Sunday, March 02, 2008


Ratcheted filament driver

I had an idea in the bath. To feed the filament, all we need are two ratcheted holes and a solenoid to drive them apart and bring them together. No gears or rotary bearings would be required, and we'd have absolute control over filament driving.

I built this working prototype from two bottle tops, one aluminium and one steel 'cos that's what I could find without having to dive into the kitchen rubbish bag. I drilled a central 2.5mm hole through in the direction I wanted the filament to go (out the top) and cut 4 radial slots out of the hole with a sharp knife about 7mm long. The aluminium one is far too soft. Move them together, filament moves forward. Move them apart or let a compression spring do it for you, filament stays put.

We can change the distance driven, change the timing, and vary the power of the solenoid to change driving pressures. A spring or split washer on top might help even things out.

A similar system might step beaded belts.

Hold on to a static filament and you can step along it...

I shall simultaneously celebrate this idea and obtain another ratcheting component.

Vik :v)

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What, like a hot glue gun? Wasn't this idea considered earlier? If not, it seems like an awful big oversight.
I'd had a similar idea that I never carried through to something real quite a while ago. I was going to use two hinged pieces of steel, one fixed and one hinged on a threaded rod drive collar with 3 mm holes in them.

A. When drive hinge swings up and binds the filament as the thrust collar drives down. The fixed hinge is thrust level and doesn't bind.

B. When the thrust collar reaches the bottom of its stroke, it reverses direction and the hinge goes level and stops binding the filament. The bottom hinge swings up slightly from the pressure in the extruder barrel and binds the filament from coming any further.


Maybe we ought to build some variations on this theme and see if we get a more efficient system than the threaded rod polymer pump.
Even though I am not building a reprap I love reading about it for clever ideas like this.
I am wondering if instead of linear motion of the ratchet to drive the filament forward, you could add a second rotating ratchet cap to provide a feed motion. A turn clockwise draws back to grab more filament and counterclockwise to push the filament forward to be grabbed by the second ratchet. That way you could use the threaded opening of the bottle. You would also be able to gain a torque advantage using the rotary motion.
bbot: Hot glue guns use a single sliding clamp and don't maintain pressure when the trigger is released. Yes they were investigated.

As to rotating the screw cap, what I'd like to do is to free us from the dependency of precision gear trains that are needed to rotate things. Solenoids would fit the bill. They're just a simple coil of wire.

Vik :v)
Are you thinking of an off the shelf solenoid or wrapping wire round repraped parts? The solenoid will need quite a apply quite a bit of force.
A little from column A, a little from B. Actually, I was thinking of a car door locking solenoid as they seem to have the power and are readily obtainable.

Vik :v)
From my extruder experiments I would guess (I haven't measured it) that the screw extruder exerts a compression force of about 200 N on the filament. But the movement required from each solenoid pulse would be very small, so you could have a long lever.

Wouldn't it be a bit noisy? :-)
If you want real noise, I was considering using two drilled-through piezoelectric sounders running at about 2kHz! The displacement per cycle would be about 4 microns. Minimal component count, theoretically reprappable if you allow ceramics, but noisy as hell.

It's almost worth doing just to see if it would work!

Vik :v)
Vik: Beware of car door parts - I used to work for a GM subsidiary making flight simulators. We were trying to do a cost reduction exercise using car parts for all sorts of things in the simulator.

One thing that surprised us in talking to the GM engineers was that car door parts (in particular) are not designed for reliability over a great many operations. Remember - car door locks are typically used maybe 10 times a day over maybe 10 years. That means that they probably have a design life of only 36,500 operations...nothing compared to the number of cycles you'd be demanding in a RepRap extruder. GM (and probably others) design for the expected life of the car in order to keep the prices low. If each cycle of the extruder solenoid drives the feedstock (say) 1cm forward then you might reasonably expect the car door lock solenoid to fail after 365 meters of feedstock has been fed...which doesn't sound like much to me.
Vik: If you want low-cost piezos, you might look into PVDF (trade name Kynar) polymer.

Sintering and poling the ceramics could theoretically be done at home, but making them gives off a fair amount of PbO vapor, which is very bad mojo in my opinion. You can use a cold trap to catch this if you sinter in a tube furnace, but that's asking a lot.

There are actually off-the-shelf piezo linear drives, if that kind of thing interests you. They work kind of like millipedes, but with a continuous sheet of material rather than discrete legs, and with shorter steps but a much faster gait. The motors in many autofocus cameras work on the same principle, but wrapped around into a ring. You could have two or three such drives bearing directly on the filament, walking it along.
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When pinch wheels were investigated, was the problem that the friction was too low? In which case, were coarse grinding wheels tried? The coarse ones have a good grip with plastic.
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