According to :
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).
Hmm ... that depends on whether you *want* it to shear to
protect the rest of the system at some point, or whether you want it to
hold on as long as possible.
If the latter, I would suggest either a hardened dowel pin, or a
solid carbide pin.
For the former, the material is selected based on the maximum
force which you want it to transmit before it fails gracefully.
examples of this are things like the shear pin in the connection of a
propeller to an outboard motor, so the propeller survives when it hits a
rock or a log. Or a pin connecting the quick-change gearbox on a lathe
to the leadscrew, so if the carriage gets wedged, the pin will shear,
instead of losing teeth from the gearing.
[ ... ]
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).
Remember -- the slot will be scalloped -- at least until you can
file it smooth -- assuming that you have a place to run the file to do
that.
[ ... ]
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?
Switch on the lathe spindle at a low speed, without planning to
make a cut. If the lathe doesn't try to walk off the table, keep
increasing the spindle speed to see where it does try to walk off the
table. The higher you can get the speed without things galloping
around, the better. If it does start bouncing, stop it, move the
counterweight either a bit more distant from the center or closer. Try
again. If the problem occurs at a lower speed, you moved it the wrong
way.
Since you're going to be drilling steel with only a 1/8"
diameter drill bit, you will probably want something close to the
maximum speed the lathe is capable of. (Check the SFM recommendations
for the drill bit material in the material you are drilling and compare
it to the results of calculation to determine the desired spindle speed.
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.
Play with it until you get the right results.
Enjoy,
DoN.
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