RepRap: Blog

No pictures to show today, as work was fairly slow in New Zealand on the weekend. But our PIC serial port does at least connect to the PC to the point where we can turn a motor on and off with it.

Attempts to drive the 5V Meccano EVA feed motor with a relay resulted in a "smoked" PIC, due to accidental non-connection of backlash diodes. These are normally integral to the buffer but wiring them got missed out.

This has prompted the move to a 12V-only EVA feed. The DVD drive shown last month has now been dismembered and the motor/gearbox from the sliding DVD tray is being pressed into service. The only snag is that it works on 5mm shafts and the Meccano EVA feed is on 4mm so there's a bit of turning on the lathe to do.

Finally, a submission for a RepRap presentation has been made to LinuxConf Dunedin for 2006: http://linux.conf.au

Vik :v)

Last night Ash and I fitted our latest EVA nozzle to the Meccano feed mechanism. We've used the nozzle of one of the wrecked glue guns, replaced the tip with a 0.5mm one, and attached the Meccano supports to flanges on the nozzle.

As we've destroyed all the heating elements and are wary of vapourising more PTFE tape we've used 5W resistors as heating elements, holding them on with a hose clamp. Using the standard 12V supply and 27 Ohm 5W cement/ceramic resistors, we generate approximately 10W of heat. If we need higher temperatures, we change the values of the resistors. It's cheaper than buying nichrome and the ceramic bodies can be re-shaped to fit the nozzle body with a tile cutter.

No controlling electronics is required, but we have added a switch and indicator LED for safety.

We still need to re-shape the extruder drive mechanism as the new nozzle is a drastically different shape and size to the old one. The EVA feed doesn't line up properly and the end of the nozzle is too far above the turntable. We'll build a better adjustable support while we're at it.

Vik :v)

We've wired up all 3 axes of the carriage and got them under control of PIC microcontrollers. The turntable is not yet callibrated, but the linear axis (X) and vertical axis (Z) do self-calibrate on power-up.

Weedy little stepper motors from floppy drives did not have enough power to directly drive the vertical screw. Nor did a larger motor liberated from an old Brother printer. But 3:1 reduction gearing did the trick, as you can see in this photo. The vertical drive stepper is at the bottom of the carriage assembly:



The existing control circuitry is starting to look a little complex, though all we have to add is the driver for the extruder and the thermal control for the nozzle heater:



The illuminated LED lower left indicates that the vertical motion has hit the upper limit sensor. There is a manual override for vertical adjustment, unclogging nozzles etc. The 4 LED cluster is for checking stepper drivers. The drivers are the ones with axis-labelled cables next to them.

The RS232 still needs connecting. But we also need a nozzle and NZ has run out of cheap glue guns. Our attempts to use a thermistor as a current limiter failed, with the heater taking 4A and burning the PTFE insulation - bad thing. Evacuate lab immediately and ventilate.

We're getting close; I can smell it. Or is that more burning insulation...

Vik :v)

We got the Z axis stepper motor working on command. We had a spare 3-way switch lying around and a limit switch from an old Amstrad printer. We hooked 'em up and used a modified stepper2 code on a broken pic to control the motor.

We've finally got round to labelling the motors, so now everyone can see what Vik's talking about.

Ran into a problem with the stepper motors: They get hot. Fast. In the testing stage, this hasn't triggered any more problems (Like Polymorph motor mounts melting) yet. I might link them to a small heat sink soon.

Ash & Vik :v)

The RepRap Meccano prototype turntable is now mounted back on the rails, using small pulley wheels rather than Polymorph sliders. A sensor magnet has been added to the turntable together with a reed switch on the carriage (blue & white wire), and a limit switch to the right-hand end of the rails (green and white wire):



A "GO!" switch (red & black wire, also front panel) has been added to allow visitors to initiate actions while I'm showing off. Actually, it's a little more useful than that; I've added a sync sequence to locate the limit switch and reposition the carriage in the centre of the rails at the same location each time the power is applied. I'll then be able to conduct a manual arming sequence for the extrusion head and start it going when I'm good and ready.

This also has the convenient side effect of moving the carriage to the far left whenever the power is cycled. Handy for getting it out from under the deposition head to clean the turntable or extract an artifact.

The white tube dangling from the extruder only contains a fat marker pen. It does not contain mice, despite the opinion of my furry feline companion Chad. The marker is used to show that the track followed is consistent, at least in 2 dimensions. Adding the vertical axis control is relatively trivial, but will involve getting another PIC going 'cos I've just run out of I/O pins. Syncing it all up is a more intricate software problem.

Suz had set the task of drawing an oval as something that would impress her, and guess what? It now draws ovals at the press of a button. We figure we're not to far off producing a RepRap logo now :)

No RS232 still. Adrian has given me his code, but I've been too busy tidying things up and drawing ovals to do the relatively simple code import. The breadboard area is much tidier though, with plugs fitted to many wires thus ensuring they don't get knocked out by fiddling experimenters or wandering cats. It might seem picky, but it leads to more repeatable experiments and more reliable results.

Vik :v)

This is probably not a new idea, but I haven't been able to find it anywhere, so I'm putting it up here.

All the powder-fusing/gluing RP machines that I have seen (Z-Corp, SLS etc) build in a powder that is held above a piston in a tube. This is all very well, but can be mechanically complicated. It also means you have to fill the entire volume to whatever depth you want no matter how small the part you're building is.

Why not have the RP machine make its own tube as it goes? Just have a descending platform and a mechanism to wipe powder flat over (part of) the width of it. Then build your part and at the same time build a dam round it to contain the powder. Excess powder would fall at the sides for recycling. The dam may have to have a slight draft angle, getting smaller as it gets higher, for this to work.

You get the big advantage of being able to build small objects without laying out a full volume of powder.

I've recovered enough from a bout of the 'flu to write a few things up. Having got control of some small stepper motors, we thought we'd try out a screw thread drive instead of belt drive. So Ash and I coupled a section of M5 studding to a stepper motor using some 2mm and 4mm PVC hydroponics tubing. For bearings we used the capstain bearings from old 5 1/4" floppy drives, and trapped them in Meccano holes as shown below:


By using small slivers of plastic tube, we were able to securely grip the M5 studding in the bearings with nuts without fouling the bearing itself.

The motor is not strong enough to drag the Polymorph sliders, so we've built a small wheeled cart while we come up with something better. A pair of angled long-nose pliers held shut with rubber bands grips an M5 nut on the studding, and pushes the cart back and forth:



We've been testing the Allegro UDN2559B buffer/driver part with good results on our stepper motors (the ULN2003 is in "last orders please" mode). It can sink up to 700mA, which we have discovered is enough to blow the output port on a PIC. It also survives being short-circuited and springs back into life when it cools down. Only 4 ports per chip, but as with the ULN2003 no other parts are required.

We've got rid of the old DC motor on the turntable now, and will be using steppers in the future except possibly for the extruder. We're considering using a separate stepper for vertical motion control as the drivers we're using make for very simple circuitry. It also cuts out the long wait between printing an artifact and waiting for the turntable to crank itself back up again in preparation for the next test.

A shortage of plain old 0.1uF capacitors and the 'flu delayed plans to get the MAX202 serial interface chip connected up. When I'm out and about we'll see if we can't control things through the USB/serial interface (blue plug in the bottom left of the control panel).

Our floppy stepper motor does seem to have enough power to shift the turntable at realistic speeds. Keith, Ash and myself set one up last Tuesday night after hacking it and its bearings out of a floppy chassis with a cutting wheel. Earlier tests were not so good, and we now suspect a mechanical problem with the prototype causing bearings to bind.

We put a worm gear on it and put that in the place of the worm on the turntable assembly. It initially moved in both directions, but very slowly and with only just enough power - it got stuck on a few occasions, even when not driving the screw shaft. Stripping down the turntable and rebuilding some bits of it resolved the problem.

The resolution seems a bit lacking, but we can live with it. The motors have an 18 degree step, and with a 100mm diameter turntable that needs to move at roughly 0.1 degree steps to get 0.1mm accuracy. At the moment we've got that but there is no room for error, and anti-backlash gearing will be needed.

My PIC programmer now refuses to erase PICs under Linux, despite having all the right volts on the pins. Moving it to Suzzy's Windows box allows it to work 50% of the time so I suspect timing issues.

Still, we can drive steppers in both directions at varying speeds with the ULN2003 and we've now got a proper USB-serial connection mounted up on the front panel with terminals leading to the breadboard area. Next step is to mount up a MAX202 chip and get some serial connectivity going.

Vik :v)

I've made a few changes to the Meccano prototype, now known locally as "Da Witch" (due to the recycled front panel from a Da[ta S]witch) as you can see from this illustration:



This picture shows a sturdy steel brace laid across the rails. If you look carefully, you'll see a protrusion from the brace resting on a small white cylinder under the turntable - this is the cam.

Tests show that a more controlled approach is needed to rotating the turntable and controlling the feed, so I've had to learn about stepper motors.

On the front left corner, you can see a fitted breadboard area that comes wired in to the prototype's power supplies. Next to that is the stripped chassais of an old 5 1/4" floppy drive, with just the stepper motor left on it. The motor is a 6-lead centre-tapped motor with 2 tapped coils running at 12V. This is handy because it allows the motor to be stepped in both directions without having to reverse current flow.

The lowest component count I've managed for the driver is 2 chips - a PIC and an ULN2003 darlington driver. Both are very common and low cost parts, and no other bits are needed; the LEDs are just for my own personal confidence building exercises :)

Vik :v)