Ball nut conversion - work in progress

I've now completed my first ball nut holder - and made all the jigs to allow the next "batch" to be made much more quickly and accurately.

Material is aluminium bar 7/8" x 1/2" grade 6082T6. A total length of 6"" (152mm) is sufficient for 4 holders and I bought mine from Forward Metals in the UK (they seem to be very good value for money).

I had some issues - yet to be resolved, when machining deep into the aluminium. This resulted in a chipped cutter (3mm) with aluminium chips welded to the cutting edges. This may have been due to tool overheating because of friction against the already cut part of the workpiece - perhaps a little lubrication might have helped here because there were some nasty noises coming from the workpiece!!

When I cut the central hole the cutting speed was set to 10mm/sec, which looked frighteningly fast to my inexperienced eye even though the cut depth works out to 1/60mm at 18000rpm on a 2 flute cutter. The profile was cut at 5mm/sec because I went chicken on the speed - maybe that was the wrong decision and the cutter just heated up through friction - hence the welded aluminium on the cutter. Cutting depth was 1mm in both cases.

Attached some pics of the making of the adapter - the final finishing was by hand because I wasn't able to complete the final profile cuts because of the tool problem.

Hi Peter.

Good work. I have been busy progressing my guitar build so have not checked into this forum in a few months.

This is one thread I’ll be following.

I recently tried to mill a 10 mm thick block of 6082 T6 aluminium with a 4 mm diameter end mill and it was partially successful but it sounds like I had similar problems to you.

I previously had no problems milling a 20 mm thick block at 0.1 mm depth per pass, 7,500 rpm spindle and something like 750-1000 mm/min so I don’t know what was different this time.

Doug

Hi Doug,
Its a long time since we communicated - hope the guitar project is progressing well towards completion.
My interest in the ballnut conversion was re-awakened by Fafinos who completed a conversion using the original ball screws and SKF ball nuts (from RS)

I think the machining problems I had were due to friction between the sides if the tool and the workpiece. I plan to make some more adapters soon, and have reverted the cutting speed to 10mm/sec at a 1mm cutting depth, however I have reduced the tool step-over to 50%. This should reduce the heat build up at the cutting edge. Additionally I plan to stop the cut every 2 or 3mm and apply some light lubrication to the machined walls - this will give the tool a chance to cool as well.

I did try water lubrication on a trial cut, but just ended up with a pudding of chips that were building up to a solid mass as depth increased - I stopped this for fear of breaking the tool, so now I tend to machine dry. Bigger machines tend to use flood cooling which also washes away all the machined swarf - not really practical for me anyway.

I'll update this thread when I've some more useful information.

Peter

If using the SKF ball nuts, be aware that even with the inner retaining sleeve in place, balls can (and do !!) escape from the tracks!!

If this can happen, the sleeve must be undersized - I'll contact SKF for comment on this.

I was handling the nut with the sleeve in place when, horror of horrors, I noticed one ball on the workbench. I wonder how many dropped out when I wasn't looking?

Don't panic!!

It is possible to replace them by carefully withdrawing the sleeve until the thread track with the pick up hole is just exposed - then any escapees can be carefully reloaded into the track on the correct side of the pick up hole and the sleeve re-centred. Small tie-wraps on the sleeve make sure it is securely captured whilst still allowing the nut to be handled and fitted into the adapter.

If this happens again I might make a jig to hold everything "just so" whilst replacing the balls - I'd hate to drop the lot out if my hand jerked.

Peter

Hi,

Major disappointment today - fitted a ball nut onto a replacement Z axis ballscrew for the first time. It was a real struggle to get assembled and the whole assembly is far too tight to be of any use!! I used the same SKF ballnut (SH 10 x 3 R) as FaFiNos did in his modification, so can only conclude that the ballscrew is different.

Guess the backlash is -0.07mm rather than the specified +0.07mm !!!!!!!!!!!!! I'm not sure how to proceed from here, but am really glad I only splashed out on one ballnut rather than a set of four.

I suppose the options are :-

1) change the ballscrews to SKF manufactured one's and have them machined to match the Stepcraft dimensions.

2) change the balls in the nuts for undersized ones - I'll need to contact SKF for this to work

3) Abandon this modification entirely - certainly I will in the short term.

Peter

I'm sorry to hear of your continuing difficulties Peter. I doubt you will take any solace from the fact that you're preventing several other people making expensive mistakes.

I hope you can get this modification installed eventually, mainly for selfish reasons because I appreciate the detail with which you describe and illustrate the processes you are working up.

Doug

Hi,

Still in a quandary as to the next step, "to convert or not to convert, that is the question" !!

I've tried running the ball nut from end to end of the screw with the help of my mini lathe, and this has had some benefit in significantly reducing the torque required. It is however way higher than one would consider acceptable. Currently it varies between 500 and 1000gm cm (I dont have a set of Newton weights 😆 ). Torque measurement was very basic - I've attached a picture of my setup, at least it allows any changes to be quantified.

I carefully measured the diameter of a few of the balls, and am going to contact SKF to see if they can supply sets which are 0.005mm or 0.01mm smaller then the 1.955mm set fitted at the moment.

Peter.

Hi Peter,

Sorry to hear that you are having difficulty with the amount of torque required to turn the completed Z-Axis, did you find that removing 2 x balls as per FaFiNos note made no difference at all?

I was potentially planning to make this change to my Steppi, and have been watching your thread with interest. I hope that you are able to resolved the difficulties with the conversion and get back to a satisfactory end result.

Pete

Hi Pete,

Afraid it made absolutely no difference at all - which is a real bummer.

With a properly fitting ball nut I would have expected virtually zero torque, so something is seriously amiss with the leadscrew dimensions - or perhaps the ball nut is out of tolerance, though this is unlikely with a reputable manufacturer such as SKF.

I have an enquiry out to SKF outlining the problem, so things will be on hold until I get some feedback from them. The other alternatives are to replace the leadscrews as well, but that is, if you will pardon the pun, a whole new ball game.:) I don't really want to go there!!

Regards,

Peter

Hi all,

I am facing similar issue. I have a stepcraft D600 v2 with performance kit. I changed X and Z axis with the SKF ball nuts. The X axis works without issues, but the Z keeps giving issues. I also even lost a ball in the nut because the delivered pipe was too small (is that a problem for functionality?) . It rotates but the torque required is much higher than on the X asis. I think this has also too do with the length. It sounds also horrible.
I also contacted supplier of Eichenberger ball nuts, and they told me that the specs of the screw has to match the nut too ensure a proper functionality. So far, I did not find any specs of the screw from Stepcraft. So maybe that is the cause that it don't work...
I am about too give up. I just want a reliable machine that doesn't loose steps and I am completely fed up with it..

Kind regards,

Peter de Graaf (other Peter -:).