I'm assuming that Delrin would be the best choice as far as plastics,
but my first choice is your idea to use the roll pins. It would seem
that they would withstand shearing forces better.

Well ... "best choice" varies with what you need it to do. I
might consider a fiberglass/epoxy to be stronger, or even a graphite
epoxy composition. But it is not yet clear how much strength you really
*need*.

That is why it is best to go with the strongest material I can find. A
material that will best withstand the kind of force generated(when a
3/32" wide area attermpts to shear off a 3/32", 1/8" diameter pin).

[ ... ]

I also have several 4" diameter Delrin cylinders that are about a foot
long, I'll be slicing them into 3/16" or 1/4" thick disks that I'll
need to do six plunging operations involving making different sized
holes/slots in each of six equal "pie" slices.

O.K.

BTW. If there is a way to do this using my lathe, the drill would have
to be at the headstock end, and I'd need something like a six-sided
collet block in something not only large enough to hold a 4" disk, but
also position it at varyng heights above the bed.

There are 3-jaw chucks, and 4-jaw chucks, available with 5C
collet style shanks -- designed for when a larger lathe needs to handle
smaller workpieces. Combine this with a collet block and a lathe based
milling attachment and you could probably make a go of this. The
question is how deep and how long the slots will be. If they are all
the way through, and continue out to near the edge of the disc, you will
have problems as the workpiece will collapse when enough material has
been removed.

I don't know of a better plastic for this. Basically any of six
holes/slots of various shapes(drilled all the way through the Delrin)
would fit within a 1-1/4" area and there will be a minimum space of
3/16"-1/4" between any of them. This is one reason why I was thinking a
drill press might be the best way to go.(Rpms would be a big guess at
this point).

For that -- perhaps replace the chuck with a faceplate, and find
positions in the workpiece which can accept bolts to secure it to the
faceplate (perhaps with some sacrificial material under the workpiece, so
you don't mill into the faceplate itself.

That's the plan. :-)

So obvious a drill press and a rotary table would be the easiest and
cheapest way to go for this.(I'd have to "overlap" drill holes to make
slots).

Smooth slots would be difficult to make this way. Drill bits
tend to bend when you're trying to partially overlap with another hole.
If you space the holes such that they are almost a full drill diameter
apart, and then make a second pass, drilling through the center of the
web between them, you will wind up with a scalloped structure which
you can then smooth out with a file.

Yes, perhaps "overlap" was a bad word to use. You described my
intentions. I've read about how drills catch the material on one side
when attempting to drill partly over another hole. The method you
described evens out those forces on opposite sides of the drill bit.

Your setup on the faceplate would work. But you would need to
add a counterweight on the faceplate to allow it to balance properly so
you could run at reasonable speeds for a 1/8" drill bit.

Hmm... My original thought was to drill out the solid end of the
holder(a little at a time) which at the start would be slightly heavier
than the side holding the work and Aluminum block. until the weight on
both sides of the center axis(drill point) was equal. This would be
determined by balancing the center axis on a straight edge. But if this
is a bad idea I'll entertain your suggestion.

Simply mount a lighter counterweight to one of the faceplate
slots, secure it with a bolt through the slot, and adjust it outwards
until it counterbalances the workpiece and the fixture. A lot easier
than repeated machining until you get close to the right size.


Yes, but how would I determine when it was balanced?

BTW. After some more thought, perhaps a holder made completely out of
Aluminum instead would be better. The holder/work combination would be
lighter as a result, but the heavier side would now be the half that
encompasses the work.

Thanks.

Darren Harris
Staten Island, New York.