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 : -

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