PM-25MV Spindle DRO Issues

alignment of the read head to the scale may be an issue - I had to so some reassembling on mine. Mainly floating the head and then figuring out how many washers and where to get it in line with the scale so the scale is not being forced out of line.
 
devils4ever said:
No, the DRO direction is correct. I was talking about the hand wheel for the Z-axis. It is backwards. It decreases as the head drops towards the table.
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I'll have to check my Tormach, but pretty sure as the head drops to the table the Z-axis number gets smaller. If I probe something on the table or in a vise, zero out the Z, the number gets more positive as the head goes up.

Bruce
 
I spent some time tonight diagnosing this issue. I took apart the entire DRO assembly and cleaned and reassembled it. I tightened all the screws. I've come to the conclusion that the plastic piece that is behind the two visible screws that attaches the spindle to the DRO scale has a tiny amount of flex in it. It is enough to cause the 0.004" backlash issue. I guess I'll have to learn to live with this.

On a positive note, it seems the overall accuracy is better now. When I drop the spindle to 0.375" as indicated on the DRO, the error is less than 0.001". So tightening and cleaning everything helped this issue.

I do plan on converting this mill to CNC in the future, so this issue will be immaterial. The Z-axis will be handled by the head moving by the newly attached ball screw and the spindle will be locked.
 
Yes, that was exactly my thought.

How hard is it to drill those two holes accurately and straight through?
 
That's probably in work coordinates. Z always decreases algebraically when you go down, but by the usual convention z=0 at the top of the travel, and the numbers on the dial are implicitly negative. If you crank z from 0 to 0.150, you're really going to -0.150.


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Ok, I made a replacement out of 1018 steel. It was very successful. I'm getting very little (~0.0005") backlash. See photo of new and original part. This was a good exercise for me since I'm still learning machining and the PM mill. I purposely made the steel version slightly thicker since I wanted more bearing surface on the scale and the spindle. I had to cut a tiny rabbet on one end for the spindle curve since this was slightly thicker.

20191221_103101_edit.jpg
 
I have a PM-25MV purchased in September 2019 and I have the exact same issue as described in the OP.

I have a z-axis DRO, but I have my z-axis locked when I engage the fine feed (I would imagine that everybody does this).

I found that I can’t use the quill DRO for precise cutting if I am to move the spindle both up and down. I can, however, manage it going in only one direction (which has always been going down while cutting deeper and deeper).

Reversing direction on the fine feed feels just like backlash, and the numeric value on the quill DRO does not change for, say, .025” even though I can see the handle moving and the cutting tool going up (or down).

Also, when I lock the quill, the value on the quill DRO will change by .001”.

Before reading this thread, I had figured that the worm gear had (considerable) backlash. I had thought the solution was just take up the lash and keep moving in one direction like you would on a handwheel on a mill table.
 
erik: So, do you plan on replacing the plastic piece with a steel one?
 
devils4ever said:
erik: So, do you plan on replacing the plastic piece with a steel one?
Click to expand...

It looks like I will need to add this to my project list. This mill needs to get me by for about 15 years, and I am always using the fine feed.
 
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