> Please do!
>> One of these days I'll have to document how to build a CHAD milling table
>> like I used for making centering rings when all I had was a disk sander.
>> The design is adaptable to making round things on so any different
The idea is stolen from a balsa *** plan in an old NFFS digest. Can't
recall if it was by Drela, Stoy, or someone else. I've described it before
in words, and the concept is simple enough that you should be able to figure
it out and adapt it to your needs. The concept is pretty simple.
You build a jig out of a sheet of plywood, hardwood, or whatever suits your
fancy. A piece of 1/4" (or other size to suit your needs) rod is used as a
pivot pin to turn the ring, much as was done with the drill bit in the root
of this thread.
The key is the adjustability, which is stolen from a metal lathe or milling
machine feed. A piece of threaded rod is chosen for the feed screw. If you
like to work in fractional inch, use 8-32 rod. If you like decimal inch use
1/4-20 or perhaps 4-40. If you're into metric find some metric threaded rod
with .5mm threads. This is your feed***to move the pivot closer or
further from your cutter. Each turn of the rod will change your radius by
the thread pitch, or your diameter by twice the thread pitch.
Next build a dial indicator. You want a disk perhaps 1-2" in diameter
calibrated around its circumference. If you used 8-32 rod, calibrate your
disk in inch fractions, each turn being 1/32" RADIUS or 1/16" DIAMETER. You
might choose to divide the disk into 8 sectors, making 1/128" diameter
calibrations. For 1/4-20, each turn is 1/10" diameter, and 10 sectors make
1/100" diameter calibrations. And for 0.5mm, each turn is 1mm diameter,
again 10 sectors making 0.1mm calibrations. You'll need a thin piece of
plywood, plastic, or G10 for the disk. Near the edge, attach a small handle,
which can be a small machine***bolted through the disk. To mount it to
the threaded rod, glue a T-nut against the CENTER of the flat plate.
Take a second T nut, and thread it on the rod with the plate facing IN
towards the long piece of the rod. Then put the dial T nut on the opposite
way, with the plate out. Jamb the two together so neither will move. Add a
drop of CA or locktite if necessary to secure them to the rod.
Now put a nut (a square nut will work better if you can find one) on the
threaded rod you've chosen. This becomes the base to hold the center point.
Weld, solder or glue the pivot pin to a flat of the nut. You might want to
add a few bushings to the threaded rod, one near the handle, and one at the
end of the rod, for support on each side of the nut/pin.
This whole assemble is put in a slot of a wood base. The slot should be just
large enough for the (square) nut to fit in, both in depth and in width.
Only the pin should stick above the base surface. Using something like a
Dremel cutof fwheel cut a slot for the second T-nut flange near the end of
the block. There must be no slop when the T-nut is put in this slot, yet the
T-nut must turn freely in the slot. This holds the handle in place.
You've now got a milling machine style adjustable pin. Clamp the jig to your
tool of choice (router bit, Rotozip, sanding disk, band saw, or whatever).
Drill a 1/4" (or whatever your pin size is) hole in the piece of wood you
want to make into a disk. Put it over the pin. Crank the handle until the
far edge is in contact with your cutter, turn and cut. You can make fine
adjustments with the calibrated handle to get just the size you need.
WARNING: Bad ASCII art follows: top view
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= threaded rod
O bushing to support threaded rod
N square nut with center point attached to one flat
[ T-nut in slot to position rod assembly in block
] T-nut with calibrated faceplate glued in place
~ handle to turn rod assembly
Turning the crank and the nut with pin (N) moves left or right. With the
recommended thread pitch choices, you can easilly get 1/100" accuracy with
Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD"
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