On 2013-04-08, DoN. Nichols wrote:
On 2013-04-08, Ermel wrote:

I need to turn an AC 1/4 HP bench motor with 1/2" shaft into a
stationary drill.

Hmm ... rather small motor for the task -- especially since it
is an AC motor.

I'm asking because I am not familiar with motor shafts and unsure about
drill chucks.
I assume I mount the 3/8" or 1/2" drill chuck on shaft with an adapter.

Or -- machine threads of the right size directly on the shaft,
using a lathe and turning between centers. (I don't think that a
hand-held die and die stock could be started straight enough to avoid
problems there.)

They sell adapters for this, and chucks that thread on them.

i

Or -- machine a Jacobs taper on the end of the shaft and mount
the drill chuck with that. Probably better centering, and less flex
compared to the other ways. And -- it has the additional benefit that
it works equally well in both directions. Generally better chucks come
with Jacobs taper mounting instead of threaded mounting.

How small will the smallest drill bit used be? How large will
the largest be? Imperfect centering can result in a lot of broken small
bits. And large bits -- especially drilling in steel -- need a lot of
torque.

Now do I need a left hand or right hand thread on the adapter?

Do you expect to drive left-hand drill bit? They are uncommon,
and more expensive, and seldom needed for most tasks. So you would want
a right-hand thread.

Also if motor shaft is 1/2" diameter and adapter is held on to shaft
with set screws, and the other end of adapter is a threaded end, how
should I purchase the drill chuck? what size and thread orientation? Do
all shafts come with a flat area on them?

No -- some come with a keyway instead.

Someone else has already posted a pointer to a kit for the
purpose.

Also how would I vary the speed of the motor?

Proably -- with great difficulty -- assuming that it is a single
phase motor. Single phase motors are optimized for a single speed and a
single power line frequency. Any attempts to vary the speed will mostly
result in very low starting torque.

A three phase motor can be varied using a VFD (Variable
Frequency Drive), but a 1/4 HP motor would be unlikely to have sufficient
torque at low speeds. (And three-phase motors as small as 1/4 HP are
hard to find.)

This is the sort of thing that a DC or a Universal motor (such
as found in hand-held electric drills) is particularly good at.

Do you really want to run this mounted to the bench? If so --
consider mounting the motor *under* the bench, and the shaft with the
chuck above it in pillow blocks -- and use a pair of step pulleys
between the two with the belt passing through the bench. This would
give you a few speeds -- not as nice as continuously variable, but
kinder to the motor.

Thanks for your help in this.

Good luck,
DoN.