Thursday, 20 March 2014

Pushing the case for a CNC pick-n-place

When Steve found out we were spending time on a CNC rather than just go heads-down and hand-solder some boards for our prototype game, he made a wager - a bet that the CNC chews up more than 20 hours (the time we reckon it'd take to hand-solder a dozen or so board game sections). The prize? A bacon butty.

It looks like Steve is likely to win the wager.
But only because he bet we'd spend more than 20 hours on the CNC - had the wager been we'd spend more time on the CNC than on soldering the boards, it might be a different outcome.

Because of the two boards we soldered by hand the other day, neither worked!
Inspecting them shows some really nasty blobby joints:

A lot of this probably comes down to dabbing on too much solder paste at the start (and so a stencil, rather than a pick-n-place machine would probably be the answer). But there were at least two instances when a shaky of a wobbly hand spread the paste around a bit, across the tracks (so perhaps a solder mask might also have been useful!)

Normally we don't worry too much about this - with plenty of flux on the board before soldering, any excess is usually burnt away, leaving a nice, clear joint. But obviously something is not quite right, with both of these boards refusing to work - and a failure rate of 100% means it'll take a lot longer than our rather conservative estimate of 20 hours to solder up enough board sections to try out a half-decent game.

So, having already arranged to meet Alex to collect some RFID keyfobs for the new BuildBrighton door access system, and being at the hackspace already, and having a few hours to hand, it seemed a shame not to spend them productively - with a view to getting a moving head on an x/y axis as quickly as possible.

Here's pretty much how we left our CNC x/y axis on Monday evening. So last night, we fixed the x-axis travelling head in place, and mounted the y-axis rails and secured everything, making sure it was as square as we could get it

We used Arthur's approach to squaring up the rails - screwing down opposite corners and pinching them tight, then backing off the screws a quarter-turn: secure enough to hold the rails, but not so much to stop them from re-aligning if necessary. We then ran the carriage up and down the rails a few times, and tightened up the rail mounts wherever they had settled.

The whole carriage assembly glides up and down the y-axis rails perfectly. We we expecting at least a little binding or resistance somewhere along its travel, to indicate that either a rod was bent, or the linear bearings were not quite aligned, or that the rails weren't quite parallel. But - amazingly - the carriage glides up and down the rails with no stickiness at all.

It's all very encouraging - and with the 2.5mm pitch belt in place, it's easy to see how steppers will pull the carriage along. The belt needs to be pulled tight and fixed solid at either end of the y-axis rail. This means that as the stepper motor rotates, it causes the carriage to move along the length of the belt.

Because this is our first belt-driven system, we decided to use two steppers - one on each end of the carriage. Normally we'd be tempted to go with the least hardware and the simplest design (i.e. a single stepper per axis) but we've no idea how this belt works when under load. If we had just one belt on one end, it means that the single stepper is driving one end of the carriage, while dragging the other end behind it (a single belt down the centre of the y-axis would be ideal, but we need to keep our working surface clear so we can place circuit boards anywhere within the x/y reach and not worry about the belt getting in the way).

If all the rails are perfectly square - the two rails on each axis are perfectly parallel, and the rails on the other axis exactly perpendicular to them - then an single-stepper-down-one-side arrangement should be just fine (it's how most laser cutters work, for example). But given that we were cobbling something together out of scraps of stainless steel rob, laser-cut mdf and some left-over chipboard for a base, there was always the chance that something wouldn't fit quite right.

So we've gone with the dual stepper approach. Both steppers will be driven exactly in sync, and this should ensure there are no dragging or twisting forces on the y-axis as it travels up and down our (rather shonkily-built) rails.

As it's BuildBrighton Open Night tonight, there's a chance that not much more will get done on it until the early hours - though perhaps we might just forgo the tea and pizza and keep cracking on with it - once the new door entry system has been fitted, of course!

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