New PM-25MV binding at top of Z Axis travel

M

mlchman

H-M Supporter - Silver Member
H-M Supporter - Silver Member
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I just purchased a PM-25MV, received it the other day. I installed a Priest Tools head lift. At the lower end of the Z axis travel it works fine. Once it gets toward the upper end of travel you can hear the motor start to labor. When the pointer on the head is near the 3 inch mark on the ruler, the lift motor stops. Working the Z axis lift handle on the machine, there is a definite difference in force required to continue up. I measured the distance from the dovetails to the lead screw, near the bottom of the lead screw, when the head was fully up and most of the way down and there is a difference of about 2mm - the lead screw is further away (in) when the head is up. To me, it seems like something is causing the Z Axis leadscrew to be to be pushed inward at the upper end of travel, either improper placement of the leadscrew mount or the Z axis leadscrew nut is a hair to long, which causes the leadscrew to be pushed out of alignment causing the binding.

Hopefully, this makes sense. Has anyone had this issue? Any suggestions?
 
P.S. as soon as I saw the binding issue, I removed the Priest Tools head lift and put the machine back in its original configuration. The issue is definitely with the machine, not the Priest Tools kit.
 
It's probably the famous Chinese quality control.

Had a similar issue with a brand new Optimum Optimill MH 35V mill last year. Ended up replacing it on warranty with a Optimill MH 50V.
 
Hi,
Can you tell if the tightness is a monotonic function of the z-axis or if it is sudden? Do it by manual motion and you maybe able to tell. I do not have your machine, but the head looks pretty heavy. If it is sudden like with in a 1/2 inch or something then there is probably a blockage.

However if it take s a few inches or more to tighten, then it probably is due to the gib getting too tight in this region. This can be caused by several issues with the ways or the machine itself. If the ways are not parallel, but are getting wider towards the top, then having adjusted the gib in the middle of travel may cause this increased gib tightness when near the top to make it hard to move the stage. However, here is the real hanger! What if the ways were cut parallel, but the machine head itself is just too heavy for the vertical column design. As the head moves into the region away from the ends and the column bracing the U shape of the column may be week and allow bending of the column towards closing the U shape. This closure can happen because the head weight is applying a torque on the column. It squeezes the column at the top of the head saddle as the head is pulling outward on the ways and at the bottom it is resting on the flat parts of the way so does not apply a squeezing pressure. This pinching results in the ways being closer together in the middle parts of the column (at the top of the saddle) and so narrower than at the top of the travel ! This bending of the U shape maybe hard to believe, but why not? Even cast iron bends a little and it only takes a few thousandth of an inch to cause this effect. How heavy is your mill head? 100#s. More? Mine is more like 275#s but I have a bigger column. We will hope that this is not your issue.

This is equivalent to not being parallel but is dynamic depending upon where you are operating along the z axis. You can determine this adjusting the gib for the top of the mill and then checking it at the middle area as well. I use backlash to determine the proper gib adjustment. My CNC mill and the built in DRO makes this somewhat quicker, but using an indicator of backlash works as well. You just have to figure out a good way to mount an indicator or block when at the various positions of Z-axis. You may wish to adjust your gib at the mill position where you expect to do most of your work and just giving up on having a full travel. It is a disappointment if this is the case. The other option is to adjust the gib at the top. If this bending of the column U shape is an issue, the next choice is to get the weight down. I am considering a counter balance or an air spring to help hold it up. But the support must reduce the tilting of the head, the torque. So just lifting at the way surface is not enough. The head needs to be supported near to its center of gravity or along that vertical line.



I just commented on using backlash to adjust the gib and on non-parallel effects at another thread. Maybe you want to look at that comment. https://www.hobby-machinist.com/threads/symptoms-of-loose-x-axis-gib-on-mill.98130/ .

Good luck
Dave L.
 
xr650rRider - Yes, I have both gib screws loosened. The head moves easily in the lower area of travel. It is just when the head gets to the upper end of travel that it gets tighter.

B2 - This is a brand new machine. It has not been used once. The tightness is a slow progression starting towards the top of travel. All is fine until the head gets to about the 4" mark on the scale and gets worse as you go up. I will check out the other thread you mentioned.
 
Ok, Good to know, but not a critical issue. My machine arrived with problems as well. Some far more serious than others.

So, maybe it is an easier problem to solve. If the lead screw is not the proper distance from the ways then it will bind up as you approach the end bearings with the 1/2 nut (ball lead screw bearings). The end bearings fix the lead screw's distance from the ways. At the same time when far from the end bearings the lead screw can bend to accommodate the incorrect placement and so turns easier. it is for this reason that they commonly do not put lead screw bearings on both ends. So we will try to move the bearing housing to free up the lead screw when near the top.

I do not know your machine's construction, but to some extent they are all the same. Sometimes the other end of the lead screw is without bearings and so can flop around. If not you may have lead screw bearings at both ends and possibly binding at both ends.

There is normally a housing into which the lead screw bearings are placed. It is at the very end (top) of the column. Hopefully there is not some additional constraint for the lead screw... like a gear for cranking. If so keep an eye on it as well. So try the following: Tighten the gib tight when the saddle is at the mid point of the z-travel. Move the head to the top. The tighten the gib should make it hard to turn at the top, but not so tight that you might crack your saddle. The purpose of this is to ensure that the saddle is tight on the ways and not flopping around. (You may want to take some of the weight off of the head so that it does not have that torque about the x-axis that I mentioned earlier. ) Loosen the screws (4 typically) that hold the lead screw bearing housing to the top of the column and let it move freely left, right, front and back to where it wants to be. It does not have to move a lot. If there is not enough play for the lead screw to move around then you may have to completely remove the screws holding the bearing housing and let the whole think slide about more.... Anyway, the idea is to let the end of the lead screw seek its position where there is no binding. ( Hopefully, the mill does not fall apart when the screws are loosened, it should not.) This may now allow you to crank the z axis without undue force to move the head up and down. If need be tighten the gib some more and then tighten the bearing housing screws back down to lock the lead screw, bearings, and housing at this new position.. You may need to readjust the gib. If they pull the lead screw back to where it was, then you will need to make the clearance screw holes bigger so that there is slop. After doing this check the travel ease by the cranking force at both the other end and in the middle. If there is a bearing and binding at the other end then you may have to adjust this bearing location.

Ok, I just looked at the PM manual. You have a ring of some sort at the top to constrain the lead screw for the crank. You may have to loosen this as well, but it looks that it can be done without pulling the lead screw out. Likewise at the bottom, but that will be harder to reach.....

Let us know how you come out.

Dave L.
 
B2 - Thank you for your thoughts. I also think it is an upper bearing/housing alignment issue. I am on my way out to the shop to take a closer look. The lead screw bearing housing/support has 4 bolts but also 2 roll pins to locate/align it. I thought I would take the head off the Z Axis saddle to make things a little easier for me.

Thanks again for your insight, I will let you know what I find.
 
Yes, my y axis also had the pins. However, the workmanship was so bad that one of them hardly aligned to the other one. It was so poorly drilled in position and too shallow to be of use. I just left it out as this was the end that needed to be free!!! My machine is CNC and the motor is on the back, with a hand crank on the front. It is ironic. You purchase a CNC machine and it is all made by hand probably will unskilled (or at least cheap) workers who are given a template to drill holes by etc.

By the way, the pins that are in mine take a metric screw to put into them so that you can have a head to pull them with. One of mine fell out and the other was pretty stubborn. Behind the screwed on housing cover was a hole in the base that the y-axis lead screw came out of so that a handle could be attached. The hole which is about 2" diameter was so far off that the leadscrew was rubbing on the side bending it. I fashion a guide and drill it to make it an oval so that the lead screw did not rub. But this was not my fundamental problem. It is in the nonparallel ways.

Good luck.
Dave.
 
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Well, after much fiddling I was able to get the binding/stiffness reduced. It now feels about equal from top to bottom. I moved the head all the way to the top of it's travel and loosened the cap screw that holds the Z Axis leadscrew nut tight to the Z Axis saddle. There was some looseness so I don't think the top leadscrew mount is misaligned. I was expecting it to be tight against the saddle. I worked with the gibs and lots of lubrication and it seems better. I also loosened the Z Axis handle which also seemed to make a difference. I will keep an eye on it.

Thanks for all your advice.
 
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