Modding a tiny chuck to fit a spindle with a smaller nose thread

[andyf]

Many years ago, I paid £25 for a ‘60s secondhand Perris baby lathe, which is still produced with few changes as the Cowells 90ME, though costing 85 times as much!!!  One change is that the spindle nose thread was ½” x 20 UNF on the Perris, M14 x 1.5 on early Cowells machines, then M14 x 1 on modern ones. My Perris came with a 4-jaw, but I’ve always hankered after a 3-jaw scroll chuck for speed when near enough is good enough. But ½” x 20 chucks seem impossible to find; even the M14x1.5 version is rare. And these 2.5” chucks are so small that there’s no meat behind the scroll to fit studs for a backplate.

I recently went to one of Chester Tool’s open days in Hawarden to buy a bandsaw, and found an M14x1 lever operated 3-jaw going cheap. Fitting a bush in it looked do-able so it followed me home. Here it is, both assembled and dismantled. It’s held together with a circlip at the back, with a washer beneath. Not exactly quality engineering.





The minor diameter of M14 x 1 is 13mm, and the major diameter of ½” UNF is 12.7mm. A bush to those dimensions would have impractically thin walls, so the chuck body needed to be bored out and rethreaded. I opted for M17x1 (minor diameter 16mm); not exactly a standard thread, but that doesn’t matter.

To get started, I mounted the body of the little chuck in the 3-jaw of my larger lathe, and bored it out to 16mm. The boring bar is home-made (obviously) and holds a bit of round 3/16” HSS. The diameter is reduced at the end, to clear the 14mm diameter which was being left at the front of the chuck body where the jaws fit. The chips came off as a powder, much coarser than cast iron gives, so I don't know what it's made of.



I have no bore gauges, but the micrometer showed the shank of a 16mm end mill to be 15.99mm, so I bored away until that was a tightish sliding fit, going to a depth of ½”



Next job was the infernal internal threading using a similar toolholder, though this could be a bit fatter now the hole has been bored out. The lathe has no back gear, so I used a spindle hand crank.



The thread ended up nearer M17.5 x 1, due to a bit of a mix-up near the end over which way to turn the top-slide handle which was towards the back of the lathe  so boring to 16mm has been a waste of time. Never mind; I’ll just make the bush to suit.

The work was gripped in a 3-jaw, so the threaded hole and the bush are probably a tad off-centre, but that doesn’t matter. What will matter is getting the ½” UNF hole though the bush in the right place, of which more anon.

Andy

[andyf]

Before going to the pub on Saturday night (this post is sent now I'm home again wearing my beer goggles, so I hope it makes sense) I made a bush with an external thread to suit my 17.5mm(ish) thread through the chuck body and a thin flange on the end. The bush was formed on the end of a 2” length of 1” round MS bar. First step was to mill two flats where the flange would be, allowing a 20mm spanner to be used to tighten the bush into place, and equally importantly to unscrew it if I made another internal threading cock up and had to start again. I used hi-tech red tape to mark where the flats needed to be, rotated the stock after milling the first and eyeballed a rule held up against it to get the second one more or less parallel.



Then, lopped it off with enough spare to hold in the lathe chuck, turn it down to 17.5mm and  screwcut it until the chuck body would screw on.



The threaded section was made 2mm or so too long, so it could be faced back to just the right length. Once that was done and while it was still in the chuck, I drilled it 10.5mm to be bored out later to 11.5mm ready for internal threading. Finally, I parted/sawed off the bit I needed. Here it is, showing it in and out of the chuck. You can see how far my parting tool reaches by where the sawcuts start. 


 
So that’s where I’m up to tonight: chuck body bored out and threaded, and an embryo bush which screws into it, with a rough outside end and a plain 10.5mm through hole. I’ll tidy up the rough end tomorrow, before boring the hole and threading it. That will be the tricky bit.

Apologies for the mixture of imperial and metric units, but I am equally disloyal to both systems, using whichever seems handier at the time.

Andy
 

[andyf]

Right, this morning I had slept off the beer, so it's time to finish this chuck off    .

Here’s a Perris spindle. I have now fitted one in better condition, and the old one will save me making a dummy spindle nose.  The first 0.375” is threaded ½” UNFis with a ½” register diameter 0.125” long. The overall length is around 5.25”/135mm, so it’s definitely miniature.



The 10.5mm hole in the bush needed boring out to 11.5mm, the minor diameter of the UNF thread. In a gesture towards concentricity, I assembled the little chuck with the bush Loctited in place, and gripped it on to a 12mm diameter silver steel (drill rod) bar which had been persuaded to run true. There it would remain until the boring and threading was done. The rough flange end was reduced with facing cuts to about 3mm thickness. With no boring tool small enough, I used one of the remaining good tips on a chipped 3/8” end mill, held at a slight angle to the lathe axis for clearance, and used the shank of an 11.5mm drill bit as a gauge. The micrometer showed the shank to be to be 0.07mm undersize, so after getting a tightish fit I took off 0.07mm more. Pics of that bit would be far too boring (sorry!).

I said in my last post that there would be a tricky bit: single-pointing the internal ½” UNF thread. It wasn’t practical to thin down the little threading tool holder I used earlier for the M17x1 thread; there would have been too little metal under the cross-hole.

A few days back, I spent what seemed like half a day pressing a 6mm tool blank against the grinding wheel to make this little threading tool.



Obviously, the tool lacks rigidity, but the thread is only about 0.6mm deep, so multiple light cuts wouldn’t take too long. A day or two ago, a trial run was made on a bit of bar, using a tap to finish off. After boring a ½” register just over 1/8” deep, the test piece was a nice fit both on the spare spindle and on the one in the lathe.

So, on to the real thing: my little chuck was still on the lathe ready for internal threading which went OK this time (phew!) followed by the tap which was supported by a tailstock centre to keep it in line.



All that now remained was to bore out the outside end of the thread to fit the 0.500” register on my spare Perris spindle, and a bit of deburring.

So, to end this saga, here’s the baby chuck in place on the baby lathe.



A quick trial holding a 12mm silver steel bar shows 0.004” TIR next to the chuck and the same at 2” out, which is reasonable for what it will be required to do.  Other diameters may exhibit different TIRs, of course; I don’t expect wonders from what is just a cheap little Asian item.


If you have been, thanks for looking.

Andy

PS I might mention that a Madmodder in the US sent me a PM yesterday, offering me a very similar M12 x 1 chuck for the cost of the postage to the UK. The one I've been working on seems to have turned out OK, so maybe someone in greater need will end up with his. But it really was a very kind offer.
 

[caskwith]

PM sent, I could certainly make use of the other chuck you mention

[mzt]

I had the opposite problem, an M14 threaded 4 jaws chuck needing a 20mm register and 4 through holes for front mounting.
As easy as facing the threaded bulge on the back of it, then machining the 3mm deep recess.
It worked well, till the day I gave a "nice, good twist" to the jaws. 
The wp. got loose: it took a while for me to understand why.    --->
I still have the four parts into the drawer, might get around making a new body in steel someday.
Marcello

[andyf]

PM sent, I could certainly make use of the other chuck you mention

Hi Caskwith,

M14x1 was a typo on my part; the chuck I was offered is actually M12x1. You would need to bore it out and re-thread it. If still interested, let me know and I'll put you in touch.


Hi Marcello,

I know it's unkind of me to laugh at your 4-part 4-jaw, but      

But I'll remember your experience and not tighten mine up too hard. Its weak points will be the scroll and the teeth on the jaws which fit into the scroll.

Andy

[Dean W]

You did a lot of good work to reach your goal, Andy.  Well done.
Just for future reference for others that may have the same Perris lathe, Sherline makes their excellent little 2.5" high
quality chuck in 1/2-20 unc thread. 

[andyf]

Hi Dean,

I did think of a Sherline chuck, because I knew that the Australian Clisby, from which it evolved, had a 1/2" x 20 nose. But looking at this Sherline page http://www.sherline.com/1040inst.htm , it seemed that this had now changed to 3/4" x 16, and further ferreting around on the site disclosed nothing in the smaller size. However, it seems I didn't look hard enough; this price list http://www.sherline.com/ChuckPlist.pdf shows one. However, it's $120; shipping/import tax/admin fees would probably jack that up to around $160, which is about 5 times what I paid. Granted, mine may not be of the same quality, but it seems to work fine, and I'm retired, so had the time to make it fit.

Regards,
Andy.

[Stilldrillin]

Andy.
That was a quality saga, with a good outcome. 

Well done Sir! 

David D