RE: [CAD_CAM_EDM_DRO] Re: machine rethink

>more TPI means even more delivered power.

Semantics of power and force aside, with stepper motors, this only holds if
you also reduce your cutting speed.

If you take some code that has a feed rate of 4"/min then change to finer
leadscrews (adjusting the cnc softwares settings to suit)
then there's a pretty good chance you'll end up with less force at the tool.


The reason for this is that the motor now has to spin faster to deliver that
4"/min and it's output torque and power drop.
If you're lucky and you're still using a coarse screw (and by coarse I mean
<5 TPI) then you'll break even.
Stepper motors deliver peak torque at 0RPM, from there it gets worse. It
took me a long time to get my head around this. Eventually a friend built a
router with 20mm pitch screws, at the time we figured it'd go 4 times faster
than my router which uses 5mm pitch screws. It goes 8 times faster because
the motors can turn slower and develop more torque than they loose from the
reduced mechanical advantage of the coarser pitch screws. They also
accelerate 8 times faster. The mass of the machine is small compared to the
rotational inertia of the motor and screw.

Here's a video http://www.caswa.com/cnc/nobott_circles.wmv (about 1Mb)
This is driven by the ubiquitous 250ish oz-in double stack nema23 motors and
a xylotex, 24v, driver.
Top speed is about 15m/min.
There's plenty of cutting force at 10m/min

Note how the circles that include Z are about half the speed of the XY
moves? The Z axis screw has a 10mm pitch, the X and Y axis have 20mm pitch.
Same motors. same driver,
For the best speed and cutting performance you should be using screws that
give a half step resolution just a bit better than your hardware can
resolve. If you go finer than this, the ONLY outcome will be a slower
machine.

PK

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