I'm looking for some ideas on how to repeatedly and hold some 20mm OD PVC
round bar in a lathe chuck. I'm turning batches of 20 pieces - they start as
a 25mm length of 20mm OD PVC rod and end up with:

a) an 11mm thru' bore
b) a 17mm x 5mm counterbore.
c) a 17mm OD step on the outside at one end.


Hopefully the cross-section below will make sense of my ramblings:

__________
_____| ____]
|___________]

11mm thru 17mm internal counterbore
bore
___________
|_____ ]____
|__________]

17mm 20mm OD
step



Is there a neat way I can fabricate some sort of jig into which I can
quickly clamp the 25 L x 20 OD PVC pieces so they will be in the same
location? Some sort of simple collet??

I need to achieve 0.5mm accuracy lengthways and 0.2mm on the turned OD and
internal counterbore diameter.
Unfortunately I dont have a milling machine - just the lathe,a CQ6230 lathe
(Chinese copy) with a 3 & 4jaw chucks + faceplates etc.

TIA
Robbo (the novice)

According to Robbo :
I'm looking for some ideas on how to repeatedly and hold some 20mm OD PVC
round bar in a lathe chuck. I'm turning batches of 20 pieces - they start as
a 25mm length of 20mm OD PVC rod and end up with:

a) an 11mm thru' bore
b) a 17mm x 5mm counterbore.
c) a 17mm OD step on the outside at one end.


Hopefully the cross-section below will make sense of my ramblings:

__________
_____| ____]
|___________]

11mm thru 17mm internal counterbore
bore
___________
|_____ ]____
|__________]

17mm 20mm OD
step

I'm not sure how the bores are meeting -- or whether they are
expected to meet.

Is there a neat way I can fabricate some sort of jig into which I can
quickly clamp the 25 L x 20 OD PVC pieces so they will be in the same
location? Some sort of simple collet??

Yep. start out with a piece of aluminum or brass round stock
large enough so you can offset a 20mm diameter bore to produce the
desired offset when the piece is chucked in a 3-jaw chuck. (You'll need
a 4-jaw chuck to bore the hole of course). Make it long enough to hold
the length (exclusive of the length to be machined on one end), and
solid for perhaps 50mm after that to give the 3-jaw chuck a good grip.
While you have it in the 4-jaw chuck, turn the outside to be concentric
with the pocket for the full 25mm length, with perhaps a 4mm wall
thickness. (Hmm ... make it shallower -- the pocket should be no deeper
than the length of the workpiece full-diameter body plus the longer of
the two turned down projections (assuming that they are not of equal
length).

Once you have the pocket machined, go to a vise, and use a
hacksaw to cut three cuts through the center of the pocket at 60 degree
angles so you have six evenly-spaced slots.

Deburr the saw slots on the ID so you don't scratch your
workpiece.

Mount the solid length in a standard 3-jaw chuck (assuming that
the accuracy of a 3-jaw is good enough. Slip the workpiece into the
pocket, and put a worm-gear hose clamp around the OD as far from the
chuck as possible. Tighten that with a screwdriver, and it will grip
the workpiece.

If you want, you can make two of these -- the second will be
shallower, and then with a hole drilled for the first projection, so the
orientation from the first to the second will be consistent.

I need to achieve 0.5mm accuracy lengthways and 0.2mm on the turned OD and
internal counterbore diameter.

What about accuracy of the offset? If this is critical, you
will have to put your fixture in the 4-jaw chuck and tune it for being
on center.

Unfortunately I dont have a milling machine - just the lathe,a CQ6230 lathe
(Chinese copy) with a 3 & 4jaw chucks + faceplates etc.

That -- plus a hacksaw, or a bandsaw -- should be all that you
need.

Good Luck,
DoN.

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