# PM-25MV Spindle DRO Issues



## devils4ever (Dec 12, 2019)

I'm having some issues on the DRO on the spindle of my PM-25MV mill. 

First, I'm seeing a backlash issue. When I lower the spindle using either the fine control or the handles, I see the DRO increase as it should. When, I reverse direction and raise the spindle, it takes 4 or more thousandths before the DRO responds. I used a dial indicator to verify this. I thought the DRO had no backlash issues. Any ideas?

Second, I think the DRO is off by 6 or more thousandths when I lower it by 0.375". I'm trying to hit 0.375" DOC for some milling operations and it seems to be off based on my dial indicator and by measuring the depth via a digital caliper. Anyone else seen this?

Thanks!


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## higgite (Dec 12, 2019)

devils4ever said:


> I'm having some issues on the DRO on the spindle of my PM-25MV mill.
> 
> First, I'm seeing a backlash issue. When I lower the spindle using either the fine control or the handles, I see the DRO increase as it should. When, I reverse direction and raise the spindle, it takes 4 or more thousandths before the DRO responds. I used a dial indicator to verify this. I thought the DRO had no backlash issues. Any ideas?


Just for clarification, are you measuring .004” of actual spindle movement or handle/knob movement before the DRO responds?



devils4ever said:


> Second, I think the DRO is off by 6 or more thousandths when I lower it by 0.375". I'm trying to hit 0.375" DOC for some milling operations and it seems to be off based on my dial indicator and by measuring the depth via a digital caliper. Anyone else seen this?


Copied from my post in a previous thread on this subject (https://www.hobby-machinist.com/threads/perplexed-a-little-help-needed.79163/#post-677151):
You may be having the same problem that I ran into with my mill before I added a quill stop. Mine isn’t a PM, but I think the same principle may apply. There is backlash in the rack and pinion that drives the quill up and down when you use the handwheel or the fine adjust knob. When I plunge milled with the fine adjust engaged and without a quill stop, I would run the the end mill down to my target depth and stop. The end mill would continue going deeper for a few thousandths until it took up the backlash. It drove me crazy until I noticed the Z-axis DRO changing after I stopped plunging. Before I installed a quill stop, I learned to sneak up on my target depth once I got close. Try plunging to a target depth with your fine adjust and then stop and watch the DRO to see if yours does the same as mine.

Tom


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## T Bredehoft (Dec 12, 2019)

Subsequent to installing my  PM25 in the shop I noticed a lag in the DRO on the quill. I fund that the two round head Allen  screws just below the head (connected to the spindle) were not absolutely tight. Once they were tightened (I disassembled them and cleanded all mounting surfaces) The DRO no longer lagged behind the movement. Check that before continuing.


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## Chewy (Dec 12, 2019)

I wonder if this is the problem on the PM 30?  I mill something and measure it.  Say it is .010 short.  I down feed .009 and lock the spindle.  The DRO goes to .010 by itself.   OK I can handle that. In theory you woul think that I hit my mark,  In reality I am usally deeper by .001-.003.  It varies. I'm having trouble sneaking up on that because it changes.  Note:  This is on the spindle DRO that comes with the machine.  When I use the Installed DRO it hits pretty good with the spindle locked. Never paid attention to backing up the spindle but i will the next time I use the mill.  I thought about calling Matt, but I haven't the time to do anything right now.  
Charles


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## devils4ever (Dec 12, 2019)

Yes, the dial indicator (and the handle) moves by about 0.004" before the DRO responds when I change direction.

I don't think there is a quill lock for this mill. 

So, for me to hit my 0.375" mark, the DRO needs to be about 0.368".

The two Allen screws on the DRO scale were loose and I thought I found my problem. I tightened them, but it does not help at all.

I'm tempted to use the Z-axis on the column instead, but one revolution is 0.083" and the scale is backwards for plunging (scale decreases). I'm sure the 0.083" per revolution is metric, but it doesn't seem to match anything normal since this is equal to 2.1082 mm.


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## Tmag8500 (Dec 13, 2019)

_the scale is backwards for plunging (scale decreases)._ 

You should be able to switch that in the parameters. They have a manual listed on their website for the DRO. Page 8 starts the parameter section.


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## devils4ever (Dec 13, 2019)

Tmag8500 said:


> _the scale is backwards for plunging (scale decreases)._
> 
> You should be able to switch that in the parameters. They have a manual listed on their website for the DRO. Page 8 starts the parameter section.




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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## yendor (Dec 13, 2019)

Backlash in the Quill RACK would allow the Quill to continue to drop after you stop moving gthe FINE Feed.
Escentially you are pushing the Quill DOWN via the Rack/Pinion Gears into the the material.
Once you stop gravity allows the Quill RACK to move down the amount of the backlash in the Rack/Pinion Gears before it settles.
So-  YEP - you'll need to stop short by what You "KNOW" is your backlash (+ some margin of error) and see where the DRO settles.
When you reverse direction with the fine feed (go up) the backlash is in the gears between the Fine Feed and the Rack. Different Gears for both instances.


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## higgite (Dec 13, 2019)

T Bredehoft said:


> Subsequent to installing my  PM25 in the shop I noticed a lag in the DRO on the quill. I fund that the two round head Allen  screws just below the head (connected to the spindle) were not absolutely tight. Once they were tightened (I disassembled them and cleanded all mounting surfaces) The DRO no longer lagged behind the movement. Check that before continuing.


I had forgotten that I had the same issue with the quill DRO lagging due to loose screws. (The DRO’s loose screws, not mine.   ) It only lagged when I changed direction of quill travel, independent of backlash. Once I tightened the screws, the DRO lag disappeared, but gear backlash remained a problem for plunge cuts until I installed a quill stop.

Tom


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## Nyala (Dec 13, 2019)

There are actually 4 screws on the spindle DRO not 2.  Two are exposed and two are not.  You must take off the metal face plate to access the other 2.  I was having issues and PM had me remove the face plate and tighten all 4 screws.  Problem solved.  I then added Locktite to everything secure.


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## tcarrington (Dec 13, 2019)

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.


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## BGHansen (Dec 13, 2019)

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.


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


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## devils4ever (Dec 13, 2019)

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.


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## T Bredehoft (Dec 13, 2019)

devils4ever said:


> the plastic piece that is behind the two visible screws



Make one from steel, it just needs two holes and two parallel ends. The one in my PM25, 3 years old is steel.


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## devils4ever (Dec 14, 2019)

Yes, that was exactly my thought. 

How hard is it to drill those two holes accurately and straight through?


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## pstemari (Dec 15, 2019)

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.


Sent from my Pixel XL using Tapatalk


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## devils4ever (Dec 21, 2019)

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.


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## erikmannie (Dec 23, 2019)

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.


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## devils4ever (Dec 23, 2019)

erik: So, do you plan on replacing the plastic piece with a steel one?


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## erikmannie (Dec 25, 2019)

devils4ever said:


> erik: So, do you plan on replacing the plastic piece with a steel one?



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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