# PM 833-T Backlash



## Bill Kirkley (Sep 29, 2019)

I am having problems correcting the Y axis backlash. PM tech support suggested tightening the split-nut screw and tightening the split-nut mounting bolts. 

After a lot of trial and error I was able to tighten the split nut screw to the point that it was hard to turn the crank handle. There was no affect on the backlash. 

Getting to that screw was difficult to say the least. I used a 1/4" drive ratchet, a mirror, and a flashlight. String was attached to the flashlight and wrench. 

The mounting bolts for the split nut could not be tightened any further. 

For all my effort the backlash is a little worse than when I started. 

Does anyone have any suggestions?  
Below is a link to a YouTube video:


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## benmychree (Sep 29, 2019)

Could the screw be moving axially in its mounting bearings?


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## Bill Kirkley (Sep 29, 2019)

I think correcting axial movement in the split nut is what tightening the split nut screw corrects. When I tightened it enough to cause more resistance to turning the Y axis handle it didn't decrease the backlash. 

I got an email from the PM tech folks. They are going to look into it further. 

Any other suggestions would be appreciated


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## davidpbest (Sep 29, 2019)

Kind of a stupid question perhaps, but are you sure the lead screw is turning over that range of backlash?   From the video it looks like the handle is loose on the lead screw shaft.


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## Bill Kirkley (Sep 29, 2019)

The handle is tight. There is a key between the handle and the dial. There is a second key between the dial and lead screw. The keys prevent circular motion between the handle and lead screw.


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## benmychree (Sep 29, 2019)

My comment on axial movement was directed to looseness in the thrust bearings that connect the screw to the table; if you've tightened the screw until it binds, that is about the only other possibility.


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## Bill Kirkley (Sep 29, 2019)

Benmychree, I'm not sure I follow. What is the part number on the attached diagram you are referring to?


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## davidpbest (Sep 29, 2019)

Same as on my PM935.  Just thought I’d ask.


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## jaek (Sep 30, 2019)

You say you have backlash - what is moving relative to what? .081 is more than a sixteenth - this should be readily visible. Leadscrew relative to splitnut? Splitnut relative to saddle? Leadscrew relative to bearing mounting to base? Dro readhead?


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## mksj (Sep 30, 2019)

Per above, take off the handle and see if there is any back/forward movement of the Y-axis leadscrew with your hand and also rotational.  Make sure the gibs are adjusted and not binding. Also at the end of the video, there seems to be a M5x5 set screw that is missing from the handle dial. Check if the handle to leadscrew key is is present. Not sure if this makes a difference, but if may effect movement of the hand wheel/leadscrew. Need to determine what is loose. Can't imagine the leadscrew nut having that much play when new, part diagram does not show it as being adjustable (split nut), you have tightened it to the saddle so there should be no movement at the nut.


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## Bill Kirkley (Sep 30, 2019)

Mksj-  I checked and there is no back and forth motion to the lead screw.

As it turns out PM's tech support suggesting the split nut is moving back and forth is the answer.

I pushed a ruler against the split nut and moved the handle back and forth. The ruler, and therefore the split nut, is sliding back and forth. I have to find a way to tighten the mounting bolts more. I attached a video to show this. I also uploaded another YouTube video

PM tech says to move the table back to visualize the bolts. I tried once and the table would not move far enough back. I'll tackle it again later.

FOR SOME REASON THE VIDEO WON'T WORK. MAYBE THE SIGHT'S ADMINISTRATOR CAN GET IT WORKING


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## Bill Kirkley (Sep 30, 2019)

I deleted the original YouTube video. Here is a link to the updated video:






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## Bill Kirkley (Sep 30, 2019)

Jaek,  if you rotate the Y axis handle counterclockwise about two revolutions until you zero out the dial, then rotate it clockwise to a dial reading of .081, the table doesn't move. It turns out the Y axis split nut is sliding on its mount.


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## mksj (Sep 30, 2019)

Based on what your have indicated, the 4 retaining bolts (50) are the same for the X axis leadscrew nut (51) and the Y axis leadscrew nut (62, see diagram). It would seem the only way to assess the problem and tighten the screws would be to remove the mill table.


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## Bill Kirkley (Sep 30, 2019)

MKSJ,  you are right. Sadly, I suppose to make a little more money, there are only 2 bolts (50), not four. In the four bolt configuration the base of the split nut would likely have more surface area, so more friction.  There would also be twice the clamping force. 

I don't think you have to completely remove the table. It just needs to be backed off enough to expose the bolts. I also think it is important to keep one of the two X axis handles attached to insure the X axis lead screw is lined up. The table needs to be properly aligned as well.


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## jaek (Sep 30, 2019)

Good news that you found the split nut moving relative to the table.

Putting the table back on shouldn't be too bad. Realigning to the saddle is automatic - the ways and the gib locate it precisely. Leadscrew alignment is done by brackets 55 and 46, you should just be able to run the table to each side and tighten down the screws for the corresponding bracket.

This video link ought to work:


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## Bill Kirkley (Oct 1, 2019)

Jaek, I plan to give it a try soon. I'll post my results.


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## Bill Kirkley (Oct 1, 2019)

It's fixed!  There is a third "magic" screw PM tech said I had to tighten. They slid a table to photograph it and told me the steps for doing the same.  Their photo is the last one. 

Not wanting to remove the table I came up with a way to tighten it with the table on.

The third screw was loose. Tightening it about 3/4 turn did the trick!  The Y backlash is now four thousands.

  I used a craftsman 6mm ratcheting wrench, a cut off 6mm hex wrench peened at one end, and a screw driver. See the photos and, a video. (The third photo is before I cut a new hex insert).  The video is the end of the tightening process.









Your browser is not able to display this video.


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## jaek (Oct 1, 2019)

Sweet! Don't forget to back off the split nut screw so you don't wear it completely away and need a new one...


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## Bill Kirkley (Oct 1, 2019)

Thanks Jaek,  I had backed off the split nut screw before tightening the third screw.


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## Bill Kirkley (Oct 2, 2019)

Here is a link to the updated video which includes the fix. I deleted the old videos so those links won't work.


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## Praeger (Nov 6, 2019)

I just discovered the same condition on my 833T.  I took the same troubleshooting path (minus calling PM) and ended up removing the X-axis hand cranks, sliding the table to the left enough that that third bolt became accessible.  It locked down in less than a full turn.  Backlash down to .005".  Exercise caution when moving the table so far as to get to this bolt - you'll be close to the tipping point for the table - it is heavy.  I used an auxiliary out feed table to provide support.    

As a PSA, anyone buying the 833T should check this during setup.  Disappointing QC, happily an easy fix.

Bill - Did you disable the spring mechanism on the Y-axis hand crank?  I find it annoying and used a zip tie to lock it in the engaged position.  In your video it looked like you weren't pushing in the hand crank before turning it.


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## Bill Kirkley (Nov 6, 2019)

Praeger-  If you look at the thread "Shipping Dispute" you will find someone in the shipping department told me the spring handles are discontinued. They replaced mine with a real handle. If you contact them they should do the same for you. It seems no one liked the spring handle, including me. 









						Shipping Dispute
					

I ordered the PM-833T mill and power feed together. It came with two handle-dial assemblies and one spring loaded handle as part of the power feed kit.   If I had ordered the two separately, I would have received three handle-dial assemblies and one spring handle.   When I contacted shipping...




					www.hobby-machinist.com


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## mksj (Nov 6, 2019)

Are there any lock washers under those bolts, one would expect that they would loosen over time otherwise with the constant back and forth motion. Might also be some consideration to using some blue 242 Loctite.


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## Bill Kirkley (Nov 6, 2019)

I don't think there are lock washers. There are a number of bolts on the machine that don't have them. I am not aware of loosening being a problem.  I think this is an case of the bolts never being fully tightened.


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## Praeger (Nov 6, 2019)

Bill Kirkley said:


> Praeger-  If you look at the thread "Shipping Dispute" you will find someone in the shipping department told me the spring handles are discontinued. They replaced mine with a real handle. If you contact them they should do the same for you. It seems no one liked the spring handle, including me.
> 
> 
> 
> ...


Thanks - That's good to hear, I'll give them a call.


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## Jason812 (Dec 8, 2019)

I'm so glad you posted this thread.  I too had .08" of backlash in the y-axis.  The 6mm ratcheting wrench, 6mm cutoff cheap allen wrench, and screwdriver worked like a charm once I was able to get the allen in the bolt.  I haven't touched the other bolts yet besides the 3rd screw and backlash is down to .013" and am afraid to tighten the bolt any more.  I have .015" in the x-axis.  Before I start trying to reach the other bolts and split screw adjustment bolt, what is the target amount of backlash?


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## Bill Kirkley (Dec 8, 2019)

I don't know if there is an ideal target. I think it is what you can live with. A few thousands maybe. Some of the more experienced guys can probably give you a better idea.


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## ahazi (Apr 16, 2020)

Praeger said:


> I just discovered the same condition on my 833T.  I took the same troubleshooting path (minus calling PM) and ended up removing the X-axis hand cranks, sliding the table to the left enough that that third bolt became accessible.  It locked down in less than a full turn.  Backlash down to .005".  Exercise caution when moving the table so far as to get to this bolt - you'll be close to the tipping point for the table - it is heavy.  I used an auxiliary out feed table to provide support.
> 
> As a PSA, anyone buying the 833T should check this during setup.  Disappointing QC, happily an easy fix.
> 
> Bill - Did you disable the spring mechanism on the Y-axis hand crank?  I find it annoying and used a zip tie to lock it in the engaged position.  In your video it looked like you weren't pushing in the hand crank before turning it.


I have the same problem on my PM833-T, about 0.055" backlash on the Y-axis, very low (less than 5 mil) backlash on the X-axis. I installed a power drive on the X-axis (on the left side).

Praeger and Bill not really clear how much disassembly of the table(s) you did? Can all this work get accomplished without removing anything and just squeezing modified allen wrench and turning the loose screws? When the X-axis table was released, did you have to take off both hand (left and right) cranks and the bearings that hold the lead screw?

I will take a closer look maybe stick in a flexible camera to get a better view. Annoying... I was expecting better QA.

Ariel


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## davidpbest (Apr 16, 2020)

ahazi said:


> I have the same problem on my PM833-T, about 0.055" backlash on the Y-axis, very low (less than 5 mil) backlash on the X-axis. I installed a power drive on the X-axis (on the left side).
> 
> Praeger and Bill not really clear how much disassembly of the table(s) you did? Can all this work get accomplished without removing anything and just squeezing modified allen wrench and turning the loose screws? When the X-axis table was released, did you have to take off both hand (left and right) cranks and the bearings that hold the lead screw?
> 
> ...


Ariel, I keep telling you, you should buy German engineering.   Taiwan is chasing the "value engineering" curve to compete with the motherland.  

 Das est gut.


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## ahazi (Apr 16, 2020)

davidpbest said:


> Ariel, I keep telling you, you should buy German engineering.   Taiwan is chasing the "value engineering" curve to compete with the motherland.
> 
> Das est gut.
> 
> View attachment 321152


Yes David, it is a super nice machine, just too much for my skills and needs... Also what will I do if everything worked out of the box, nothing to fix and the factory technician will fly from Germany to set it up?

Anyway, I fixed my machine's problem, Y-axis backlash is now 0.005", same problem as the other posts. *No need to disassemble anything* in the machine. 99% of the time was spent building the tool to turn the loose Allen screw (6 mm tip) from underneath the table.

See pictures of the tool and also few more pictures and description in the PDF document attached. I hope this will help others facing the same problem.

























Ariel


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## Bill Kirkley (Apr 16, 2020)

Ariel,  It's good to see you got it to work. It has been awhile but I think I just moved the table to the right to access the third screw.

Bill


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## davidpbest (Apr 16, 2020)

Ariel, you know I was just jerking your chain about the Deckel, right?   I love the tool you came up with.   Might be a new business opportunity there.


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## ahazi (Apr 16, 2020)

davidpbest said:


> Ariel, you know I was just jerking your chain about the Deckel, right?   I love the tool you came up with.   Might be a new business opportunity there.


Of course I know. I also know that this is the Silicon Valley shop of David C. from the FOG forum. Beautiful machines with amazing engineering and manufacturing that probably are less relevant now with today's new multi-axis CNC and metal 3D printers taking over even for prototype work. 

I was very happy and proud of my little tool at 2AM... it solved the problem. My son also made fun of me "...lol... you used a welder to make that tool?..."

Ariel


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