# PM-940M CNC Z axis bearings?



## merb7880 (Jun 3, 2017)

Relocated our PM-940M CNC from Western PA to FL.   When we pulled the machine from its base, we found this bearing and nuts on the floor inside of the base.  See attached image.  My assumption is they came off the Z axis somewhere/somehow, and now I'm on a mission to put them back in the correct order.  I have no idea what that order may be.  Does anyone have any info on this, pictures, descriptions, anything?  

When I asked Matt over at PM his only guess is "it may have came off a ball screw end from somewhere"

I'm pretty sure that's the case, but I was hoping for some diagrams or something.

I've been trying to find any info I could on this, but to no avail.  I did see a picture once of an assembled z axis that would have probably helped,  but can't find it anymore.

Does anyone know where these belong, and how to get them back to their home?  Also, we have no idea how long they were off, but were still using the machine to make parts. Hopefully it's not a big deal, the machine still seemed to function properly. 

Thanks!


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## JimDawson (Jun 3, 2017)

Based on the condition of the parts in the picture, it looks like they have never been installed.  I'm guessing you may have gotten some ''bonus parts''.  If the nuts were not installed, I would expect massive backlash in one of the axes.


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## cut2cut (Jun 3, 2017)

merb7880 said:


> Relocated our PM-940M CNC from Western PA to FL.   When we pulled the machine from its base, we found this bearing and nuts on the floor inside of the base.  See attached image.  My assumption is they came off the Z axis somewhere/somehow.....
> Does anyone know where these belong, and how to get them back to their home?  Also, we have no idea how long they were off, but were still using the machine to make parts. Hopefully it's not a big deal, the machine still seemed to function properly.
> 
> Thanks!



It probably came from the lower end of the Z axis.  I removed them because my Z axis made noise when it would "whip" and bind a bit against the hand crank gears on every rotation of the screw, creating a noise I didn't like hearing all the time.    Either someone forgot to install yours, or they were extras.  This is what mine looked like BEFORE i did anything ( see pictures ).   I suppose it being there would act as a failsafe if your upper Z bearing nut ever failed or was unscrewed it would result in the entire Z ball screw crashing down with the head.

I had to lift the Z screw about 7 inches to remove the hand crank assembly entirely.  Wasn't a "fun" easy task.


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## merb7880 (Jun 3, 2017)

Thanks guys.  From the pictures, i can definitely tell its from the z axis ball screw end.  I can't tell the order they would sit on the screw, and I really wish this electrical cabinet wasn't attached to the rear column, blocking access to the screw.  I'm wondering how big of a deal it would be if I just left them off.   cut2cut you said you removed yours because of a noise, do you think it has impacted the machine in a negative way at all since removing>?  How fast do you rapid on your axes?   The only way I think I can access it would be to try to reach from underneath inside of the base.... maybe....


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## cut2cut (Jun 3, 2017)

merb7880 said:


> Thanks guys.  From the pictures, i can definitely tell its from the z axis ball screw end.  I can't tell the order they would sit on the screw, and I really wish this electrical cabinet wasn't attached to the rear column, blocking access to the screw.  I'm wondering how big of a deal it would be if I just left them off.   cut2cut you said you removed yours because of a noise, do you think it has impacted the machine in a negative way at all since removing>?  How fast do you rapid on your axes?   The only way I think I can access it would be to try to reach from underneath inside of the base.... maybe....



I don't think it is an issue if its not there, unless you use the hand crank.

will respond tonight further...

Jake


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## cut2cut (Jun 15, 2017)

cut2cut said:


> I don't think it is an issue if its not there, unless you use the hand crank.
> 
> will respond tonight further...
> 
> Jake



One negative to removing this lower set of bearings and hand crank gears is the head can creep down on its own weight because the hand crank gears/assembly acts as friction.   I've removed the quill and transmission gears, and oil so the head is not as heavy so I don't have to worry as much about the head creeping down when the machine is off.   It was marginally too heavy before I removed the weight. 

I only use 50 inches per second rapids in the Z.  Someday I hope to replace the steppers with bigger servo motors.  Can't say what the standard steppers can handle but I keep it on the very conservative side.

Jake


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## B2 (Sep 2, 2018)

Yes, I too purchased a PM-940M CNC VSD (Pre-assembled) with all 4 axes.   

I am still learning to use the mill as I am pretty new at this. I have an engineering background with lots of hands on over the years, but this is my first mill and milling experience.  

I too found the extra nut and custom lock washer under the mill when I picked it up from the pallet.  No it was not extra parts!  The the tabs on the lock washer had not been bent over correctly into the key-ways and so they  all just came loose and fell off.  I had already run the mill, while still on the pallet, as I built a 4 wheel dolly for it.  I had drilled about 80 holes in 3/8 steel plates for bolts and dolly wheel plates.  The dolly is for the base, but as part of my process of moving the mill into the basement from the garage I also used it on the mill bottom (moved the dolly wheels over a bit).  

During this build I also found that not only did the z-axis drift when powered down (in fact at one point the breaker tripped and the drift was so fast that the falling head crashed a dill bit into my work!).  Also, the z axis motion would miss steps on the way up but not on the way down. I ran a three inch up-down test for about a dozen times and found that the zero had moved down about 0.2in!   The head is indeed very heavy.  Later I weight it in place, with motor, oil etc and it come in at about 200 lbs with only the drill chuck for tooling!

Like everyone else I found that the z-axis crank worm gear arrangement causes knocking during travel.  I thought about removing it, but after observing the power down vertical drift (fall!) I did not want to give up have some method of stopping the drift when a power failure might occur!  So it is still on the mill.

Long story short.  After I finally got the mill installed and the big, heavy electrical control box moved to the right side of the mill, I had access to the z-axis screw and the gears from the access hole that is normally hidden by the electrical box.  Your picture does not really show the parts on the z-axis screw, but there is a set of these  bearings on both sides of the bracket bracket that holes the z-axis lead screw next to the z-axis crank gears.  The top set are trapped and would not fall off without pulling the entire lead screw up.   The ones that you have shown fell off the very end of the shaft.  You show three flat washers, the locking washer and the two nuts.  Two of the flat washer fit on either side of the ball bearing and show a designed wear track for the balls of the bearing.  The order they should be put on the z-axis lead screw is the heavy flat washer, the flat washer with the wear track for the balls of the bearing against the bearing, the bearing, then the second flat washer with the wear track up toward the balls of the bearing, then the first nut, the lock washer, and the final nut.  after these are all put on and the tightness of the top nut is adjusted properly, the lock washer inside tab is bent to fit into the keyway of the z-axis screw shaft and the two of the outside tabs are bent up and down to fit into the slots of the two nut slots.  This is not what the picture of your locking washer looks like.  This way the two nuts cannot turn on the shaft or relative to each other.  

I do not think that this up and down bending of the lock washer tabs was the design. You will note in your picture that all 6 of the outside tabs are bent just a little bit upward.  This would then work like a spring to hold only the top nut in place relative to the shaft.  However, the bottom nut can then vibrate loose, fall off, and then the lock washer can fall off, followed by the rest of the parts!   

Yes, I was concerned that if I bent the tabs over harder that they might break off, and probably will if I had to straighten them more than once.  This is a design flaw if one cannot find replacement washer!  But for me it worked the first time.  I did not bend them to a complete 90 degrees, but they are close, greater than 60 degrees.  I figured it the tabs did break off, Matt at PM would be able to get me another... or I would just have to make one.

 I adjusted the first nut to a proper tightness by using the z-axis crank.  I would apply just a little pressure to it so see if it was about to drift down.  If so I tighten the nut a bit more until it did not.  Then added just a little more.  The lock washer tabs and the shaft key way have limited adjustment, but I assume that is the reason for there being 6 tabs.   Three for the upper nut and three for the lower nut.  I then put the lockwasher on, bent the tab a bit to hold the nut in place and then added the lower nut and adjusted the lower nut until lightly tight and bent its lock washer tab.   I then checked the drift again and things seem to be OK.  So I bent the tabs to final position.  

I found that not only did the power off drift stop but most of the knocking caused by the lousy fit of the z-axis crank gears went away.   I have repeated my up-down test with even more cycles multiple times and found that the z-position returns to zero within the hysteresis, which is about 0.002 in.  I am only sure of my measurements to about 0.001".  Hurray!  

I think the fundamental problem is that the stepper motor is none too large for all this weight, but does it the job.  When the z-axis screw is not tight on the end the head weight causes the z-axis screw to tilt back and forth just a little causing the coging of the stepper to unlock, lock and unlock repeatedly allowing it to turn.  This goes into a resonance and speeds up.... allowing the power down drift.   The downward drift would also happen when the motor was trying to pull the head up and so the inaccuracy in up motion.  If this concept is correct then this maybe an actual flexing of the z-axis screw which also means that it is applying a small thrust to the z-axis crank gear causing the noise.  A better design would be that there is another bearing above the z-axis gear. 

Hope this helps you or others, or at least reassures you that you had fixed it in a manor that works.  

By the way, I am pretty happy with all of my interactions with PM.  Matt and his gang are pretty helpful.  Now if we could just get good, detailed manuals, especially of the electronic parts and.... in English.    By the way, I will attach a picture of my Mill in place in my tight basement spot right after I got it put back together ...  somewhat.  Electrical cabinet on the right and the dolly wheels.  I will block the base off the  dolly wheels and level it when I am sure that this is exactly where I want it.  For now it is working.  

Dave


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