Cutting a Register on a Chuck Back Plate

[Nick Hacking]

Dear All,

I have a nice chuck that I bought on eBay a year or so ago: it was cheap because it doesn't have a back plate fitted. I plan to use it on my Myford lathe.

One of the hobby engineering companies in the UK sells blank back plates threaded for a Myford spindle, so I've bought one of those. Now I need to cut a register on the face to accept my chuck.

The most precise and easy way to centre the back plate is to simply screw it on the spindle. But: there's quite a gap between the end of the ways and the headstock so any tool is going to stick out from the compound by several inches. My little lathe seems to work best when everything's really rigid.

Plan "B" is to grip it in the 4-jaw chuck. I'll be honest, I hate the 4-jaw because no matter how I try halving the error on the clock, it always takes me ages to get things running true. That's one reason that I bought a new self-centering chuck (lazy, I know). This said, if it's the better method, it is do-able.

What's the group's opinion please? Or is there another way that I've overlooked?

All help and guidance gladly received...

Kind wishes,

Nick

 

[mikey]

Hey Nick. I have never even seen a Myford but the pics do show the gap so I can imagine the issue. I agree that you should machine the plate with it spun on to the spindle; do not use the 4-jaw.

I would use the biggest lathe tool you have and make sure it is sharp. HSS will work well, although carbide works, too. You are mostly going to be doing facing cuts and you do not need to take huge cuts to do that. Get as close as you can to the work. You may need to extend the compound a little bit, then snug the compound and saddle locks and face with the cross slide.

You may be able to do the job with a boring bar if you have to. A carbide shank boring bar would be ideal.

Another option is to bolt a heavy solid bar to the cross slide in place of the compound and bolt your tool post to it. Called a plinth, this would allow you to extend the position of the tool post so you can get on with it.

Let's hope the other guys can come up with some better ideas.

 

[Nick Hacking]

Mikey, thanks so much for a solution that doesn't involve an hour of turning the chuck, loosening, turning back, tightening, re-checking the run-out, turning again...

A plinth? I've never heard of one on a lathe but it sounds like a good idea. The Myford cross-slide is slotted so it's easy to bolt things on to it. I wonder if I could build one with a vertical support at the headstock end? Running on a couple of bearings perhaps? Do you think that might reduce chatter?

Much obliged,

Nick

 

[mikey]

A vertical support under the headstock end would be an excellent idea except that the support would have to move with the cross slide. I suppose it could be done if you laid a bar across the ways. Not sure its worth the effort unless just the plinth isn't sufficient. If you make the plinth substantial, maybe 30mm thick or more (maybe as thick as your compound is high), then it shouldn't flex enough to make a difference provided you take light cuts.

Replacing the compound with a plinth is a common mod nowadays, and you may wish to use it instead of the compound for everything except taper turning and threading. It is much more rigid than the compound and will improve the function of your lathe. Do a search here and on the net and you'll find a wealth of info.

 

[P. Waller]

I often use a right hand boring bar held upside down with the spindle in reverse to cross the gap.
3" diameter inset bar hanging 10" out to cross the gap, this is not ideal but the gap on this machine is 15" wide.

Removing the gap

 

[Nick Hacking]

Thanks to both of you, you've given me some ideas to work with.

I have two schemes in mind for a sort of "extension" - either a pair of sliding dovetail joints, one above the other, at 90 degrees to each other, with a tool-holder on top (shims to adjust the height of the tool) or maybe a threaded bar to use the QCTP... OR the same idea but sliding on polished rods in long brass bushes. I've pondered on the idea of simple bearings but on reflexion, it seems too sketchy. There would be a a lot of lateral force on them and that wouldn't really do, would it?

I think I'll try the rigid tool, upside down and in reverse idea first. The thing that worries me a little is that my backplate fits to the spindle with a thread so that could go very wrong, very quickly. Some sort of mandrel (is that the right word?) down the bore might stop the backplate from unscrewing itself but then machining the centre, where the mandrel restrains the backplate, is going to be problematic. I suppose that could be finished off in the mill...

a lot to think about!

Kind wishes,

Nick

 

[mikey]

I would be leery about applying any cutting force to a plate mounted to a threaded spindle running in reverse.

 

[P. Waller]

I think I'll try the rigid tool, upside down and in reverse idea first. The thing that worries me a little is that my backplate fits to the spindle with a thread so that could go very wrong, very quickly.

Nick

If not facing to -center use the tail stock to push the back plate against the spindle, it will not unscrew and does not have to be accurately centered as it is only pushing the part into the spindle. I do this often with large diameter yet thin parts that are only lightly held in chuck jaws.

This is not the same operation but the same idea, had to part 6"OD X 3" ID X 1/8" thick aluminum washers from 6"+ solid bar.
Drill then bored stock to 3" Diameter X 2" deep, placed a center plug in the bore and pushed the part against the chuck with a live center and parted off 3 or 4 parts, bore again then part another 3-4 parts, worked a charm.
Even if the parting gets out of hand it is not leaving the machine when pinned between the tail stock and chuck, the worst that will happen is that the tool breaks, parting tools are cheap but ripping a piece of 6" stock out of the chuck and flinging it across the shop is not as cheap (-: