Sunday, 7 September 2014

More pick-n-place testing: feed roller

There was a bit of debate about how to do the tape feeding mechanism - and whether we should even have one at all. At first we were going to have a manual tape advance (i.e. you placed the tape on the base of the machine and after each component had been taken out, simply pulled it by hand to get it into position, ready for the next time). But this was deemed a bit lame.

So then we thought about having pinch rollers - simply feed one end of the tape into a mangle-like pair of rollers, and drive the bottom roller a set number of revolutions, in order to move the tape forward. This seemed like a great idea, except if the tape is not fed exactly straight into the rollers, by the time five or six components have been dispensed, the tape could have wandered laterally across the rollers.

So, the more complex, tractor-feed device was designed and cut from mdf.

We found a website which demonstrated the "standard" measurements for SMT tape reels. Apparently, there was a standard, but it got complicated adhering to it, with all the different tape and component sizes, so it was abandoned. But most SMT component manufacturers still use the most commonly used pitch widths and hole spacings, so we stuck with those.

We created a simple gear, with 24 teeth (it seemed sensible to go with a multiple of 8 for some reason and 16 created a very small gear) and set the tooth pitch at 4mm. Because the gear has to fit inside holes not more than 1.5mm across, we had to adjust the image in Inkscape a little bit, and cut the final gear, not from 3mm mdf, but from 0.8mm birch.

A crude test, but it looks promising - the strip of 1206 resistors fit perfectly around our feed wheel.
The idea is to have the tape enter and exit from the same side of the feeder, so we built a simple track around which the tape would bend.

The other side of the feeder is then clamped in place, holding the tape between the "runners" built into the frame. At present we have only washers and bolts to set the spacing between the two sides, but it seems to work well enough.

One of our trust 28BYJ-48 super-cheap motors will provide the driving force. These motors, from memory, have a large number of steps per revolution (internal gearing means it's something crazy like 2048 or 4096 steps per revolution!). Since we're always driving in the same direction, we don't have to worry so much about backlash, like we did when we used these motors on our super-cheap cnc-drill.

Here's our SMT parts feeder during the very first trial run

The spacing isn't quite right for the tape - our slot is much wider than the tape, allowing it to "wander" and not quite line up with the teeth on the gear. But for a first test, it's very encouraging.

Here's the feeder advancing by approx. 4mm per command.
Since our tape has a single resistor placed between every hole, we know that they are exactly 4mm apart. Other components and other tape manufacturers may vary, so we need to allow the number of steps to advance to the next part to be variable, as well as to cater for different tape widths.

Thanks to the 28BYJ motor, advancing to the next part in the reel is actually very quiet. Not something we were even considering, but it's a pleasant surprise!