How would I extend the life of my mills z axis gib? I have.09" (measured on a fin of the motor) of nod when I loosen the z axis locks, I can see the head move when I tighten them. I've tightened the z axis gib until I only have about two threads left on the bottom screw. But it still moves by hand and the z axis motor just fine. The mill is a 2017 model and only used for hobby stuff every couple of weeks. Seems odd that the gib adjustment would be used up. Can I solder shims behind the gib and re-insert it? Does PM have a user manual that goes into detail about this level of disassembly? I squirt oil on the z axis every couple uses. Thanks for any help.
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Pm940pdf is out of z axis gib adjustment
- Thread starter FE427TP
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- Jan 7, 2016
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I don't have the PM940 but the Z-axis gib on what seems like many of their machines is quite simply too small. I ended up cutting about 3/4" off the small end of mine so that the top adjusting screw could push it down further and yet still have the bottom adjusting screw installed.
Maybe get ahold of PM and have them send you a new gib, but I would only do that after they (and you) verify that it is larger than the current one in your mill.
Maybe get ahold of PM and have them send you a new gib, but I would only do that after they (and you) verify that it is larger than the current one in your mill.
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Is the bottom screw the narrow side of gib, or the fat side?
Edit - I think I understand what you are saying.
Edit - I think I understand what you are saying.
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If the gib is still a bit long, I.e. room to advance the gib with the top screw, I’d trim a half inch off the narrow end. If it’s pretty much all the way in I’d shim. But I’m a newbie.
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I would shim it before cutting any more off.
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Agreed. On one of my machines the gib was a tad long, so I trimmed it. If a gib still has room to tighten with one screw, but no threads left to keep it captive, I’d think a trimming would be a good idea though.
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What I have done on other machines is make a shim from brass shim stock that goes behind the gib along its entire length and width and maybe 0.005" or 0.010" thick. You can bend the ends over a bit so it stays in place better. Note that the shim should go against the stationary side of the gib, not the sliding surface.
That's kind of what I was thinking. I was thinking soldering on a couple of 3" pieces that would be behind the lock screws but that would better support the whole length of the gib. Thanks for the advise guys.
Hi @FE427TP
I have the PM940M CNC VS purchased about the same time as yours ~late 2017. I have mentioned the issues with my gib before and you can search for those if you like, but quickly. The Z-axis gib was cut off too short as it was probably not inserted all the way when the measurement was made at the factory. It is my understanding that they make them extra long, fit them to the machine, and then cut off the extra. Due to this error it is also not thick enough. I spent a long time measuring it and the hole that it fits into and have the tapper in all three directions pretty well determined. I was going to make a new one, but never got to it. I found that I could make it all work if I shimmed it . I used Aluminum Duck tape. You know the old fashioned kind that they used to tape the joints in your house force air heating and cooling system. Old fashioned means it really uses Al, not the mylar coated with evaporated Al. It was about 005" -007" thick with the sticky glue. I clamped the head saddle to the column using a large C-clamp, but could have used a wood gluing pipe clamp so that I could remove the gib without fear of dropping the head. I found the gib was not even flat in width or length and the small end was so small that it did not really hit well on the clamping screw. The clamping screw would wedge the small end into the way and so would make the factory think it was in place. Anyway, I stuck the shim to the back side, not the way side, and it works well. Then I made a washer which has a finger sticking out on one side that sticks into the gib slot. This washer then is used to catch onto the small end of the gib. Between these too things it now works.
In spite of this I still find that when adjusted correctly to prevent head nod and backlash, the head will fall if the power to the stepper is shut off. If I tighten up the gib I can prevent this but then the ways are way to tight at the top position. Either the ways are not cut the same width from top to bottom or they bend toward each other in the center position from the torque that is exerted on the ways by the saddle with the full 250-300# of the hanging head. I think it is this torque that is the problem and so the best way to solve this problem is to help to support the weight! I have some concepts for this but this is a project that has been on hold for some time now.
By the way, I suspect that your gib taper is the same as mine. If you want the numbers I can sent them. Otherwise, I suggest you just measure the taper while you have the gib out. Then using the thread TPI you can figure out how thick a shim you really need. But it sounds like a 5 to 7 thousands would be about right to move the tightening screw up about 1/2 inch.
Good luck. Just shim it!
Dave L.
I have the PM940M CNC VS purchased about the same time as yours ~late 2017. I have mentioned the issues with my gib before and you can search for those if you like, but quickly. The Z-axis gib was cut off too short as it was probably not inserted all the way when the measurement was made at the factory. It is my understanding that they make them extra long, fit them to the machine, and then cut off the extra. Due to this error it is also not thick enough. I spent a long time measuring it and the hole that it fits into and have the tapper in all three directions pretty well determined. I was going to make a new one, but never got to it. I found that I could make it all work if I shimmed it . I used Aluminum Duck tape. You know the old fashioned kind that they used to tape the joints in your house force air heating and cooling system. Old fashioned means it really uses Al, not the mylar coated with evaporated Al. It was about 005" -007" thick with the sticky glue. I clamped the head saddle to the column using a large C-clamp, but could have used a wood gluing pipe clamp so that I could remove the gib without fear of dropping the head. I found the gib was not even flat in width or length and the small end was so small that it did not really hit well on the clamping screw. The clamping screw would wedge the small end into the way and so would make the factory think it was in place. Anyway, I stuck the shim to the back side, not the way side, and it works well. Then I made a washer which has a finger sticking out on one side that sticks into the gib slot. This washer then is used to catch onto the small end of the gib. Between these too things it now works.
In spite of this I still find that when adjusted correctly to prevent head nod and backlash, the head will fall if the power to the stepper is shut off. If I tighten up the gib I can prevent this but then the ways are way to tight at the top position. Either the ways are not cut the same width from top to bottom or they bend toward each other in the center position from the torque that is exerted on the ways by the saddle with the full 250-300# of the hanging head. I think it is this torque that is the problem and so the best way to solve this problem is to help to support the weight! I have some concepts for this but this is a project that has been on hold for some time now.
By the way, I suspect that your gib taper is the same as mine. If you want the numbers I can sent them. Otherwise, I suggest you just measure the taper while you have the gib out. Then using the thread TPI you can figure out how thick a shim you really need. But it sounds like a 5 to 7 thousands would be about right to move the tightening screw up about 1/2 inch.
Good luck. Just shim it!
Dave L.