Wednesday, September 17, 2008

Arduino Rebrain Project.

After getting an Arduino Decimila for my repstrap, Zach at RRRF developed the bigger-brained (RAM&ROM) & additionally-fingered (digital input/output) Sanguano.

Oh, and Zach has done a lot of clever work on the software side of things too.

Anyway, I assumed I could get one of these larger Atmega microcontrollers and stuff it into the Arduino so it uses all the PSU, USB etc. that are already there. This would need a new board with a header to go into the old brain's socket, and pins to carry the extra IO lines.

I use 'EasyPC' which produced the 3D. The 'old' brain is shown as a DIL package but the pins would be soldered in to project out of the bottom of the PCB. The grey square is a 44 pin surface mount version of the 644P microcontroller. The 3 sets of four pins are the extra 12 I/O.

Then it got a bit more complicated. The big hole in the board is there to clear the Decimmila's in-circuit-programming connector. Unfortunately, it is connected to different pins on the new brain, so I have added a new connector on my PCB.

Also, I am unsure of the performance of the crystal oscillator - the PCB track length between the original XTAL and up to the new brain may cause problems. I have added pads for a new XTAL and a pair of capacitors close to the '644P and a pair of vias in the tracks so they can be drilled to disconnect. This isn't shown above 'cos I only thought of it last night.

The schematic is a bit tricky to read - I don't like tags that go places that I cant find, but when I connected all the points by lines on this one it is just as tricky to trace. It was originally a quick 'n' dirty hack anyway. I also haven't put the effort into finding out how to make EasyPC schematics look good. Anyway, this is a 44 pin chip so there is a lot going on anyway.

The aim of course is to make a USB sanguano by reusing as much of what I have already got. I shouldn't really have bothered because of high parts cost in small quantities and all my time (I am better then most lawyers I have met, so that makes my hours for inventing very expensive indeed!) (I do seem to enjoy myself much more than many lawyers I have met though!!).

I could make these rebrain boards for under ten pounds if I get several tens of them. Anyone interested ?

Monday, September 8, 2008

Roll On !

I got the nice needle roller bearings, opened up the roller bores and trimmed them to my original design length. Beautiful, smooth.

In fact, even with my heavy springs fitted they roll well on an oval filament. Grip is good, but needs more so I will experiment with heavier springs, and maybe bigger ribs on the rollers. I also think the pitch angle needs revising. The photo shows the rollers in their 'nut' with radial mounted compression springs, all mounted in the housing. I am using red 'fibre washers' as thrust bearings mainly because I had them to hand. As the rollers are smaller than the tile cutters I was able to make up the brass tube spacers too so everything stays tight, but not tooooo tight and the rollers roll well.

On a different tack - I have been cutting toooooobs to make my repstrap bot structure:

I have some of the 22x22mm aluminium extrusions left over from an old project. It is nice stuff to work with with basic tools. The bearings (not skate bearings, but 70pence each from RS) are on M5 bolts, 3 washers between them and pull down onto special 'T' nuts in the aluminium slots. This allows pressure adjustment.

The bearings run on 6mm steel rods, simply sitting in the slots. The plastic cable tie each end help hold the rods in place whilst I fiddle/assemble. (the carriage is made from 2-slot and the rail is 3 slot extrusions)

This reveals an interesting thing with the bearings - they are designed for radial loads. In this arrangement, they are being loaded partially between radial and axial. Normally ball bearings can run with axial load, but in this case the axial component is ALL on one side which causes the outer race to cant over slightly - there is often a bit of axial slack.

This causes the bearings to 'cog' for some reason (like when you hand-turn a unenergised stepper motor) . It could be the balls passing the loading slot, or something to do with the cages. Anyway, it is very bad when I put 4 washers beetween the bearings and almost perfect with only 2 washers. 2 washers reduce the contact angle on the rods and the carriage shown popped off with only slight side loads when running along. Not good.

I have settled on 3 washers for my Mk.1 and I have skeched a layout with proper 45 degree shafts and radial loading for the Mk.1.5 if it needs to be smoother. 45 degree angles add considerably to the tooling requirements, but now my Dad has got his mill motor re-wound I could do it.

I assume this 'cogging' issue applies to Timothy Schmidt's chunky pipework design too.

Progress is a little slow this week - too much time spent in Jazz clubs and visiting friends for vegetarian curries. Nice.

Friday, August 29, 2008

Rolling Progress.

I have made some nice new rollers.

The heatshrink test made the plain rollers about twice as grippy, but still not good enough. Also, the heatshrink skin started to 'ruck' up so I would have to use a glue lined shrink. Move on to the ribbed rollers.

I tried them last night, and they grip the filament really well. The problem was I made them in aluminium (quick, easy, available barstock, soft so prototype only - brass will be the material of choice). The 4mm axle holes are a bit rough - simple drillings - and don't run well on the stainless axles so I ordered some nice needle roller bearings from RS that arrived today. I nipped over to the workshop, opened the roller bores to 8mm and slipped in the bearings.

The result? wonderful grip, smooth rolling action. Questionable feed rate under load. Hmm.

Encouraged by the grip etc, I assembled the 'nut' assembly into the 'fusion reactor' housing for the first time to give it a spin. This revealed one or two minor problems. Minor problem no.1: the radial springs crash into the frame where I added ribs to carry the alternate bearing system so no revolving can occur until I cut them off. Minor problem no.2 : the frame squashes the nut which stops the rollers rolling.

Solution to problem 2 will involve A: machining the rollers to the correct length (I had left them slightly long because the way I parted them off the barstock left a nice extra grippy rib. Now I know that 3 grip so well, the original 2 will be ok). B: possibly getting the correct flanged ball bearings to take radial and axial thrust; or C: making up some spacer tubes for the teeny M2.5 screws that sort of hold it all together.

Option A and C is the best/lowest cost but B is the costliest but correct design. Anyway i will have to wait until next week for B's bearings do A/C it is !

On a related front - last night I also pulled apart a metal can stepper motor from an old printer paper feed - I will mod it for hollow shaft operation; I have a plan, a drilled out rotor and no idea at all if it will have enough torque. It will be super compact.

The stripped motor is a unipolar winding but runs the rollers through a long geartrain. The printer carriage motor is identical except it is bipolar but is direct onto the printhead with a toothed belt. I suspect the latter has more torque.

When I get this back together I will run it with the filament lifting a 1.5 litre water bottle. My calcs indicate a 20 bar nozzle pressure (from the forum) needs 14N thrust on the 3.2mm (worst case) filament. I may need to increase my nut 'thread' angle a bit to reduce the chance of the mini grooves I am making becoming munged together. My rollers are already have the ribs set 0.16 progressively offset to compensate for them each being 1/3 of a rev along the thread on the 2.7 degree angle I am attempting.

Much more testing to do . . .

PS. I have machined a small groove near both ends of each 4mm axle - the springs no longer go ping-boing.

Saturday, August 23, 2008

Ping, Boing.

Progress, of some sort.
I wish I could say it works great, but there is still a lot of work to do. The picture shows the assembled roller carrier - or 'nut'. The fitted rollers are the tile cutter discs - two on each axle separated by a 4mm ID x 6 OD x 9 long sintered brass bush. All these parts are standard and no machining is required.
The axle rods are 4mm steel, with notches files in each end.
Compression is provided by standard circlips resting in the grooves.

So what doesn't work? The brass rollers shown are M8 plastic inserts - usually moulded in when injection molding parts. They really grip the filament well and with the thread drilled out and the 4mm to 8mm bushes lightly finger pushed in (also standard parts) they roll nicely on the 4mm axles. Alas, they don't roll nicely on the filament. The knurling pattern isn't random or angled enough, so with the massive circlip pressure they 'bump bump' round. No good.

Next up is the tile cutter build. This is nice - the edges are about 90 degrees and whilst not rounded, they arn't sharp like a knife either. They grip the filament really well at low pressures. Shows promise if I can get the loading right.

Also failed the experimental progress is the circlip system. The un-sprung ID of the clips is 18.5mm. At 18.7mm ID the pressure is too much and the filament and the tile discs send the free end of the filament pinging over my shoulder!

So on to spiral wire compression springs. These also go pinging over my shoulder but mainly due to my eagerness to install and operate the nut without securing them properly. Patience required.

The heavy ones are too heavy, but now I have the range of motion to adapt to varying filament diameter. I have a roll of HDPE (the slipperiest one) which is oval - 3mm max, 2.8mm min.

The light ones ('recovered' from a dismantled inkjet printer paper feed path) are not strong enough. I have been to Dad's to get some alternative ones from his 'spring box'. More tests to follow but whilst no quite as simply elegant as the circlips, I think this will work.

One drawback with the tile cutters is their expense - about £1.50 each from B&Q and 6 are required. Not too bad and being tungsten carbide they will last for ever running on plastic. And that is another problem. Sometimes they make a groove that is just far too deep and all feeding stops. To solve this I have machined a couple of plain aluminium cylinders - 12.7 OD (as bar stock) with 4mm bore and 12.5 long.

These plain rollers roll - even on oval filament, but grip is a problem and if I increase the spring pressure then ovality may become a problem. I will try this, but I also want to try some other ideas -

1. Heatshrink on the rollers. This is easy to fit and repair and may just add the grip I need.
2. Machine (eek, not ideal) the roller OD to include pointy ribs much like the tile cutters, but leaving flat section to prevent the ribs penetrating more than 0.1mm or so into the filament.
3. Using shorter plain rollers and the tile cutter roller in a stack (OD will need careful matching/machining).
4. Putting a short taper section on the roller to aid initial filament feed-in.
5. all of the above together.

I am trying to design this to be reprapped (Plastic housing, nut etc in place of Laser profiles) and hand made from 'standard' parts. I have access to my Dad's workshop - ML7 lathe - nice, Milling machine etc. But I do have to drive 12 miles to get there. Proto and test parts from this source are ok and when (IF) it works then I can think about making it home-makeable later.

And the pencil sharpener? to make the filament end a bit pointy to help feeding of course !

Sunday, August 17, 2008

Filament Feeder Hardware.

On Thursday morning I collected the laser cut bits from Acorn laser. I also collected some parts for my customer, had to test them, rush them to his place, whiz to Tesco, cook stuff, buy a marquee 'cos it was peeing with rain, setup the BBQ and do the host thing.

The rain did stop and it all went very well. But I was itching to get at my metal parts all the time.

Friday too was fully occupied with my Brother & family. Then Saturday I was over at my Sister's place so finally I have got the hardware tweaked and test assembled.

The top photo shows the motor mount before bending the tabs. Yes, it does look like a small dog who's parachute has failed to open.

This picture shows the filament feeder chassis with the motor mount clipped in and the gib key wedges holding it all together. (I always wanted to do that since crawling under my mum's kitchen table and realising how it was held together).

The two roller carry plates will be stacked with the three rollers between - note the way the inner parts are angled 2.5 degrees. There are some fiddly M2.5 screws to hold them together. Oh yes, there will also be a couple of bearings to take the thrust and keep the gears centralised too.

The third large disk is the spring carrier - three springs press in to push the rollers against the filament.

Ah yes, the filament - It runs vertically right through the centre.

The chassis is inspired by the design of the fusion reactor that will allow everyone to live in a first-world economy without windmills (which will never work anyway) OR destroying the environment any more. see : WWW.ITER.ORG

The chassis is upside down here (I clipped the motor mount in upside down, but it will actually all work that way too). The three slotted extensions are where the heater will clamp in using a worm-drive hose clamp.

Wednesday, August 13, 2008

Filament Feeder Design

Here it is so far.

This is my 'everything in one drawing with different views and different options all on different layers in different colours' mode of design.

Yes its 2d - but I am really productive in this environment and mostly my output is for laser profiling or simple turned parts. Obviously in order to feed designs into my repstrap I am going to need to do more 3D.

It must look like a total mess to anyone but me - but I have been promised the metal parts will be ready tomorrow AM, so I hope to post photos very soon.

Tuesday, August 12, 2008

Kick off.

Hi, and welcome to my third blog. This one is for my Tech Stuff.

I am currently working on my RepStrap. Whats one of those ? see
It will hopefully lay down layers of hot plastic with precision under computer control in order to make things of almost any shape. As long as they are plastic. I started a design, changed my design, got sidetracked, ordered a load of driver electronics from America ( assembled and tested electronics, ordered a complete plastic extruder from and then set about designing my own design extruder.

As I blog, the laser cut parts I designed are about to be cut by a local service ( and I am hoping to collect the parts tomorrow. I also need to order mundane stuff like bearings and bushings from RS Components.

I have a Arduino to drive this lot - but I am in great admaration of the RRRF chap who has created an upgraded version called Sanguino ( and I have a plant to bung a Atmega644 SMT on a mini PCB that will plug into the Arduino where its old brain was. The mini PCB will pick up the old brains clock, IO, power etc and have a header for the extra I/O lines.

Looks like I will need a online shop so I can share this hardware with others.

Other possible projects :
My brother-in-law recently purchased a hand held plasma cutter which I need to automate sometime soon. I will also develop a system for engraving things on the outside face of wafer-mount check valves - I design the valves for a local manufacturer but they need a good way to mark up batches.