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  1. #21
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    Default Re: Adjustments for accuracy?

    > How does approximately 200# (in-lb? ft-lb?) at the stepper relate to force at the cutter...?

    I was referring to the force driving the table into the cutter (lbs), a portion of which deflects the cutter. I'm totally wrong on the 200# number, here's the reference for the 1100 that says it has 850# (Technical Support FAQs | Tormach Inc. providers of personal small CNC machines, CNC tooling, and many more CNC items.), so the 770 should be in that neighborhood.

    --Bryan



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    Default Re: Adjustments for accuracy?

    I am very interested in the outcome of this thread.
    The Tormach has a bit of flex as its not a large machine, so therefore it wont be as rigid as one.
    All cutters flex, my reason for using the shortest fattest one possible/practical.
    side milling will accentuate the problem.
    there is a reason finish cuts are called "spring passes"

    It isnt surprising to me that the bottom of the square boss is wider at the bottom than at the top, the reverse is true for a pocket of equal depth.

    I put an indicator in the vise, indicated a 3/4 piece of shafting in the spindle about an inch and a half down, with hand pressure of about 10 / 15 pounds I could flex the machine .0002 to .0003 in X, I didnt check Y.

    my opinion, and everyone has one of those, is that a lot of the things mentioned will make it better but never eliminate it completely. Hopefully you can get it down to where it will meet the specs you need for this job............

    mike sr


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    Default Re: Adjustments for accuracy?

    Quote Originally Posted by cncoperator View Post
    How does approximately 200# (in-lb? ft-lb?) at the stepper relate to force at the cutter...?
    The Tormach Series I and II PCNC1100 have X and Y steppers with 640 inch-ounces of holding torque and the Z axis stepper has 1208 inch-ounces. To convert that to holding force at the table in the X or Y axis direction or the head in the Z axis direction, you simply use the lever principle. Assume a ball-screw with a 1 inch radius for convenience (the actual radius or diameter does not matter since the result is independent of ball-screw radius or diameter). In one rotation of the ball-screw a point on the outside of the ball-screw will move 2*PI*1" while the ball-nut advances 0.2" (all of the Tormach ball-screws are a 5 pitch). So the leverage is 2*PI*1/0.2 = 31.4159... Since we selected a 1" radius and the torque is given in inch-ounces that conveniently translates to 640 ounces * 31.4159 = 20,106.19... ounces, which is about 1,257 pounds of holding force. The moving force will be less than holding force since the motor rotor has to move away from its previously preferred position towards a new preferred position and the magnetic field covers a longer path until the rotor actually reaches the new preferred position. This results in a reduced moving torque, which then remains approximately constant until the back EMF starts reducing the applied winding voltage to where the winding current starts decreasing due to the stator winding inductance. The ball-screw is also typically around 90% or so efficient. Using 90% efficiency for the ball screw and 75% for the running vs holding torque at an upper mid range RPM results in 848 pounds of force on the X and Y axes and essentially twice that for the Z axis. Tormach's stated 850 pounds of force is a very safe and conservative number.

    I do not know the torque ratings for the Tormach Series III steppers since I don't have the data sheets for those motors. However, Tormach states that the Series III motors and drivers produce higher torque at both lower and higher speeds than the Series I and II versions.



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    Default Re: Adjustments for accuracy?

    All this math is great, but it's pretty obvious that a 5 thou finish pass doesn't actually bend the tool by 2 thou, unless there was something very wrong about the tool holding practices involved. There's certainly a number of factors contributing to the taper, and the Z gib was evidently one of them.

    I look forward to seeing the results of your continued experimentation, Bryan.

    Remember, the goal is to eliminate taper and make the X and Y axes equal in their size deviation. Once you have that, cutting accurate parts is as easy as adjusting part program coordinates, or better yet incorporating cutter comp into your toolpaths.



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    Default Re: Adjustments for accuracy?

    So let's start here:
    Quote Originally Posted by flick View Post
    There's certainly a number of factors contributing to the taper, and the Z gib was evidently one of them.
    Please read my last post and explain to us how the Z gib can be responsible for any significant amount of this taper without massively affecting the dimensions of the part. Bear in mind that the original post listed tolerances at the top of the test piece being "only" 4-5 thou oversize.
    Step



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    Default Re: Adjustments for accuracy?

    Quote Originally Posted by TurboStep View Post
    So let's start here:

    Please read my last post and explain to us how the Z gib can be responsible for any significant amount of this taper without massively affecting the dimensions of the part. Bear in mind that the original post listed tolerances at the top of the test piece being "only" 4-5 thou oversize.
    Step
    I read your post, and I'm aware of the sizes involved, and I understand the math. I don't think you're necessarily wrong, but there are certain assumptions involved that may or may not be correct - assumptions such as the fulcrum being halfway along the gib. So my explanation is that the head might be tipping a lot closer to the tool than was previously assumed. Maybe even right at the bottom of the gib, rather than in the middle.

    Now if you think the taper is caused by tool deflection, then why don't you explain how other machines have been making nice straight walls all day long with the same type of tool and cutting conditions for as long as there's been mills and cutting tools?



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    Default Re: Adjustments for accuracy?

    Quote Originally Posted by flick View Post
    ...why don't you explain how other machines have been making nice straight walls all day long with the same type of tool and cutting conditions for as long as there's been mills and cutting tools?
    Sure:
    http://www.cnczone.com/forums/tormac...ml#post1966012
    You'll find the recipe at the end of the post.

    Bryan's calculator indicates feed per tooth at 0.0023 but the actual chip width will reduced by chip thinning to a fraction of this. A new, sharp cutter could manage ok but a cutter that has seen better days may just burnish the surface.
    Step



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    Default Re: Adjustments for accuracy?

    Quote Originally Posted by TurboStep View Post
    Sure:
    http://www.cnczone.com/forums/tormac...ml#post1966012
    You'll find the recipe at the end of the post.

    Bryan's calculator indicates feed per tooth at 0.0023 but the actual chip width will reduced by chip thinning to a fraction of this. A new, sharp cutter could manage ok but a cutter that has seen better days may just burnish the surface.
    Step
    Yeah... especially if the ways are so sloppy that they aren't rigidly pushing said tool into the cut.

    Look, you seem pretty married to your idea, so if you think gib adjustment is "insignificant" then feel free to loosen them up on your machine and make chips to your hearts content. I'm quite ready to agree to disagree, so I'll stick to my original contention that there are a number of factors (including the gibs and, yes, tool sharpness and, yes tool rigidity) that all contribute to the tapered walls.



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    Default Re: Adjustments for accuracy?

    Why does anyone need to explain anything? Lots of smart dudes here. The OP can take the info and figure out what works IMO.



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    Default Re: Adjustments for accuracy?

    Hey folks, sorry for the absence!

    I'm not sure I did any favors to my machine by adjusting the X & Y axis gibs...

    X-Axis: Backlash 0.0005", no change from previous.
    ---------------
    So this was interesting, I kept tightening the X-gib (left side screw tightening, right side screw loosening) with no change to the backlash until I couldn't screw the left side in any tighter. Was my gib installed backwards? It's easy to get to, so I might try going the other direction to see if it changes anything.


    Y-Axis: Backlash 0.0007", down from 0.0009"
    ---------------
    Y-gib is seriously annoying to access! Got it dialed in, about 3 turns tighter than it was before.


    Test Cuts:
    ---------------
    Square #1: X 0.7537, Y 0.7525
    Square #2: X 0.7540, Y 0.7522

    :-(

    Well, at least it's consistent for these two test pieces.

    Any suggestions?

    --Bryan



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    Default Re: Adjustments for accuracy?

    So re-run your CAM software with your tool diameter 0.006" undersize. Voila: 0.751x0.749 blocks. In spec.

    Maybe the thrust bearing on your X axis is loose? Maybe both X & Y?

    Current version of the manual on page 166, Section 9.3.4, has instructions for "Angular Contact Bearing Preload Adjustment". If this is insufficient, than you would see what you are seeing.

    I don't see this mentioned in the discussion above.

    Tim
    Tormach 1100-3 mill, Grizzly G0709 lathe, PM935 mill, SolidWorks, HSMWorks.


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    Default Re: Adjustments for accuracy?

    Quote Originally Posted by Bryan Turner View Post
    ---------------
    So this was interesting, I kept tightening the X-gib (left side screw tightening, right side screw loosening) with no change to the backlash until I couldn't screw the left side in any tighter. Was my gib installed backwards? It's easy to get to, so I might try going the other direction to see if it changes anything.
    --Bryan
    Brian, you can't put the gib in backwards. It won't fit. Tormach had originally stated that on the 770, tightening left hand adjustment screw and loosening the right hand screw tightened the gib. It is in fact the reverse. I pointed that error out to them during their second open house in a maintenance seminar. It has been corrected in the new manual.

    R J



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    Default Re: Adjustments for accuracy?

    CountrySmith,

    Thanks indeed! You're right, I was working from the original manual. Just downloaded the new manual and it has the sides reversed. Well, at least that's the easy gib to get to!

    tmarks11; I'll check into the bearing preload next. Need to find a pair of spanner wrenches..

    --Bryan



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    Default Re: Adjustments for accuracy?

    Quote Originally Posted by Bryan Turner View Post
    CountrySmith,

    Thanks indeed! You're right, I was working from the original manual. Just downloaded the new manual and it has the sides reversed. Well, at least that's the easy gib to get to!

    tmarks11; I'll check into the bearing preload next. Need to find a pair of spanner wrenches..

    --Bryan
    I bought my spanner wrenches at the Tormach open house two years ago. As I recall, they were under $20 for the pair. I also made a torque indicator which I used rather than adjusting by feel. I wrote it yup on another forum. Measuring Ball And Lead Screw Torque | The Hobby-Machinist - The FRIENDLY Machinist Forum

    If you can't find any locally, PM me and maybe we can work something out.

    R J



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    Default Re: Adjustments for accuracy?

    I think people underestimate how much flex there is when you're cutting hard! The tool deflection, machine frame, backlash, gibs, workpiece, fixturing, it all adds up. By leaving .010" nominal for a finish pass, I have been able to get +/- .001" consistently. Without that, it's all over the place.

    However, this is different than a spring pass, which I first learned about while boring on the lathe. You have a tiny boring bar reaching far into a hole and the further in it goes it gets pushed outward. Therefore you take a spring pass which is the exact same path twice, to make sure the hole is straight through and not tapered. The same principle applies to an end mill, but I can see getting a lot of rubbing on the upper part of the cut, no matter how sharp your cutter is. I wonder what the best way to do a spring pass is...



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    Default Re: Adjustments for accuracy?

    Quote Originally Posted by CountrySmith View Post
    I bought my spanner wrenches at the Tormach open house two years ago. As I recall, they were under $20 for the pair. I also made a torque indicator which I used rather than adjusting by feel. I wrote it yup on another forum. Measuring Ball And Lead Screw Torque | The Hobby-Machinist - The FRIENDLY Machinist Forum

    If you can't find any locally, PM me and maybe we can work something out.

    R J
    Using a torque wrench is a great idea. I bought an inch-lb torque wrench for this purpose when adjusting my own gibs. By tightening the gibs while moving the table with the torque wrench you can find the point at which the gib adjustment begins to affect the drag. I chose to do it this way because it's how I was shown to adjust pinion bearing preload in a rear axle. It yielded excellent results with the mill.



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    Default Re: Adjustments for accuracy?

    Quote Originally Posted by cobrakai View Post
    I think people underestimate how much flex there is when you're cutting hard! The tool deflection, machine frame, backlash, gibs, workpiece, fixturing, it all adds up. By leaving .010" nominal for a finish pass, I have been able to get +/- .001" consistently. Without that, it's all over the place..
    The OP had a 0.005" finish pass, and was still is missing the mark by 0.004" or so.

    Tim
    Tormach 1100-3 mill, Grizzly G0709 lathe, PM935 mill, SolidWorks, HSMWorks.


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    Default Re: Adjustments for accuracy?

    Quote Originally Posted by tmarks11 View Post
    The OP had a 0.005" finish pass, and was still is missing the mark by 0.004" or so.
    Oh yikes... didn't see that he had a finish pass.



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    Default Re: Adjustments for accuracy?

    Quote Originally Posted by Bryan Turner View Post
    >
    0.375" nominal diameter, 3 flute, HSS
    Overhang is about 0.8"
    0.05" DOC
    0.375" WOC
    6100 RPM
    42 IPM
    Do you regularly take such shallow cuts with all of your endmills? I'm not saying this is causing your taper error, but if you're wearing out the tips of your endmills, it's not going to help your taper problem. I would take the full depth of cut that your endmill is capable of, and adjust your WOC accordingly. For what you have above, I'd almost swap the WOC and DOC.



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Adjustments for accuracy?

Adjustments for accuracy?