Wednesday 5 November 2008

Sky Hook

I have found that the most convenient way to provide the filament feed is to throw the whole reel into a large hanging basket and suspend it over the machine with a ball bearing. The machine pulls the filament from the centre of the coil through a hole in the bottom of the basket. As it does this the basket rotates to prevent the filament being twisted.

When the basket contains a full 5lb or 2Kg coil then it does take some force to pull the filament from under the weight of it. Since I sharpened and angled my drive screw and moved to an HDPE filament guide I have not had problems with grip, so this works flawlessly.

I have even started stacking the baskets two deep to save on storage space for different colours of plastic. The top filament can be used simply by passing it through the centre of the lower basket. Obviously only one can be used at a time.

It was easy to attach a frame over HydraRaptor but harder to do over a Darwin. I decided to attach it to the ceiling instead with a "Sky Hook", as suggested by Adrian Bowyer here.

I designed a ceiling mount to hold a skate bearing. These seem to be the cheapest ball bearings to buy. My first idea was to put an M8 bolt through the bearing and attach a plastic hook, but I didn't have any bolts long enough, so I made some plastic parts instead.

The barbs just go through the centre of the bearing, but only when they are twisted over each other.

More by good luck than design, but that means that they can't be pulled out because they pull towards each other and meet in the middle before they can clear the M8 hole.

Just to make sure I made a small locking piece which slots in afterwards to keep the barbs fully splayed.

The bearing then drops into the housing and is held in place by gravity and the ceiling.

The eye and barbs are tensioned along their strong direction, so can be quite thin, but the housing is stressed in the direction where de-lamination can occur, so I made it 4mm thick. It would be tragic if it gave way and dropped a few Kg of plastic onto the machine from a height, so I tested it with a lot more weight to get some confidence.

These 13 wine bottles weigh 16.5Kg, which is 8 times more than the plastic I intend to suspend. It held up for 24 hours with no sign of stress to the parts. Using plastic ties with barbs on the end could be a useful technique for making RepRappable fasteners.

When I came to mount it on the ceiling I realised that the three screw holes were too far apart to all screw into the ceiling joists, which are only 1.25" wide, so I made a two lug version. I made this with 50% fill rather than 25% to ensure it was as strong as the three lug version.

Here you can see a trick I did to get support for the overhanging lip. I just put a one layer thick diaphragm across the hole and cut it out with a penknife afterwards.

Here it is installed : -

Tuesday 4 November 2008

Black Heart

I made this tiny heart shaped box as a little gift for my wife's birthday.

Being such a simple shape, with no extruder off moves, it came out perfect, just like my wife.

Thursday 9 October 2008

Pot black

Having tried green ABS and found it a bit disappointing, I had a go with black ABS and was even more disappointed. I got it to try and make objects a bit more aesthetically pleasing but it was even harder to get working than green and has aesthetic problems as well.

The temperatures seem to be the same as those for green, i.e. higher than I have used for plain, but it is harder to get it to stick to the raft and it de-laminates more readily.

To make it stick I had to make the first layer a bit lower than normal. With plain ABS I can have it at the normal layer height above the raft (filament diameter * 0.8) but with HDPE and green ABS I had dropped it 0.1mm. With black I had to drop it 0.15mm as 0.1mm does not stick enough and 0.2mm sticks too much. 0.05mm makes all the difference and has about the same effect as changing the temperature by 10°C.

Another problem is that the extruded filament is not smooth. Close examination reveals that it has small craters in the surface where it has out gassed. It is particularly noticeable when laying the thick filament for the first layer of the raft.

It could be the effect of water absorption but, as all my ABS is stored in the same room, I think it is more likely a volatile component of the black dye. Another effect is that if the filament is stretched while molten, so that it is drawn into a fine thread, then it looks like a string of beads. I think that is an indication the black dye does not mix well with the ABS.

Fortunately, for some reason I don't understand the crater effect is less noticeable when making objects.

These are Darwin y-belt-clamps, a nice small part good for a quick test. The small one is made with 0.3mm filament and shrunk by 3/5. As you can see it is far less shiny. All I can think is that is related to the fact the filament is being stretched more.

Another downside of coloured ABS is that ABS turns white if it is stressed. This shows up far more when it is coloured and particularly with black. Because the base of the object is weakly welded to the raft then it gets bruised when it is peeled off.

This is the bottom of another screw topped pot.

As you can see it has white highlights where the welds to the raft have been broken. One way of fixing it is to wave a hot air gun over it to relax the stress points. That also flattened the base, which was a little convex due to warping.

I expect rubbing it with a little solvent like MEK would also solve the problem.

I now have a little family of pots!

I would not recommend coloured ABS as the dye introduces more variables and generally seems detrimental to strength and aesthetics.