table rocks when reversing x travel

[dahui]

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?

[NC Cams]

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.

[Walt@SGS.Inc]

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

[dahui]

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

[NC Cams]

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....

[HuFlungDung]

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.

[PoppaBear10]

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

[dahui]

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.

[P40Sortie]

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.

[dahui]

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.

[P40Sortie]

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

[PoppaBear10]

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