Belts & pulleys & advice please

**Robin Hewitt** · 26-04-2012

Nowt wrong with an optical head so long as you have someone there to kick it when it reaches that awkward corner

I'm just drawing up a 2.5m x 1.25m plasma table and Gary has promised the 2.7m profile rail will arrive unbent.

I plan to use roller chain. I know it's supposed to be a big no-no because of the cogging but the price of the 04B 6mm chain has plummeted, it doesn't stretch and with a 20t sprocket my drawing says the cogging error will be around 0.076mm, about 3 thou.

3:1 down on to the sprocket gives me 40mm travel per motor rev which divides conveniently down to 0.1mm per half step and seems optimal for both rapids and slow cutting.

You only need large motors for a heavy gantry if you need large accelerations. If you don't need large accelerations to avoid a plasma torch blowing the corners off on thin sheet then NEMA23's have a much better speed range than NEMA34's.

If it is free rolling, and you only have the inertia to contend with, then the maths is easy. If the gantry weighs 100 lbs then you know that a force of 100 lbf will accelerate it at 1G because that is what you would happen if you dropped it. If you driving force was half the weight then you would have 0.5G and so on.

To calculate the driving force you need to know how far the motor will move the gantry in one revolution then divide that in to 2 pi.

eg: If I fitted a 0.5Nm motor to move my gantry 4cm/rev the driving force is 0.5 * 6.28/0.04 = 78.5N = 17.6 lbf

Meaning, if I can keep the gantry weight below 17.6 lbs I might hope for 1G acceleration from a 0.5Nm motor.

Play with the numbers, see what you can come up with.

**Jonathan** · 26-04-2012

What feedrate do you currently get/need on this machine? Knowing that and the mass of the gantry along with what Robin said you'll be able to work back to find an appropriate motor.