Re: Turning speeds re-visited
- From: "Don Young" <notme@xxxxxxxxxxxx>
- Date: Tue, 2 Dec 2008 20:36:10 -0600
"Michael Koblic" <mkoblic@xxxxxxxxxxxx> wrote in message
news:gh4po7$8om$1@xxxxxxxxxxxxxxxxxxxxxx
Steve Lusardi wrote:I neglected to mention that for large diameter work at slow speeds you don't
Michael,
Very good observation. It is amazing how few make the same one. It is
typical of mainland Chinese lathes. Even my 10" SB goes down to 33
RPM and is very handy in winding springs and electric coils if you
are so inclined, not just large diameter turning jobs. You may note
that Taiwan made machines are 12 speed and the mainland machines are
typically 9 speed in the 13/14 x 40 class. Another observation to
make is thread count from the lead screw gearbox. The cheap machines
are very limited, even the better Taiwanese machines have that
shortcoming. (30 to 40 threads), as opposed to my 18 x 54 L & S at
over 120 plus leads for hobs. You get what you pay for.
Much of this stuff is so much black magic to me.
1) The torque of DC motors with speed reduction. My understanding was
that the whole point of using DC in these applications was that you did
not lose an appreciable amount of torque. How much do you lose? And when?
I looked at some graphs, some implying that the relation is in fact
inverse. Jim Cox in his book "Electric motors" states in the chapter on
commutator motors "the maximum torque is constant and only slightly
affected by speed".
2) Motor ratings. Often I see the motor specs given as voltage (120V AC
etc.), horse power (usually a fraction) or current in Amps. Often one
finds all three. In those cases it seems pretty clear that the VA multiple
signifies the input power and the HP the output. In those cases where only
voltage and HP is given I am not sure what to think. BTW is 60% efficiency
of a lathe motor usual?
There is an interesting comparison with routers: many are marketed as
"3-1/4 HP". To get this kind of mechanical output one would have to have
one's 120 V outlets rewired to 20 amps *and* have a 100% efficiency!
From reading this and some of the stuff elsewhere one has to conclude that
for turning thin steel objects up to a 7" diameter and cylinders up to 4"
(but no longer than 14" or so) one is actually better off with a
mini-lathe than with a 9X20. One proviso being that not all the minis go
below 100 rpm with their speed either. There is however, at least one
source available which shows how to reduce the speed in a mini further.
I am ignoring the rigidity issues. I am only beginning to understand those
in practical terms on my mini-mill.
Although in general terms I agree that you get what you pay for, it is
important to determine the shape of the diminishing return curve for each
application. I am still trying to define mine :-)
--
Michael Koblic,
Campbell River, BC
need the same torque as for small high speed work, you need more torque.
Don Young
.
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- From: Michael Koblic
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