3/4 shaft at 1300 rpm, can not currently run faster with the LVDT, I had a 55 db signal at 21.698 Hz (once per revolution), and 14 db at 9.399 Hz (assumed from the cage) ( 21.698/9.399 = 2.309). Obviously the frequencies are not as accurate as the decimal places imply, but this is the FFT result. This is quite a small signal compared to the fundamantal. This was done with an 4 minute average of the inout signal.

On the other hand we do need better runout on some of our toolholders. I doubt that reduced toolholder runout would solve our cuurent problem. But since I do not know the cause it can not be totally ruled out. However, the same tool and toolholder, or different tools and tool holders produce the same results in this 1998 VF3, but produce good results in other rmachines.

I would like to know more about your lapping procedure. If you are lapping the spindle, then how do you force the lapped cone to be concentric with the centerline of spindle rotation? Also the same question relative to the toolholder, and its relation to its collet?

MR_NC --- there is nothing so far that I can pin on the spindle. If I run a vibration test with the accelerometer mounted on the spindle bearing housing measuring the radial vector and no toolholder, then I can not see the fundamental or the 1/2.3 components at 7500 rpm, but there is a strong 100 Hz component. By not see I mean they are not identifiable as meaningful. Is this 100 hz from the motor? I do not know. Adding heavy unbalance then the fundamental component shows up.

I do not think the clamp force is a factor because HAAS previously used a lower value, and the toolholder appears to be locked in the spindle by the amount of force to remove it.

Force vs displacement measurements vertically on the spindle do not indicate a lack of preload on the bearings.

We have used different toolholders and tools.

We have done heavy unbalanced tests with two 1/4-20 screws radially. One screw head is 2.5 from center and the other is 1.5". At 7500 rpm this shakes the machine. With this unbalance the once per rev fundamental shows a strong signal at 125.099 Hz and 60 db. The 101.292 Hz component is about 46 db. 89.626 is 53 db, 21.537 is 50 db, and 13.442 is 51 db.

Below 6000 rpm the surface finish problem is mostly gone.

Have not been able to find a difference between initial startup and after running.

Most tests have been run with X, Y and Z still.

Over the last weekend I ran comparative tests cutting 6061 over a straight path 5" long. Side milling .25" in Z and DOC usually .02" some times .04". These were done at both 120 in/min and 30 in/min. Accelerometer tests were done at 30 in/min.

At 7500 rpm, 30 in/min, .02 cut with accelerometer on spindle housing perpendicular to x-axis we get a strong component at 25.05 Hz (69.3 db), but the fundamental at 124.97 is only 54.5 db. 100.94 is stronger at 63.6 db.

Shifting to 6900 rpm and we do not get what would be expected, yet the pattern on the cut is as previously seen. Then I moved the accelerometer to the vise, both perpendicular and parallel to the x-axis, and no correlating components showed up.

I need to drop this experiment for a while. But later I want to revisit power measurement to the motor, a way to measure speed and runout at 7500 rpm on the shank of the tool while cutting.

Thank you everyone.

Do not not hesistate with new ideas. Your various comments have caused me to redo experiments and to think of new ones.

.

I love this thread!

Gar - you have compiled some heavy duty information there - awesome analytical work! I like your approach. With testing of this caliber, you will isolate the problem I am sure.

Could this be a line frequency/motor/machine harmonics thing? Or do you feel it is straight mechanical?

Keep us posted, please. Very interesting. Now if you'll excuse me, I need to go re-read most of your posts.

Scott

most of this is over my head but as far as frequencies go, It could be a harmonic of you 100/125hz couldn't it?

I mean a fly landing on the spindle causes some deflection right so a small loosness somewhere will cause a small harmonic and when you hit that sweet spot all hell breaks loose (or in your case a small surface finish defiation). Hell it could probably be something as minor as a loose encoder belt on the spindle setting an osilation up in the spindle controler.

just throwing some random stuff your way to maybe give you some ideas. Just making you think this guy don't know $h!t might bust something loose! ;)

050830-1521 est usa

mxtras and miljnor:

Both good comments.

I have constantly swung back and forth on whether this is a mechanical or electrical problem. It should be noted that the same DC supply, contained in the spindle vector drive, is used to supply the brushless servos.

I am looking for a measurable signal that is a predictor of the end results. Frequency analysis is extremely useful, especially when I can average the signal over a long time, and the signal is stationary. Stationary means that the statistical properties do not change over the interval of interest.

Anything in the gear or belt train that would cause this problem should not be particularly sensistive to the speed (meaning amplitude with speed), unless there was a resonance. Its frequency should be directly related to speed.

When I am running at a spindle speed of 7500 rpm the once per revolution frequency is 125 Hz derived from 7500/60 = 125 rev/sec. This in itself won't generate 100 Hz. Further when there is nothing hanging on the spindle (it is very well balanced) I do not see a 125 HZ signal from the accelerometer. When I put a major unbalance on the spindle, then the 125 Hz signal pops right out.

But running the spindle at 7500 rpm I see a strong 100 Hz signal. If total gearing from the motor to the spindle is 1 to 1.25, then the motor is running at 6000 rpm for a 7500 rpm spindle speed. Now I have a source of 100 Hz. Likely the motor is not as well balanced as the spindle.

From the magnitude of the unbalance I added to the spindle to get a good solid 125 Hz signal I do not think that a minor cutter unbalace would have any effect on my problem, and unbalance does not correlate with the 25 Hz.

On my above assumption, the 1.25 ratio of gears, the motor is running at 100 Hz for 7500 rpm. 25 Hz is a subharmonic of this and it might originate in the motor control.

If I had a small speed variation in the motor could it produce the surface pattern I get? I have no idea.

A cage velocity of 1/5 would be a far better explanation, but then the surface finish problem should not drop away around 6000 rpm.

.

Are you possibly experiencing something like belt slap? The motor/spindle drive is a toothed belt I believe; this could generate several fundamentals from the tooth spacing, the different pulley diameters and the different natural frequencies for the tight and loose sides and could give rise to beat frequencies that could come and go over a narrow rpm range.

And just a question for interest sake; have you done any tests with a lefthanded cutting running the spindle counterclockwise?

Could it possibly be just the vibration of the whole head on the Z axis rails. Is there some method to calculate the resonant frequency that the head would have?

It might be a good idea to take the spindle motor out and test it alone. Might be time for some new bearings in it anyways :)

Just another shot in the dark but could it be a hiccup in the carrier frequency of the drive to the motor. Thus slower rpm equals small change in each pulse of dc so hiccup is not seen at higher speed the change in pulses of dc is greater which might make the hiccup stronger. This hiccup might translate into a ever so slight change in chipload which would cause a mark on the part.
Just a thought.

JP

050830-1943 EST USA

Geof:

The HAAS serviceman tightened the belt with no change.

My unloaded tests on spindle rpm were made with a 100 tooth encoder on the 3/4 test shaft at 7500 rpm. I first counted down by 5, then 2 using a 7490 to generate a balanced square wave. Then I used the Tektronix expaned sweep and looked at the tenth count. There was no jitter. Next time I will count by 50, then two and average for 4 minutes, then do the FFT. This should do better than my viewing the scope.

I currently do not think there is an unloaded belt or drive train problem.

Next time I need to measure from the cutter as I previously mentioned. Then I can get data while cutting.


Hu:

That is a thought. I tried tapping the head with a soft hammer and the damped frequencies generated were higher than 25 Hz. But this may not represent the head on the rails. My guess is that the head on rails will resonant lower than 25 Hz, there is a lot of mass in the head.

When I had the accelerometer on the vise I tapped the coolant catch tray attached to the X Y table and got as much signal as when I was cutting the sample material.


JPMach:

This is the area I keep thinking about. The power meter I used to monitor the moter was a three phase unit using Hall devices. This does not have the bandwidth that is really required, but it was interesting and I did see a low frequency component. I can not run it at 7500 rpm because it is saturated with just the spindle power losses.


I will report back when I get some photos on my web site.

.

I would side with Hu....I suspect an imbalance in a rotating part.

050901-1449 EST USA

Hu and ViperTX:

An unbalanced member rotating at constant speed will produce a sinusoidal force vector at one fixed reference angle. At any other angle relative to the reference angle the vector magnitude will be the same but the timing will have a phase shift.

A tooth on an end mill will produce a force pulse which is not a sine wave. It roughly can be defined as an impulse.

Since at 7500 rpm and 30 in/min I see what appears to be a 25 Hz variation on the surface. This won't result from an unbalanced member unless it is about 1500 rpm. There is nothing in the drive train that results in a frequency this low unless it is the length of the belt. The gear box idler shaft is lower than 6000 at an output of 7500, but not as low as 1500.

However, as Hu has suggested does something resonant in the 25 Hz range that can be excited by a higher frequency source.

Hu's comment has forced me to try to find head resonance that might relate to the finish problem. I was wondering where I would find a suitable exciter. Then it became self evident even though it is not the best.

25 Hz corresponds to 1500 rpm. Thus, if I use my heavy weight unbalance test detail, then I can run the spindle at various speed and produce a sinusoidal force excitation above and below this 25 Hz. A resonant circuit, whether electrical or mechanical, will have a higher amplitude at resonance than on either side.

I am in the process of running some tests. So far I have found little at 25 Hz, but a relatively large 20.032 Hz signal that does not change with motor rpm. The various frequency components are different, for the same excitation frequency, between vectors in the x axis direction vs y axis. The 20.032 Hz signal is almost always large, but is it some unrelated instrumentation artifact.

At an excitation in the 1000 rpm range I have generated a very large 5 Hz component. Later I will supply the exact rpm.

.

Your first post has this sentence: "The side milling surface is identical whether X or Y axis is viewed." Which I take to mean you see the same effect whether you are passing the cutter along the X or Y axis. If this is the correct interpretation then if the source is a resonant head vibration it implies the vibration has the same frequency and amplitude in both directions. That would be surprising. Possibly I am not following things correctly.

On another tack; have you tried boring a hole with a single point tool? Granted it might be difficult to get the tool dynamically balanced but this may not be essential. You bore down taking a very light cut then retract rapidly with the spindle still running; the tool trace during retraction will not be even if the head is vibrating.

050901-2056 EST USA

Geof:

When I initially said the effect was the same in both X and Y this was not a precise statement in the sense of the greater analysis I have done cutting along the X axis. Looking at the part that started this analysis the patterns look very similar. I count 5 cycles per 1/10". That is 25 Hz at 30 in/sec, and I do not think that I am off by 20% to get to 20 Hz.

The resonant frequencies of the head are unlikely to be the same in both the X and Y directions. And the measurements I have made show different patterns.

I have revisited test speeds above and below 1500 rpm and nothing significant occurs at 25 Hz. Interesting that at 1500 rpm I got a very large spike at 0.208 Hz ( one cycle every 5 seconds ). Nothing like this at +/- 1% of 1500. The rest of the spectrum at 1500 rpm had no noticable spikes.

No I have not tried a hole boring experiment.

.

050905-2104 EST USA

Have run a lot of tests.

Does not look like a 325 vdc supply coupling between vector drive and brushless servos is our problem. The 325 v is fairly clean.

It is very clear that there is a 25 Hz modulation of the surface finish at 7500 rpm.

I have now created a crude mechanical exciter for 50 hz down. With the machine unpowered I can generate a strong resonance around 50 Hz and some other frequencies. This is providing an excitation in the x-axis direction directly to the table. The excitation is sinusodial. The accelerometer is mounted on the vise sequentially in both the x and y axes. I do not have a triaxial accelerometer, thus separate runs are required.

It appears the cutter is exciting the lower frequency resonances. Possibly the 50 Hz resonance is not dominate from the 7500 rpm input because this is a 2.5 divisor, rather than an integer divisor.

.

So what is the solution, do you think? Retorquing the X guideway and ballcars?

Are you saying it is the table behaving like a big tuning fork? Let me try another stab in the dark. I bought a Super MiniMill when they first came out (mid 2002 I think) and it had the annoying habit of the servo motors oscillating. I noticed the noise first and then found I could feel the vibration by placing my hand on the table or spindle. Sometimes I could stop the oscillation just by twitching the handle jog and othet times I banged the offending axis with a plastic hammer. I found I could either stop or start the oscillation depending on how hard and in which direction I hit. The machine was still in warranty and I am very close to the local Haas dealer so I was able to coerce them into sending a technician around many times to fiddle with parameters and eventually we found a sweet spot and it stopped. Actually it did not stop completely; sometimes after handle jogging it would give a little buzz and then stabilize. I have no idea of the true frequency but the buzz was comparable to AC hum; actually at first I thought it was a noisy fluorescent ballast. This puts it in the correct region for your phenomenon; the untrained human ear is wonderfully incapable of distinguishing between even frequency multiples. I remember the technician mentioned he had encountered other machines with the same thing and all of them had high speed rapids.

050906-2049 EST USA

Geof:

Any mass and spring produces a tuned circuit like and electrical RLC circuit. So yes it has the same result as a tuning fork. A tuning fork is designed to have one major resonance. However, in one physical structure such as the CNC machine or even just the X-Y table mechanism there may be many resonant frequencies. That is the case here.

I have suspected the servos might be causing the surface finish problem, but now I am leaning toward just a mechanical problem. But I have to learn much more and maybe my present ideas may change.

It is interesting to note that HAAS was not able to stablize a new machine for you.


Hu:

Last night's tests were with the machine without power on anything as I mentioned. Thus, I am looking at a pure mechanical system. What is the determining spring constant for this frequency I do not know yet.

My exciter is working quite well. However, I need better frequency control and a somewhat higher frequency capability.

It is beginning to look like the X-Y table has a major resonance around 50 Hz.

Tonight I setup excitation in the Y axis direction and I got a higher response at 50 Hz in this direction than in the X-direction.

I do not think we want to tear into the machine yet. Lets see if our HAAS App person will respond with known information on expected resonances in the table area.

Also I have seen much higher resonant frequencies, in the kiloHz range.

Another observation when I was doing the cutting tests at 7500 rpm was a variation in overall amplitude as I passed thru the 5" cut path and then not cutting for about 12". Total X travel was 1" no cut, 5" cut, and 12" no cut all at 7500 rpm and 30 in/min. The 5" of cut was the largest amplitude. The 1" before, and about 2" after the cut is lowest amplitude. The center of the Kurt vise is located about 15" from the left edge of the table. After said 2" the amplitude gets somewhat larger. Also a large low frequency component, about 1/5 Hz.

.

hi gar,

I have been following you post with interest (and geat difficulty ;) ) most of it is well beyond my scope of knowledge. But I was rereading the post and noticed on the firest one the surface finish variations were in the micron range. Could you post some pictures of the finish????

As I personally would like to see what someone with your advanced knowledge of harmonics and the like would spend so much time trying to correct. I personaly think you probably have done more work than haas has with machine diagnosis! I would be supprised if any normal haas technician could even come close to answering any questions for you and I doubt if the run of the mill (use a pun go to jail! ;) ) engineer would either.

ps: just trying to make a verbose post shorter :D

050907-0817 EST USA

miljnor:

Yes I believe the variation in the surface finish is on the order of a micron (0.000,04"). We do not have a way to measure the variation, but optically it is not satisfactory. Visually a customer looking at this surface would not consider it satisfactory. We have four mills, the newest VF3 of 98 is our problem, and only this one has this pronounced effect. Two mills are VF3's, oldest is VF2, and newest is VF0.

We may have a torsional vibration as distinguished from a linear one. The next experiment is to block the Y-axis to change the spring rate and see if this moves the frequency. It is hard to load enough mass on the table to produce a significant frequency change that I can be sure of cause and effect.

At some point I will provide some pictures.

.

Just out of curiosity how 'old' is your '98 VF3 in Feed Cutting hours? In an earlier post you mentioned this phenomenon has been becoming more pronounced over time and I wondered what your time period is.

I also had a thought about your weight on the table; you have a lot of room inside a VF3 to put weights on a long arm to perturb the angular moment of inertia.

050907-1150 EST USA

Geof:

Our machines are not heavily used. These are not production machines. Feed time is very low for the machine age.

The judgement that surface finish has changed over time is very qualitative because one does not know ahead of time that they need to make tests for reference for this type of problem.

I like your torsional idea. I have moved the table around with no significant results to point in a given direction yet. The long moment arm may tell me something. The exciter is very useful, but needs development to make it easy to position. Using the exciter instead of the machine head eliminates a lot of erroneous signals. It also allows changing the direction of the forcing vector.

.

"... because one does not know ahead of time that they need to make tests for reference for this type of problem..."

Hindsight is very accurate and I suppose if one wanted to be as accurate with foresight the first thing to do on a new machine would be to machine some squares, circles, flat faces and interpolate a few holes all at known machine settings and keep them for future reference.

If it is a factor of resonance, couldn't you try to change the resonant frequency of the table and see if it has any effect? Have you tried maybe bolting something heavy to the table to see if it changes anything?

Just an observers thought.

As an extension to the weight on a lever arm idea there is a technique used for seismic stabilizing on tall building which uses heavy masses at the top of the building attached via elastomeric blocks. The idea as I understand it is to have the resonant frequency of the mass and the attachment system an odd multiple of the building's natural frequency so you get destructive interference with any excitatory oscillation. You could build something and test it against your exciter.

A similar principal is also used in long boring bars on lathes; the bar has a hollow head which is almost filled with leadshot and this damps resonances over a wide frequency range preventing chatter building up.

So you get to put a beanbag in you mill! what a ride! :D

050908-1335 EST USA

It is called a "tuned damper" in the automotive axle business, but seldom used because of cost. If placed as a cyclinderical unit on a drive shaft it can be quite effective.

Ford never solved a 70 mph resonance on the Aerostar 4 wheel drive. Dana claimed to have a gear set to solve the problem, but Ford Sterling would not consider this. Nor did they implement a tuned damper. Dana had destroyed all the samples prior to my realization of the noise problem. Otherwise I would have tried to get a sample set. Some people never hear gear noise, but others, especially ones that test axles, never hear a quiet axle. Whether a tuned damper would have solved the Aerostar problem I do not know. A change from a steel drive shaft to an aluminum one did not make any significant effect.

My contacts at Sterling said the problem was never solved. I had the axle rebuilt 3 different times, and last an entire new axle was installed. Virtually no change.

Axle and gear noise is one of the major prolems in building axles.

.

I forgot about the axle dampers. I live on the west coast in an earthquake zone and years ago I worked on big lathes boring big holes in sprockets, hence my focus.

Well, for starters you problem is related to the fact that IT'S A HAAS! Your trying to squeeze water from a stone here pal. Pick one of the two-

cheap machine
accurate & smooth

You can't have both. If you want a smooth spindle take a look at a Mori. The windings are directly around the spindle. Makes the Haas feel like it's got gravel for bearings.

p.s. I run a few Haas, one of which is a brand new vibrational POS. .1 mils/p at 12,000 k , and they call it " still within spec."

I am a HAAS man and with what I've read in this post (most goes over my head) I would tend to agree with Mishikwest (as much as it hurts me to say it). I will be realy supprised if you guys find 1 cause to end the problem your having.

But on the positive side what you guys are doing is probably going to benifit HAAS in a good way as I doubt they go this indepth with their testing. So if you do find whats causing your issue Gar I bet it will be a great insight to their machining and build process at the Factory!.

now us who don't have any constructive ideas should probably stop posting to this thread! ;) specialy the "purchase a better machine" posts" everyone already knows you get what you pay for and most people (me included) don't realy want to hear it.

050912-1351 EST USA

mishikwest:

We have four HAAS mills. Only one of these has this particular surface finish problem. For many applications a HAAS is an excellent machine. In some ways it may be better than more expensive machines. In fact the HAAS control interface is vastly better than most, if not all, other controls.

I have no evidence at all that our spindle is the problem.


miljnor:

In any problem the real necessity is to figure out the correct questions to ask. Your responses, whether you know the answer or not, are useful because they either cause me to reconsider a question, or make me think of new questions.

At the moment I think I have learned more about the machine, but nothing I want to report on now.

.

050923-1908 EST USA

There are lots of resonances in our machine, and gaging artifacts from my sampling rate and filtering. I have gone to a higher sampling rate and more filtering to reduce the artifacts. An artifact is something generated by the gaging system that does not exist in the item being gaged.

It is fairly clear that we have strong accelerometer signals at once per revolution (the spindle), 100/125 of once per revolution (the spindle drive motor), and approximately 1/2.3 of once per revolution ( this I believe is the spindle ball bearing cage velocity). Strong only means that these signals standout from the background. It does not mean these are strong enough to cause surface finish variations.

Spindle speed is basically rock stable under our test condition while cutting with no evidence of any modulation anywhere near 1/5 of the spiindle speed. Our cutting load is very light.

Remember from previous posts I am looking for a problem that causes variations that relate to something about 1/5 of spindle speed.

From tests so far I can not draw a logical conclusion for the source of this 1/5 effect.

Today I created a new interesting test. Changed the test cutter to an essentially new high helix carbide 1/2" cutter. This has 1" of useful cutting length, and a 1" pitch for the helix. The cutter has very few nicks. (edit 050923-1945) The pitch on the 1/2 cutter is 2". (end edit)

The test sample is 2" x 0.5" 6061 several inches long. Three tests were run at 7500 rpm ( our max speed ). These were 30, 120, and 240 in/min. The side cutting contact was 1" and the cut depth was 20/1000".

We know there is once per revolution cutter runout. That is not what we are investigating, because it mostly visually disappears at 30 in/min and 7500 rpm.

The 120 in/min provides a very interesting pattern. There are vertical lines spaced 16/1000" apart. This you calculate from 120 in/min = 2.0 in/sec, and 7500 rpm is 125 rps. So the pitch is 2000/125 = 16 or 16/1000". These are verifiable with a scale and magnifing glass.

Someplace vertically you will see the vertical lines cross over to a mid position of the first said vertical lines. The cross over points occur at cutter rotation angles of X+0 and X+180 degrees where the radial distance from the true centerline of rotation of the spindle to the cutter edge is equal for both cutter edges. This is a neutral point for runout. This of course is for a cutter centerline that is not parallel to the true axis of rotation. The vertical position of the crossover is a function of the angular position of the cutter in the tool holder.

This is an interesting way to do a qualitative evaluation of runout.

On our machine being investigated we can see a vertical motion of the crossover pattern approximately every 5 revolutions.

So I am leaning back toward spindle bearing cage problems, but I can not explain 5 vs 2.3.

It would be interesting if others ran this same test.

.

It would be interesting if you could find some way to hook an dc alternator up to the spindle, and see if there was a power drop or spike every 5 revs. At least then you'd know for sure whether it's a spindle power issue or a tracking issue. It would tell you definitively if the problem existed even when you wheren't attempting to move the head at all.

These are the test conditions? "The test sample is 2" x 0.5" 6061 several inches long. Three tests were run at 7500 rpm ( our max speed ). These were 30, 120, and 240 in/min. The side cutting contact was 1" and the cut depth was 20/1000"."

How many is several inches? I assume you have the material held with the 2" dimension vertical.

I will see if I can get my VF-2 and VF-0 free for me to play with sometime during the next week or two. VF-2 is 10krpm, VF-0 is 7.5k rpm. I will also play on one of my Super Minis with a 10k spindle.

Can you take pictures including close-ups with a macro lens?

050923-2122 EST USA

Chris Welton:

The spindle speed test was with a special encoder that I made to fit on the tool holder. This consisted of two angular 90 deg slots at approximately a 3" diameter. This provided a double frequency square wave.

Looking at this on a scope before, during , and after cutting it was rock stable. Also FFT analysis of this signal showed nothing but the double frequency signal.

.

050923-0924 EST USA

Geof:

I used scrap 6061. Yes the 2" is vertical, and 1/4 thick would be ok. This is positioned so 1.25" is above the vise jaw. This easily provides for the 1" cut. Thus, the 2" stock width could be anything to allow gripping in the vise. Length --- 2" is more than adequate. The gripping space I use to identify the machine and operating conditions. The 7500 rpm 120 in/min gives a good pattern.

Pictures I am working on. The angle of lighting has a lot to do with how the 1/5 once per rev shows up, and 30 in/min is better to view the 1/5th pattern with your eye.

(edit) We use climb cut so when you look at the milled surface time runs from bottom to top, and from right to left. At 240 in/min and with a square you can see the pattern tilted to the left at the top as you would expect. In a sense the pattern is an oscillogram vs time with scan lines from bottom to top.

At 7100 rpm vs 7500 rpm I have a synchronous bump in the nearly vertical lines, both at 30 in/min. This bump may be on the order of 2/1000 to 4/1000. This was present throughout an 8" test cut. Its vertical length implies a duration of 1 ms or less. It is precisely synchronized with once per revolution. (end edit)

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Gar,
Have you run your battery of tests on one of your "good machines"? It might be useful to compare and rule out whatever is the same about both the good and the bad.

I still think that no matter what machine is running, they all have certain resonances. Whether they actually build up or not could be due to miniscule differences in fits of the slideways, etc. Think how far the spindle is from the Z axis linears. There is a lot of leverage. Even .0001 inch looseness in the ball cars is going to translate into some measurable deflection out at the spindle. When something is "loose" in this manner, its pretty difficult to determine when and where it's going to wobble, and if the wobble happens to coincide with certain cutting conditions.

050923-1026 EST USA

Hu:

I have run limited tests on other machines. This machine, our id 103, is mostly available to play with because of the surface finish problem. We run our machines considerably below 100% capacity on average so playing with it is no big problem.

To tie up the other machines is not feasible, but limited testing can be done. I am later today going to do this pattern test on our oldest machine. Whether this test pattern approach is going to be useful I do not know yet. One has to learn how to interpert this data.

The big problem with resonances is that I can not find something that associates with the effect I see on the pattern.

.

If it wasn't such a PITA to set it up, an FEA would be nice - I wonder how involved it would be for HAAS to do that for you or if they even would entertain the idea? Or do you think this is beyond the current state of FEA capability?

Scott

050923-1408 EST USA

mxtras:

I can not get HAAS to tell me what the cage velocity is on the spindle bearings. Thus I doubt any FEA information would be available because that really would be proprietary.

From my excititing the machine I can not find anything to realisticly correlate with the surface information. But that brings up an idea for the back burner. If I ran my exciter while machining then would it be sufficient to affect the surface?

.

I really, really wish I was there in person to participate in this. I am far from an expert on anything that you are doing to find the problem but I love working around people that are wise enough to know they don't know.

I wonder if you could cancel out the annoying frequency with an opposing oscillation from your exciter....hummmm...

Scott

050923-1945 EST USA

My 1/2" cutter has a pitch of 2", not 1".

.

050926-1615 EST USA

mxtras:

When trying to solve a problem one has to ask many questions, and one has to ask the correct questions, but one may not know what the correct questions are. Thus, you need to probe and go around the tree 20 ways.

There are ideas that I have tried on the surface finish problem. Some from participants here. Some I may reject out of hand, but may come back to later.

I have a hangup on the 25 Hz or approximately that value. It does does not seem to be a stable value. Additionally to get cancellation one must have phase correlation. So far even though the pattern on surface finish seems to imply the 25 Hz factor I can not yet find the source.

New things.

I switched to using a single flute cutter.

Ran some LVDT measurements --- resolution is better than 10/1,000,000" (1/4 micron) and RMS noise is about that level. On my plots 2 major divisions are 0.000,1" and full scale is 0.001".

I used a program that stepped an axis 0.000,1" from a zero reference to 0.000,9" every 500 ms, and then back. This was a repeating loop. To my surprise at each change of direction the move was about 0.000,2". HAAS had done a ball bar test on the machine and classified it as very good. But in reality that means it meets some maximum criteria, but it does not mean that the machine is the best that it can be.

Further tests with the LVDT showed that both X and Y have virtually no backlash. But in the HAAS parameters ( the values are their default values ) the backlash compensation was set at 14 for each of the axes. 14 is about 0.000,1". These I changed to 0 and now the step curve looks correct.

In the process I made a rough determination that when the servo motor is on that displacement along the axis of the lead screw is about 0.000,15" for 75# force. (edit) This is a spring rate of 500,000 #/in. This includes the thrust bearing, ball nut, lead screw, and mounts. Whether it is linear or not I do not know. (end edit) Thus, this becomes the spring rate for oscillation in that axis.

(edit) I found about the same spring rate for a side force (x direction) on the head.

These measurement are crude but are not in error by more than 10x or 1/10x. Thus, the spring rates is certainly witin the range of 50,000 to 5,000,000 #/in. (end edit)

Changing the backlash compensation had no affect on the surface finish.

However, on the X axis there was a low frequency oscillation of about 1/3 Hz with a peak to peak amplitude of about 40/1,000,000". This was a nice sine wave. After modifying the backlash compensation this disappeared.

Useful signal points are hard to access on the HAAS machine.

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Gar,
See your private messages.

Gar,
Having just recently gotten into the guts of my Haas, by opening up the holy of holies, or should I say the "spindle of spindle-ease" :D, I made a few observations that may relate to your analysis.

First off, this habit the setup people have of "shimming the cartridge" to correct the sweep of the spindle may bear closer examination. When I removed my spindle cartridge, there were 3 pieces of .001" shimstock clustered around one bolt.

In my mind, this could destabilize the spindle. Machine rebuilders go to great lengths to scrape a lathe carriage in to the bed so that it has so and so many points of contact and provide good bearing.

But in a modern precision machining center, we are happy to just "jack the spindle up and throw a block under it", tighten it down and run it that way! If the shimming is not properly spaced to create at least a 3 point support, then it is not properly done, IMO

In my machine, the effect would be to create a two point support: the shims and the far edge of the spindle flange. Now, tightening those 6 tiny bolts is not going to bend that flange! So either it will tip this way, or it will tip that way. When reassembling my machine, I will be sure to use two widely spaced shims to acheive decent 3 point support over a known footprint. I was even thinking of spreading a film of Loctite on the flange to really fill the gap.

Secondly, the fits of the bearings inside my spindle cartridge makes me wonder if your problem machine might have slightly loose fits that permits the spindle to shake. The main bearings in mine had a half thousandth of clearance. While this makes for dead-easy assembly, it sure doesn't seem like it would contribute much to spindle stability. One would have to rely heavily on the clamp effect of the spindle end cap to keep those outer raceways locked in place.

060603-1350 EST USA

Hu:

Thanks for the information. Good luck on your spindle rebuild. Even though we have a bearing expert next door I would not attempt a spndle rebuild.

Since last fall I have not done anything more on the surface finish problem. We have decided to use this machine for jobs not as critical on finish.

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Hu; I like your "throw a block under it" comment. I think adjustment with shims is a perfectly legitimate way to do final alignment on assembly but when this procedure is going to be used the least that can be done is have a selection of specifically prepared shims and also incorporate appropriate locations on the mating surfaces for the shims to seat. What you describe is just plain sloppy design and execution.

I just found this thread. I didn't see if the fit of the taper was ever addressed. If not, try using a good holder and some high spot bluing. Not the liquid or spray dykem. Use prussian blue. A thin thin layer on the holder. Clamp it in and unclamp. Look at the contact. Tapers wear over time by bell-mouthing. The big end gets bigger. I saw someone say lap it. The problem there is the lap wears as does the taper. Both things change a little. THe ideal way to fix it is to have the spindle ground in place on the machine. Gives the best fix possible. Perfect contact and minimal runout and no teardown of the machine. We fix problems like yours everyday. It surprises me sometimes the things we help. Since the problem has not been fixed yet, why not try it. Nothing to lose. Check us out.
www.spindlegrinding.com
Over 20 years fixing tapers.

Rocky

060603-1951 EST USA

Rockyr49:

What logical explanation do you have for a worn taper to produce a surface finish problem that has a relation of approximately 1/5 the frequency of once per revolution of the spindle. 7500 rpm is 125 Hz, and our surface finish problem is about 25 Hz.

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Gar:

Logical explanation? I couldn't tell you. I've never concerned myself with the frequency of the vibrations that we fix. It just seems that since the problem has been going on for almost a year and so far no fix, maybe a quick check of the taper could show something. I could be wrong. But for the last 15 years that I've worked for these people, I've been to everything from 2 machinie mom and pop shops to the biggest companys in north america and repaired problems such as repeatability, excessive tool wear and breakage, tools sticking, and yes finish.
2 Haas VF3s last week. '98 models. Over time the tapers wear and it does cause vibration. Haas said the drawbar was slightly below spec. How much? These 2 had 750 lbs. Should be 1400. The problem seemed to happen over a 2 year period if I read right. Tapers wear slowly. I'm just throwing ideas at you. Check if you want. But frequency? Can't comment on that.

Rocky

On our okuma, we had an odd noice. Couldn't really place it at first, but it turned out that a hydraulic er.. hose/pipe, was laying just ontop of the spindleoil box, which caused resonance and vibration of low frequency. Now, i don't know if your Haas have hydraulics, but it may be worth a check if it does.

Hello there, I'll keep this vey simple ! Your computation on resonance frequency may be a simptom NOT a cause ! concider anything causing a frequency may effect a weekness down line.

I am a milling Machine programmer using masterCamX. I have a couple of questions regarding 3d surfacing.
1) How to calculate scallop Height? ex. if milling a profile, material cast alum. using 1/4 ballnose. What is the Scllop height or stepover?
2) What is the diffident between Flowline and paralle?

060605-1902 EST USA

Cruiser:

Yes the approximately 25 Hz component that I see in the surface funish with the spindle at 7500 RPM and feed at 30 IPM with a 2 flute cutter is a symptom, and what I am looking for is the cause. But I am not actively working on this at the current time, rather we use the machine for less critical work. Just thinking about causes until more free time.


To everyone interested in spindles.

Today I had a vist from Walt, Jr., and Walt III of Spindle Grinding Service ( www.spindlegrinding.com ). Walt had contacted me and indicated that they would be in Ann Arbor and would like to stop by. Note under the name Rockyr49 Walt has posted some comments here.

They checked the tapper, tool clamp force, and runout on our problem machine. These measurements were all quite good, and thus no indication that any of these would be the source of the problem.

I am very impressed with Walt and Walt. They know what they are doing, have good technique, and tools. They have seen many problems with spindle tappers, and spindles. They can visually spot certain types of problems even without tools. Thus, I want to recommend them to anyone that might need their services. Their web page is short, but describes what they do.

(060605-2129 edit spell error).

Hey! Wait just a minute! I would never consider myself to be as smart as Walt himself. He is what we call the "guru" of spindles. I'm just one of his lowly employees. But seriously folks....we just do the best we can to help the customer understand the importance of the fit of the taper. We fix 'em to the best of our ability. And if we can teach you something new, even better. Thanks for the kind words. We all appreciate them.

Rocky

I am new to this foum good thread by the way. My buisness is manufacturing mountain mod and race snowmobile engines out of 6061. as a engine builder every shaft and crankshaft have a harmonic vibration and not always will it tell you the freqency it can fool you. What i have learned that one seadoo engine break the crank
at only 7800 rpm and it does vibrate but it is vilent and not per revoution but skips 4-5
rpm's the crank is doing the snake efect and then the crank pines will breake.
Try and check if all the legs of your mill have the same tourque .just a thought.

Art

Hello Gar and all included in this conversation. To my knowledge, Coolant type (flood or mist) or vacuum has to be mentioned yet. Am I a noob or is any of this in use?

A whale can communicate to other whales using frequencies that bounce off the ocean floors miles and miles away. Liquid, depending on density can allow or deny this- a whale cannot communicate in Mercury (easily.)

Can you experiment with various coolants, or at minimum use a pool to verify spindle inconsistencies via ripple?

Hello Gar,

I am also pretty new to this forum and have just read this thread from end to end; quite a lesson in practical diagnosis! The thought that occurred to me, though, was: maybe the reason you are having a hard time finding your 25Hz source is that it's not there. What you are seeing is the "beat" caused by constructive interference between the 125Hz of the spindle and the 100Hz of the motor. As every musician should know, the beat frequency is equal to the difference between the two frequencies that cause it. See the attachment I knocked up quickly in Excel to demonstrate - the red curve is just the sum of the other two. Note the strong 25Hz modulation. There is obviously some other part of the structure which is resonating at this frequency, but maybe only as a harmonic of its base freqency.

It sounds like your motor is causing more vibration than it ought to, and I suspect that by addressing that - replace bearings, rebalance motor spindle, whatever is needed - your 25Hz resonance will be reduced as well.

060610-1525 EST USA

Rabies:

If the approximately 25 Hz component at a spindle speed of 7500 rpm is the result of the difference frequency of the motor and spindle, then as I lower the motor speed I should see a related difference frequency in the surface finish. There is some evidence of this.

However, for there to be surface finish variations there has to be physical motion of something at the observed frequency. This motion then has to be associated with an acceleration signal of something at that pattern frequency.

Side subject. --- I should note, if I already have not, that in one experiment I observed a low frequency motion of the table in the 1 Hz or lower range when under cutter load, but not when the cutter was away from the test part. This was one straight motion in the X axis. This is a very strong acceleration signal easily observed on the acceleration vs time plot even without using FET analysis. But this is of such a low frequency that it is not visible in the surface finish with the other variations that exist. Most likely this is the X axis servo response under load.

On my problem surface finish I get a much more definitive signal in the surface finish than in the accelerometer measurements.

At some time I plan to look at the belt as a possible source, and the motor mounts. At the moment, as I said a while back, we are just using the machine for less critical purposes. Most tests were run in the X direction.

The recent tests by Walt, Jr., and Walt III of Spindle Grinding Service ( www.spindlegrinding.com ) did not show anything wrong with the tapper or tool clamp force.

.

OK, it was just a thought -- I'm no expert, and I wasn't sure if your analysis would pick up components like this - I figured the surface artefacts could be due to a 100/125Hz vibration increasing and decreasing at 25Hz (so, for instance, cutting at 60IPM there would be more vibration at .040" and .080" into the cut than at .020" or .060", which would produce a markedly uneven finish.) But if you're looking at acceleration/time plots as well as FFT results, I suppose you would have seen it by now :( Oh well, good luck in your search anyway!

I'm digging this thread up from the past hoping that there was some positive outcome. We've got a Toyoda fa400 horizontal mill that is doing the same thing. Finish cuts are being run much, much slower than they should be because the finish is so bad running at any kind of speed. We've spent $1000's with toyoda and even had their spindle tech come out and check out the spindle with no luck. After seeing the tests done here, i'm pretty confidant that calling a service tech would result in nothing more than wasted money.