On Fri, 14 Nov 2003 09:03:11 -0400, jt wrote:

Toolpost thingy to mill/drill/grind/etcetera:

1.5 x 2 x 3.5 inch block mounted on the toolpost stud, spindle hole drilled
on the centreline of the lathe, small motor mounted on the other end and a
few pulleys to connect it to the spindle.

I'll need a simple collet to hold mill cutters with a 3/8" shank. Right now
I'm thinking of a nut with an internal cone and a collet that sits in a hole
drilled in the end of the spindle, like a large version of a pin-vise; but
should I be using the flat which is ground on the cutter shanks?
Suggestions on the whole arrangement are welcome.

What kind of bearings and arrangement of same should I use for the spindle
that will turn the cutter?

I'm no expert but obviously you'll want to use bearings that can handle thrust
as well as rotational and side forces. Tapered roller bearings fit the bill here.
Although brass bearings are cheaper and easier to replace (machine your own from
stock), I don't know if they'd hold up adequately in this application. Any bearings
will need to be rated for the top speed that you expect the spindle to be rotating,
which will be on the order of 10,000 rpm for grinder mode.

The toughest part of fitting tapered roller bearings to the spindle carrier
will be in machining the taper in the end of the carrier. It'll need to be
very precise as well as being well aligned to get good results.

In any case the spindle will need to have a shoulder at the collet end which
will ride on the bearing. Imagine what a shaft with a thick washer slipped onto
one end would look like. That's the approximate shape you'd want. Actually, in
an effort to keep killer dust out of the bearings, I'd machine a cup shaped
shoulder into the spindle which would be large enough to fit around the entire
bearing assembly, and then a mating groove in the end of the spindle carrier,
so that when assembled the cup part of the shoulder fits into the groove (without
actually contacting it). I'd also insert a felt O ring in the bottom of the
groove to act as a wiper.

Since you'll be using this tool as a toolpost grinder then you'll surely want
the bearings to be sealed against abrasive dust from the grinding wheel.
For safety you'll also want to design in a removable grinding wheel guard
which attaches to the spindle carrier.

Anything else I should figure out before the hacksaw stage?

Your pulley set up. The speeds needed for drilling/milling are
radically different than for grinding. 100 - 500 rpm for the
former, and 10,000 rpm or so for the latter. That's going to
be pretty difficult to solve with a single speed motor.


It's an interesting project, but it gets pretty complicated
to do it right and have it last. I guess that's why professionally
made toolpost grinders are so expensive. :-)