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I am currently bidding on a pre-machined 8mm ballscrew on eBay (one of the Marchant-Dice auctions). The shaft end is machined to 6mm OD. Looking for support bearings to suit this in the bearing catalogues I am rather drawing a blank. I see that MD sell a pre-assembled block, but I won't be able to use this due to some rather tight space constraints in my retrofit project. (I can't use a bearing with an OD>18mm)

Would Magneto bearings work OK in this application? Magneto bearings are only available with 5 or 7mm bores but this is a surmountable difficulty.

Can a pair of deep-groove ball bearings be preloaded to work similarly to angular contact bearings with enough care and shimming? The geometry is not that different, after all.

In the end I gave up on the preloaded deep-groove balls, there wasn't really room. I found a way to fit some 7200 angular contact bearings instead by moving them outboard of the saddle to the handle housing, and using a needle-roller bearing adjacent to the belt pulley to take the belt tension forces.

I have now installed the ballscrew (pictures below). The shape of the ballscrew housing is an idea I stole from the Smart and Brown Model M (1946 model) which allows a spring steel shim to pass through a tunnel over the nut to protect the leadscrew.

Anyway, to the point of this post: It appears to be slightly more "springy" than I was expecting (about 0.1mm movement on the feed knob). This may prove not to be a problem and I  might yet find it to be in the thrust bearings,  but in the event that it is the ball-nut;
Is this likely to be a consequence of the small size of the ballscrew, 8mm is pretty small compared to most CNC conversions I see, or might I expect a significant improvement by upgrading to a C5 class ballnut?
Is it possible to just swap the balls for oversize ones to convert a C7 to a C5? I have seen instructions on the web on re-filling ball nuts, and it seems like it is fiddly and tedious more than difficult.

"Is it possible to just swap the balls for oversize ones to convert a C7 to a C5? I have seen instructions on the web on re-filling ball nuts, and it seems like it is fiddly and tedious more than difficult."

Class 7 or Class 5 bear no relation ship to the balls.....Class is the accuracy of the Ballscrew spindle per 300mm travel

JIS Standard:
C7 = 0.052mm/300mm
C5 = 0.018mm/300mm

Upgrading the ballnut may improve the run (feel) of the assembly...Class 5 having a ground balltrack rather than the class 7 which is screw cut.

It will be interesting to see how long the assembly survives....8mm is rather small?

Kevin Marchant:

Class 7 or Class 5 bear no relation ship to the balls.....Class is the accuracy of the Ballscrew spindle per 300mm travel

I was going by what you say at http://www.worldofcnc.com/images/RSW%20Ballnut.pdf which suggests that the difference with these nuts is in the balls.

Kevin Marchant:

It will be interesting to see how long the assembly survives....8mm is rather small?

It is, indeed. There isn't any room for anything bigger, though, and it has a greater OD than the original leadscrew. Not necessarily stronger, I realise, as the load bearing area is smaller, but made of stronger materials. (the original nut seems to be a Mazak casting)

Thinking further about the size of the ballscrew. It seems to me that there is a lot of wasted material in the ballscrew I have used. In principle I think it would be possible to grind a larger size square, leaving the ball-return tube in one corner. Possibly a very stupid thing to try. I assume that the whole housing is bearing-hard and unmachinable? 

That train of thought, however, led me to look at the ISEL combined ballscrew and housings. It is not clear from the drawings, but are the ball-tracks in those a separate part or integral with the housings? I Think with some ingenuity (and too much spare time on my hands) it might be possible to make a custom housing for those internals to allow me to use a usefully larger ballscrew. 

Making a whole new saddle and cross slide might be easier, I realise. 

I to had space limitations leading me to buy one of the ISEL type 2 integral ball nuts through Marchant Dice.  The ball race is integral to the housing as you suspect.  The stock item I got had the lubrication port pointed in the wrong direction, right into the mounting plane so I asked for advice on how it was assembled so I didn't damage anything while trying to turn it to the orientation to suit my application. 
Attempts at getting support through their German headquarters were totally useless and the US support appeared marginally competent at best.

Bottom line, the ball nut looked promising but the lack of application support from the manufacturers was stunning to say the least. 

If I figure it out before I design them out, I'll let you know.

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Kevin's Reply......The ISEL Ballnut is a standard product....if we had received an email, The housing / ballnut design is simple and robust, I would have advised to work around the issue, tampering with ballnuts is something customers should not attempt! I understand that many custmers like to "tinker" with products that took months to develop...however this does invalidate the warrantee..

Andy,

Since the stock ball nut was not as I had invisaged and several emails to ISEL went unanswered, I took a wrench to the oil fitting and found it was just pressed in.  No "work around" needed.  Since I was there, I removed the oil port and put in an oil line fitting for a one shot lubrication system.  The same holds true for the type three ball housing.

Andy, 

Someone is changing my posts!.  If you want my true reply, contact me directly at pbarratt@sover.net before this post gets changed/deleted.

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This site is for constructive comment only, not a place to rant....so being constructive: you have resolved the issue and we can all learn from your efforts! many thanks / Kevin