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

[TomS]

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.

 

[jumps4]

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

 

[TomS]

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.

 

[TomS]

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!

 

[spumco]

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.

 

[TomS]

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.

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.

 

[cs900]

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.

 

[TomS]

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.

 

[TomS]

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?

 

[cs900]

it's a 25mm diameter x 5mm pitch