"Jon Elson" wrote in message
...


William G Darby wrote:

I also like the idea of trepanning!

You could drill a hole at the center of the hole location and use a bolt
through the hole into a Tee slot anchor with a threaded hole to receive
it,
in order to form a pivot for your plate to rotate on. (like a rotary
table)
Pick up the hole center and traverse your table two inches away from it
(making allowance for the diameter of a slot cutter)

Take great care not to let the plate get away on you. Also a bolt with a
smooth shank would be best and cut to a length such that it bottoms out
and
locks up at the right depth. A bar clamped to the plate to provide better
rotational control would provide additional safety.


This sounds EXTREMELY dangerous!


Yes it could be dangerous!!!

But I would not use a big end mill and I would not try to hog into a 3/4"
cut either. That would be asking for trouble. I would go with a 1/4 slot
cutting EM and I would start with about an eighth inch depth and more if the
set-up felt right******

OK I just got back from the mill. I cut a 4.25 " dia hole in a bit of scrap
half inch plate and once I got the bugs out, it went along fairly smoothly.
The first thing I will say is that this method is a lot of work and
dangerous, but you do not need much equipment to get a big round hole or
disc.

I used:
1/2" drill
1/4" slot cutting EM at 5000 RPM
one Tee slot nut
one 3" piece of 1/2" threaded rod
two 1/2" nuts
one short heavy coil spring

I anchored the threaded rod in the center of the bed in the Tee slot nut.
Drilled a half inch hole where I wanted the center of the hole on the plate
to be. Then placed the plate over the threaded rod followed by a washer, the
spring another washer and two nuts locked against each other.

Picked up the center of the threaded rod and did an X move of about 2.25
inches

On my first attempt I had set the speed at 5000 but was getting only 1000
RPM. This had the effect of severely shaking the set up and unknown to me
the threaded rod slacked off and allowed my center to move enough for me to
notice it.

On the next and final go I got the RPM up to speed and planted that threaded
rod deep enough into the Tee slot nut sos it'll never come out. After that
it was clear sailing there was very little rotational pressure on the plate,
so it was not in the least difficult to control the rotation. At the same
time I was fully aware that the rotation had to be controlled at all times
least it be allowed to free wheel. (BAD) )^:

So the method is doable, and yes it is dangerous but I at no time was the
least bit concerned for my safety. Somewhat like operating anything else:
most anything can tear your hands off if you do not know what you are doing
or are inattentive.

I am including a few pictures of the set-up to you Jon and anyone else that
cares to see it.

Bill Darby






With a 3/4" plate, and a big end mill
hogging into a 3/4" deep groove, I doubt you could hang onto it.

Trepanning with a hand-carved single-point cutter in a boring head will
be a LOT
safer, and all you have to do is feed the quill.

Jon