Chuck adapter/backing plate

[mickri]

I recently got an 8" 4 jaw chuck. The backing plate on the chuck has 2 3/8 x 8 tpi threads. The spindle on my lathe is 1 1/2 x 8 tpi. Here are the backing plates off my old and new chucks



The outside diameter is not critical. The raised center section is a very tight fit in both chucks. I think that this is a what keeps the chuck true to the backing plate. I was hoping that the diameter of the bolt circle would be the same but they are not. The bolt circle diameter of my old chuck is 0.120 bigger than the new chuck. Too many choices and don't know which path to follow.

The easiest would be to make a simple adapter with 1 1/2 x 8 tpi on the inside and 2 3/8 x 8 tpi on the outside. I have a piece of 3" round bar that I could make the adapter out of.

Using what I have on hand I thought of drilling and taping news holes in the old backing plate and making a bushing to fit between the raised center section of the old backing plate and the hole in the new chuck. This is the least desirable alternative because I would not be able to use the old chuck.

Or I could make a new backing plate with 1 1/2 x 8 tpi that fits the new chuck. I would have to buy a piece of round bar 5" diameter x 1" thick. That would probably cost me more than what I paid for the chuck when you add in the cost of shipping. I bought the chuck at an auction for $40.

What are the pros and cons of my choices?

 

[mmcmdl]

Possibly trading the new oversize backing plate for the one you need an option ? There's many of us on here in the same boat .

I have a 4" 3 jaw chuck with 1 1/2 mount from a Atlas lathe i f you can use it. ( this and many more are in our trading section )

Plenty of these types trades on this site .

 

[mickri]

Thanks for the trade offer. I really don't need more chucks. I have 3", 5" & 6" 3 jaw chucks and a 6" and now two 8" 4 jaw chucks. The only reason I got this new 8" 4 jaw is because my old 8" 4 jaw is in fair condition with lots of threads on the jaws broken out. After every use I take all of the jaws out to clean out any wayward chips to hopefully prevent losing anymore of the threads.

 

[mmcmdl]

This wasn't a trade offer from me . I was suggesting you checked thru that section and see if you and someone else could strike a deal . Many on board have extras of everything we no longer have a need for , and are willing to trade for something we could put to use even up .

 

[BtoVin83]

Are you going to set the old 4 jaw aside in favor of the new one?

 

[benmychree]

Bottom line, make a new backing plate for the chuck, first make an exact plug gage of your spindle threads using 3 wire or thread wires, use it to thread a new back plate from an iron casting, screw it onto the spindle and machine it for a tight fit onto the chuck, drill and tap for bolts, assemble; Bob's your uncle!

 

[MontanaLon]

If the register of the old back plate fits tightly into the new chuck just use the old back plate on the new chuck and call it good. It is a 4j so runout isn't the issue it would be with a 3j where you can't adjust it out.

 

[mickri]

The center section of the old backing plate is roughly 2" OD. The center section of the new backing plate is roughly 3" OD. Also the bolt circle diameter of the old backing plate is .120 bigger than the diameter of the new backing plate bolt circle. So they don't match and I would have to modify the old backing plate. I don't think that trying to modify the old backing plate is the way to go.

I researched thread plug gages and don't understand why I would need to make one when I can check the threads for fit on my lathe spindle. When I was making my ER32 collet chuck I would remove the chuck holding the collet chuck to check the fit of the threads. My research in making the ER32 was that you wanted a slightly loose fit so that the chuck would register on the spindle and not on the threads. My knowledge of this is so limited that I don't know what would be the right way to do this. What is the benefit of cast iron over other types of steel?

I do plan to use the new chuck most of the time. I can envision having the new chuck on the lathe and using the old chuck on my mill/drill and vice versa.

 

[mmcmdl]

I researched thread plug gages and don't understand why I would need to make one when I can check the threads for fit on my lathe spindle.

The adaptor that you're threading may just possibly be for the lathe you're machining it on . Pretty hard to check for pitch if you have to remove your part and chuck and fit it to your spindle .

The center section of the old backing plate is roughly 2" OD. The center section of the new backing plate is roughly 3" OD. Also the bolt circle diameter of the old backing plate is .120 bigger than the diameter of the new backing plate bolt circle. So they don't match and I would have to modify the old backing plate. I don't think that trying to modify the old backing plate is the way to go.

I agree . Sounds like a large conundrum to me . Backplates are cheap on the bay .

 

[MontanaLon]

You don't need to modify the old plate other than drilling new bolt holes in it while it is attached to the chuck with the register fully seated. Those holes don't really need to be in a perfect pattern either. Close is good enough because you will put it together and then put witness marks on it so you know what holes line up with each other. One of my chucks has 2 sets of holes, one that matches the current back plate and another that matches some other back plate in the world.

When you say "center section" are you talking about the hub on the headstock side or the register on the chuck? Hub side won't matter, register side is a no go as there won't be enough thickness to create a new register in all likelihood.