The £200 CNC

[irving2008]

Sorry for the long time with no updates but life took over! However I have now ordered some more parts, horray!

SY60STH86-3008BF Nema 23, 1.8 degree hybrid stepper motor with a holding torque of 3Nm (425 Oz-in) £88 from ebay

and

3 Axis TB6560 Stepper Motor Driver £47

As soon as they arrive show my progress

What voltage are you going to run that driver on? And what leadscrews have you got?

[Wotsit]

Only 18V with a trapezoidal lead screws, i was going for 8mm bar and derelin nuts but think it might be worth an upgrade. Cost is the major factor for this project, just over the £200 mark now and hopeing to stay there for this one.

[irving2008]

Only 18V with a trapezoidal lead screws, i was going for 8mm bar and derelin nuts but think it might be worth an upgrade. Cost is the major factor for this project, just over the £200 mark now and hopeing to stay there for this one.

18v with that motor/driver combo and 1mm pitch screws will give you a top traverse speed of around 150mm/min assuming direct drive... but then speed and cost are mutually exclusive!

[Wotsit]

Speed is not that important for this project, just trying to use what i have available at the moment. I do also have a 100VA transformer that supplies 34v open circuit but i think that might be to low current and too high voltage for my driver.

Really hope i can cut alu even with a tiny feed in the end.

[irving2008]

Speed is not that important for this project, just trying to use what i have available at the moment. I do also have a 100VA transformer that supplies 34v open circuit but i think that might be to low current and too high voltage for my driver.

Really hope i can cut alu even with a tiny feed in the end.

The maximum you can run those boards to is 30 - 32v, and with 3 motors like that you'll need a supply capable of delivering about 10A. Those motors will run at 4.2A bipolar parallel wired for max torque/speed but your drivers will current limit at about 2.5A RMS, 3.5A peak.

Cutting ali is not directly a function of feed rate but a combination of factors includng # of flutes on the cutter, cutter dia, spindle speed, spindle power and the depth of cut (both radially and axially) as well as the feed rate. For a cutter to work efficiently it must actually cut, not rub, so too high a spindle speed combined with too low a feed rate will just wear out cutters and generate a lot of heat without performing much work. If you keep the depth of cut small and can turn the spindle speed down and use a single flute, small dia cutter then you will cut ali.. just anything worthwhile is going to take a long time and you need to consider whether your spindle motor's duty cycle will allow it.

As a rule of thumb the cutting rate for ali is 100m/min and therefore spindle RPM should be 100 x 320/d rpm, where d is cutter dia in mm, so for a 6mm cutter spindle speed you are aiming for 100 x 320/6 = 5333rpm.

Another rule of thumb is that feed rate for ali is around 0.3mm/rev per tooth so with a single flute cutter and a spindle speed of 5000 rpm, you need a feed rate of 5000 * .3 = 1500mm/min which you cant get close to. If you can turn the spindle speed down to 1000rpm say then you are more likely to cut the work than just burn up the cutting tool.

The other consideration is spindle power. Aluminium requires about 17W per cubic centimetre/min. So for a 6mm cutter edge milling a 3mm slot 1mm deep at 100mm/min you need 51W of spindle power at the cutter, or about 100W input. An 80W Proxon or Dremmel wont cut it here... (excuse the pun)