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 !