There are a variety of machines and ways to attack this problem. Smaller
cutters are very fragile and so I have made up 7/16 drills to fit the
rotabroach.
If I remember correctly the shank of the cutters is 3/4 diameter. I would
center drill a piece of cold finish about an inch long to match the 7/16 bit
then braze a HSS drill bit in the sleeve. I would cut the bit in an
abrasive chop saw since I only needed the end of the drill bit. If you are
only drilling one or two standard sizes this is one approach.
The lightest rotabroach mag drills will not accept the adaptation of a
Jacobs chuck. The larger, heavier, and expensive magnetic base drills such
as Milwaukee will operate either way and have a Morse taper in the spindle.
They come with variable speed.
A 3mm hole is only 1/8th. I cannot see why you would bother since I can
push a 3/16th hole through one inch plate in less than a minute with a hand
drill.

Randy


"Christopher Tidy" wrote in message
...
Hi all,

I've been thinking about buying a magnetic drilling machine for a while,
and recently I've seen a few at affordable prices on eBay. I need one
for drilling steel plates which are too big to fit in my drill press,
and also for drilling frameworks which must be drilled after they've
been welded together for reasons of alignment. I've seen Rotabroach
cutters in sizes from 6 mm upwards, but suppose I want to drill a 3 mm
hole? Can I fit a Jacobs chuck and a twist drill to a magnetic drilling
machine?

Many thanks,

Chris Tidy