New (to me) chuck, and making a backplate

[smfr]

I'm sure this is all old hat for you experienced machinists, but this was only my second chuck backplate that I've made so a couple of things had me scratching my head.

So I got a pretty good deal on this Bison 5" 3-jaw on eBay:



and you can see beside it the backplate that I ordered. My lathe is a 1962 Emco Maximat "Standard", which has a Myford-style spindle thread: 1 1/8 12TPI. Now backplates with that thread are as rare as hen's teeth in the US. Bob at New England Brass and Tool can supply one, but for $155! So I went with Arc Euro Trade in England, and got two for £16 each, and with shipping it was still only about $100. They are a nice fit on the spindle nose, too.

Once the backplates arrived it was time to turn one down for the chuck. I skimmed the face, then turned down the edge about 1/8", leaving a boss in the middle which was a close fit to the chuck body. I managed to get a nice fit. The steel of the backplate machined very nicely, leaving some pretty blue chips:



Now the next part was to locate the bolt holes. I don't have a DRO, and since the backplate has a hole in the center, it's tricky to mark out with dividers. So I ended up sticking a bit of Al sheet over the center with superglue. Then a small tap on that with a center drill gave me a center I could work from:



Next problem. Machining the backplate had tightened it on the spindle pretty well, and there was nothing to grab onto to get it off! That required a trip to the hardware store to get a strap wrench which did the trick!



For marking out the bolt holes I got the hole pattern from the Bison documents (54mm radius), and used dividers to mark that radius in six spots around the edge. The center point is raised above the edge by just over 1/8", but some quick math showed that it wasn't necessary to worry about how that affected the radius I just marked out. Then, without changing the dividers, I marked off six divisions around the circumference of the scribed circle; that marks off a hexagon, so the alternate points give me the 3 bolt hole locations.

I then clamped the backplate down on the milling table and drilled the 3 holes, cleaned up the burrs, and tested the fit. It was good!

And finally the acid test: how bad was the runout?



I get a max of 0.0012" (after zeroing on the opposite site, which I assume is the normal way to measure TIR?) , which I'm pretty happy with. I also checked the steps on the jaws, and they seem quite accurate, which is better than my Bison 4-jaw.

 

[mzetati]

I'd have marked the bolt circle using the cross slide dial, having it zeroed when a pointed tool (scriber) was aligned with a center set into the spindle.

Marcello

 

[Blogwitch]

I have used those very same backplates, and can attest that they do machine and fit very nicely indeed. I use them for my interchangeable tooling between lathe and mill.

I always drill my mounting holes 0.5mm larger, just to make sure there are no binding forces acting on any part of either the backplate or chuck, and your final runout is about the norm for self centering chucks. In fact anything up to about 0.003" is about right, unless you can get hold of very expensive or truing types.

If it is up to your expectations after the job is finished, then it is a job well done.


John

 

[lazylathe]

Nice and informative for me!!! ;D

I have often wondered about how to go about fitting a backing plate.
You make it sound easy!
I will have to give it a go one of these days!

Andrew

 

[lordedmond]

do as John ( bogs ) has said but reduce the register a little bit about 5 thou


then put in a test bar of your most used size of stock slack off the bolts a tad tap thae chuck with a hide hammer until true nip up the bolts , your own set true 3 jaw



nice work BTW


Stuart

 

[Blogwitch]

Just as an aside, I also used one of those cheap backplates to make a clamp plate for my rotary table, which also now has a Myford nose on it.

http://madmodder.net/index.php?topic=2002.msg22748#msg22748

But it could easily have been used on the lathe instead of a faceplate or drive plate.


John

 

[steamer]

.0012" is great for a 3 jaw! That'll work just fine.

It will be different with the jaws at different diameters, but that's to be expected from a 3 jaw.

If you want the limit of runout, then you need a 4 jaw or good collets.

Great Score!

Dave

 

[Hopefuldave]

If you have the register for the chuck turned to size (I usually go a little on the tight side), marking the bolt locations is easy - You need Transfer Screws

Take a few capscrews the correct thread for the chuck, and turn or grind to a point (it needs to be nicely concentric with the screwthread) then file or grind a pair of flats on the pointed ends so you can grip 'em with a pair of pliers - then cut off the bolt heads and clean up the threads, screw them into the chuck so about 1mm / 40 thou" of the point is projecting, marry up chuck and backplate and administer a thump with a mallet / deadblow hammer - hey presto, three (or however many you need) centre-punch marks on the axis of the screw holes in the chuck.

Much, much easier than trying to mark out the hole locations, and a damn sight faster. Works well when you need to mark out a cover for an existing part, too!

My Grandad (a toolmaker) taught me this, as I sat on his workbench, aged 10...

Dave H. (the other one)

 

[gmac]

smfr;
How long did it take to receive the order from Arc Euro Trade? How were they shipped? Like you I balked at the price for a ready made chuck adapter on this side of the pond, but at that price for two....hmm. Thanks for the post!
Cheers
Garry

 

[krv3000]

HI a job wel dun

 

[xo18thfa]

0.0012"!! Doesn't get any better then that. TIR on my 3 jaw is all over the place switching between the regular and reverse jaws.

 

[Blogwitch]

You can sometimes get the runout to be a little better by tightening up using the same chuck key hole each time.

Sometimes there is a mark on the chuck, that is the master hole used by the factory to set the chuck up, but if you don't have one, just go around each hole in turn and find which one gives the most stable and best results by opening and closing a few time and check the results with a clock. Then mark that particular hole, and only use that one.


John

 

[smfr]

gmac: it took about 6 days from ordering to receiving. They ship via TNT Express which you can track on the web, and it came in via Newark, NJ. Shipping was £37. I'd certainly use them again.

Simon

 

[smfr]

I made up some Al soft jaws for this puppy. It was pretty easy to find the two-part jaw dimensions online.



I would have made them longer if I were doing it again, but these get some use anyway.

Simon

 

[gmac]

Thanks Simon. It can cost me more money and take more time to just ship across the country. I think Arc Euro will get my business!
Cheers
Garry