Saturday, 17 October 2009

Extruder plans

MakerBot are sending me fresh capacitors (labelled, this time, I hope) and those parts that were missing from my PCB kits. These should be coming in the next week.

Meanwhile I've been reading up on the various designs and options for constructing an extruder. Looking at the heater details, I was faced with use of copper or stainless steel for the heater barrel and/or nozzle, separate or combined barrel and nozzle, PTFE to insulate or a heatsink block of metal to dissipate heat, uninsulated or insulated nichrome wire or resistors to provide the heating,....
The drive for the plastic rod was originally intended to be a screw-thread, but has been superceded in most cases by a pinch-wheel. There is also a move away from DC motors to a stepper motor.

I already have my DC motor and its RF noise-suppressor board and its controller board. I am planning to make a pinch-wheel extruder, but I am unsure of the rest of the design features to use.
As ever, I posted on the forum and Chris (Nophead) quickly replied explaining the main reasons for different choices.

Since I don't have machining capability for chunks of aluminium into which to push resistors, my heating element will be Nichrome wire. I shall order the insulated Nichrome from Makerbot, perhaps 2 feet, for $1 + P&P. I shall wrap this in Kapton (polyimide) tape, which I shall order from DealExtreme $2.92 (free shipping worldwide, apparently), following BodgeIt's tip. This one claims to work up to 300 oC, whereas the UK supplies state 260 oC. However, I did find reference to one rated up to 260 which stated it would withstand 300 oC briefly. The difference in price works out at about 50 p, so I'll try the one from the States' site, and see what the packaging says when it arrives. I wonder if the cheapest supplier for the UK hapens to be the same company anyway.... their sites are uncannily similar in many respects.

I shall try using my B&Q welding tip to make a combined nozzle/barrel. According to Nophead's work, the nozzle narrow should be around 2 mm or more in length to prevent the nozzle dribbling much. I would like to have a go at Frank's method (August 29 2009 entry on builders' blog) of using a brass nut to conduct heat away from the upper end to an aluminium heatsink. However, I may struggle to get the top (entry end) of the copper welding tip to be cool enough. The rule of thumb is that the heatsink should be not too hot to touch after a time in operation. From what I have read, with a long warm path for the plastic filament to travel before the zone where it melts, the plastic expands too much, which produces a lot of friction, ultimately making it difficult to drive the filament through the extruder.

I do not have a heatsink. However, I may have some aluminium I could use as a heatsink: the flat of the furniture leg I bought in order to attach a horizontal timber arm onto the MDF "vertical base". This would require the purchase of a second one just for heatsink purposes, but is probably the cheapest way of doing this, as it only cost around £6.
The flat has mass 164 g, and is 8x8x1 cm, ie 64 cm cubed. Density of aluminium is approx 2.5 to 2.9 g/cm cubed, therefore this piece of aluminium should weigh (say) 2.7x64 = 172 g. So at 164 g, it IS probably aluminium.
The surface area of the flat is 16000 mm square, so it should act as a good heatsink.

Further information from Nophead concerned the amount of gearing that would be needed to make my little GM3 motor rotate a pinchwheel slowly enough to provide a sensibly slow feedrate for the plastic filament. I don't think it is insurmountable. But I have been hunting a small gear, a large gear, and a tiny pinchwheel.
I have been doing a lot of maths with the various gear ratios, considering what is available and what I need to achieve.

Although this may not be adjustable in use, I have settled on a worm gear, matching large gear onto which to mount a small gear meshing onto another large gear, which will have the pinchwheel mounted on it. I know it'sa lot of gearing.

I shall have to come up with some way of mounting all of these, and I haven't yet decided on what to use as my pinchwheel.

Until I started looking into this, I had no idea there were different sorts of gears, and so many, all for different purposes: worm, bevel, ratchet, rack, spur, helical, pinion, contrate..... and a sprocket is something different!

My design will be a cobble-together, as is typical of repstraps and it may not work....
but I'll have learnt some more along the way, and I can always change components or features as necessary. If I don't try it, I'll never know....

Tuesday, 29 September 2009

More postponement

Cheered by my recent board assembly successes, I wanted to rush ahead and assemble at least one of the remaining motherboard or extruder controller boards today. No such luck.

I had ordered these as complete kits (from Makerbot), so I got out all of the pieces for the motherboard. When I had ordered this, it was the most up-to-date version, so I duly had the webpage open for the most up-to-date version of the motherboard, 1.2. Something seemed wrong; I didn't have all the parts. So I re-checked, and I had ordered version 1.1!
After this false start, I was checking all the components, and I still didn't have all the parts! I am missing five 1.8 k ohm resistors and one red LED, and the IC1, the RS485 communication chip. But I have 5 excess 472 resistors, and a spare green LED!

Moving on to the extruder controller, I am missing a 180 ohm resistor. Also, the eight 100 nF ceramic capacitors, the two 16 pF ceramic capacitors and the two 0.22 uF ceramic capacitors are all packaged in pairs in unlabelled packaging, so I have no idea which is which! Not helpful.

I have used the "Contact us" facility on the Makerbot store website to request the missing parts and some advice or perhaps new labelled parts? for the confusing capacitors.

So for now, I can't work on either board.

Three step(per)s forward

I "fried" the last stepper motor driver board last night, and touched it up in a couple of places and removed solder balls before hand-soldering the rest. I cleaned it and checked it this morning, plugged it in and it all works. Now I have a triplet of boards at last. Yay!

Although I can understand the routing of the circuit on the boards themselves, I still haven't learnt much about the circuit diagrams. I know that the white dots on the board denote the negative ends of the LEDs, because I worked out the negative end of the circuit diagram, and I now know that VC+ is the 12 V supply to the board and that VCC, the voltage common collector, is the adjusted voltage, which is 5 V on this board. I came to realise that the arrow heads on the diagram don't have anything to do with direction of electron flow
or what is thought of as current; they just mean this part of the circuit carries on somewhere else! Oh, and I now know the symbols for LEDs, resistors, variable resistors, electrolytic and ordinary capacitors. And that's about it.

But I don't know how to count the pin numbers on the chip, for example. And I am confused by the power LED on this circuit - on my boards it is connected straight from the 5 V leg of the voltage regulator, and yet the diagram places a capacitor before it, either an electrolytic or ceramic, depending on which route you take.

Hmm, that really doesn't look like what's actually in front of me.

........................................unless this (below).......................really means this
And if so, why wasn't it drawn like that for clarity?

Finally, as promised, a costings update:

Farnell sub-total from separate orders = £54.88
  • A3982 driver chip, part number 1521717, £4.60 and
  • chip-leg cutter, part number 7256449, £39.90, combined VAT of £6.68
  • IDC header, straight, 10-way, £1.61 each (x 2), VAT of £0.48
No P&P.

Running total (excluding set of washers and screws from B&Q) = £487.51

Monday, 28 September 2009

Boarding school, term 3. Muddling through the end of year tests....

Hoorah! Hooray! Yippee! Wonderful! Marvellous! Brilliant! Fantastic! (Can you tell I'm happy?)

After a whole week of head-scratching, web browsing, and advice from my brother, my partner and Nophead, I have finally FIXED my bad stepper motor driver board. Unfortunately, this was the board with near-perfect soldering, of which I was very proud. Now it looks a mess! Anyway, here is the proof it works:

OK, it's not pretty.

The voltage regulator had been tested whilst still fixed to the board (but only by its ground pad). The regulator was definitely faulty. Unfortunately, once I'd removed that, everything else in the 5V path gave readings of zero ohm when tested on a 20 k ohm setting on the multimeter with the +ve probe on the path and the -ve probe on the ground pin of the Molex-style connector. This meant there was a short to ground somewhere, still.

The first thing I checked was the power-indicating LED, as this was what I had replaced twice, once because the first one was dead (although it turned out that that was incidental to the board not working), and once because I checked it for correct orientation when the board still didn't work. The result was that this part of the circuit certainly didn't look tidy, and was a cause for suspicion. I cleaned it very carefully and checked the gap between it and its protective resistor, but it looked separated. I wondered if I had damaged the pad on the board, but without disconnecting it from the circuit there was no way to check.

Next candidate was the damaged track alongside the regulator's space. I decided to cut the track close by the 5V pin' pad,so I scraped off the protective Araldite I had applied, and scraped back to the copper track. Then I very carefully cut the track, and made sure there was no continuity across the cut. Now, using the diode setting on the multimeter, I could light the power LED, so that side of the circuit was sound.
I cut the track in a similar way towards the rear of the regulator's space. Checking on resistance setting 20 k ohm again, the part of the track alongside the regulator gave reading of 1 against ground, meaning no connection to ground, ie no short there.
O.K., they were my top candidates! And they were fine. And I now had 2 unnecessary cuts in my track!

Oh well, nothing ventured, nothing gained.....
Following the track, it goes from the regulator to the electrolytic capacitor from where it splits, with part then heading to the 100 nF ceramic capacitor and to the second RJ45 socket, and the rest going down a "via" under the electrolytic capacitor and off to various other parts of the board.
In order to test this, I cut the track in a clear section underneath the board. Testing the chip end of the circuit to ground, the meter gave 1.
Ah-ha, now the problem is narrowed down to 2 capacitors, the "via" or the RJ45 socket.

The RJ45 socket seemed unlikely to have a problem, and ditto the large capacitor. Having plenty of spare 100 nF ceramic capacitors, thanks to supplies from Nophead after I cracked (a different) one on this board right at the start, I decided to simply swap it.
Some little desoldering and resoldering later, and voila! we have readings of 1 from everywhere on the 5V path to ground. So the problem WAS the capacitor.

Now my problem was the breaks I'd deliberately put in the track....
First I used the conductive pen to fill in the gap in the track on the underside of the board. It takes 10 minutes to get conductivity through the "ink", and testing afterwards showed I had good conductivity between the copper ends.
Unfortunately, when I tried this on the top of the board, I found that I also had re-shorting to ground! Obviously the cuts on the upper side of the board reached the ground connection in the board. So, using a trusty cocktail stick (which should also be added to the parts requirements!) I could paste Araldite (ditto) into the cuts in the copper track, but while that dried, I used the conductive pen to rejoin the breaks going around the cut section. If the conductive pen only works temporarily, I shall solder wires between the track breaks.

Then I soldered a new voltage regulator in place, with the help of Blu-Tack, again. Unfortunately, I knocked the electrolytic capacitor loose, causing more burning plastic due to access issues with my soldering iron as I fixed it back on. I tested it, and the voltage regulator, and then resoldered the first (removed) RJ45 socket. I tested everything exhaustively.

And now we have Blackpool at Christmas! (For those non-UKers, that means all the lights came on.)

My partner thought the board was a gonner. I thought if it didn't work anyway, there was nothing to lose by trying to resuscitate it, and it was going to take another 3 weeks to order a fresh board from the States anyway. And it works.


Saturday, 26 September 2009

Switching the lights on

I swapped back to "playing" with my believed-to-be working stepper motor driver board. Looking back, I found Nophead's comment about connecting the enable pin to 0V in order to get the indicating LEDs working. I established from the instructions that the enable pin was pin 5 on the IDC socket, but I didn't know which was pin 5.

After some internet hunting, I eventually managed to confirm the identification of the pins relative to the ribbon cable wires. I thought. I tried that, and got nothing. Hmmm. After some more hunting, I found that my reference website had been wrong - but they're in good company, as Asus apparently got it wrong in their manuals and on their own website! Luckily, this being a communication type connection rather than power, nothing drastic happened when I'd connected the wrong pin.

Using this website, I got the right diagram.
Wire 1 on the ribbon cable goes to pin 1 on the IDC, which is at the end of the row adjacent to the tab on the male connector, shown here as on the top left of the connector, and pin 5 is marked with a wire.

In order to test the stepper motor driver board's indicating LEDs, the pin on the board's connector corresponding to pin 5 on the ribbon cable is connected to ground.
Here is a photo' showing the connector pins 5 (enable) and 9 (ground) linked with a wire.

After arranging this, I plugged in and switched on power to the working board - sure enough the LEDs lit, but one green on one side and one red on the other. Is this correct? Anyone know which ones are supposed to light up?

Friday, 25 September 2009

Second term at boarding school!

Just when you think you've got everything settled..........

.....My brother pointed out that just because I wasn't measuring 5V out of the voltage regulator whilst it was connected in circuit on the stepper motor driver board didn't mean it wasn't working; with a short-circuit anywhere in the 5V section of the circuit, the current would flow straight to ground, meaning no voltage to measure. However, there were no obvious shorts, no solder bridging two components and I had previously checked that all the resistors apart from the 0.25 ohm ones were sound. After Nophead's advive on checking low value resistors, I noted that my multimeter read 0.5 ohm when the probes were touched together on the lowest resistance range setting, 200 ohm. The resistors gave a reading of 0.8, meaning they are rated at ~0.3 ohm, which is about right allowing for slight inaccuracy of the meter at such low levels compared with range setting.
In circuit, there was no reliable way of measuring the capacitance of any of the small capacitors, so I was going on guesswork that the problem still lay, in fact, with the regulator.

I desoldered one pin on the regulator, and then the other.
I am getting on fine now using the solder braid with flux drawn onto it from a pen. This helps the solder to reflow. I placed the braid onto the top of the pin, acrosswise if possible, and applied the soldering iron tip on top of the braid for a few seconds. I found it easiest to position the braid and hold it in exactly the right orientation, with no twisting etc, with my hand, but once held in place by the soldering iron, I swapped to holding the braid with tweezers so that I didn't burn my hand when it got hot. Access was awkward, and I decided to desolder one of the RJ45 connectors to reduce the amount of melting and burning plastic where the large screws on the soldering iron shaft would catch! I still caught the edge of the Molex connector from time to time. I cut off the used end of desoldering braid holding the absorbed solder and repeated with clean and freshly-fluxed braid until the regulator's pins looked to be clear of solder. The pins still seemed tacked to the board, possibly with sticky burnt flux, so I cleaned around them and prised them up off the board. The pads below are fine.
Desoldering the through-hole pins of the connector was easier, still, after cutting the end of the braid into a point and using the fine iron tip, pressing the braid slightly down and into the hole as the solder wicked up.

Next I tried to desolder the regulator's large ground pad - that was just too tough! I managed to remove most of the solder, but there was some visible under the centre of the pad, but the gap between the part and the board was too thin (top to bottom) to be able to slide the desoldering braid in, and although I could melt some of it, because of the proximity of other components I couldn't use the big flat soldering iron tip, which would have been ideal to melt the lot all at once and allow the regulator to be released.
Eventually my partner went for the brute force option! and created more damage than was intended! Some of the board's contact pad lifted clean away, but the remainder still has contact to ground, and there is damage to the green coating over the track that leads from the 5V pin adjacent to the side of the regulator. There still seems to be continuity everywhere (there should be), though, so I have Araldited a thin protective layer over the shiny copper that is visible.
We tested the removed regulator - it is most definitely dead! Luckily, the minimum order for these had been 5, when I (thought I) needed 3, so there are a couple of spares after all (which is a relief because this was the part that came from RS, who were absolutely useless, and from whom I have no wish to order again)!

My brother rang just as all this was done, and said that I could test this board before putting another regulator on (just in case there is still a short on the board as well) by connecting the working board to power, and connecting a 5V contact on that board to a 5V contact on this board, and a ground contact on that board to a ground contact on this one. The regulator on the first board should easily be able to handle the current for both boards together. At least the power LED should come on for both boards. And if I do Nophead's thing of getting the other LEDs to work, too, I should get proper indication on both boards as well if this board is working. I need to look that up again, but meanwhile I'm letting the Araldite cure.

Thursday, 24 September 2009

Boarding School

I had advice (again) from Nophead (again) to forego the desoldering braid for removal of the dead chip on the first stepper motor driver board, and to cut the legs off as high, i.e. as close to the chip body, as possible. This I did - with the end legs, anyway, but I couldn't find anything fine enough to fit between and cut the other legs. After considering what was available that would definitely do the job with limited manoeuvre room, I decided to buy a chip-leg-cutter. You never know, I may need it in future.
This did the job well. When I lifted the broken chip off the board, a portion of the green board coating came away with it, exposing the copper underneath. The patch didn't extend to any of the pads. Carefully, I coated this with a thin protective layer of Araldite.
This was ancient - I got it in 1991, when I worked for the company! The lids were pretty stuck on, so that trying to unscrew them from the tubes resulted in highly twisted tubes, but putting some grip onto the lids worked. It is good to have some glue you know is still going to work years after first being opened.

Nophead's advice to brush the cut chip legs away with the soldering iron tip worked a treat. Each leg came away easily, and I wiped it from the iron. I used the flat tip to flatten the remaining solder on each pad, and put a thin line of paste along the row of pads on each side before soldering the new chip in place. The right way round!

I checked for bridges, and there were only bridges across legs that are connected with the trace anyway, so I didn't remove those bridges. I checked that there was continuity where there should be, and a lack of continuity where there should be, comparing with the working board. I checked the resistors, and they were all fine, too.

So I plugged in. Oops, no power LED and a very slight smoky smell. I switched straight off.
I took the power LED off, and checked it. Using my new flux pen to wet the component side of the desoldering braid, I was able to remove the solder quite easily. The LED had been fitted the right way around, and it still worked, so I soldered it back on. Hmm.

Since then, I have spent 2 days checking, rechecking and generally scratching my head. I checked all the vias going from top to bottom and did notice a couple without shiny metal rings like the working board has. At the stage I noticed, I didn't know what "vias" in electronics were!
I inserted the probe from my multimeter into the tops of the holes, and turned it around a few times. A thin green film came off, revealing shiny metal once more. I checked there was continuity between the top and bottom, and made sure there was for all such vias.

I checked the resistance for all the resistors, and they were all as rated. The only ones I couldn't check were the 0.25 ohm ones because my meter only went down as low as 200 ohm on its lowest setting.

At this stage, still scratching my head, I posted for more advice on the forum, specifically if the voltage regulator could be checked, and how.

I talked to my brother, who works in electronics, as he happened to 'phone, and he advised that I should check anywhere that a trace has been placed between the pads for a surface-mount component, because there may be a short here. I knew there were a couple of these on the boards, so I checked these, too. No problems found. He also suggested checking every pin, on the headers having 2 rows, against its neighbours. So I did that. No different from the working board. He also recommended connecting the power supply via a 12V bulb rated for 5 or 6 W to the board. If the board were shorted, the bulb would light more brightly compared with doing the same thing on the working board. I put this off because I couldn't locate a suitable bulb or pair of bulbs.

The advice from Nophead was simply to connect 12V power to the regulator and check that 5V came out.
Referring to this website, the power supplied to a 7805 regulator need only be 7V or more; a-ha, say a 9V battery, then, since I didn't want to connect the whole circuit again.
I roped in assistance, and connected wires to a PP3 battery with the ol' trusty Blu-Tack and held the +ve end against the reglator pin next to the "rrrf" text on the board, with the -ve end against the flat plate of the regulator, whilst my assistant measured the voltage off the pin next to the "make" text against the ground pin on the Molex-style connector. Oh. No voltage output. That would be the problem, then. No wonder the power LED didn't light, as it wasn't getting any power at all, it was all just going straight to ground!
That means the rest of the circuit hasn't been tested under voltage since the chip blew!

So for my next order, a new 7805DT voltage regulator..... And I said I might need the chip-leg cutter again, looks like it was sooner than I expected/hoped!

Nophead also explained that I could check the 0.25 ohm resistors by reading the resistance on the meter with the probes touched together, and subtracting that reading from the one I get on the low-value resistor. I'm going to try that next.

In the last 3 days, I have learnt a huge amount about checking boards.
I shall update the project costs soon.