Tuesday, 6 September 2011

CNC pick-and-place machine needed!

After spending hours and hours last night assembling and soldering just a couple of PCBs, the need for some sort of automation is growing - especially if we're going to realise the dream of actually making and selling a few miniature instruments.

So we're back to investigating a miniature CNC-type pick-and-place machine.
We've already got some stepper motors working and pulled apart a few printers and scanners, and have had no luck in finding exactly the types of steppers, belts and pulleys we were hoping to use.

Which has lead us down a slightly different path - instead of determining which types of stepper motors and belt-drive system we're going to use up-front, we're going to build a system which anyone else can build too - but using parts that can easily be scavenged from old computer hardware.

We dismantled an old Lexmark Z73 and found some useful looking stuff - stepper motors, carriage rods, belts and so on. None of these match our original cnc requirements (1.8deg steppers, 20-tooth pulley, 5mm pitch belts) but we've decided to change our approach, and build a machine using the parts we can get hold of. We'll write some software to drive our custom-made stepper board, so that you can simply enter a few parameters and let the computer do all the tricky calculations.

This sounds like we're heading towards Mach3/traditional CNC type ground - the original plan was to just build something that would work "out-of-the-box" without lots of difficult setting up and parameter fiddling. But then again, buying all new hardware is going to get quite costly for us, or anyone else wanting to make a similar machine, whereas re-using and recycling old computer hardware is a much more eco-friendly way to go about making stuff in general.

Here's our starting point - a stepper motor and a timing belt.

The stepper motor is a Mitsumi M42SP-6NK.
A quick look on Google returns the datasheet, telling us that it's a 7.5 degree motor, runs at 12V and has a peak current of 400mA. We marked one of the teeth on the cog, then counted them clockwise, and discovered that this motor is fitted with a 15-tooth pulley

The timing belt didn't reveal much - the serial number OPM 300766 returned nothing of interest, so we had to do a bit of investigating....

To find out the pitch of the belt, we need to measure from the centre of one tooth to the centre of another. This belt has tiny teeth, so we marked out 20 teeth using some masking tape and measured across the tops of the teeth with a steel rule (marked in 0.5mm spacing). Despite the photo's appearance, we made it 24mm across 20 teeth, making the belt pitch 1.2mm

This may or may not be correct. The belt may even use imperial measurement (e.g. 1.2mm = 0.0472 inches - it may be a 0.05" pitch belt and we've just not measured it properly!) All this can hopefully be corrected in software once we've actually got the machine built, entered a few parameters and calibrated everything fully!