RE: [CAD_CAM_EDM_DRO] Musings on screw pitches

Hi Paul,

First of thanks for posting that great video and for not being rude by only
showing the tool bit motion. I've always thought this group was about being
helpful and passing on information when posting; which you have been!!!

>
> All this talk of fast machines, and some recent events, have
> got me thinking
> about stepper based systems.
>
> Maybe it's from reading the web published stuff, or maybe
> it's from looking
> at pictures of other peoples hardware, or maybe it's because I've been
> confusing torque and power, but I have long had it in my head
> that a 4 or
> 5TPI screw was "about right" for an NEMA 23 double stack
> motor driving a
> small machine.
> Certainly there are a lot of machines with ballscrews of
> about that pitch
> (or finer) out there.

I agree with you that a small machine could easily be run with high quality
size 23's. You just have to look at a printer or plotter with an even
smaller motor and the speed at which everything moves. But they tend to use
belt, wire, metal tape drives rather than a ball screw.

If you've ever had the chance to watch a pick and place machine stuffing a
board or a wire die bonder the speed is really quite something. But the one
I was watching the other day uses what looks to be a 4 start thread of about
1 turn every 50mm. With a 10,000 step per rev motor that's 0.005mm
resolution; of course assuming a stepper could be made to resolve that is
another issue. I'm not sure how the carriage followed the screw as it was
hidden behind all the pick and place hardware.

>
> A buddy recently built a small router (envelope about 400mm x
> 400mm x 100mm)
> the screws he had were about 20mm pitch. I speculated early
> on that it would
> go "a bit" faster than my, very similar, machines 2500mm/min.
> And that this
> is because the motors would have more torque at the lower
> rotational speed.
> Nb that's a metric "bit".

Nice.

>
> Well it went 6 times faster. http://www.caswa.com/nobott_on_speed.wmv
> (cutting the road runner)
> Not only that, it accelerated 6 times faster. Same drives,
> same motors.
>
> The acceleration is the clincher. It's clear that, on
> machines of this size,
> that the mechanical advantage of the ballscrew reduces the
> mass that the
> motor "sees" to a point well bellow the inertia of the motor+ screw.

Where did he get the ballscrews with that pitch?

>
> When I compare the performance of my friends new machine to
> mine in terms of
> RPM an RPM/sec, they are pretty close.
> OK, the rest of the discussion (resolution, microstepping
> etc) pans out from
> here.
>
> My question is, why is it not in the FAQ that you should use
> 20mm pitch
> screws on small machines (particularly routers, where 0.05mm
> for a half step
> is plenty of resolution)??
> Opinions..

My XY system for programming a panel of processors ended up being so slow
with a 20TPI thread that it was faster to do it by hand. My next approach
was going to be to use belts. But 20mm ball screws sounds like a good idea.

John Dammeyer

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