Cross slide assembly - balls dropped and balls up!!

Hopefully that's most of the machining done for now. Let's try it out:

Once I'd deburred the edges, I've got a perfect sliding fit in the slot. It needs to be sliding so I can slide the cross slide body into place with the ballscrew / ballnut / yoke assembly in place. Well that's a reasonable start.

This lot fits pretty nicely, although the coupling is very slightly oversize. I'll need to shave that down at some point.

My balls have dropped finally:

However, after playing with the movement for a while, I found something was interfering with the coupling, getting caught between the coupling and the saddle. Turned out to be some of the bearings from the ballnut. WTF??

Obviously these can't escape once the ballscrew has been inserted, so clearly this must have happened during the insertion process itself. The displaced balls were held behind the plastic seal, which finally gave way and released its captives as I played. Bugger.

I've always suspected I'd have enjoy the pleasure / challenge of repacking a ballnut before too long. And today was the day it seemed. The wait is over.

Each of the end seals was held in with a couple of tiny pointed grub screws. Luckily I have a set of tiny hex keys. Although this isn't the smallest one, it's getting close to an inhalation hazard.

99% certain I recovered all the balls, as most were caught in the saddle cavity, with a few in the chip tray directly underneath. Lucky I cleaned the tray recently before starting on this. Now I need to get the buggers back in....

It's very similar in construction to the Chinesium ballnuts I used on the Bridgeport conversion. 

There are quite a few Pootube vids showing how to do it. This one isn't the best quality but actually shows how to do it if you don't have a bucket of new balls to hand:

Looks simple enough. What could possibly go wrong? Well plenty of things come to mind....

Not easy to see but there's a circlip in there (not sure what for) and beyond that there's a plastic diverter. The balls need to be fed in just after / below that. And in order for that to be possible, the screw needs to be unscrewed far enough to expose that section of the ballnut body. Then a pair of pointy tweezers is all you need to push them into place. Actually quite straightforward. Phew.

Finally, replace the seal. Some seals have a groove around the circumference. These have a couple of holes, so the seal needs to be aligned carefully with the grub screws, hence the black markers.

Next - the balls up:

I'm hoping this ground ballscrew will show relatively little axial backlash - at least when compared with the Chinesium ballscrews used on the Bridgeport. Let's assemble it and see what we've got.

Bollocks - that's a 32t pulley and I have around 1 tooth's worth of lost motion before the cross slide actually moves. With a 5mm pitch ballscrew, that's 5000/32  microns or ~150um. WTF???

Using a slightly more scientific approach, I am measuring ~130um of actual axial movement of the ballscrew. Well that's a relief - it's not the legendary Japanese ground ballscrew - but what is causing it?

It's quite simple - the 6200 ball bearing itself has ~130um of axial backlash. These are genuine Nachi bearings, so it's unlikely to be a faulty part. More likely The Stupid Fat Bloke has fucked up again.

Sure enough, only a cursory look through the app notes from the likes of Nachi, Koyo and Schaeffler (INA FAG etc) tells you why. Talk about beginner's mistakes...

• The deep groove (6200) ball bearings don't have magical zero internal clearances. In fact, they have the best part of (wait for it).....125um of radial clearance. This translates into something similar when seen as axial clearance. So they come with some initial backlash.
• To make matters worse, Fat Boy thought he was being clever specifying "C3" tolerance bearings, thinking these would be somehow "better". In fact, they have looser tolerances to allow for operation over a wider (=higher) temperature range. The so-called "standard" clearances would have been better. Oh well...
• To get anything approaching zero backlash, you need to have a pair of back-to-back angular contact bearings. They are designed to have a controlled preload when clamped together in a pair. You can get several steps of preload, depending on the application. However, for my needs, the "standard" zero clearance would most likely suffice. Time to get more stuff ordered up. This is going to require a pair of 9mm angular thrust bearings, rather than one deep groove bearing. I can't see that a double row angular contact bearing would do the trick, although you'd guess it would, otherwise what would be the point? 

From Koyo, you can see that a C6 bearing would be rattling like a sausage in an alleyway. The table only extends to C5, by which point we have 30-40um radial clearance. Not sure exactly how that translates to axial but either way it's not good.

So I've ordered up a pair of Nachi 7200 bearings which will arrive while I get on with other stuff. They will take up more axial length than I'd planned but at a push I can simply  fit 2 of them by using longer fixings for the bearing retainer plate.

So there you go - balls dropped and balls up.