Plasma Attempt #2

[Jonathan]

You work to three decimal places and we disagree

Not quite - we disagreed because we're talking about fundamentally different things. I should have explained myself more clearly. It's three significant figures anyway :whistling:
To accelerate the gantry the motor needs to impart translational kinetic energy into the whole gantry and rotational kinetic energy into the rotating parts. Therefore we must consider the sum of the torque contribution from each. So far you've only considered the translational kinetic energy, so I worked out the torque contribution from rotation. For that instead of calculating the inertia, you find the moment of inertia (I), which is a measure of an objects resistance to changes in its angular velocity and use the formula torque=angular acceleration * moment of inertia, where acceleration is in rad/s^2.

In post #4 I calculated the moment of inertia of your 10g (well not quite, see below) tube using the formula, courtesy of Wikipedia:


In post #5 I calculated the acceleration of the tube and stuck that in the above formula for torque to get 0.45Nm and subsequently 0.135Nm. With the mass of the tube included your estimate for the torque due linear motion is now 0.55Nm, so just add that to the figures I get to find the overall system, i.e. 1.0Nm or 0.68Nm. Or if you used 12mm steel bar it's <0.01Nm added... I should also calculate the moment of inertia of each pulley and add that to the system, but it's not going to change the conclusion. Either way the 3Nm motors even on 50V drivers (although clearly 70V preferable) will be a good match and surely the best option since they are the best price/Nm!

Incidentally, what thickness are you using for gauge? Once you get past tiny the OD goes up in multiples of 1/8". People like tubing to fit snug one inside another so the wall thickness is usually 1/16" for 16g and 1/8" for 10g.

I used:
http://www.engineeringtoolbox.com/ga...eet-d_915.html

Is this some silly American/English difference in the gauge system (like with wire gauges)?

I got some 15mm wide, steel reinforced belting which is probably massive overkill but I'd rather it stretched as little as possible.

It would be interesting to know how much it actually stretches - i.e. find out the spring constant. You could hang up a length of it and attach something heavy, put DTI underneath and zero it, then add known mass and see by how much it stretches. I've spent ages trying to find the spring constant of different types of timing belts, but just can't find anything.