The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)

[awemawson]

The replacement 35 degree diamond insert boring bar has arrived so I did a test cut on the poor old EN8 blank that's been ripped out of the chuck a couple of times by the previous bar.


I  turned the blank to the major diameter and length of the taper, then marker blued the parallel blank.

The program progressively forms the taper, reducing diameter by 25 microns each pass. As the taper forms the blue area decreases showing progress. In the final part it will be very securely bolted to the spindle allowing me to make test fits at stages without disturbing it. Here the blank is only in a manual 3 jaw and the weight of the collet chuck would undoubtedly move it in the jaws.

The finish with these tips is not as good as the previous one being slightly 'torn' so I need to experiment with speeds and feeds again.

[Sea.dog]

Might it not have bee easier to grind some relief on the previous insert? OK, you sacrifice an insert, but you get the job done and with a better finish.

[mattinker]

Might it not have bee easier to grind some relief on the previous insert? OK, you sacrifice an insert, but you get the job done and with a better finish.

Inserts are sacrificial by nature.........................

[awemawson]

I did consider that Graham, in fact I have two diamond disks sitting on my desk awaiting 3D printed centre adaptors to go on my Clarkson T&C grinder spindle but in the end I decided that the advantage of being able to make an undercut if needed, and the advantage of being able to swap inserts on the fly and remain consistent was worth having.

I've just re-run it with the feeds somewhat reduced and the finish is very acceptable. It'll all change again when I mount the adaptor, as that's EN19T not EN8 !

[awemawson]

At long last the taper on the adapter is DONE

This afternoon I mounted up the part finished adapter and crept up on the final size. I may still have to do a bit of adjustment but it's pretty well bang on with the rear flange and the taper closing simultaneously. I may have to trim a tiddly bit off the flange to make sure the mounting bolts pull the taper home for the last tiny bit - not sure.


I used a 'chuck catcher' (just a bit of gas pipe pushed up the spout) to save my toes, and it worked quite well 

A slightly stressful operation with people bursting into the workshop at just the wrong moment  but I got there in the end

. . . enough for today . . . . I still need to check concentricity and check the hydraulic side of things but that's for tomorrow

[Sea.dog]

     

[awemawson]

It's only taken FOUR MONTHS Graham !

Now - why did I want a collet chuck ? Oh yes, because it floated by on eBay 

[tom osselton]

Woohoo! 

[mattinker]

 

[RussellT]

 

[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 ! 

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 

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


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

[awemawson]

So next thing is to dismantle the chuck body and try and find out if I re-assembled it previously with the wrong orientation.

The body is in two halves held together with nine cap head screws. Gingerly undoing them the two parts start to separate by the pressure of the internal springs so I carefully marked the current orientation in case it had to either go back as was or rotated depending on what I find.

Well happily (I suppose!) there are three tapped hole to force the two halves apart if it gets stuck, and I could distinctly see the ghost of the holes in the flange that the screws would bear on, and they are orientated as they should be. So no, I didn't cock up the orientation.

Further proof was shown by a tiny locator hole for a pin in both halves with an equally tiny O ring to retain the pin, however the pin was missing !

So I carefully cleaned up the mating surfaces, removed a minor amount of debris and put it back together  exactly as it was, obviously as assembled in the factory, as I thought there was little point in ringing the changes having found the pin thingy.

All back together, and refitted - needless to say the run out is still there but this is to be expected

So next the big leap: Re-couple the hydraulic lines that I had had to blank off at the far end of the spindle, and see if the chuck actually works or whether it spews hydraulic oil everywhere 

Hoo-blooming-ray it actually works. OK there is a run out issue, but perhaps my expectations of a thirty year old heavily used chuck are too high 

[carlquib]

I'm sure you will have it sorted in no time and the 30 year old chuck will be performing like new.

Sent from my SM-N976U using Tapatalk

[awemawson]

Well never take anything for granted.

Just 'cos I could, I swapped the 10 mm collet for a 25 mm collet, and guess what - run out reduced to 20 microns 

I reckon someone has whacked the 10 mm one that I was using, though it looks OK'ish

[AdeV]

 

Nice work Andrew!

It's hard to say from the video, but is that piece in the last one a bit of raw bar stock? How about turning an oversized piece to fit the collet (possibly on another lathe, unless the collets are easy to swap), turn the oversized end true, then flip it end-for-end & see what the runout is then? I'm just wondering if your raw bar (if that's what it is) is not actually perfectly round.

[awemawson]

Yes Ade you are right (as usual) it IS raw bar, but a pretty good finish. The collets are the '1st Operation Serrated' type, and when I closely examined the 10 mm one there were bits stuck in the serrations that won't help.

When I get a bit of time i'll dismantle them and give them a clean up. They are only held together by an encircling spring clip, with rubber bits between the sections.

 

[AdeV]

Ah, ok; one wonders then, would they necessarily be "gnat's cock" accurate, if they're effectively roughing collets? Can you get 2nd op/finishing collets in that size?

[awemawson]

Oh yes, and they are £140 each new 

[AdeV]

Yikes!

Time to figure out a program to make your own?! 

[awemawson]

Yes a bit pricey aren't they !

It was suggested quite correctly by Ade that the surface of the previous test bar could be the cause or at least contributing to the observed run out of the collet chuck. After all it was just a bit of MS bar stock.

So this morning I mounted up a piece of 30 mm bar that I had parallel ground to a good finish in the J&S 1300 cylindrical grinder and measured it's run out. It's a different collet of course being a different diameter but this probably is typical of what the chuck can achieve at the moment with the rather old collets that I have, and it's state of wear. Perhaps 15 microns run out.

Now I reckon that most of this is going to be in the taper that the collet is pushed into to close, so I'm scratching my head how I can possible grind or lap this in situ on the lathe.

As always, suggestions would be welcomed.

[awemawson]

I'm trying to find figures for what degree of concentricty I should expect from these Crawford Multibore M673 collets but have drawn a blank. However I have found one manufacturer for collets who quotes 30 micron for 'standard' and 20 micron for 'precision' in the 30 mm diameter of the last test where I was getting 15 microns.

Am I chasing ghosts - it seems a lot of run out to me.