Tuesday, 21 July 2009

Production issues

Since the beginning of the year HydraRaptor has been fairly reliable. I optimistically thought that I could get the 100 hours of printing done in a week. I set one build off before I go to bed and another before I set off for work. I use these slots to print the large parts, in multiples if possible. I use the evenings and weekends to print the many smaller parts while I am around to remove them.

I have a script that can print multiple copies of the same object. It works out how many can fit on the bed and spaces them out so that the head can get in between them. It then prints them, one at a time. I have to remember to tweak the script whenever I change the extruder shape. It simplistically works out the bounding box of the object and then uses the object height to decide how fat the extruder is at that height and spaces the objects so the extruder clears their bounding box. A more sophisticated approach would be to do collision detection between 3D models of the extruder and the object. That would get them a bit closer together in a lot of cases, but I don't consider the gain worth the extra complexity.

I could get a lot more on the bed if all the objects were printed one layer at a time. The easiest way to do that is to load them all into a CAD program, place them like a jigsaw and then join the bases with a very thin membrane that is too thin to actually print, but makes them into one object so they can be sliced together. The reason I don't do that is that the chance of a breakdown in a build that would take tens of hours, possibly days, is too high at the moment. Also, if it did go wrong it could waste a lot of plastic. The objects would also end up with a lot more string on them as the extruder has to flit between them on every layer. Perhaps when the system is more reliable, and ooze control is better, I will switch to this approach.

Right from the start things did not go to plan and in the end it took two weeks to complete the build and used almost all of my free time during those two weeks. Not something I would choose to do again until I can get it much less labour intensive.

The first time I made these parts the extruder kept breaking due to parts wearing out: the flexible drive cable, the JB-Weld, the 6V GM3 motor, the PTFE barrel and the bearings. Having eliminated all those causes, the breakdowns were more due to human error and bad luck.

This is the extruder I used: -

A recurring problem I had was the extruder jamming due to the heatsink getting too hot, allowing the plastic to melt inside causing the plug effect I have detailed before. It can then no longer be pushed forward, so I have to remove the pump parts and pull it backwards while it is hot.

I have disassembled it and reassembled it so many time that the M3 threaded rods started to lose their threads. I keep meaning to make a wider extruder with M5 bolts but never get round to it. Once the extruder has broken you can't print new parts so you have to fix it some other way, then you don't need the new parts! I should really keep a spare working extruder.

When the threads had stripped, rather than replacing them, I swapped the bottom two wingnuts for threaded brass spacers. They have a much longer thread engagement area, so get round the problem. They are also blind, so they only go on so far. I found with them fully on, I got the right spring tension for ABS using a pair of M4 nuts as spacers. For HDPE I added a second nut each side. Being able to reset the tension consistently each time I put it back together was a big bonus.

The reasons it got too hot were various, but fundamentally it needs a better heatsink or a fan to give more margin between the running temperature and the glass transition of the plastic. The new extruder controller I have built but not tested yet has a second fan drive and a second thermistor input to allow the cool zone to be regulated. A simpler solution in the case of HydraRaptor would be to make the extruder base / clamp out of aluminium. That would conduct the heat to the z-carriage, which is all aluminium, so could dissipate hundreds of watts .

I eventually tracked down the first reason for over heating to a bad four pin connector in the wires to the heater and thermistor. They are rated at 3A in the Maplin catalogue, but I have had problems with them on the extruder controller before and I am only putting 2A through them. The connections go high resistance for no apparent reason. If it is the heater connection then the heater cools down and the connector gets hot. I have had one de-solder itself from the board.

This time the thermistor connection intermittently went high resistance causing a low temperature reading. I also had the extruder motor stop during a build, again it appeared to be due to its connector failing. I don't know why they do this. If I re-seat them then they work for a while and then fail again. Perhaps they are not rated for the number of insertions they have had due to constantly rebuilding the extruder for 2 years! I have built my new controller with Tyco connectors rather than these unbranded ones.

I switched from the 0.3mm nozzle I have been using recently to 0.5mm so that I could use the same g-code I made the first set of parts from. I can do that because I only use the g-code for tool path information. All the machine settings like temperature and feed rate are in my script.

After the nozzle swap, molten plastic started oozing out of the side of the extruder. The bolts which clamp the compression joint had worked themselves loose after many heat cycles. Changing the nozzle broke the seal and let the plastic out. Tightening them again fixed it.

With the older 0.5mm nozzle I had trouble getting its PTFE cover to stay on. This is essential for making the nozzle wipe work and prevents burnt bits of plastic getting incorporated into the parts leaving brown marks. I had a daft idea of taping it on with Kapton tape. That did not work, but when putting it on I forgot the thermistor wires round back and broke one off.

I had to drill another hole and stick a new thermistor in with Cerastil, a 24 hour job. Worse than that I must have mixed the Cerastil with too little water because it started to come out a day later. Because it was out of sight I did not notice, but the objects started to get very hard to remove from the raft and the raft from the bed. It was only when I broke a hole in the surface of the bed that I realised. So another 24 hours of repairs!

I fixed the PTFE cover with two tiny set screws into indentations in the nozzle.

Other times the heatsink seemed to get too hot for no apparent reason, the hot weather did not help. Generally it failed near the end of a large object, very annoying. In the end I used a mains fan about 1m away to keep the heatsink cool.

Apart from the reliability problems the other issues I had were as follows: -

The corner blocks have hair line cracks through the narrow bits half way up the middle of the vertical edges.

This happened on some of the blocks on my original Darwin and does not seem to matter. I think it was worse this time round because the rafts I used held them down better, leading to less curl up of the bottom corners, so more stress through the edges. I made these with 90% fill, rather than 25%, to ensure they were strong enough. It made absolutely no difference to the cracking but added 8 hours to the print time. It makes the thin bits stronger, but it also increases the warping forces by the same amount.

The rafts I use at the moment are very well bonded. On the up side that reduces warping by holding the object down, but it is quite difficult and time consuming to remove. The only object that pulled away from its raft was the extruder drive block.

This is the worst case in the set because it is both wide and thick. Fortunately it does not need to be flat.

This part came out a bit bockety where it gets thin near the top: -

The problem is the layer is so small it does not have time to cool before the next layer arrives on top. I fixed it by adding some logic which halves the extrusion speed if a layer would take less than 10 seconds at full speed. You can see it completely fixed the problem: -

It was not sufficient for the very thin opto tabs though, so I also cooled them with a fan. Here are the with and without fan versions side by side: -

I won't be printing another set until I get my own Darwin up and running. I hope to double the extrusion speed and reduce the ooze with a stepper driven extruder. The bed is much bigger so perhaps it will only need two batches taking about 1 day each.

Monday, 20 July 2009

HydraRaptor's second child

Back in March I had a visit from Marcin Jakubowski, the founder of Open Source Ecology. He was over here in Manchester presenting at a conference and asked if he could come and see HydraRaptor, as he wants to use RepRap machines on Factor e Farm. Like RepRap, his project also aims to change the world.

He asked lots of questions and made a couple of videos of my answers for his blog, which you can see here.

I volunteered to print a set of Darwin parts to help get Factor e Farm up and running with 3D printing. I was confident that I would have my Darwin running in time to churn out the parts. However, because I spent a lot of time experimenting with extruder designs in an attempt to get something more reliable, I ran out of time and had to print the parts on HydraRaptor.

Here they are, all 109 of them: -

All the parts were printed with 0.5mm filament at 16mm/s with 32mm/s moves. Most were sliced with Skeinforge set to 25% fill and larger objects have double outlines to maintain strength.

Here are some stats: -

Build time Plastic volume Quantity required Total build time Total plastic Weight Cost Percentage of total

Corner bracket @ 90%
02:44:44 29.1 cc 8 21:57:49 233.1 cc 291 g $5.83 29%
Diagonal tie bracket-chris

00:27:00 4.8 cc 20 09:00:06 96.4 cc 120 g $2.41 12%
Bed corner
01:32:06 15.5 cc 4 06:08:25 62.1 cc 78 g $1.55 8%
01:23:12 14.6 cc 4 05:32:47 58.2 cc 73 g $1.46 7%
X motor bracket
03:56:35 37.2 cc 1 03:56:35 37.2 cc 46 g $0.93 5%

03:51:39 40.2 cc 1 03:51:39 40.2 cc 50 g $1.00 5%
Y housing
00:56:36 9.9 cc 3 02:49:47 29.8 cc 37 g $0.74 4%
Extruder drive block
02:30:44 26.5 cc 1 02:30:44 26.5 cc 33 g $0.66 3%
X idler bracket
02:28:06 25.4 cc 1 02:28:06 25.4 cc 32 g $0.64 3%
Y motor bracket
01:51:06 19.6 cc 1 01:51:06 19.6 cc 25 g $0.49 2%
Bed constraint
00:43:20 7.5 cc 2 01:26:39 15.1 cc 19 g $0.38 2%
Bed clamp
00:21:39 3.7 cc 4 01:26:36 14.7 cc 18 g $0.37 2%
Extruder base
01:13:19 13.1 cc 1 01:13:19 13.1 cc 16 g $0.33 2%
00:14:21 2.6 cc 4 00:57:26 10.2 cc 13 g $0.26 1%
Opto bracket @ 50%
00:19:00 3.1 cc 3 00:56:59 9.4 cc 12 g $0.23 1%
00:10:46 1.9 cc 5 00:53:50 9.5 cc 12 g $0.24 1%
00:43:50 7.6 cc 1 00:43:50 7.6 cc 9 g $0.19 1%
00:13:24 2.3 cc 3 00:40:11 6.9 cc 9 g $0.17 1%
00:38:10 6.6 cc 1 00:38:10 6.6 cc 8 g $0.17 1%
00:12:35 2.2 cc 3 00:37:44 6.7 cc 8 g $0.17 1%
Bolt plug
00:04:36 0.8 cc 7 00:32:11 5.8 cc 7 g $0.14 1%
Tall foot
00:14:27 2.6 cc 2 00:28:54 5.3 cc 7 g $0.13 1%
Y motor coupling
00:25:02 4.5 cc 1 00:25:02 4.5 cc 6 g $0.11 1%
00:24:12 4.1 cc 1 00:24:12 4.1 cc 5 g $0.10 1%
Short foot
00:11:21 2.1 cc 2 00:22:42 4.2 cc 5 g $0.10 1%
Fan base
00:22:29 4.0 cc 1 00:22:29 4.0 cc 5 g $0.10 1%
Y belt clamp
00:03:43 0.7 cc 4 00:14:50 2.6 cc 3 g $0.07 0%
00:15:48 2.8 cc 1 00:15:48 2.8 cc 4 g $0.07 0%
X-motor washer
00:15:27 2.8 cc 1 00:15:27 2.8 cc 3 g $0.07 0%
00:13:00 2.3 cc 1 00:13:00 2.3 cc 3 g $0.06 0%
Bearing 360 run
00:02:47 0.5 cc 4 00:11:09 2.0 cc 3 g $0.05 0% HDPE
Extruder PCB holder
00:09:45 1.7 cc 1 00:09:45 1.7 cc 2 g $0.04 0%
00:08:45 1.6 cc 1 00:08:45 1.6 cc 2 g $0.04 0% Black ABS
00:08:39 1.1 cc 1 00:08:39 1.1 cc 1 g $0.03 0% HDPE
00:07:43 1.4 cc 1 00:07:43 1.4 cc 2 g $0.03 0% Black ABS
Bearing 360 jam
00:02:49 0.5 cc 2 00:05:38 1.0 cc 1 g $0.03 0% Black ABS
00:04:43 0.8 cc 1 00:04:43 0.8 cc 1 g $0.02 0% Black ABS
00:04:26 0.7 cc 1 00:04:26 0.7 cc 1 g $0.02 0%
00:03:20 0.6 cc 1 00:03:20 0.6 cc 1 g $0.01 0%
00:01:26 0.3 cc 2 00:02:53 0.5 cc 1 g $0.01 0%
Bearing 180-x
00:02:38 0.5 cc 1 00:02:38 0.5 cc 1 g $0.01 0% HDPE
Bearing 180-z
00:02:03 0.4 cc 1 00:02:03 0.4 cc 0 g $0.01 0% ABS

109 74:28:04 778 cc
973 g
$19.47 100.00%

The times and weights are calculated, and don't include the raft time, which is significant, or the time waiting for temperature changes and raft cooling. I weighed the parts on kitchen scales and they came out at 931g, so pretty close to the calculation. The cost shown is on the basis of ABS at $20 / Kg.

I save all the rafts for the day when we get recycling working. I weighed them in at ~ 200g, that is about 20% wastage and will bring the actual printing time up to about 100 hours.

I also wasted 150g in failed prints, for silly reasons, more on that later. It gives a measure of the reliability I am achieving at the moment, i.e. 8 parts failed out of 117 prints so 93% success rate. Of course the bigger the part is, the more chance something will go wrong, so by weight and time it is much worse .

I used plain ABS for most of the parts because it seems to bond better than coloured. I used black for the opto tabs. No guarantee that they will be opaque to IR, but I think black ABS usually is. The green parts are just ones I had left over from experiments.

I made some of the bearings in HDPE as that should be a better bearing material than ABS, lower friction and longer lasting. The black ones are "jam" bearings so I left them in ABS as they want maximum friction.

Some of the parts are my own design. Most significant are the z-axis parts described in the previous post. Here is a list of the other design tweaks, with links to the article describing them:- simplified diagonal tie brackets, X-motor washer, x-carriage bearing and the feet.

Some parts I had never printed before. The Pinch wheel extruder: -

The nozzle wiper assembly has appeared in the latest Darwin release but I can't find any assembly instructions. I leave it as a puzzle for Edward Miller, the guy who is actually going to build this machine.

Similarly the new adjustable z-opto flag assembly: -

I aimed to print these parts over the course of a week, three batches a day, but the machine had other plans and it actually took me two weeks. I will give more details tomorrow.

Sunday, 19 July 2009

$8 Z-axis

About a year ago I blogged an alternative Z-axis for Darwin using four tin can steppers instead of one expensive stepper and a belt drive. The only thing missing was a source of cheap motors to make it economically viable. Some time ago Forrest Higgs pointed out a source of cheap 15° motors for $2.50 made by Airpax. I also found them available for $2 at Surplus Shed. That makes a z-axis for $8 possible, which is much cheaper than original motor, let alone the belt.

They are surplus stock, so when they are gone they are gone, but there does seem to be a lot of them around. Unfortunately it costs more than $40 to ship them from the US, so the economics don't look nearly so good this side of the pond.

They are 12V 0.4A per coil, so four wired in parallel will take 1.6A, well withing the 2A capability of the RepRap electronics. They are six wire unipolar motors, but they can also be driven from a bipolar drive by using the red and orange wires as one coil and the green and brown as the other.

The pull in rate seems to be about 200pps, which would give 200 × 15 × 1.25 / 360 = ~10 mm/s with M8×1.25 threaded rod.

The boss on the back of the motor is a bit bigger than the motors I used before so I have updated the bracket design accordingly. The motors come with a spiral drive screw on the shaft. I could not find a way of getting it off, so I made a new coupling piece that clamps over it. It has a pointer so that it is visually obvious if the motors get out of step with each other.

I have uploaded both of these to Thingiverse. The other parts needed are shown below: -

And this is how they go together: -