Free hobbing

[camdigger]

I was asked to reproduce a plastic worm wheel the other day.

The easiest way I can think of would be to generate a worm to match and cut flutes to make a free hob. The plastic used for the original seems to be an injection mouldable nylon or PE. Fairly hard as plastics go. Do plastics lend themselves to free hobbing?
https://youtu.be/vghcnXPki5s

The teeth are quite small, roughly 0.090" pitch. The whole gear is only 2.100" OD. The gear thickness is 0.500".

[Ironman]

Originally Posted by camdigger

I was asked to reproduce a plastic worm wheel the other day.

The easiest way I can think of would be to generate a worm to match and cut flutes to make a free hob. The plastic used for the original seems to be an injection mouldable nylon or PE. Fairly hard as plastics go. Do plastics lend themselves to free hobbing?
https://youtu.be/vghcnXPki5s

The teeth are quite small, roughly 0.090" pitch. The whole gear is only 2.100" OD. The gear thickness is 0.500".

This is the easiest way to do it, but I would have a washer and nut on top so the gear does not wobble around like in the video. And he never showed the fact that you have to center the gear so the lathe center line bisects the gear width center line that you are cutting.

[Lew Hartswick]

Tht example wasn't centered very well :-) A simple heavy weight on the top of the wheel would have helped a lot and a captivated restraint would have been even better. :-) But the idea is there anyway.
...lew...

[clive]

The diameter would need to be devisable by the thread pitch but it’s a keeper.
Already thinking of a use.

[camdigger]

Originally Posted by clive

The diameter would need to be devisable by the thread pitch but it’s a keeper.
Already thinking of a use.

I am hoping that since I am copying an existing part, most of the design questions would be answered with some careful measuring.

I have heard and read about freehobbing before. I understand from that research that an Acme tap will give something very Close to an involute shaped tooth. I have seen and read about many examples done in metals, especially Brass, Bronze, and Aluminum. I am still a bit curious how it will work with plastic because it is so stringy.

[LKeithR]

The basic idea in that video is sound but the execution is really poor. As others have said, a better fit on the pin and something to keep the blank from flopping around would have produced a much more accurate result. Notice when he takes the gear off the pin that the teeth are cut way off centre because the blank was tipped up.

Originally Posted by camdigger

...I am still a bit curious how it will work with plastic because it is so stringy...

Not all plastics are stringy; some of the harder delrin materials will break a pretty good chip. Also consider that if the gear you're producing is fairly narrow like the one in the video you're going to have an interrupted cut as the hob moves across the blank--each tooth is only in the material for a short distance...