VDE-200 (Schnekenstruder Rev. 2)

So MarksAlot got me thinking, and it's not like I don't have some spare parts kicking around. Attached is a photo of the drive mechanism for the Rev.2 Schnekenstruder.

The housing is PLA, and the drive is a 10mmx4mmx4mm shielded bearing, mounted at 15 degrees to the motor axis. Press fit into the bearing bore is a Grade 5 titanium sleeve with a knife edge half way up the bore. The knife edge was machined by mounting the 4mm diameter, 4mm length titanium rod in a lathe, and using a countersink from both ends. The internal bore is 1.60mm.

The idea is that the outer race will rotate with the hollow motor shaft, and the inner race with the knife edge will wobble like a helicopter swash plate, while slowly rotating due to the ratio of the diameters of the bearing balls and the bearing race. The whole thing weighs 2 grams.

Yes, it works. Tested with the drive module mounted in a hand held drill. Because the knife edge completely surrounds the filament, it has a ferocious grip. There is practically no torsion on the filament. As far as I can tell, it appears to be very repeatable; at least the "thread" pattern on the filament is consistent and symmetric with what little testing I have done with various filament materials and diameters.

BUT...because the ball to race ratio is so large (roughly 40:1, admittedly an educated guess on my part), it also has terrific mechanical advantage, which means it has to spin at 1000's of RPM's to get any kind of useable filament feed.

Plus, I'm sure the balls are slipping, which doesn't help at all. As a micrometric rotary to linear converter, it's great. As an extruder drive, not so much. But it was fun and cost nothing but time.

Back to the original VDE-100 Schnekenstruder, which is permanently mounted on my delta. I just received the CNC machined bearing carriers (which are gorgeous), and eagerly await the NEMA 11 stepper, which flew into Kennedy airport two days ago, and is now in Atlanta, GA awaiting the attention of the US Postal Service. I'm in New England. Go figure.

Edited 3 time(s). Last edit at 02/24/2022 06:19PM by rq3.

Hi!
I'm curious about this design; do you mind sharing the model or explain better about how the titanium insert was made? I don't really understand how it can have an internal bore of 1.60mm if the filament diameter is 1.75mm.

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cyber_v1
Hi!
I'm curious about this design; do you mind sharing the model or explain better about how the titanium insert was made? I don't really understand how it can have an internal bore of 1.60mm if the filament diameter is 1.75mm.

The design is basically a swashplate, with the friction of the bearing balls providing the drive ratio between the inner hollow drive shaft, and the outer bearing drive diameter.
The inner titanium insert is a press fit in to the bearing bore. In a lathe, a counterbore cutter is used to make the internal knife edge by plunging from both ends.

A 1.60 mm bore on the knife edge gives a roughly .075 mm bite of the knife edge into a 1.75 mm diameter filament.

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rq3

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cyber_v1
Hi!
I'm curious about this design; do you mind sharing the model or explain better about how the titanium insert was made? I don't really understand how it can have an internal bore of 1.60mm if the filament diameter is 1.75mm.

The design is basically a swashplate, with the friction of the bearing balls providing the drive ratio between the inner hollow drive shaft, and the outer bearing drive diameter.
The inner titanium insert is a press fit in to the bearing bore. In a lathe, a counterbore cutter is used to make the internal knife edge by plunging from both ends.

A 1.60 mm bore on the knife edge gives a roughly .075 mm bite of the knife edge into a 1.75 mm diameter filament.

Thanks! I think I understand the first part. But I don't understand about how to make the internal knife edge with a counterbore cutter. Do you bore at an angle? What is the resulting shape of the knife? I'm sorry for dumb questions. I'll try to sketch something after work to make it more clear :-]

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cyber_v1

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rq3

Quote
cyber_v1
Hi!
I'm curious about this design; do you mind sharing the model or explain better about how the titanium insert was made? I don't really understand how it can have an internal bore of 1.60mm if the filament diameter is 1.75mm.

The design is basically a swashplate, with the friction of the bearing balls providing the drive ratio between the inner hollow drive shaft, and the outer bearing drive diameter.
The inner titanium insert is a press fit in to the bearing bore. In a lathe, a counterbore cutter is used to make the internal knife edge by plunging from both ends.

A 1.60 mm bore on the knife edge gives a roughly .075 mm bite of the knife edge into a 1.75 mm diameter filament.

Thanks! I think I understand the first part. But I don't understand about how to make the internal knife edge with a counterbore cutter. Do you bore at an angle? What is the resulting shape of the knife? I'm sorry for dumb questions. I'll try to sketch something after work to make it more clear :-]

Put a thick wall tube in a lathe. Plunge a countersink into the tube. Retract the countersink. Reverse the tube in the lathe. Plunge the countersink until the bore is good.

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rq3
Put a thick wall tube in a lathe. Plunge a countersink into the tube. Retract the countersink. Reverse the tube in the lathe. Plunge the countersink until the bore is good.

That's what I thought! But now, if the bore is 1.60mm and it's put at an angle to the filament, it's less than 1.60mm in the plane perpendicular to the filament, isn't it? It should be 1.60mm/cos(15°)=1.66mm. But maybe I miss something.

Is this what it should look like?

It looks like the 1.60mm bore won't work as the knife then intersects the filament on all sides.
I also don't really understand how it may work since the part is totally symmetrical and it can't "prefer" to which side to push the filament.

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cyber_v1
It looks like the 1.60mm bore won't work as the knife then intersects the filament on all sides.
I also don't really understand how it may work since the part is totally symmetrical and it can't "prefer" to which side to push the filament.
[attachment 120741 intersect.png]

Yes, and a nut intersects its bolt on all sides. Yet observe, grasshopper, that it goes up and down the bolt when either is rotated. Or sometimes even both. Amazing.
The universe is stranger than you can imagine.

Edited 1 time(s). Last edit at 01/12/2024 07:29PM by rq3.