1 1/2 -8 spindle actual size

AGCB97

AGCB97

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I'm looking to fit a chuck on a dividing head. The thread on the head measures 1.572" (max diameter) and 8 TPI. Is this the actual size of a 1 1/2"-8 spindle or some type of collet nut thread?
Thanks
Aaron
 
1.572" is very close to 40mm (39.93). Are you sure it's an 8 tpi thread, or could it be M40 x 3, which would be a standard metric fine thread. The OD is too big for 1 1/2" thread and 1.572 isn't a nice round fraction (closest I can get is 1 37/64, which is 1.578).

Rob
 
Are you sure the pitch is 8TPI and not 3mm? There's only about .006 difference. The reason I ask is the shop I worked in had a Summit lathe with a 40mm x 3mm spindle. 1.572 is very close to the actual major diameter of 40mm thread. If the dividing head is Asian it could easily be metric.
 
I checked it with a thread gauge and it is 8 TPI.

There is an inside taper 3" long in the spindle which I measure at 1.074 large end and .956 small end. This is not MT or anything else I can find. Too much taper (calculated) for B&S.
 
I thought a friend told me he had an very early Jet lathe that had a very odd spindle thread configuration, of 40 mm x 8 tpi. Made that way, the way he thought, so you had to go back to the manufacturer to get another chuck.

Another friend had British cars, that had odd thread configuration, metric diameters and fractional thread spacing.
 
I guess I'll either make a threaded backing plate to fit or make a bolt-on one. I'd like to also make an adapter for ER40 collets since I already have those and 5C collets. If I can't figure out what the taper is, that will also have to be shop made.
 
It should be easy to set the taper, swing the compound to where it looks good, put a Dial Test Indicator on the exact vertical center of the taper and adjust the compound until there's no indicator movement when cranking the compound in and out.
 
I was curious about 8 tpi v 3mm pitch:

IMG_3135.JPG

There is a variance but it isn't much.

Craig
 
Just because the OD of the thread is odd size does not mean that it is an odd size, lathe and dividing head spindles have truncated threads so they are not so easily damaged as a sharp thread, it could be 1 5/8" - 8 TPI. You could check it with thread wires and look up what the measurement should be for that thread. What I always do is measure the thread, either with wires or a thread mike, and make a plug gage to the exact size, and use it to try the thread fit when making a new chuck backplate or other spindle accessory.
 
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