Need guidance or procedure to adjust the gibs on my PM940-cnc

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rodjava

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Hi,
Is there a detailed guide or procedure on how to adjust the gibs on my PM940-cnc?
Perhaps there is a member in San Francisco that would help me adjust my gibs?
I would compensate for their time.

Update:
I may have miss spoke about adjusting the gibs.
I really need to make adjustments for backlash in the x and y axis and not sure how to go about it.
My circles are no longer cutting round.
So I need to adjust the lead screws for correction.


Thank you in advance.

Rod in San Francisco
 
Last edited:
The only empirical method I have seen documented for gib adjustment of a knee mill is found in the user manual for a Wells-Index milling machine. All other methods I’ve seen documented are by feel rather than measurement. I have put up a series of four videos on how Wells-Index suggests to do the adjustments empirically here. This is on my PM935 which is functionally identical to you machine for this purpose. Hope this helps.
 
rodjava said:
Hi,
Is there a detailed guide or procedure on how to adjust the gibs on my PM940-cnc?
Perhaps there is a member in San Francisco that would help me adjust my gibs?
I would compensate for their time.

Update:
I may have miss spoke about adjusting the gibs.
I really need to make adjustments for backlash in the x and y axis and not sure how to go about it.
My circles are no longer cutting round.
So I need to adjust the lead screws for correction.


Thank you in advance.

Rod in San Francisco
Click to expand...

Backlash on a CNC can be attributed to multiple factors. Play in the lead screw or ball screw nut, lead screw/ball screw bearing preload, loose gibs, play in the drive coupling, and excessively tight gibs, requiring additional torque from the motor, resulting in winding up the lead/ball screw.

Loose gibs affect the perpendicular axis. Tight gibs affect the same axis as the gib. Tormach has a method for adjusting gibs using a dial indicator to measure lost motion. The process is basically to loosen the gibs and mount a block on the table and an dial or dial test indicator to come in contact with it. Moving in one direction, zero out the indicator and the DRO. Move past the zero enough to exceed the amount of lash and then reverse until the indicator again zeroes. The DRO will read the amount of lash. Now tighten the gib slightly and repeat the procedure. The lash should be about the same. As you continue this procedure, you will reach a point where the lash will start to increase. This is due to additional friction winding up the lead/ball screw.back the gib adjustment off to the point where the lash just started to increase. This would be the optimal setting.

But, before that, the lead screw nut and bearing preload should be adjusted. My Tormach ball nuts are not user adjustable but bearing preload is. It requires removing the motor and coupling. I developed a method of measuring oz-in. levels of torque. It can also be used to adjust the gibs and the lead screw nut.
www.hobby-machinist.com

Measuring Ball And Lead Screw Torque

I have recently begun the task of tuning up my Tormach PCNC 770. Part of the process is adjusting the ball screw thrust bearing preloads. Two methods for adjusting are recommended: measuring backlash or free play as the bearing play is removed, and estimating the amount of torque needed to...
www.hobby-machinist.com www.hobby-machinist.com

If you have an unexplained amount lash, another cause could be a loose coupling. A loose set screw could permit a small amount of rotation in the in the coupling and it would rotate back when the motor was reversed. Even a few degrees of play.
 
Thank you to everyone that chimed in.
All very useful information.
I've been running my Pm-940 for almost 2 years now without any issues.
Then suddenly, the circles turned out -"out of round."

I haven't taken any measurements, but there seems to be excessive backlash in the x axis.

I found some information on how to adjust the gibs and lead screw nuts on the PM website and will give it a try.
Right now I have a extra long 4 mm hex wrench on order from Amazon which is needed to make the ball screw adjustments per the manual.

Rod in San Francisco
 

@rodjava​

rodjava said:
Is there a detailed guide or procedure on how to adjust the gibs on my PM940-cnc?
Perhaps there is a member in San Francisco that would help me adjust my gibs?
I would compensate for their time.

Update:
I may have miss spoke about adjusting the gibs.
I really need to make adjustments for backlash in the x and y axis and not sure how to go about it.
Click to expand...

Did you ever solve this issue? I read the post and string, but it appears that it just stopped when @phazertwo ask you to share the PM info that you found. If you have some PM documents please share them.
I too have a PM-940M CNC back in 2017 and am once again re-investigating the backlash. So I am asking questions.

The only documents I got were for the PM940M not the PM940M CNC. I find my PM940M CNC to be different from the PM940M.

Anyway, where did you come out on backlash?
 
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