Re:sherline motors/handwheels was: first post
Posted by
ballendo@y...
on 2000-09-06 04:18:55 UTC
Alan wrote:
Taking off your handwheels may have improved your mills' ACCURACY.
But for many, leaving those off can/will REDUCE accuracy!
If you have powerful double shaft motors, you may want to put the
handwheels back on.
You don't have a lot of torque to begin with, as you mentioned in
that post. Step motors act like an electric spring. (technically,
they are a spring-damped system) At the beginning and end of each
step, there is a "springy" gettin-goin' and a "springy" getting-
stopped as the electromagnetics of the motor work to start and stop
the rotor.
The sherline leadscrew is small diameter, small mass. When the CNC
changes direction, the stepper "spring" can combine with the low mass
of the leadscrew and cause missed steps. Retro-fit ballscrews can be
worse. I have seen this on MANY MFR's models of sherline retro-fit.
Why don't we hear about it?
1.A good machinist can make excellent parts with a marginal tool. Not
that he or she wants to, mind you, but his or her intuition and
experience makes up for what the tool may lack.
2.If the parts are good enough(to tolerance),it doesn't matter.
3.The measuring tools(or techniques) are poor.
4.Combining the three above, it may not be noticed.
After all, when we set up the sherline (before Joe Martin's new cnc
mods), we have a typical .003 clearance(backlash). And if we 1/2
step, we have .000125(8K steps per inch) resolution. This means worst
case, we can miss up to 24 steps and MAY not even notice! I know many
who turn the B/L comp off on sherline mills, so the arcs' finish
looks better.
When you took the wheels off, you reduced the inertia of the system.
With your weak motors, this may have put the system in better balance.
Others with more powerful motors may want to increase the inertia
(leave the handwheels on).
Or install lanchester dampeners (talk to a stepper manufacturer)This
is not usually needed, but it is a standard solution in the motion
industy.
The z axis is less affected than xy since gravity keeps a somewhat
constant force against the leadscrew rotation, where in xy the
leadscrew at times is the ONLY thing moving.(besides the stepper)
You can use belts to drive the sherline.(just like the big boys do
it) This enables the belt friction to damp the ringing of the
steppers,lets you keep the handwheels in nearly original position,
and reduces the overhang moment of the stepper weight.
Ballendo
P.S. There are Many sherline mills doing excellent service for many
people. If yours is working for you, great! But if you are having
problems holding tight tolerances, Get a hardinge!(joke) No really,
you may want to look into it along these lines.
Heavily greased leadscrew can also damp the system. Tightening the
ways won't help because it's when ONLY the leadscrew is turning that
steps are lost.
>In my case,the handwheels I removed from my Sherline!Alan,
>
>For example, My little Sherline now as 70 oz/in (direct), and that's
>not really enough!
Taking off your handwheels may have improved your mills' ACCURACY.
But for many, leaving those off can/will REDUCE accuracy!
If you have powerful double shaft motors, you may want to put the
handwheels back on.
You don't have a lot of torque to begin with, as you mentioned in
that post. Step motors act like an electric spring. (technically,
they are a spring-damped system) At the beginning and end of each
step, there is a "springy" gettin-goin' and a "springy" getting-
stopped as the electromagnetics of the motor work to start and stop
the rotor.
The sherline leadscrew is small diameter, small mass. When the CNC
changes direction, the stepper "spring" can combine with the low mass
of the leadscrew and cause missed steps. Retro-fit ballscrews can be
worse. I have seen this on MANY MFR's models of sherline retro-fit.
Why don't we hear about it?
1.A good machinist can make excellent parts with a marginal tool. Not
that he or she wants to, mind you, but his or her intuition and
experience makes up for what the tool may lack.
2.If the parts are good enough(to tolerance),it doesn't matter.
3.The measuring tools(or techniques) are poor.
4.Combining the three above, it may not be noticed.
After all, when we set up the sherline (before Joe Martin's new cnc
mods), we have a typical .003 clearance(backlash). And if we 1/2
step, we have .000125(8K steps per inch) resolution. This means worst
case, we can miss up to 24 steps and MAY not even notice! I know many
who turn the B/L comp off on sherline mills, so the arcs' finish
looks better.
When you took the wheels off, you reduced the inertia of the system.
With your weak motors, this may have put the system in better balance.
Others with more powerful motors may want to increase the inertia
(leave the handwheels on).
Or install lanchester dampeners (talk to a stepper manufacturer)This
is not usually needed, but it is a standard solution in the motion
industy.
The z axis is less affected than xy since gravity keeps a somewhat
constant force against the leadscrew rotation, where in xy the
leadscrew at times is the ONLY thing moving.(besides the stepper)
You can use belts to drive the sherline.(just like the big boys do
it) This enables the belt friction to damp the ringing of the
steppers,lets you keep the handwheels in nearly original position,
and reduces the overhang moment of the stepper weight.
Ballendo
P.S. There are Many sherline mills doing excellent service for many
people. If yours is working for you, great! But if you are having
problems holding tight tolerances, Get a hardinge!(joke) No really,
you may want to look into it along these lines.
Heavily greased leadscrew can also damp the system. Tightening the
ways won't help because it's when ONLY the leadscrew is turning that
steps are lost.