Test 1-1/2”-8 TPI Back Plate?

[whydontu]

Anyone have a 1-1/2”-8 TPI back plate that I can buy cheap or borrow? My Barnes lathe looks to have an 1-1/2”-8 spindle, but when I measure it seems to be over 1-1/2” o.d.. I need to get a new chuck for the lathe (the century-old Cushman chuck is beat-up and best I can get is 0.015” run-out) but I can’t justify spending a couple of hundred on a chuck that might not fit.

So if I can find a test back plate or chuck I can confirm if a current back plate will fit, then I can decide if it’s worth buying a new chuck.

Any options put there?

 

[Arbutus]

I have one. I’ll drop it off next week

 

[Arbutus]

Or - I could model a threaded spigot and you could 3D print it..... Saves a trip into the city.
🙂

 

[SightlessSeer]

One thing to remember, the way most lathes with threaded spindle noses are setup, there is a threaded section and behind that a plain cylindrical section. The cylindrical section is the actual radial locating feature for the chuck and it can vary in both length and diameter for a given spindle thread.



There are some lathes with the same thread for the spindle nose, but with differently sized radial locating sections. The example I remember was South Bend vs Atlas 1 1/2-8 spindle nose. In that case I seem to recall the difference in diameter was only something like 0.010", so in one direction the chuck won't fit and in the other direction the chuck will screw on just fine, but wouldn't necessarily repeat positionally. This isn't the thread that I originally read it from, but it was the first example of someone talking about this that I could find. The fix for this is either boring the backplate out to match your spindle nose, or sleeving it, then reboring it if it's already bigger.

Another issue that you can run into is that the unthreaded section isn't necessarily the same length for all machines of the same spindle thread. I actually encountered this one myself with a NOS Bison 1 1/2-8 chuck that I bought for a 10" Atlas that I used to own. The backplate on that chuck had an unthreaded locating section that was excessively long for my Atlas' spindle nose. As a result I could only screw it on something like a turn and a half, or maybe two turns before it bottomed out. It worked fine like that, but I worried, so I wanted to fix it. The fix was pretty simple. I held the chuck backwards (backplate facing the tailstock) in a second chuck and face off the excess length (something like 0.200" in that case). You have to take make sure that you get the chuck properly square to the plane of the spindle though because that back face is also a registration feature that sets the plane that the chuck lies in when mounted.

tl;dr Even if the thread matches it still might not fit properly, though that can generally be rectified.

 

[140mower]

I have an embarrassing number of dog drive plates in 1 1/2 - 8 tpi I do believe, should one of those fancy you as well.... I think it was 7 but might be down to five or so...

 

[whydontu]

Thanks for the replies. I'm prepared to do machining as needed, this is for my secondary goofing-around lathe and I can use my newer lathe to modify whatever I end up getting. My main concern is just to know if the existing thread matches current thread form and dimensions, then I can be reasonably confident that a chuck from Vevor or eBay will mostly fit.

The spindle nose is quite short, with a very shallow bearing surface that's not much larger than the thread o.d.. The existing Cushman chuck just sits tight up against the spindle end plate, with no overlap. 'm assuming whatever I end up with will need some machining and shimming to get running true.

 

[whydontu]

I have an embarrassing number of dog drive plates in 1 1/2 - 8 tpi I do believe, should one of those fancy you as well.... I think it was 7 but might be down to five or so...

Apparently the original Barnes drive plates is worthy of a stern warning from the folks at lathes.co.uk:

"In the picture above, the slotted cruciform-style backplate on the floor by the pedals was used as a "driver" or "catch" plate to turn work between centres - this particular design must be the most dangerous ever invented - perfect for grabbing the sleeve of an unsuspecting operator and winding him into the mechanism. If you have one with your antique lathe - bin it, or frame it, but don't use it; ever."

Lathes.co.uk doesn't offer any warnings about the exposed gears, chains, and belts.

I have a fan spacer from a 1961 GMC 283 V8, looks just like the Barnes backplate. Once I know the proper thread form, I might just machine the fan spacer to stay authentic.

 

[SightlessSeer]

Interesting too that they don't mention the standard form of drive dog as being a particular danger. Surely the square headed screw that it clamps it to the workpiece is much more likely to grab a sleeve than a closed ended drive plate like that

 

[140mower]

Apparently the original Barnes drive plates is worthy of a stern warning from the folks at lathes.co.uk:

"In the picture above, the slotted cruciform-style backplate on the floor by the pedals was used as a "driver" or "catch" plate to turn work between centres - this particular design must be the most dangerous ever invented - perfect for grabbing the sleeve of an unsuspecting operator and winding him into the mechanism. If you have one with your antique lathe - bin it, or frame it, but don't use it; ever."

Lathes.co.uk doesn't offer any warnings about the exposed gears, chains, and belts.

I have a fan spacer from a 1961 GMC 283 V8, looks just like the Barnes backplate. Once I know the proper thread form, I might just machine the fan spacer to stay authentic.

View attachment 87261

Ummmm..... if a guy started getting caught up, wouldn't he stop pedaling? 🙄
Asking for a friend of course..... perhaps the startled turner goes faster to escape?😳