jacob's collett chuck

Pretty sure I have seen them for large threaded spindles, like over 2 inchs.

The big Jacobs collet chucks, the ones that take No. 91 collets, did come in models to fit threaded lathe spindles but I've never seen one for a 1½-8 spindle. They made a smaller one for that spindle but they're really rare. Another solution you might be interested in is what I use on my South Bend

9" lathe. I have an ER48-3MT adapter for which I made a drawbar. I mount it in my lathe spindle and it holds ER48 collets, which go up to 1". They don't allow stock through the collet but on the other hand they're dead concentric (mine is, anyway, ETM brand). You can also use your ER-48 collets on other machines with other adapters if you care to. At one time I used them on my drill press (3MT spindle with drawbar) and my surface grinder (5MT toolhead) and I also have an R8 adapter in case I ever decide to use them on the mill.

Grant

The 91-A6 or 91-F6 came with a 4" flange mount that was designed for special applications or threaded spindles. Apparently the user was expected to fabricate his/her own adapter. The A6 was recommended for lathes with

2-1/16" or larger threaded spindles; the F1 for 2" or less.

There was another Jacobs chuck (Model 50?) that might have been designed for threaded spindles. At least the ones I've seen on Ebay all seemed to be designed for lathes with threaded spindles. They featured a slightly different collet design that had a lower maximum capacity - might have been

1", compared to 1-3/8" for the 91 series.

Mike