Hi folks,

I've noticed that my table rotates/rocks slightly when I change directions while traversing the X axis. Okay okay, adjust the gibs right? I've posted on the shoptask delphi forum as well but despite some good advice I can't solve the problem.

Okay here is what I've tried...I pulled the gib strip and checked it with a parallel. I found the center was higher than the ends. I put a steel rod in the mill, blocked up the gib at both ends, and used the rod to push down in the center of the gib (gently) untill checking with a parallel showed the gib to be more or less flat by eye, looking for a gap or rocking of the parallel.

I sanded/polished the gib and looks like I'm getting about 80% contact. I reinstalled the gib with the locking handle totally loose. Then I would put tension on the adjustment screw and work the table in the +/- X direction while snugging the screw. I went back and forth between the two adjustment screws with this method.

The best I could do is a .002 variance between +/- X travel. To test this I chucked a steel rod in the lathe and attached my dial indicator to the table and put the indicator up against the rod. I used this while adjusting to see if I was making any progress. Even by tightening the gibs "a lot" I couldn't get the wobble out. It seemed that I could achieve the same .002 variance with light pressure on the adjustment screws and the table moving very smoothly.

At one point I cranked the locking handle down hard, loosened it and then moved the table +/- X. I was shocked to find that now I was seeing almost .030 variance. I went back and adjusted the gibs with "firm" pressure on the locking handle instead of loose. I can get back to .002 variance but no better. I was wondering why the ball turner I made slides perfecly in the slot and then gets stuck...this would explain it.

Any thoughts?! Thanks in advance. One thing I did notice is that if I look at each end of the gib (when it's mounted) one end is flush with the casting on the bottom and the other end of the gib rides higher than the bottom of the casting. To me this says the indents for the adjustment screws and the locking handle are not parallel to the edge of the gib...could this be the problem?

Probably a case of simple geometry of the BIG picture as opposed to a microscopic/focused look at the system.

At any individual point, 0.002" isn't much. But when you do some math over larger lengths, you see some interesing magnification issues become quite apparent.

0.002 clearance is 2 parts in 1000. Or zero clearance at X=0.000 and 0.002 at 1".

OK, now go out 12 inches. O.000" at X= 0.000, at 1" it is 0.002 but at 12" it beomes 0.024" - simple proportion when you use the law of Tangents.

What you have to do is slowly start lapping the gibbs over the full length of the table. As you remove the high spots and local deviations, you'll reduce the clearance without encountering localized binding.

Long steady process that takes time, care, patience, fastidious attention to details and strong, durable arms.

Before you try to lap the gib to fit, you might want to look
at what the gib slides on. The contact surface should be
flat. If it is not flat, and the gib is lapped to fit it, the slide
will be always traveling in an arc.
You might want to start with scraping the machine base flat.
Once this is accomplished, then blue up the gib and see what
it looks like.
I find it necessary to regrind my slides about every 6 or 8 months.
The gibs seem to wear to fit almost anything if it is straight.
If the machine base is not flat, it is difficult at best to adjust the
gib so it is tight enough and still not binding in the high spots.
Good Luck Walt

Hi guys, thanks for the replies. I've read that scraping ways should be done by a professional (read "not me"). As far as scraping the bottom of the machine...that sounds like an exciting project.

On a machine like the shoptask, should I expect .002 table movement or should it really be nil? Right now this seems like a pretty serious limitation and I don't see how you could effectively run CNC with that much slop in the setup.

I guess I'm wondering if other people have this problem, and if so, how they fixed it.

thanks again

The amount of slop you can tolerate determines of 0.002 is too much or not.

Yes, others do have the problem

Try rereading posts #2 and #3 for starter ideas...

There are only so many ways to boil a 3 minute egg....

I'm not sure if your X axis piggybacks on top of the Y, but if so, slop in the Y axis has an effect on the stability of the X axis. So you might want to check the Y gib, if you haven't already.

The trouble usually lies in the X/Y saddle itself, and/or the X-ways. If you can get a Machine straight edge, (borrow or beg), and lay it up againts the x-ways, and then check the X saddle ways, you might find your problem.

I had a simular problem as you described when I initally got the machine (sometimes the castings are bad and slip through the questionably QC at the china plant). If you machine is still under warrenty, you can get John to replace the X axis way, and/or Saddle if one or the other or both is off. For me, I hired a guy who does machine ways, and had him scrap/fix mine ran me about $200.00... But they are smooth now.

A "poor mans" way of cheaking the quality of your ways/saddle is tighten up your X axis way till you get a slight drag on the saddle around the middle of the machine, then move it right and left all the way to the endes of travel, "Feel" the X axis while you slowly turn it. If you feel it "bind" here and there then you know the fit is bad.

scott

Thanks again for all the advice. I'm moving in a couple weeks, so when I get there I will try and find someone to take a look at the machine who's got more experience than I do.

The Y-axis seems to be right on so I can slot in that direction but it seems pretty limiting if I plan to do anything that requires parallelism...like dovetails.

I tried getting the gib pretty tight and traversing the X axis to the limits. I get a tiny increase in drag at both ends...maybe more towards the headstock...but it doesn't seem that severe.

How exactly do you lap the ways? I'm having the same problem and need to do something. I'm developing grooves in my x axis ways that are not pretty. Do you use emory cloth or some type of stone? Is there a specific procedure that you follow? If you look at the picture it would seem that I need to surface my gibbs as well. My father told me about some type of metal epoxy that you spread over the length of the way then slowly sand it down until it's smooth. but he did not know much more than that.

Hi P40,

I'm sure there is a proper way to do it, but I don't know what it is. However, I can tell you what I did...for what that's worth. I just used some 400 and 600 grit wet sand paper and a little oil. I sanded the ways very lightly...more of a polish.

As for the gib strip, I clamped a strip of sand paper to the mill table, oiled it and ran the gib over it. I figured the table surface would be pretty flat. Even without using layout dye you can see where the paper is hitting the gib. I also checked the X axis gib with the edge of a parallel to make sure it was straight. Mine was bowed slightly so I bent it as flat as I could and kept sanding. Not ideal but better than nothing.

The tailstock gib was harder because you can't bend it at all. Just apply lots of elbow grease.

Of course this light hand finishing can't really account for more "serious" deviation or defect in the ways but it certainly helped smooth things out.

Here is what I found on the net. I guess I should have looked a bit more before asking a stupid question. I found two nice sites describing what to do. Looks easy enough. I purchased some lapping compund from ENCO for 13 bucks. I will try it this weekend when my shippment arrives. I'll let you know how it goes. I have already taken the Y-Axis apart. Here are some before hand pics of the gouges in the ways and gibb.

www.mini-lathe.com/Lapping/mt_lap.htm
http://home.inter.net/mthomas/Lapping1.htm

Well I will probably start a firestorm, but, don't lap your ways, you will only increase your problems, the action will be smoother but you will loose accuracy. Seriously, find a "Machine tool rebuilder", in or close enough to your area, and bring the X way, saddle, and Y way to him to have him hand scrape/fit them, you will be much, much more pleased!!!

Remember Lapping avereages out high and low spots in relation to each other but that does not equal Flat and properly fitting bearing surfaces.

scott

I don't understand. How do you lose accuracy from Lapping? I'll check to see if there is a machine tool rebuilder in my area and see how much he charges. However, don't you have to scrap the lathe bed as well? I don't see the sense in only scrap fitting the gibb and carrage ways in the Y-Axis and not the X-Axis ways in the lathe bed. I guess I don't understand what you are talking about.

Casey

NOTE:

My note says take the x-way, saddle, and y-way to your scraper.
that means your X base (lathe slides), saddle (the pivoting part between the X way and Y slides), and the Y slides (that includes the y table).

so the summery of the above is, bring the parts of the machine that slide againts each other to the scraper to be scraped and fitted together correclty.

Or not, your choice, Lapping (other than very, very light for finish), will not do what you want it to do.

I've also read in several places that a light lap will give you a nice glide but doesn't really do anything to straighten/true things up. Basically I lapped my ways and gibs so I could tighten down the adjustment screws a bit more. This helped some but deffinitely didn't "solve the problem."

Looks like I'll be looking for someone to scrape my ways as well. Is this something that all of you experienced Shoptask users have done/had done? I'm curious to know if I got a monday machine or if you just shouldn't expect that level of fit/finish on a shoptask. So, if someone could tell us novices about their experiences with scraping that would be great. Anyone know a reputable place in the Fresno area?

Hey Dahui, I have the same problem but mine is a compound sliding table that I,ve used for my manual mill project. I have also noticed that if the table is centered that it is within .001 but if I move it back it goes out by .0025. Side to side it stays within .001 but front to backit goes out. I also noticed that the way the gibs get tightened it can cause the table to go out. But I,m going to take it all apart and clean it up real good and see if that helps some. As far as lapping goes I agree with you a little isn,t going to hurt but I also agree with Poppabear that having the ways scraped is the way to go for precision fit. Unfortunetly scraping is an artform and you,ll be lucky to find a good scrape master near you.

I found a guy in my area that scrapes for 75 per hour. He also said that if the machine is just over a year old (I purchased it last year) then it probably does not need anything done to it. He told me to check the gib adjustment. I had already lapped the Y-Axis by the time we stopped playing phone tag. I did not lap it too heavy, per everyone’s advise, only enough to clean it a bit. It seemed to help, I'm able to tighten the Y gib down more and yet it has easier action. Here are the before and after pics.

Hi P40,

Any progress with your machine? I haven't had a chance to do anything with mine. I moved from the east coasst to the west coast and my shop is on a truck somewhere. If you are having any luck with the scraping I'll try and find someone in my area. Have you measured the amount of rock you get on your table? I just wanted to compare. I got mine down to about .002 (if I remember correctly...might have been .020)which seems pretty good but not good enough for mill work like slotting or cutting a dovetail. My Y axis is rock solid.

Has anyone else measured the table rock on their machine? Maybe I just shouldn't expect it to run that true...but I can't see how you could ever use this machine for CNC being as sloppy as it is.

dahui,

I have been busy as well. I have not measured my table rock yet, however, I did manage to make a few parts that shows the problem. I really need to get this fixed somehow. I have taken the gibs out and cleaned them and checked them for straightness. I cannot tell anything is wrong. Any thoughts? I guess I need to bite the bullet and pay for someone with experience to come take a look at it.
Here are some pics of the parts showing the mismatch between start and finish.

Yep, that doesn't even look close enough for government work. I'd be very happy with the machine if I can get it to cut straight. Right now it's still sitting in it's crate on the garage floor waiting for me to put the rest of the boxes away.

It seems like it would be a lot better if one of the ways was raised (like on a normal lathe) instead of just flat. Driving that table from way over on the side is just asking for rotation problems. When I get the thing unpacked again I'm going to take a surface grinder to the gib strip...though I don't think that's where the problem is.

Let me know if you have any progress.

Dahui,
I figured out something helpfull on my Tri-Power for reducing the table rotation around the z-axis. Having noticed the rotation when boring a hole on the lathe and then reading this thread I decided to try and quantify how much rotation my machine had and why it happens.

I set up a test indicator on a magnetic base on the table indicating on the outside circumference of the lathe 3-jaw chuck at the lathe center-line above the table so I could move the table in x and read the y movement.

Starting at the right edge of the chuck I moved the table left about .1 inch and zero'd the indicator. Further movement to the left showed no indicator movement as expected. Reversing the table movement to the right showed about .005 movement away from the chuck that then stayed at .005 with further rightward movement. Moving the table back in the leftward direction brought the indicator back to zero repeatably. The reason for this movement is rotation of the saddle in the x dovetails driven by torque from the offset lead screw reaction with the friction of the dovetails. Properly adjusted gibs SHOULD eliminate this rataion.

Note that when I measured the .005 displacement the left edge of the table was about 2 inches from the face of the chuck. The reason I point this out is because I believe the table is rotating about the intersection of a line drawn from the back left to the front right with a line from the front left to the back right of the dovetails. So if you move the table under the chuck and make the same measurement the displacement is much less because you are closer to the center of the rotation and the y component of the rotation is less because of the angle.

Now for the hopefully helpfull tip. When you move the table from right to left the offset acme screw causes the table to rotate clockwise. The rotation is limited when the rear right dovetail edge and the front left gib strip edge come in contact with the ways. At the same time, the rear left dovetail and front right gib strip are actually pulled away from the dovetail. When you reverse the table direction the opposite happens. This can be used to advantage when adjusting the gibbs!

So the trick is to tighten the right gib strip after moving the table to the left and the left gib strip after moving the table to the right. Using this method I didn't have to snug up the gib at all to get less than about .0003 of displacement using the above measuring method. In retrospect I can see why I had trouble adjusting the gibbs before. If you adjust the left gib after moving the table to the left it will be to loose and vise versa.

I do have to mention that after using this method the table will run very smoothly and correctly for much of its travel but the last 5 inches or so by the lathe chuck will be a little tight. Tightness causes excessive x slop. Not to impressive for a brand new machine.

good luck!

Hi,

Thanks for the tips. That's actually the technique I used to adjust my gib but I can only get it down to about 1.5 thousandths and I have to tighten the screws so much sometimes the table sticks and jumps when I move it.

So, I went back to take a look at the actual gib. I've already straightened it and lightly lapped it...just a polish. However, when I pulled the gib this time I noticed that the countersinks are drilled on an angle. I would imagine this could cause a problem and prevent the gib from being adjusted correctly. Any thoughts? Looks like my next project is to make a new gib and see if that helps.

http://www.the-alchemist.com/Images/Shoptask/Issues/X-axis-gib.jpg

.... However, when I pulled the gib this time I noticed that the countersinks are drilled on an angle. I would imagine this could cause a problem and prevent the gib from being adjusted correctly. Any thoughts? Looks like my next project is to make a new gib and see if that helps.

Probably making a new gib would be a good idea anyway.
But, the countersinks need to be inline and square with the adjusting screws...
-not perpendicular to the gib.

Once you get the gib machined (with the 55-60 deg edges) its probably best to locate each adjusting screw, one at a time, to ensure your countersink lines up properly.
i.e. its gotta be right under the screw - no matter where the screw is located.

also, consider using brass or bronze instead of steel?

Found some interesting things in my research the past few weeks. First, I found that most of my play was in the end nuts just up from the X-Axis handle. (Picture Shown). It was loose just enough to be aggravating. Next, I measured all the ball bearings in my ball screw and found that they were all 0.001" or more shy of 0.125" (Picture #2 shown). I purchased 70 new ball bearings from a bearing supplier here in town that was much closer. I measured a consistent 0.1248" on each ball. This seems to have fixed my error on my cuts. I still have great tolerance on the Y and Z axis. I have a practice part made of wood shown below, showing better tolerance in each pass of the cuts.

Hi folks,

I've noticed that my table rotates/rocks slightly when I change directions while traversing the X axis. Okay okay, adjust the gibs right? I've posted on the shoptask delphi forum as well but despite some good advice I can't solve the problem.

Okay here is what I've tried...I pulled the gib strip and checked it with a parallel. I found the center was higher than the ends. I put a steel rod in the mill, blocked up the gib at both ends, and used the rod to push down in the center of the gib (gently) untill checking with a parallel showed the gib to be more or less flat by eye, looking for a gap or rocking of the parallel.

I sanded/polished the gib and looks like I'm getting about 80% contact. I reinstalled the gib with the locking handle totally loose. Then I would put tension on the adjustment screw and work the table in the +/- X direction while snugging the screw. I went back and forth between the two adjustment screws with this method.

The best I could do is a .002 variance between +/- X travel. To test this I chucked a steel rod in the lathe and attached my dial indicator to the table and put the indicator up against the rod. I used this while adjusting to see if I was making any progress. Even by tightening the gibs "a lot" I couldn't get the wobble out. It seemed that I could achieve the same .002 variance with light pressure on the adjustment screws and the table moving very smoothly.

At one point I cranked the locking handle down hard, loosened it and then moved the table +/- X. I was shocked to find that now I was seeing almost .030 variance. I went back and adjusted the gibs with "firm" pressure on the locking handle instead of loose. I can get back to .002 variance but no better. I was wondering why the ball turner I made slides perfecly in the slot and then gets stuck...this would explain it.

Any thoughts?! Thanks in advance. One thing I did notice is that if I look at each end of the gib (when it's mounted) one end is flush with the casting on the bottom and the other end of the gib rides higher than the bottom of the casting. To me this says the indents for the adjustment screws and the locking handle are not parallel to the edge of the gib...could this be the problem?
Dahui, you aren't the only one with this issue, I've been struggling with it for a year.

If I move the crosstable forwards towards the spindle and stop, put indicators on both ends of the crossbed and then move backwards in X, I can measure the rotation - similar to yours of about .002 to .004 Once the rotation is "complete", the table moves smoothly down the x -axis. If I stop and reverse, again you have rotation, it stops and now you are moving the other way. I've tried everythin I can think of and while I've made it better, I can't get rid of it, and if you are doing precision work, it can ruin your piece. John has been little help, just keeps telling me to straighten the gib (which is dead straight at this point) and look for high spots on the ways. Also, if I try using the gibs to solve the problem, movement becomes difficult and worsens the other problem I have of the movement of the crosstable in X (down towards the spindle) is uneven, easy at the tailstock, harder near the chuck.

Now for the experts (Ie; not me), what do I do next? Are my ways warped/twisted? That would explain the problem in feel as I move from 1 end to another, but how do I tell for sure?
And what's causing the rotation? The gib strip is dead flat, checked against a granite plate. In order to pivot, something has to be allowing the crosstable to "wriggle" where the design would suggest such rotation should be zero.

If anyone can help me figure this out, I'm more than willing to put time and effort into it, I need some info on where to put my effort. All assistance greatly appreciated!!

It might be that your dove tail on the lathe bed is machined wrong. Or that at the bottom of the dovetail their is some part of the casting that is causing the problem. Today I removed all the paint from my lathe bed then I could see where they had to ground away metal from the bottom of the dove tail of the casting. They had body filler then a coat of primer and the several coats of the yellow lead paint. They didn't even take time to remove the casting sand. It was imbedded in the paint. I had to use paint stripper and then oven cleaner to get everthing off. I am priming the casting and I will post pictures as soon as I can and show where they had ground to prevent the ways from touching the casting. You might try blueing the ways to see if their is high spots. You can buy persian blueing at Napa . It come in handy when you have these kind of problems.

Bprager,
I had a similar issue with the used machine I bought. I went through the gib strip inspection processes with JT's help on the phone, but I found the problem was a hard spot in the casting where the dovetail was machined. If you are experiencing the same rocking through the full carriage travel, then the source of the problem would be in the moving part. On my machine there was a small spot about the size of a dime that was really hard- possibly a piece of hard material that was not fully melted during the casting process. When they machined the dovetail, this spot simply didn't cut like the rest of the casting and made a high spot where the table would rock around it. I was going to send it out to be ground, but decided that it represented only a small percentage of the total surface area, so I just used a carbide burr to grind it down. Using some blue and emery cloth over a file I made sure the high spot was gone and the dovetail was flat.
That took care of the problem.

PUT A MATCH BOOK UNDER THE SHORT LEG