"Jim Wilkins" wrote in message
...
On Feb 13, 9:22 pm, "Michael Koblic" wrote:
...
Granted. But if I followed that I would be cutting nothing bigger than
1"
diameters. The frightening thing about it is that to face the piece
(1018)
in question (8" diameter) one should have a lathe with low speed of 60.
Back to cranking the handle...
Michael Koblic,
I've been reluctant to suggest the way I machine large arcs, but you
seem to have some experience now.
I attach a pivot post to the table of the mill, drill the blank to fit
on it, attach a long handle and manually rotate the blank into the end
mill. Yes, the end mill is likely to grab and try to spin the work,
more so if there is any play at the pivot. That isn't too serious
while the blank is still a polygon because the end mill quickly
reaches a gap. Once the arc is nearly continuous the depth of cut has
to be very small. I use a small endmill at low speed, extend the quill
all the way down and don't lock it so I can knock the handle upward,
and set the drive belts quite loose, but it's still as dangerous as
working freehand on a drill press. The worst one I've done this way
was a circular tee slot, after the milling force damaged my undersized
rotary table.
I mostly do this to round the ends of linkage bars and stop before the
arc is tangent to the straight edge.
http://picasaweb.google.com/KB1DAL/H...33136395165634
The wider hinge part was first clamped upright in the milling vise,
with a shaft through the hole to position it atop the jaws, and milled
almost to size across the top. A larger diameter piece could be
clamped to an angle plate. The disk will vibrate if not well supported
close to the cut.
Those would be the GT18 steering sectors?
If the disk can be clamped firmly to the mill table, raised on
spacers, it can be cut nearly to size much more safely by rotating it
slightly between cuts. The frequent reclamping is incredibly tedious
but it would get your 8" disk close enough to finish with a light cut
on the 9" lathe.
I have to digest this...
For even larger circles I set them up on a shaft and round them with
an angle grinder, held so it crosses at an angle and spins the disk as
it cuts. That's how I shaped the front tire for the sawmill. I haven't
tried yet, but I think a disk could be turned fairly accurately on a
wooden frame with a lathe bit on an X-Y table which moves crosswise,
parallel to the axle. Think of an old foot-pedal grindstone frame. You
could spin the disk with a sanding drum in a drill.
I think you mentioned this to me before. In fact I believe it was the
inspiration for this marvel of technology:
http://www.flickr.com/photos/2768312...7607743618739/
I did not have as much luck facing with the grinder. I made some plastic
jaws for a chuck on my woodlathe and used an angle grinder with a sanding
attachment but for some reason it did not work too well either. I have a
feeling that the thing has to spin at a lower rate than 300 rpm to give the
grinder chance to work (I am talking face work here).
Still, the on-going problem is the *inside* of the doughnut.
The right way is probably to buy the largest rotary table that fits on
your mill. The disadvantage compared to a gap-bed lathe is that you
can't detail the edges as much, you'll be limited to end mill profiles
like vee grooves and corner rounders.
I am certainly considering it as a viable option. Realistically I cannot get
anything bigger than 6" rotary table on my mill and even that is probably
pushing it. OTOH if one could do things that way it is a $1000 difference in
capital expenditure :-)
BTW can you put a motor drive on a rotary table so it turns at a constant
rate?
--
Michael Koblic,
Campbell River, BC