On 2009-07-03, Michael Koblic wrote:
DoN. Nichols wrote:

[...]

Shimming the workpiece has two disadvantages, depending on how
you do it all.

1) If they are kept behind the workpiece while turning, they are
likely to fling out at high speed.

2) If they are tapped out after the jaws are tightened, the
workpiece is likely to work its way in towards the chuck body as
you turn.


Ah. Did not think of No.1.

You should *always* consider what might fly out when you could
be in the path.

You *could* make the shims in the form of turned rings which go
around the last jaw step so there is nowhere for them to go even if
the workpiece is no longer firmly gripping them.

Noted for next time I have to do it with a 4-jaw.

Note that I have never tried that design -- just thought of it
as a possibility -- and one which does not require different size rings
for each size of workpiece, since they are trapped by going around the
jaw ends.

[...]

And you could replace the motor with a three phase or a DC motor
and connect an appropriate controller to it to get speeds perhaps
1/6th your current speed. That would take you down to 100 RPM or a
bit slower.

A DC motor, a Variac, and a rectifier would do a nice job there.
If you really care about spindle speed precision, a servo motor and
servo amplifier would be guilding the lily. :-)

I don't, but even the bare bones are not a cheap option as I found out
yesterday looking around eBay.

Someone else suggested a light dimmer which is typically quite
inexpensive -- and as for motors -- a cheap hand held electric drill
could be clamped onto a shaft in bearings to drive at a lower speed.
Or -- you could salvage a motor out of a vacuum cleaner or a kitchen
blender or anything else with a DC/universal motor. (Just look for
brush holders to verify that it is not AC only -- and the holders may
be hidden inside, as is common in cheap hand held electric drills.)

[...]

The countershaft might be a good idea. Two pulley steps (made
on the Taig) -- one for about 1:1 ratio, and the other for about 6:1
ratio. Or -- if you could pick up a slower motor, make the pulleys
identical, so they step up as much as they step down. A ratio of
2.45:1 (5:2 would be close enough) and a motor whose speed is about
600 RPM would be nice (though 900 RPM is going to be easier to find,
I think.

I was thinking of just getting two more Taig pulleys ($24.49) and making
*two* countershafts.

Two extra pulleys (and an extra belt) would make *one*
countershaft, not two.

The lowest speed would be just over 60 rpm. Even with
just one pulley the speed would be under 200. And the theoretical maximum
speed of 47,250 rpm. I wonder what the 3-jaw chuck sounds like at that
speed...

Hmm ... what *is* the sound of a 3-jaw chuck flying into pieces.
That speed (if the motor could put out the needed torque) would
certainly explode the chuck. A 12" chuck is considered dangerous at a
bit over 3000 RPM IIRC.

The two questions that bother me about the concept of a countershaft a

1) Is the 3M belt big enough withstand the torque at the lowest speeds? Or
would one have to go to a completely different transmission for the final
step (countershaft 2 to spindle), e.g. timing pulleys and belt?

I would suggest that you make a timing pulleys and belt for the
step from the countershaft to the spindle to handle the extra torque.
Look for pulleys for about a 6:1 ratio which will get you down near 100
RPM or a bit more.

2) If one were to make a pulley, how critical are the groove dimensions?

The angle between the walls is critical, and the spacing has to
be tight enough so the belt does not touch the bottom.

The
only pulley I made was the wooden one (and it works just fine) but the
process can hardly be repeated with aluminium (or can it?)

It could be -- but a lot of careful filing. And it will take a
lot longer than doing it on the lathe. You will either need a compound
set to make the proper angle for the walls, or a form tool ground to cut
both angles at the same time. And I don't think that your machine can
handle the amount of metal removal involved in a form tool, even for
that small a belt groove.

And bear in mind that the speeds I give above are with no
slip. It will be slower with normal slip. Fore example, the 1650 RPM
motor would be 1800 RPM with no slip. (900 with no slip becomes
something like 825 RPM with slip, and 600 with no slip becomes 550
RPM with slip.) That 550 RPM would get you down to 316 RPM with
your existing gearing.

Ah, with slip I can get down to zero, no problem...

Yes -- you can get down to zero -- but between zero and a bit
below the nominal speed with slip you will have no torque to work with,
so the descent from normal slip to zero will be very sudden.

But a DC motor a Variac, and a rectifier would be more flexible,
giving you a much wider range of speeds, and the ability to change the
speed in mid cut without having to stop the motor and change the
belts.

That is clearly the preferred option but the prices I have seen so far
really make it uneconomical.

An electric drill motor (a hand-held electric drill is called a
"drill motor") with variable speed hooked to a shaft mounted in pulleys
should do it -- until the drill motor burns out. They typically don't
handle long run times well.

[...]

OTOH the second (re-worked) parting tool works just fine. Once you
get past the squealing and screeching of the bigger OD down to
about 3/8" :-)

Keeping the amount or projection down to a minimum helps a lot.
Making sure that the sides of the parting tool are parallel and
perpendicular to the axis of the workpiece helps. Having the
parting happening as close to the chuck's jaws as possible helps.
Getting lubricant to the bottom of the groove helps. And ideally,
not having a compound as part of the stackup will help minimize the
flex of the system.

Done all of the above. Still squeals like a banshee.

Interesting. I get rather quiet parting even with 3" diameter
stock -- but I'll probably be running at about 210 RPM.

Maybe I should reduce speed...:-)

And increase rigidity too -- which is another feature of my 12"
lathe which you don't have.

That is another reason to consider the rear-mounted parting
tool. Instead of digging in, it disengages a little when things flex.

Would it then get rid of the chatter by allowing an increased feed rate? At
$5 that is the truly cheapest solution.

You would still have chatter -- it just would not be as
destructive, and perhaps sound somewhat different.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---