Wobblycogs CNC Mk1

[Wobblycogs]

Ok, so I stayed up to the wee small hours trying to figure out what size ballscrew and stepper I should be using on the X-axis but there's some things I don't think I'm understanding. I'd like this machine to be able to cut aluminium so my calculations are based around this. My thinking is that if it can cut aluminium then wood shouldn't be a problem (hope that's right).

In the post describing the motor calculation it says that for cutting aluminium a speed of 1800mm/min or better is desirable but the best I can achieve, even with the largest Nema 34 motor, seems to be about 1600mm/min. In this post though Jazz talks about cutting at around 700mm/min which is well within the realms of possibility. What speed should I enter for Max linear speed (B22) when cutting?

From trial and error I've found two setups that appear to work
Nema 24 (Zapp, SY57STH76) + RM1610

Nema 34 (Zapp, SY85STH80) + RM2510

I wasn't sure exactly what Screw Fixing (B19) meant so I left it as supported-supported. Would bearing blocks be considered fixed though?

What I don't understand though is why changing from a 1610 ballscrew to a 2510 nearly triples the total torque but doubling the mass of the gantry to 70kg makes only a small difference (in fact in the scenario with 24's is still ok with a super heavy gantry but the 34's are out of the safe range).

Am I doing something wrong here? The smaller motors and ballscrew seems to be a much better choice which feels totally counter intuitive to me.

Cheers

[Jonathan]

The common bearing blocks are somewhere between supported-supported and fixed-supported since the bearings are not sufficiently spaced to truly count as supported:
Fixed-end, supported-end, free-end are ball-screw support conditions you need to understand in order to design the best actuator system for your equipment

You need to increase the motor voltage - 70-75V is common for Nema 23/24 motors and Nema 34 motors will work on the same, but can be operated from mains voltage with expensive drivers, which shouldn't be necessary here.

The reason the increasing the ballscrew diameter from 16mm to 25mm makes such a large difference is because the moment of inertia of the ballscrew is proportional to the radius to the power 4, so you need (25/16)^4=5.96 times more torque to accelerate it at a given rate. The gantry is driven via a huge mechanical advantage, and increasing it's mass only increases the inertia proportional to the mass (no power), so it doesn't make such a big difference.