PM-728V-T table stiction

K

kmanuele

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The y-axis table has a fair amount of stiction, but only when moving toward the operator. It jumps 2 or 3 thou as I slowly turn the crank.

The other direction is very smooth and controllable.

I loosened the gibs as a test, but it made no difference. Plenty of lube on the ways.

Ideas, suggestions?

thx

K
 
Hi @kmanuele

It is strange that the stiction is in only one direction! It sort of implies that the stiction mechanism is sort of a ratchet or that the system gets bound up in one direction only? Via the PM web page, I see that the PM-728VT has split nuts not ball nuts. I suppose the surfaces of the threads in the spit nuts could have an asymmetry, i.e. forming a bit of a ratcheting effect. But this seems like a long shot.
Does the crank tend to have spring to its motion, but maybe you would not even notice it on this scale? I see this sort of binding when things get a bit tight and this effect is not always symmetric in direction. This binding can occur if the gib is too tight, but you have said you loosened the gib up completely. I suppose that also means that you completely loosened the locks completely! So I am going to guess the following:

When loosening the gib, you must still lock the gib down, i.e. the two end screws are both tight against the gib so that it cannot move. (If not the gib will slide back and forth as you go one direction and then the other. In one direction the gib will get looser will in the other direction it will get tighter and and tend to bind. Since there is no tendency for the table to fall off the machine, you can completely remove the gib and then note the stiction. Gibs are not always well made, so can malfunction. I had one where the tip is broken off causing the locking nut to actually clamp the gib against the corner of the dovetail. This is sort of equivalent to clocking the way. The travel in one direction was cleaner than it was in the opposite direction. This caused more backlash in the one direction. By the way, backlash is not always the same at all points along the travel!!!! (You may not have this problem, but if the ways are not parallel this can happen.)

Other thoughts.
1.) I suggest that you measure the backlash in both directions and see if there is a difference. I am not for sure how to interpret this result, but it would be interesting to see. By the way, what is the backlash. I measure the backlash in the following manner. Dial gauge in the spindle indicating off of a 1,2,3 block that is held in position via a clamping magnetic base. Move the saddle in one direction until you are sure that the motion is accurate and linear. Reverse the motion (crank) and observe the dial gauge to have indicated a moved of a known distance (say 0.005") at this point observe how far you had to crank to achieve this distance. Lets say it reads 0.008", then the backlash would be 0.003" . At this point any further motion of the dial gauge indicator and the crank reading should be linear and equal changes. My machine is a CNC and so I can just tell the table stepper motor to move so many steps, as long as it is several greater than the backlash and get this sort of measurement. More recently I have been using a digital dial gauge which has a resolution of 0.001mm (1micron), or on the inch scale a resolution of 0.00005" (1.25microns). I recently purchased one on Amazon that seems to be decent for this ($64). There are lots varieties advertised, but this one came with a linearity calibration scale over the full inch and I compared it to my PM lathe magnetic DRO. It was pretty close to the calibration provided, but not as good as I would like to see. So the question arises as to whether it or my DRO is off a bit!?). I am pretty sure all of these have pretty good resolution, but I am not so sure about their full scale linearity and accuracy. Google this: "Clockwise Tools DITR-0105 Electronic Digital Dial Indicator Gage Gauge Inch/Metric Conversion 0-1 Inch/25.4 mm 0.00005 Inch/0.001mm Resolution with Back Lug Auto Off Featured Measuring Tool" or Amazon Digital Gauge Site Link
By the way, they also sell a cable which will dump the dial reading directly into Excel or Word by pushing a button on a small box in the center of the cable. My cable seems to work and can be handy. It is a bit slow to respond to the button (fraction of a second).

2.) (I would only try this after exhausting all other possibilities.) Polish the split nut threads: Remove all lubricant from the split nut and lead screw and run it back and forth for a while. Don't stay on one spot on the lead screw, move around. This is not a good thing to do, but will cause a bit of wear on the split nut, polishing its threads, and to a lesser degree some wear on the lead screw. Does the stiction difference improve?

3.) Is the stiction effect observable at all points along the stage travel distance. That is, is it a function of position along the y-axis?

4.) I assume this is a new machine. If so PM should stand behind it and help you figure this out.

Good luck.

Dave L.
 
I suggest you take out the gibs on both X and Y, check them for flatness, stone them to remove any burrs, reinstall and adjust. My guess is the gib is binding in one direction of movement which suggests the gib is poorly fitted to the dovetail ways.
 
B2 said:
Good luck.

Dave L.
Click to expand...

Wow! Thanks for the lengthy reply and ideas.

After going down some rabbit holes, this is where I've landed:

Note that the table normally has a big vise mounted, and power feed hanging on the left end.

I was able to remove a lot of the stiction by loosening the gib significantly (and locking it in place), but there was too much play in the table. Could move it around by hand almost 0.002", compared to 0.0005" for the x-axis table.

Returned the gib setting for a tighter table. Got repeatable stiction, as before.

Moving the table with the crank till it tightens (meaning it will jump), removed the gib. Continued to crank and it jumped -- with no gib.

Put the gib back in.

Removed the vice and the power feed. Centered the table.

Almost no stiction.

Put the vice back on the table, stiction.

Hang the power feed, stiction but less.

If I just lean on the table, stiction.

Take the gib out, apply vertical load as above -- stiction.

All stictions are still directional.

Haven't ruled out the anti-backlash nut yet, but this is only device that would have any directionality -- right ??

Should mention that this problem has existed from the beginning, but I was always able to back up and move forward to position. Recent project didn't allow that, so it's time to fix.

Still under warranty, so will contact PM for advice if I can't fix it in a few days.

Ironically, my PM lathe has a similar problem with the carriage. Stiction while trying to manually position along the y/z axis.
 
I would follow what David Best outlined, and also note that the gibbs need to go back in in the same orientation/direction as you pulled them out. So mark the end and one side so you put them back in the same way. They may either be a bit rough, have a bur or are slightly distorted (not flat). In addition the adjustments screws on either end of the gibbs need to be snug on both ends but not so tight to distort the gibb. If the table movement binds in one direction, it usually indicates that the gibb is moving slightly and tightening in the direction. Also check that the lubrication is pumping to those locations.
 
I agree with @mksj. Still sounds like the gibb not fitting or adjusted properly, and likely biding in the dovetail in only one direction.
 
Why not let Precision Mathews give the advise if the machine is still under warranty from the get go. Messing with it with-out asking them could void the warranty.
 
Hi K,

Of the items you mentioned, the the torque on the table provided by the weight of the vise is the most interesting to me. It seems that when you load one end of the table the pressure at one dovetail would be more at the underside of the dovetail while at the other dovetail it would be downward toward the larger way surface. Without this off balance load or with a more uniform load there is more force on the bottom surfaces of both sides of the way. So for me this still points at the gib as a culprit.

However, I must admit I did not understand the part of your description of the testing process:
kmanuele said:
Moving the table with the crank till it tightens (meaning it will jump), removed the gib. Continued to crank and it jumped -- with no gib.
Click to expand...


Gibs can be really difficult to figure out, especially on the cheap machines, but usually the Taiwan machines are made better. What I have found on my Chinese mill is that the gibs are poorly made and probably do not fit all that well. They are crooked in both directions and they are probably not tapered properly to fit in the gib hole. Also, improperly fit at the factory. If you pull it and measure its thickness as a function of the length you can get a feel for how well it is made. Is it uniformly tapered with length. I did this and then fit the resulting data to try to figure out what the taper really is suppose to be. However, who is to say that the gib hole has the same taper or even is made properly. For example if the corner of the dovetail is not cleared out properly the edged of the gib can dig in when traveling in one direction, but not the other. I found this and it seemed that removing a bit of the gib sharp edge freed this up a bit. If the gib locking screw, on the small gib end, does not properly engage then it can force the gib edge into this dovetail corner. I found this to be significant on the z-axis of my mill and improved it by shaping a washer with a finger set to fit catch more of the small gib end. The washer went under the clamping screw head.

By the way, if you move the vise to the opposite end of the table does it help? This would shift which dovetail is load bearing surface was in play. You could even load it more. I happen to have a brick made out of lead and I have used this at the end of the table. Also, with the vice and power feed in place you could add weight to the opposing table end to re-balance and see if this is equivalent to removing the load.

Anyway, as you can see I seem to be focused on the small end of the gib digging in.

You can lay the gib on the flat table surface and see if there is uniform friction. Does it spin about a center high spot? Try both sides.

Other Gib thoughts. A four legged chair only rests on three feet until it flexes to fit the floor. Two almost parallel "straight" lines do not contact except in a couple of spots, so a gib and the gib hole must be absolutely perfect to make contact in more than 2 (or maybe 3 spots). So one would expect the scraping marks to wear off at the contacting surfaces first. Does it mater that there are only a few contacting surfaces? As the gib and ways wear doesn't the wearing cause more surface area to contact.... unless the gib is adjusted or is clamped differently.

The gib concept calls for a trapezoid shape. This means that if you know the length taper angle, and the thickness at the start, and the dovetail angle, then you can calculate all of the dimensions along the lenght gib and so you can see if it was cut correctly on not! However, it is very difficult to make the same measurement inside the gib hole. I spent many days trying to do this on the Z-axis of my mill to see if the gib and the hole were in agreement. Even after doing all of this it was not very conclusive.

Dave L.
 
Check out the recent YouTube videos by Max Grant, The Swan Valley Machine Shop. He is fixing the fit of a gib on the cross slide of a lathe.
 
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