View Full Version : Chuck Backing Plates - Threaded Spindle

04-24-2006, 12:21 AM
What percent of thread engagement should be used on chuck backing plates for threaded spindles?

I need to make one for the Atlas/Craftsman that is 1.5 - 8.

Thanks, Ken

04-24-2006, 01:19 AM
Short answer; I don't know. But the alignment is going to key off the counterbore and collar. I've made all mine (so far) so they spin up without too much fuss, class 3 maybe. Soon as it hits the unthreated part of the spindle nose, it takes a tommy bar to coax to the rest of the way to seat. I could be quite wrong about this, advice refunds available upon request.

04-24-2006, 09:43 AM
I'm gonna agree, and say that they do not need to be a super fit, as long as the back plate bottoms out against the spindle boss. That will almost guarantee alignment every time.

04-24-2006, 10:22 AM
Don't confuse "percent of thread" and "closeness of fit" (i.e. "class of thread"). They aren't the same thing. You could have a 50% thread that is a very close fit.

Class of thread is, I think, more important in this context than percentage of thread, but like Argo and gizmo2, I don't think you need to worry over-much about it. Get a "good fit" on the threads and good uniform contact between the flat surface of the back of the plate and the spindle shoulder it bears against, and you'll be fine.

04-24-2006, 10:34 PM
I made an Alu. face plate for a friend who bought a South Bend 10L with out a chuck. My lathe jumped out of gear and messes up the threads. I ended up cutting the sloppest threads of my life but when you tighten the face plate on the lathe everything works out just fine. I got a chuck off Ebay for him without a back plate. I told him he could use the face plate I made him to attach a backplate blank to and machine it to fit his new chuck. The next time I went over to his shop he was using the alu. face plate as a back plate for the chuck. He had attached his chuck the faceplate with four 3/8-16tpi socket head screws. He asked me what I thought and I said I did not think it would hold up but who knows. He said he would make a proper backplate out of steel when he found the time and the right size hunk of steel. Gary P. Hansen

04-24-2006, 10:39 PM
I should have added the best way to turn up a I.D. thread for a backplate is to first turn a O.D. that mathes your spindle and use it as a go-no go gauge when you thread your back plate. Gary P. Hansen

Tin Falcon
04-25-2006, 08:14 AM
POI www.littlemachinshop.com hase 4" and 6" back plates pre machined for $20 and $30 respectfully not the best material and workmanship but not bad. buying the material will likely cost more .

I just did up a couple with a 1'-16 thread from cast irion and the theads ar a little cleaner than the ones from littlemachineshop.com . I second Gary's suggestion of making a gage first also if your lead screw is an 8 pitch you do not need to wory about a thread dial.

Norman Atkinson
04-25-2006, 08:24 AM
Sorry folks- but I disagree!

It is the register of the collar( for want of a better word) that matters.
All the thread does is to mount and screw the chuck and then hold it in position- on the register.

That's the way it works on my Myfords.



J Tiers
04-25-2006, 10:14 AM
Here we go again..............................

Are you a Shiite, or a Sunni?

04-25-2006, 10:30 AM
It may be the way it works on Myfords, but my South Bend lathe doesn't have a registration collar...and it does fine.

04-25-2006, 10:33 AM
It has been thrashed over here before, and there are as many adherents to the value of the register for alignment as there are who feel it is of no value. Some manufacturers do not use a register at all.

There is a theory that the taper of the threads will force the backplate into center. I can neither prove or disprove this, but I am a register guy.

Rudy Kouhoupt and Frank McClean both stressed the importance of the register. Scott Logan has said it has no value, yet his website refers to Tony's website for directions in making faceplates. Tony's site stresses the importance of the register fit.

At any rate, the best advice I have found is to carefully make a stub spindle that matches the spindle of your lathe, and use that to get the best fit you can when making a backplate.

You will have the backplate and lathe for quite a while, and spending a few more minutes in making the best fit possible will not be wasted.

BTW, here's a link to Tony's website directions for making a backplate;

And here is a link to FAQ's on the Logan site recommending fitting the register to match a Logan lathe. There appears to be some inconsistancy between Scott's advice here a few years ago and what their website offers;

04-25-2006, 01:55 PM
Ken, Now look what you have done.

Bicker , bicker, bicker.;)

Norman Atkinson
04-25-2006, 02:53 PM

As South Bends are bringing only £100 sterling- or two tanks of gas on the UK market, would you like to give an honest price of what these comedians are prattling about.

I have followed SGW's edict to the letter.
" Don't believe everything you know"

JT- do you really mean that instead of having one of my cars in the garage that I could have 300 South Bends instead.

Happy me


04-25-2006, 07:36 PM
Thanks for all the tips and links. The A/C has a mounting face/flat on the spindle for back plate registration. Even with this registration, the original chuck seems to be a snug fit when installing over the threads. I've got a Bisson 6 1/4" that I want to use so I may just spend the $40-60 and buy one.

I'm wondering, other than the galling threat, why Tony prefers cast iron back plates over steel? The cast iron versions are cheaper but I can't see why they're better than a steel back plate. It seems the other way around.

Any opinions without kicking up more dust? ;)

04-25-2006, 08:30 PM
According to Tony's website; "At all costs avoid steel backplates; they will bruise or damage the spindle nose and, and if they become stuck, will be much more difficult to remove."

04-25-2006, 09:04 PM
Yeah, I've read Tony's opinion but I don't agree with it. There's got to be more to it than that. I suppose if you let a monkey mount your chucks, it could give rise for concern. I'm still trying to figure out what a "bruised spindle nose" might be or why would it damage the nose? It doesn't even touch the nose.

04-25-2006, 09:16 PM
A bruised spindle nose can result if a chip or bit of trash is trapped between the backplate and the spindle nose, the nose can be "bruised" or dented or otherwise damaged. If the cast iron absorbs the punishment, it is cheaper to repair or replace.

The spindle nose is that part that the backplate mounts on and contacts directly. In the case of your Atlas/Craftsman it is the threaded portion, plain register area and the flange that the backplate bottoms out on. A softer material in the backplate will absorb any damage, leaving this area in the original state, preserving it's accuracy.

04-25-2006, 09:32 PM
Must be the same people that buy cast iron screw drivers and wrenches. ;)

This just emphasises the implication of poor habits and practices. It plainly states in the manual to make sure this area is clean and oiled. I've seen pictures of lots of chucks being stored laying on the back too. Bad idea for the screw on chucks. I wonder how many people drag their chucks across a shelf?

I guess it depends on whether you care for your tools or not. Thanks for the feedback though.

04-26-2006, 05:18 AM
In my opinion it i think it is the regester that is important. I currently have 2 atlas/craftsman and a 9 in southbend all the spindles are the same size however the chucks seem to fit differently the atlas threads seem looser when i am intalling a chuck but tighten up fine. The sb was a recent purchase , soon after i got it i tried to put on a chuck from my atlas and it did not want to go on all the way so i looked at it found some crud in it not much just a very little it would not fit the sb till it was super clean then it still fit the treads snugly like the 2 chucks that came with the sb. Then i swaped chucks back and forth some and came to the conclusion (i might be right or wrong i am sorta new at this stuff)that the atlas did have loose threads and relied on the larger register to make work correctly and the southbend chose a combination of spindle thread fit being much closer fitting and a smaller register . If you are concerned about messing up a backing plate by cutting the threads wrong, find some 2 inch round stock bore it out and practice threading it and see how it fits . The feb/march Machinists Workshop has an article about making an a spindle collet holder for an atlas It shows most of what is needed to do what you want to do as far as inside threading you don't even need to do the math its in the article ......... enough of my rambling and my .02$

J Tiers
04-26-2006, 10:12 AM
In fact, SB seems to be different. SB parts don't seem to fit Atlas or Logan well.

For my Logan, an SB faceplate had to be scraped out a bit to fit the threads. Interestingly, the "register" was quite loose (but it repeats well) even though the threads were too tight.

Paul Alciatore
04-26-2006, 10:29 AM
I am not going to get in the threads vs register thing except to say that I have enough doubt and enough faith about both sides to pay attention to both of these details when making a back plate.

I have made several backplates and other fixtures that mount to my SB spindle and so far I have not made a matching gauge piece to check the fit. I carefully bore the IDs to an accurate size and then rely on the compound feed dial and a little math to get close. I stop threading a few thousanths short of the calculated value. Then, I simply remove the chuck that is holding the work, WITH THE NEW BACKPLATE STILL IN IT, and just try the fit. The existing chuck always seems to go back on within a few tenths so I can continue cutting the thread if more is needed.

I feel I can get a better fit this way than relying on a gauge piece that may not exactly match my spindle. It seems to work fairly well. What is fairly interesting is that when I am approaching the final size, the new backplate seens to start going on for a few threads at first and then will go further and further on successive cuts. I have to conclude that my spindle thread is slightly tapered or worn on the outer threads.

Paul A.

04-26-2006, 10:57 AM
I agree with Paul that the choice of register vs non-register is up to the individual, but that careful work will produce the best accuracy and repeatability.

The stub spindle may not be needed with a smaller machine, but when the chuck gets over 10", it is difficult to switch around for purposes of checking fit.

I made a stub spindle for my 13" Sheldon, and used the factory dogplate to check fit the threads. It is serially numbered to the lathe.

I have also noticed the tendency of the threads to go on farther with successive cuts. I think this is more likely spring in the cutter, and I will take a couple of spring passes at one setting when getting close to the final pass on fussy work.