Preload Nuts

[pzzamakr1980]

Im trying to eliminate backlash in my cncfusion old deluxe setup. The first thing was to replace both of the lovejoy couplings with the slightly more expensive aluminum no backlash couplings. Worked great, especially for the x-axis which was toast. The next thing Im at are the preload nuts. How much backlash can the nuts eliminate. Would it be worth it to replace the stock thrust bearings that the kit uses to support the screws with something nicer. Similar to what mrm mcmodels did. The ones he purchased from mscdirect are what I am referring to. What difference would that make in the operation of the mill. Any other suggestions. I am not willing to spend the money to replace the screws and ballnuts, so what else can I do.

The ballnuts are already preloaded with larger balls.

[Robin Hewitt]

Pre-load as in two ball circuits sprung together? If so it eliminates all backlash so long as you don't exceed the preload. You may still get a wobble if the nut isn't perfectly aligned to the screw and accuracy is limited to tolerance/stretch of the screw, but no back-lash.

To eliminate play on the shaft, maybe fit an angular contact race at one end and a spring at other so you pre-load that to.

Then all you have to worry about is flex in the connection between bed and tool tip. Hope you have plenty of iron in there

[cyclestart]

Originally Posted by pzzamakr1980 The ballnuts are already preloaded with larger balls.

And that's all that can be done with a single nut probably.

NC_cams (sp?) has many posts discussing how bearings effect backlash. The prices he mentions are in the ouch range. It seems squeezing out the last bit can get expensive. My X2 benchtop holds ~0.001 to .0015 on a dial indicator with an unaltered cncfusion setup and preloaded Nook screws. Cutting accuracy can be another matter

What type of backlash numbers are you hoping for? I can't give real help with this, just giving the thread a ^^bump^^

[pzzamakr1980]

Well, I've really beaten up all the axis. Doing stupid things of course in my learning curve and the backlash has gone from .002/.003 all the way up to about .006 x,y. The z is the worst, which is odd, and that is around .01. I would just like to get back to around .003. Ill try to rebuild the ballnuts and see if that will help.

[cyclestart]

Originally Posted by pzzamakr1980 Ill try to rebuild the ballnuts and see if that will help.

From another thread it sounds like you're doing some commercial work. Suppose that kit has piled up quite a few hours. However those screws/nuts give the appearance of ruggedness and long life. If these components are truly the cause of the increased lash.....well, that's a bit disappointing. Not enough lube possibly?

Seeing as you're headed to disassembly anyway, take a close look at everything. All the sliding and rotating parts.

[cadmonkey]

I doubt the ballnuts and screw are the immediate source of your problems unless the balls were too far oversize to start and you've really been beating the crap out of it. The hardness of the screw and nut are pretty well up there. The balls in the nuts could be worn as they're the weakest link in the system (if I recall the hardness specs on the balls, screws and nuts accurately from my Nook literature). I'm not sure what screws/nuts he was using on the "old deluxe" setup though. A way to possibly check the screws/nuts is to move to an area not highly used (ie at the near and far extents of all the travels) and check the backlash there. If it's still there, chances are the nuts/screws are ok and you've got bearing/shaft fixivity issues.

I would definitely look at utilizing AC bearings in tandem and a locknut to adjust the preload. My guess is the thrust bearings have seen the end of their usefull life. How was that kit setup to load the thrust bearings? Collars or nuts? Depending on the shaft and journal sizes you should be able to modify to use some of the 10mm VXB 40° AC bearings.

I wish my conversion was complete so I could speak with a bit more authority on the performance but my design has been 'reviewed' by a soon to be PE and he agrees my ≤.001" target resolution should be easily hit with my materials selections (Nook XPR screws, SBN series cut [not ground] ballnuts, single preloaded nut on Y, double nut on y with adjustable rigid preload [not a spring holding two nuts apart but two nuts held together with screws at 4 points] and opposing VXB 40° AC bearings on the driven/fixed end standard radial bearing at the supported/free end. Build thread to come when I get the time to compile all my photos and narrative.)

[LongRat]

Cadmonkey.. to give your opinion a little more weight, I am using double nuts rigidly preloaded against each other on my X2 (X and Z only, not enough space on Y). This works very nicely. I have racked up several thousand hours on this and my X showed increased backlash after a while, which was easily removed by retightening the nuts into the carrier and resetting the preload. Here is my X axis:



I am pretty sure the source of the increased backlash was wear on the balls.

[cadmonkey]

Thanks for adding some backup The screw looks nice - don't see any galling or skid marks (that sounds bad...) so I think I'd have to agree.

Minor hijack - What are the wires hanging down and onto the nut? Is that your limit switch? How is it attached? You can PM so we don't send this thread OT.

pzzamakr1980 let us know what you end up finding out.

[pzzamakr1980]

Thx I will. Also, feel free to post what the wires are and how the limit switches are setup if thats what they are. I always like to see novel ideas.

[pzzamakr1980]

Hey cad, I am going to purchase replacement balls for the ballnuts. Do you know, or anyone know, what type of balls I need to purchase. I looked at mcmaster carr and they have a million varieties. I know the size, just not the type. Thx

[LongRat]

Those wires are indeed from the limit switch. It is glued onto the flat on the ballnut with epoxy resin.
There are definitely no skid marks on my screws.

[cadmonkey]

Avoiding the obviously gutter minded response to the last post...

As for finding balls (still avoiding...) http://stores.ebay.com/ToolSupply_Ba...QQftidZ2QQtZkm is where I am planning on getting my oversize balls from. I found another place too that had balls too and guides on rebuilding - think it was in a thread in the ballscrew and linear motion discussion forum.

Need to remachine a part or two on my work, overbored the nut mount so the tap barely had anything to bite to. I decided to undershoot on the next one.

Ingenious way to mount the limits. I can't wait to get mine going.