Monday, 22 August 2011

What now? More CNC goodness!

The whole miniature instruments project grinds to a halt, while we wait for the postie to bring us a whole load of goodies to finish them off.
The drums need piezos for the underside of the drumheads (to send a strike signal back to the microcontroller when each drum is hit), the guitars are awaiting double-sided PCBs for the guitar neck, and the synth needs a miniature rotary pot and some LEDs.

So while we're waiting for all these bits and bobs to arrive from a variety of sources (mainly Farnell and eBay) we're starting to think about production. While it's easy enough to get things made cheaply overseas, we're a bit nervous about committing thousands of pounds to get hundreds of miniature instruments made. That's money that would be better spent on tools and equipment for other projects! Also, by committing to having loads of the same thing made, the options for customising them is greatly reduced. By manufacturing the little instruments ourselves, we can offer a full customisation service - and keep initial costs down (even if it means the cost to manufacture each one increases).

What we'd love to have/make is a pick-n-place machine for putting all those fiddly little SMT components in place on the PCB. If the same machine could also drop blobs of solder paste in the right place too, all the better.

We've already had a play with CNC machinery and on the face of it, creating and controlling a CNC machine shouldn't be too great a problem. Unlike a CNC router, we don't have to move large, heavy, cutting heads, or worry too much about external/twisting stresses on the machine joints.

Thanks to the guys at Oomlout, we've got pretty much all we need as far as servos and control gear goes. Stepper motors can be salvaged from old printers and scanners (we will be re-subscribing to the Freecycle mailing list to see what comes up!). We're considering a belt-drive rather than a threaded-rod for driving each axis. Lead-screws are great for low-speed, high-precision work, where high torque and large stresses and strains are used. Belt-driven control means faster rates of movement.

Although this is still in the early stages, here's what we've come up with:
Firstly, we're going to use stepper motors for controlling each axis. This is a no-brainer! BUT - CNC software (Mach3, MasterCAM, RouteOut etc) is notoriously difficult to get started with. So while using a Mach3-compatible control board (there are loads on eBay) means we can have something up and running very quickly, the finished product is then dependent on Mach3 (or similar software) which needs lots of parameters setting to configure correctly. Great if you're into that sort of thing, buy we'd just like to make a device you can plug in and drive!
The other issue with most CNC homebrew software is that it requires a "real" parallel port. Something our little laptops don't have! And we'd like everyone at nerd club to be able to use the machine, without having to have a dedicated CNC controller PC.

All this means that we're looking at creating our own (PIC-based, USB driven) stepper motor control board, as well as our own custom controller software. Because we don't want to draw fancy shapes, circles and arcs - we just want to control an x and y axis - we're pretty confident that our own custom drivers should be feasible. For this project, ease of use (and ease of understanding) is key to everything!

We're already familiar with the ULN2003A darlington array chips, so we'll use those to actually drive each stepper motor.


Now a belt-driven approach is a pretty new concept at Nerd Towers, so we're deliberately choosing values and ratios that make things easy to understand. They may not be the most efficient, or "best" choice for a particular purpose, but because we're building the driver board and driving software ourselves, it needs to be easy to understand (and easy to debug when things go wrong - as they invariably will!)

We're using 1.8 degree stepper motors.
This means it takes 200 steps to perform a single rotation (360 / 1.8 = 200).
We're also going to be using 5mm pitch belt (because it's easy to get hold of)



So if we have 5mm pitch belt, and use a gear/cog with 20 teeth, this means that in one rotation (200 steps) we can move our belt (5x20) 100mm. Which gives our CNC type machine 200 steps to move 100mm - or 1 step moves (100/200) = 0.5mm.
This is pretty good precision. If we drive the motors in "half-step" mode, we can get this precision down to 0.25mm

Many belt-driven CNC machines (laser cutters for example) can acheive precision of 0.1mm, using micro-stepping. While that level of precison would be a nice aim, our core driving principle is "easy-to-understand" so we'll accept slightly less precision for a much simpler machine! After all, a quarter of a millimetre is pretty precise when it comes to dropping SMT components onto a PCB.