# Has Anyone Modified a RF/ZX Z Axis Lead Screw Nut Mount?



## TomS (Jul 7, 2019)

I've been chasing Z axis ball screw/ball nut backlash for quite some time on my PM-932 CNC mill and have narrowed the main source as the ball nut mount/ball screw sleeve (items 14 and 15 in the attached assembly drawing) and the attachment to the headstock base casting (item 4).  As you can see the lead screw nut sleeve slips into a oval slot in the headstock base casting and is fastened in place with a socket head cap screw and a thick flat washer (items 26 and 27).  The lead screw nut, which I've modified to accept a ball nut (photo attached), has a cylindrical spigot that slips into the lead screw sleeve.  As best as I can determine this setup is similar across most, if not all, RF-45 and ZX-45 type vertical milling machines.  It may be applicable to other styles of milling machines but I cannot say with any certainty.  While this works well on manual mills the slip fit interfaces are a source of backlash when converting to a ball nut and ball screw. 

Before anyone asks I've installed a Z axis double ball nut that I've fine tuned by adding a .002" shim between the two nuts to take up any pre-existing backlash, made sure the AC bearings are properly preloaded and lubricated, and installed a double disc coupling to eliminate coupling flex.  I've attached a picture of my Z axis bearing/motor mount.  As you can see it's stout so no flex there.  

I have a conceptual design in my head where a one piece mount with a flange will bolt to the back side of the headstock base casting thus eliminating clearance between the lead screw nut sleeve and the oval slot and the clearance between the sleeve and the lead screw nut.  I realize I will no longer be able to tilt the head but I am willing to accept this limitation if it eliminates the backlash.  I've also thought about drilling through the sleeve and nut mount and reaming for a taper pin but I'm not convinced it will be a long term fix.    

So on to my question.  As the title of this thread says I'm searching for any lead screw nut mount modifications on these styles of mills.  I've done a Google search but didn't find anything relevant.  Rather than start from ground zero I thought I'd query the resident brain trust before diving in.

Thanks for listening.




The cylindrical spigot slips into the lead screw sleeve.  It is a snug slip fit.



This is the sleeve mounted to the back side of the headstock base casting.



The base plate is 1" thick aluminum plate and the box is made from 3/4" thick plate.  The box bolts to the plate with eight 1/4"-20 socket head cap screws.


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## jumps4 (Jul 8, 2019)

Hi Tom
 On your part 15 in your drawing, I added 2 set screws on the bottom to lock the pivot point on part 14. They come in from opposite sides at an angle
pointing up. I raised Z to its upper point then locked the screws down to be in proper alignment.
Steve


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## TomS (Jul 8, 2019)

jumps4 said:


> Hi Tom
> On your part 15 in your drawing, I added 2 set screws on the bottom to lock the pivot point on part 14. They come in from opposite sides at an angle
> pointing up. I raised Z to its upper point then locked the screws down to be in proper alignment.
> Steve



Steve - thanks for the tip.  I'll give that a try.


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## TomS (Jul 8, 2019)

Here's another twist.  The leadscrew nut sleeve (item 15) is about .010" loose in the headstock base casting slot.  I did put a shim in when I converted the mill but all that was left was a piece about 1/4" x 1/2".  Originally the shim was about 1-1/8" x 3/4".  I could put another shim in but that isn't a permanent fix.  The sleeve needs to slide side to side so the ball nut/ball screw runs absolutely vertical.  Anyone have any ideas? 

Here's a shot of the brass shim.  Hindsight I should have used stainless shim.



This is the lead screw nut sleeve.  I measured the flats and found they are not parallel.



Here's a closeup of the back side of the leadscrew nut sleeve.  When I did the conversion the nut sleeve was a loose fit to the lead screw nut spigot.  I bored out the nut sleeve and made a bushing with a snug fit to the lead screw nut spigot.   



And a shot of the nut and sleeve assembly.  The nut spigot is able to rotate and slide in and out of the nut sleeve which can also slide side to side.  All this movement accommodates alignment of the lead screw/ball nut to the vertical sliding motion of the headstock base casting.  Way too many moving parts!


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## spumco (Jul 8, 2019)

Use epoxy to grout the nut sleeve in to the headstock slot.  Reassemble the headstock casting, ball screw, nut, and so forth on the column.  As Jumps4 suggested run the assembly all the way to the top of the stroke and set the alignment there.

Get the alignment as close as you can and fill the cavity with epoxy paste/putty before tightening up the screw.  If you grout just the upper side and let it cure, then preload the casting a bit downward you should be able to pack putty in the non-parallel lower gap.

The clearances look pretty close and epoxy has a pretty impressive compression strength.

If you can get it all snugged up without extruding the epoxy out you'll have significantly better contact area than shims - brass or stainless.  And that will also take care of the non-parallel surfaces.  

If you want to take it apart again you can polish the nut spigot and put some mold release/wax on it before applying the epoxy and it should tap out if you need to disassemble in the future.  Worst case you chip the epoxy on the way out and just do it again.

Good video on machine mating surface grout:






Same principles apply here, although a bit tricky with the two opposing surfaces being fixed.


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## TomS (Jul 8, 2019)

spumco said:


> Use epoxy to grout the nut sleeve in to the headstock slot.  Reassemble the headstock casting, ball screw, nut, and so forth on the column.  As Jumps4 suggested run the assembly all the way to the top of the stroke and set the alignment there.
> 
> Get the alignment as close as you can and fill the cavity with epoxy paste/putty before tightening up the screw.  If you grout just the upper side and let it cure, then preload the casting a bit downward you should be able to pack putty in the non-parallel lower gap.
> 
> ...



I appreciate your thoughts on this.  I've been chasing numerous gremlins for years on this mill.  Some I fixed and others were design issues.  I'll fix what I can then put this one up for sale with full disclosure.  Been looking at the new Tormach 1100MX and used Haas mini mills and TM1 mills.  We'll see where this goes.


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## cs900 (Aug 1, 2019)

I made a new piece altogether. It was an interference fit, and I used the screw in the tip to pull in into place. Zero backlash guaranteed.


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## TomS (Aug 2, 2019)

cs900 said:


> I made a new piece altogether. It was an interference fit, and I used the screw in the tip to pull in into place. Zero backlash guaranteed.



I like it!  I thought about making a one piece ball nut mount that bolted to the back of the Z axis slide.  I'm still trying to make this work as I got no response from my CL ad.


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## TomS (Aug 2, 2019)

cs900 said:


> I made a new piece altogether. It was an interference fit, and I used the screw in the tip to pull in into place. Zero backlash guaranteed.



BTW - what size ball screw are you using?


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## cs900 (Aug 2, 2019)

it's a 25mm diameter x 5mm pitch


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## TomS (Aug 2, 2019)

cs900 said:


> it's a 25mm diameter x 5mm pitch



Thanks


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## cs900 (Aug 4, 2019)

you are most welcome!


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## TomS (Aug 5, 2019)

Here's a screen shot of the Z axis ball nut carrier I came up with.  Thanks cs900 for sharing your design.  It inspired me to move forward.  I still have some fine tuning to do but it's really close.


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## cs900 (Aug 5, 2019)

you are very welcome   You may want to move the "nub" down as far as you can. the bracket, if it's like mine, will hit the column casting before anything. Moving it down gives you the most z travel possible. That's also why mine has chamfers on the bottom edges...more travel!


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## TomS (Aug 5, 2019)

cs900 said:


> you are very welcome   You may want to move the "nub" down as far as you can. the bracket, if it's like mine, will hit the column casting before anything. Moving it down gives you the most z travel possible. That's also why mine has chamfers on the bottom edges...more travel!



I'll take a look at that.  I did a quick measurement before modeling it and by my calcs I will lose about 3/4" of travel.  Doesn't hurt to double check.


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## TomS (Aug 13, 2019)

Thank you cs900 for posting your design.  This is what I came up with.  I chose to go with a bolt on design rather than using the original design lug.  I was concerned that over time the lug would fret and become loose.  The corner notch in the left picture is wrench clearance for installing the ballnut lube fitting.  




Here is the ballnut carrier bolted to the Z axis slide.  I'll head to the hardware store in the morning to get the proper length bolts.


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## jumps4 (Aug 13, 2019)

That sure looks to be an upgrade.
Nice work
Steve


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## TomS (Aug 13, 2019)

jumps4 said:


> That sure looks to be an upgrade.
> Nice work
> Steve



Thanks Steve.  I'm hoping to get the mill together tomorrow so I can test it out.


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## TomS (Aug 15, 2019)

Yesterday I got the Z axis put together.  Wow!  It works beautifully.  With the original style ballnut carrier I had +.020 Z backlash.  After this upgrade it's less than .002".


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## cs900 (Aug 21, 2019)

looks good TomS, and that's a heck of a lot better backlash too!


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## TomS (Aug 22, 2019)

cs900 said:


> looks good TomS, and that's a heck of a lot better backlash too!



Thanks.  I wasn't sure how it would work out.  Your design was the push I needed to move forward.


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## jbolt (Sep 16, 2019)

Nice work Tom. Mine is done similar to cs900 and has worked out okay. At some point I was planning on making another out of steel. I like your additional bolt idea. Most of my play comes from the dovetail. Once I get settled into the new shop I have plans to counterbalance the head and add linear rails to the sides of the column that will engage the dovetail plate to take the tilt load off the dove tail. That is assuming I have any money left after the move and remodel.


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## TomS (Sep 16, 2019)

jbolt said:


> Nice work Tom. Mine is done similar to cs900 and has worked out okay. At some point I was planning on making another out of steel. I like your additional bolt idea. Most of my play comes from the dovetail. Once I get settled into the new shop I have plans to counterbalance the head and add linear rails to the sides of the column that will engage the dovetail plate to take the tilt load off the dove tail. That is assuming I have any money left after the move and remodel.



Thanks Jay.  I had an engraving  job that that required very accurate Z axis positioning.  The Z axis had been giving me fits for quite some time so it was time for me to do something about it.  It worked out great!

I was also experiencing column flex (I was able to push up and down on the head and watch the DI move +/- .010") and came up with the Rube Goldberg contraption in the picture below.  It's not pretty and didn't solve the flex issue because it wasn't the column flexing but the base that the column bolts to.  However, by turning the upper bolt that retains the vertical bar I was able to adjust the Y axis head tram.  No shimming required.

Good luck with your move and remodel.


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## jbolt (Sep 17, 2019)

TomS said:


> Thanks Jay.  I had an engraving  job that that required very accurate Z axis positioning.  The Z axis had been giving me fits for quite some time so it was time for me to do something about it.  It worked out great!
> 
> I was also experiencing column flex (I was able to push up and down on the head and watch the DI move +/- .010") and came up with the Rube Goldberg contraption in the picture below.  It's not pretty and didn't solve the flex issue because it wasn't the column flexing but the base that the column bolts to.  However, by turning the upper bolt that retains the vertical bar I was able to adjust the Y axis head tram.  No shimming required.
> 
> ...


Looks like a medieval torture device there.

That is interesting about the flex at the base. I will have to look into that at some point.

We took our first load to the new place last week. Many more to go over the next year and a half. Not really looking forward to moving my machines.


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## TomS (Sep 17, 2019)

jbolt said:


> Looks like a medieval torture device there.
> 
> That is interesting about the flex at the base. I will have to look into that at some point.
> 
> We took our first load to the new place last week. Many more to go over the next year and a half. Not really looking forward to moving my machines.



The base flexing was a surprise to me.  I am wondering what effect it has on the Y axis dovetails.

Have you seen or used these?  The movers had my lathe and two mills loaded in less than an hour.


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## jbolt (Sep 17, 2019)

TomS said:


> The base flexing was a surprise to me.  I am wondering what effect it has on the Y axis dovetails.
> 
> Have you seen or used these?  The movers had my lathe and two mills loaded in less than an hour.
> View attachment 302274
> View attachment 302275


Ha. I just picked up a pair of something similar to those. 

I purchased a 14' cargo trailer a few weeks ago to move our things as we make trips back and forth to work on the property. Last week I had a 1000 lbs in the bed of the truck and 3600 lbs in the trailer which was no where near full. Everything I own seems to be heavy....LOL

Tricky part is getting the machines in and out of the trailer. I plan on bolting 4' long 4x4's to the base of each machine for stability. Thinking about mounting an electric winch in the trailer and getting some ramps. I still have a year to figure it out.


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## TomS (Sep 18, 2019)

jbolt said:


> Ha. I just picked up a pair of something similar to those.
> 
> I purchased a 14' cargo trailer a few weeks ago to move our things as we make trips back and forth to work on the property. Last week I had a 1000 lbs in the bed of the truck and 3600 lbs in the trailer which was no where near full. Everything I own seems to be heavy....LOL
> 
> Tricky part is getting the machines in and out of the trailer. I plan on bolting 4' long 4x4's to the base of each machine for stability. Thinking about mounting an electric winch in the trailer and getting some ramps. I still have a year to figure it out.



A friend of mine had a enclosed car trailer with a winch mounted at the front.  He could drive the car in but couldn't open the door far enough to get out.  He would winch the car in and tie it down and do the inverse when unloading.  Worked great.

Agreed, moving is not fun.  I'm glad the latest move is behind me.


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