SPI precision chuck question

We recently got an 1/2 inch SPI precision chuck at work. It is marked "JT6". We then bought a Jacobs brand chuck arbor marked "6J.T.".

The arbor taper does not seem to fit the chuck very tight. I can put the arbor in, give it a good rap on the end with a lead hammer, and then pull the arbor back out with my hand.

Now I will admit all my experience in this area is with older Jacobs keyed chucks used on manual machinery and this expensive super precision stuff is geared toward VMCs. So am I missing something here? I've just never seen a taper fit that I could pull apart with my bare hands and it just does not seem right. I'm afraid that the chuck will fall off the arbor during a rapid tool change.

None of our existing VMC tooling is like this but I do not know who set them up. Should I use a press or something to press the arbor in or should I call the supplier and complain?

Either the chuck or the arbor is incorrect. Try another arbor? You can also put spotting blue (correct name?) to check fits. I've used a sharpie (very quickly) before to confirm a morse taper.

Maybe the arbor is too long (or the chuck too shallow, but the arbor is easy to machine)

If its a really good fitting system it could be trapped air pressure preventing the taper seating. Been told that seating with a lead hammer or similar is bad practice for various reasons including not allowing air to escape. My mentor said the correct way is to lightly seat the taper by hand then rap the chuck nose (jaws retracted) on an alloy block a couple of times so the taper inertia seats things properly. Works for me but I drill all my chucks to take an extractor punch which avoids trapped air problems.

Check that the brand of the taper arbor is reliable. These days Jacobs has a less than stellar reputation due to outsourcing.

A metric B18* taper is sufficiently close to a JT6 as to be indistinguishable by casual measurement and the mismatched pair will frequently hold after a fashion. Half angle taper differs by under 2 thou per inch, nominal lengths are 25 mm and 1 inch with the small end of the B18* being around 10 thou bigger.

I've had the misfortune to be lumbered by a cheapskate mate with sorting out a male and female taper pair clearly made just far enough out of spec to be sort-of satisfactory with a correctly specified mating taper of either JT6 or B18* size. Unfortunately the mismatched pair didn't work as well together being, probably, slightly worse fitting than yours. A right game sorting out what was what, especially as I'd not heard of a B18* then and the misguided genius who designed them had put slight curves on the taper sides!

Clive

Since you purchased a Jacobs arbor I think you can eleminate that aspect,however SPI doesn't mean (Swiss Precision Instruments)as it once did.I bought one of their precision digital mics and from the little brochsure that came with it I was able to deduce that it was made in cheena.Works fine when it works.Leave a new battery in it overnight and it will be dead the next morning even after pushing the off button.Internal short is my guess.