"DoN. Nichols" wrote in message
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[ ... ]

Hmm ... for the drilling, you could make a special holder which
held a thrust bearing assembly with a bore larger than the drill which
you would use to start the hole. You could crank the carriage into
position (with the bearing well centered) to maintain extra pressure
while drilling, since you don't need the carriage for the actual
drilling.

That is getting tricky...

Hmm ... another way to hold it fairly concentric (assuming that
your stock is truly round and near the desired size) would be a ring
which slips over the OD of the workpiece and (with less than 1/2" radial
size around the workpiece) and drill and tap four holes at 12:00
o'clock, 3:00 o'clock, 6:00 o'clock, and 9:00 o'clock and clamp it to
the faceplate with two steel bars going from 12:00 o'clock to 3:00
o'clock and from 6:00 o'clock to 9:00 o'clock so they don't interfere
with the spindle -- and the ends have to be cut to clear the bed. This,
perhaps augmented by the double-sided tape, would hold things firmly in
place.

I like that.

Hmm ... two rings -- one which is a fit on the faceplate, and
the other on the (larger) workpiece, held together with bolts might make
getting things centered a bit easier.

Anyway -- with these approaches, you could use higher spindle
speeds than you would find safe with just the tape.

In any case -- use alcohol or acetone to scrub down the surface
of both the workpiece and the faceplate to make sure that they are both
free of oil and grease for maximum adhesion of the tape.

Acetone for the faceplate. Alcohol for me...

[ ... ]

It depends. super glue forms a rather brittle joint, so if you
have an interrupted cut or otherwise bump things you could break the
bond. The double-sided tape -- especially sufficient area of
double-sided foam tape -- will give a very good grip, and be more immune
to shocks.

I have not considered that aspect...

[ ... ]

Think of the special soft jaws available for the Taig/Peatol
lathe.

I do. All the time :-)

Those will handle a workpiece significantly larger than the
chuck body. For a 5" chuck -- just make large pie jaws to replace the
hardened top jaws -- *if* the machine comes with a chuck with two-part
jaws.

The problem arises with the Taig also. None of the chucks will hold a 4"
piece. I do the insides with a boring head on the mill and finish on the
Taig.

[ ... ]

Thanks a lot. I shall try this on a smaller scale (4") and be ready to
duck
if things fly at 550 rpm :-)

The larger the diameter, the more grip surface you have between
the double-sided tape and the workpiece and faceplate.

But in any case, I would not consider 550 RPM to be a good idea.

First -- if the tape does let go, it could throw things rather
hard.

Second, 6.5" diameter means 935 SFM, which strikes me as a bit
fast unless you are using something like carbide tooling and a free
cutting mild steel like 12L14.

No, no! I did say I shall try with *4"*. Yes, 550 rpm (the lowest the Taig
will go) is way too fast for that too, but I have managed so far. The only
way it works for facing is a sharp HSS tool with a small point radius. And a
very slow and steady hand as one gets to the outside. To do the inside I
need to bore only about 2" diameter. OTOH there is probably no point doing
that as the mill method works just fine and it will provide me with only
limited information about how the big piece would behave.

Based on the cutting speeds chart for HSS tooling in the
Jorgensen steel book:

Steel SFM
---------------
1018 125
12L14 300
4140 120 (annealed)

Stainless steels below
Steel SFM
---------------
304 75
416 160

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,
Campbell River, BC