The Craftmans Shop > New from Old
The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
awemawson:
Today I started looking at the inevitable run out. This is a well used collet chuck.
Firstly I put a DTI on the internal operating taper of the closer and was appalled at the run out:
Then I realised of course that the taper slides in and out using the internal hydraulics - there is no collet in place - not even the front plate that retains the collets so until I've sorted the hydraulics that's not going to be a fair test.
But what about the body of the collet chuck - it was probably made to be pretty true though not the primary accuracy element. Sure enough it was dramatically better than that internal taper. However I'm still not convinced that the mounting taper is seated fully into the chuck as there should be a microscopic gap between the rear of the chuck and the mounting flange, and there isn't.
So I dismounted the chuck to skim a tad off the flange and measure the concentricity of the A2-5 mounting taper while it was off. Dead nuts ON as it should be having been turned on the lathe.
Measuring the chuck body run out it was marginally reduced but not zero, but there again I've probably not skimmed enough off the flange as I still can't see a gap at all
So it's still a work in progress !
awemawson:
In a flash of inspiration I realised that I COULD fit a collet without the hydraulics, as the internal bias springs are pushing closed and pushing the collet retaining plate off. By bolting it back on it closes a collet, though not as forcibly as the hydraulics will.
So suitable collet fitted and 10 mm ground pin inserted.
I'm afraid the news is not good - still showing 100 micron / (4 thou) run out
awemawson:
As respite from mowing rather a lot of wet grass (10 acres, 7 hours, 36 litres red diesel) I got back in the workshop to look again at the chuck location.
I needed to make and fit a driving peg which meant taking the adapter off the spindle, but first I did a 'sanity check' on the taper runout -zero as before. OK then I removed the adaptor and with it on the bench blued it's taper with a light smear of hi-spot and checked the taper fit. Very nice surface contact was revealed. Then I made up the actual drive dog - 15.9 mm mild steel drilled 6.1 mm and countersunk for an M6 CSK screw. I also made a drilling jig from the same 15.9 mm bar drilled 5.0 mm to help align the drilled and tapped hole that I needed to make in the drive dog recess.
Now either this adapter is more flexible than I would have thought or some other phenomenon is occurring. With the chuck sitting on the adapter and bolted hand tight I can measure a gap of 30 microns between the flange and the chuck body. If I tighten the six M10 screws murder tight (as they should be) it closes the gap so not only will the feeler not enter but I can't see light through the gap. So the taper has been engaged a full 30 microns further 'on'.
I re-mounted the adapter - checked the taper run out which pleasingly was still zero and skimmed a further 30 microns off the flange and remounted the chuck. Same thing, if bolts hand tight a gap, but if driven fully tight then the gap closes to zero.
Now I am assuming that the major concentricity alignment is the actual taper faces, the purpose of the flange and bolts being to hold the taper in engagement and if it 'bottoms out' this is a good thing for rigidity and load capacity. Well it certainly bottoms out when torqued up!
Chuck body run out was unaffected by these shenanigans and I didn't try fitting a collet as there is no reason to suppose it will have magically improved.
I think then that the next action is to whip off the chuck and adapter, fit the hydraulics O ring seals and oil way bungs and try it 'for real' I may dismantle and clean the chuck in this process but it is crawling with springs and pistons and seals and goodness knows what else.
awemawson:
Contemplating dismantling the collet chuck got me thinking. The rear end of the collet chuck isn't a backplate as such but performs the same function. Just maybe it is OK and the quasi-backplate to rest of the collet chuck interface is the issue!
So I steamed the machine up again and this time ran my DTI on the quasi-backplate. Well I never - the chuck backplate has zero runout so maybe the chuck body to backplate interface is the problem ! :ddb:
Now when I got this chuck and was thrashing about trying to find a way of mounting it, I managed to have a conversation with one of the engineering Directors of the late Balding Engineering company that made the beaver, and he said that some of the collet chucks had detachable true backplates. Well as an investigation I pulled it apart when he was on the phone, found that it was not a true back plate and put it back together - all one handed while holding the 'phone. Did I possibly not get the rotational position correct and is this the issue?
Hopefully a more controlled dismantling and cleaning will reveal all :scratch:
Meanwhile see the proof in this video :
awemawson:
So my conclusion is that my chuck adaptor is as concentric as is reasonable to expect and the run out that I'm seeing is in the chuck itself.
(There is always the tendency to suspect your own work first I suppose!)
Thus my plan today is to once more remove the chuck AND the adaptor, and ready the adaptor for hydraulics. This involves loctiting grub screws in the oil galleries to block them off after a thorough cleaning, and refit the adaptor with O rings.
I'm pleased to say that both tapers were the very devil to separate so they must be good fits :ddb:
Once the adaptor was back on the spindle I again checked its runout just in case the O rings were misplaced or too fat etc, but glad to say all was well
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