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Porter McLeod Chuck BackPlate

eaglehatman

Plastic
Joined
May 3, 2020
I recently acquired a Porter McLeod 16" Model L lathe. The past month has been spent taking it apart, cleaning, lubing, replacing missing fasteners, generally taking care of upkeep and maintenance that had been neglected for decades. My goal is to use it for puttering around with jobs too big to fit on my 9" South Bend Model C.

The spindle is threaded at the EVER-POPULAR 2-3/16 - 7tpi. Of the (2) backplates that I have cleaned up (I got 3 chucks with it… still waiting for the guy to bring me the face-plate, and the third chuck weighs over 100 pounds - so I haven't really dug into it), one is the correct size (the one that came mounted to the spindle that I struggled for 2 weeks to remove). The second one is the slightly more common 2-1/4 - 8tpi. It jams up onto the spindle threads after (3) turns, about 1/2 inch from the shoulder. Total number of threads available on the spindle is 6... so really, half engagement is not terrible, but jam-up before contacting the shoulder does not provide good alignment or structural support of the chuck, and would probably make removal problematic if I try to use as-is. I cannot buy the correct back-plate because it does not exist (that I have found), and I have never tried to single-point machine an internal thread (lacking both tooling and experience). The mis-matched back-plate really has two other out-of-spec conditions... the plain-section of the bore is too short and also too large in diameter. My thought is that the most reasonable first-pass compromise to make it useable is to simply plain-bore a section of threading to the proper diameter, as deep as necessary to allow contact with the shoulder. If I were tooled up in a real machine shop with some professional machinist skillz, I might also try to line up and re-cut the threads to the proper pitch - at least deep enough to get the full 6-thread engagement instead of three. But really, without some way to measure or go/no-go the threads, it would be difficult to know if I even got that right. I think I need to duplicate the spindle geometry on a piece of round-stock that I can then use for assessing any future back-plate that I may try to machine. That's one step toward a long-term solution...

Any thoughts or advice?


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Joe Michaels

Diamond
Joined
Apr 3, 2004
Location
Shandaken, NY, USA
Making a spacer to allow the use of the back-plate with the short hub length would work in theory. The plain (unthreaded) diameter on the spindle of the lathe is the 'register' diameter, intended to hold the backplate so it runs true (concentric) with the spindle. The 'register bore' in a backplate is usually bore with a very close fit, maybe 0.001"-0.002" clearance. The threads, while cut to a good close fit, are usually not what establishes concentricity between a backplate and the lathe spindle.

A backplate is often machined on the lathe it is going to be run on. After being bored and threaded, it is then screwed onto the lathe spindle, and facing including any 'steps' (known as 'spigots') to match the back of the chuck to be mounted are machined. That establishes the backplate as not only running true with the spindle, but also squared to it.

If you introduce a spacer, you risk taking the backplate out of true and out of square. If you do want to make a spacer, the obvious and easiest method is to make a 'loose spacer'. I'd bore a piece of steel round stock to a close fit on the spindle, so close you have to 'wring' it on (twisting motion). I'd machine a small 'relief' or counterbore are each end, maybe 1/16" wide x 1/16" deep. I'd also tap the spacer for two setscrews at 90 degrees to each other. For setscrews, I'd use setscrews with a nylon tip (McMaster has them in small quantities, I think). This is so the setscrews do not mar the spindle.

Place the spacer on the spindle and drive it hard against the shoulder of the spindle with a piece of brass and a hammer. Tighten the setscrews. Then, take a very light facing cut, and the 1/16" wide counterbore is the space for 'tool runoff', so your toolbit does not touch or cut into the spindle. Chamfer the outer edge and run a very fine oil stone on the faced surface while it is turning in the lathe (an "Arkansas Hard" small oil stone or "India Medium Hard" stone are what is used).
This breaks any burrs and ridges.

Remove the spacer from the spindle, turn end for end, and repeat the facing and stoning. If you have big enough micrometers, take a set of mike readings of the length of the spacer at different locations to check for parallelism of the end faces.

Mark which face of the spacer faces in towards the headstock or towards the tailstock. A light punching with letter/number punches, or a set of prick punch marks is what I'd use as this is 'indelible' and won't wash off with oil or similar.

You now have a spacer to use that shorty backplate. Make the spacer about 1/16" longer than the distance between the end of the shorty backplate and the shoulder of the spindle. This will prevent 'bottoming' of the threads and will use the register and shoulder of the spindle to establish concentricity and square of the backplate.

Try to use what I call 'good steel' for this spacer. Something like Stressproof or 1045, or 4140. A steel a bit harder and stronger than the normal lower carbon steels such as cold rolled.
 

Joe Michaels

Diamond
Joined
Apr 3, 2004
Location
Shandaken, NY, USA
Addendum:

Remove the two setscrews and use the spacer without them, so it can move on the spindle when clamped by the shorty backplate. You could tap two (2) holes in the end face of the spacer for 'jacking screws'. These are thru-tappings, and the same pair of nylon-tipped setscrews can be re-used there. The jacking screws are to remove the spacer from the spindle register when you want to mount some other chuck or faceplate.
 

eaglehatman

Plastic
Joined
May 3, 2020
I may be able to work with that concept. Unfortunately, the close-fit radial register you speak of is completely absent here... there is a 0.030 clearance. I have not put a dial indicator on this - I am skeptical whether or not this was ever run on this spindle. When I picked this lathe up form the PO, he gave me all of the chucks that did not fit the 16' South Bend that is going to his Grandson... it is possible that this chuck went to lathe he offloaded years ago. If I go the spacer route, I think that i may put a step in it, and bore the plain section of the backplate a little bigger, so I have positive radial register as well as squareness. when done, I plan on taking a skim pass on the backplate to re-true it before I mount the chuck.
 

Froneck

Titanium
Joined
Dec 4, 2010
Location
McClure, PA 17059
If the counterbore/register is .030 larger then you should defiantly bore it and make a bushing. Try screwing the back plates on backwards to determine if there is a burr of some type on the threads possibly when the register was bored. Also check the threads on the spindle for slight damage. If it still don't fit and it's the correct threads you will have to re-cut them so they fit. Then make a threaded arbor with a shoulder so the back plate can be screwed on so the the inserted bushing can be bored. If you don't have micrometers you should get some, a lathe is worthless without micrometers! No a digital caliper is not good enough, A good Starrett Vernier Caliper is better than Digital but still not good enough for .001 to .002 fit. Also get a good set of Telescope gauges. You can find used on ebay. For you lathe don't worry about the accuracy of the micrometer as long as it looks in good shape and someone didn't use it as a graduated C-clamp! It can be as much a .050 off (exaggerated error to make a point) since you will use it to measure with so no matter what the error is it will be the same as measuring the spindle register and the telescope gauge for the bore. Naturally later you would want to calibrate it.
 

eaglehatman

Plastic
Joined
May 3, 2020
Froneck, the crux of the problem is the PO gave me this chuck, purported to be for this lathe, but the back-plate is a 2-1/4 -8tpi, and the spindle is a 2-3/16 -7tpi. The long term solution, that will probably take me a few months to get the tooling and skills, is to machine the correct backplate from a plain blank plate. Looking for quick and dirty advice to make it useable, even if with restrictions, in the meantime. Sorry if I wasn't clear in my original post.
 

Rudd

Stainless
Joined
Jul 30, 2003
Location
savannah, jaw-ja
You aren't going to be able to rethread something from 2 1/4 x 8 to 2 3/16 x 7.
I think you have it sorted out you should not use the BP with the wrong threads. Think about how little contact there will be to resist turning and lateral forces. Something will give, either the threads on the spindle, the threads on the backplate, or both.
You are correct about needing to make a dummy spindle. My hard earned advice is to measure the threads over wires when you make this dummy spindle. Don't just trust that it will go into a backplate with the same threads. DAMHIKT.
Here's a handy tool to determine measurements over wires. Note you can adjust the wire diameter to suit something you have on hand if it is close to the recommended size.
UN imperial screw thread calculator

I started right where you are with one chuck and an oddball spindle thread. It really isn't as hard as you would think to make a backplate.
 

Froneck

Titanium
Joined
Dec 4, 2010
Location
McClure, PA 17059
Froneck, the crux of the problem is the PO gave me this chuck, purported to be for this lathe, but the back-plate is a 2-1/4 -8tpi, and the spindle is a 2-3/16 -7tpi. The long term solution, that will probably take me a few months to get the tooling and skills, is to machine the correct backplate from a plain blank plate. Looking for quick and dirty advice to make it useable, even if with restrictions, in the meantime. Sorry if I wasn't clear in my original post.
No problem! Probably my fault as I didn't read your post carefully! If you were near me I'd make that back plate for you, maybe someone here in your area will help you out!
 

eaglehatman

Plastic
Joined
May 3, 2020
Thanx for the advice gentlemen. I can see now, re-reading Joe's recommendation of a spacer, that there was a mis-understanding that the backplate is somehow short, rather than the actual problem of a mis-matched thread size. The good news is that last night I managed to get the back-plate off of the third chuck - a big 4-jaw independent - and it is the correct size/threading for this spindle. So between that and the smaller 3-jaw scroll chuck, I should be able to be productive with this lathe until I can get this problematic back-plate replaced.

Here is a picture I snapped when I first started bringing pieces home from the guy's house last month. The little 5-inch chuck is the original that came with my 9" South Bend Model C - for comparison. The larger 4-jaw is the one that has a correct back-plate (haven't measured it... 14 inch maybe?), while the 8-inch 4-jaw to the right is the one with the mis-matched back-plate... a much more convenient size for many of the projects I have in-queue, which is why I started looking at making it work before the others...

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Because someone will ask, here is a photo of the 3-jaw that came on the lathe:

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eaglehatman

Plastic
Joined
May 3, 2020
You aren't going to be able to rethread something from 2 1/4 x 8 to 2 3/16 x 7.
I think you have it sorted out you should not use the BP with the wrong threads. Think about how little contact there will be to resist turning and lateral forces. Something will give, either the threads on the spindle, the threads on the backplate, or both.
You are correct about needing to make a dummy spindle. My hard earned advice is to measure the threads over wires when you make this dummy spindle. Don't just trust that it will go into a backplate with the same threads. DAMHIKT.
Here's a handy tool to determine measurements over wires. Note you can adjust the wire diameter to suit something you have on hand if it is close to the recommended size.
UN imperial screw thread calculator

I started right where you are with one chuck and an oddball spindle thread. It really isn't as hard as you would think to make a backplate.

Thank you sir. That thread calculator is GOLD. I have never used the wire method, but that makes it easy. I make Chaine Maille armour as a hobby, so I have lots of different wire sizes available in my basement, just not an official set with precision sizes for measuring... so it's nice to have this flexibility. I think I'm gonna use this to check the 1-1/2 - 8tpi spindle I made a few years ago to duplicate the spindle of my 9" S-B... see how good I did <|;O)
 

johnoder

Diamond
Joined
Jul 16, 2004
Location
Houston, TX USA
No. 45 drills are close to the .0825 best - but you do want the wires big enough where you are measuring them - not the crests of the threads :D
 

eaglehatman

Plastic
Joined
May 3, 2020
No. 45 drills are close to the .0825 best - but you want the wires big enough where you are measuring them - not the crests of the threads :D

Yup. Makes sense. I have some 0.0840 stainless wire... should work here, according to the calculator.
 








 
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