[How-To] Threading a chuck backplate

I think I have a internal threading tool that will do 8 TPI.
The one I got for my collet drawbar is too small, it is 1/4" HSS.
 
mmcmdl said:
The flat is irrelevent . The pitch diameter is what matters .
Click to expand...

Not really so (that the flat is irrelevant). It could be that your spindle has a true full 60º thread, made before thread standards were adopted in the 1940's. With a full 60º thread, all your back plates would also need a full 60º thread. If you have a back plate that meets the "new" thread standard, it won't fit on your spindle, since the spindle's threads are not compatible with it.

Is your spindle major diameter 1.500"?
 
SLK001 said:
Not really so (that the flat is irrelevant). It could be that your spindle has a true full 60º thread, made before thread standards were adopted in the 1940's. With a full 60º thread, all your back plates would also need a full 60º thread. If you have a back plate that meets the "new" thread standard, it won't fit on your spindle, since the spindle's threads are not compatible with it.

Is your spindle major diameter 1.500"?
Click to expand...
Yes, I did check that.
My lathe is a 1947 SB9.
The new chuck is a Horton S36S.
 
My thread pitch gauge fits my chucks and the spindle like it should, the new chuck, the side are good, but it does not fit down enough, there is a gap at the top where the flat is.
 
If this helps .........................class 3 external thread has a max major diameter of 1.500 as you noted yours was . Class 2 internal thread has a min minor diameter of 1.365 up to 1.390 max . You should have a min of of .0022 allowance on the pitch diameters . Your root diameter on the internal thread may be small due to a broken or dull tool . If you can , just pick it up with a sharp v tool and drag it thru . From what you're saying with the thread pitch gage , it sounds like your problem .

The only thing I know after quite a few years in the shops .........................a 1 1/2"-8 internal thread has to mate to a 1 1/2"-8 external thread . If not , summpins $^*$@'d up ! :pickaxe:
 
So I should indicate the spindle register when I chase the threads ?
 
Jim F said:
So I should indicate the spindle register when I chase the threads ?
Click to expand...
Yep . It was most likely turned true to the threads in the same operation . Make sure your face is running dead nuts also . :)
 
mmcmdl said:
If this helps .........................class 3 external thread has a max major diameter of 1.500 as you noted yours was . Class 2 internal thread has a min minor diameter of 1.365 up to 1.390 max . You should have a min of of .0022 allowance on the pitch diameters . Your root diameter on the internal thread may be small due to a broken or dull tool . If you can , just pick it up with a sharp v tool and drag it thru . From what you're saying with the thread pitch gage , it sounds like your problem .

The only thing I know after quite a few years in the shops .........................a 1 1/2"-8 internal thread has to mate to a 1 1/2"-8 external thread . If not , summpins $^*$@'d up ! :pickaxe:
Click to expand...
I just checked and I do have an internal threading tool that will work.
Now if we have the 43 high temp. they are calling for, I might be able get to it tomorrow.
 
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