# Revolve with a twist?

Hi, I am trying to make a cutting tool to make a double enveloping worm gear. I need to take a gear profile and revolve it and twist by one tooth, in this case 18 deg for a 360 deg rotation.

It doesn’t appear I can do a revolve and twist unless I am missing something (quite likely)

Use polar array to put a toothed section every 20 degrees around your base revolve diameter. Then using the gumball or rotate command twist the first one 1 degree, the second one 2 degrees, etc. Then do a sweep 1 with the base revolve diameter and 18 sections. Simplified example below.

Another way to do this is to Extrude your gear profile straight along a line that is the equivalent length of your half-circle through the center. Then use Flow with History on and Copy=Yes to flow the straight extrusion from the centerline line to the half circle. Then use Twist to twist the original straight extrusion around its axis by one tooth. The flowed extrusion should update with the twist.

You can also do it without history, Extrude/Twist/Flow, but I like to see the object update with the history…

Its also not necessary to have the extrusion line exactly the same length as the half circle if you check Stretch=Yes in Flow.

–Mitch

Thanks guy’s I think I have a lot of reading as I just can’t do as you both describe. My aim is to make a full torus with gear profile on it. I managed to loft a straight section with the twist in it and hoped I could bend it 360 degree so it joined up.

Feeling pretty thick at the moment 8-/

I just seem to be ending up with kinked torus doing the above. When I tried twist it kept the ends straight and put the twist in the middle, hence my loft and shifting the loft points 1 tooth apart generated the straight twist I needed. I am now just scratching my head how I can bend this end to end.

Early on I had looked at drawing two helical lines and sweeping the gear shape along two lines to get the torus and twist in one go.

I was trying the lofted (sorry that should have been sweep1) sections every 20 degrees. LOL Rhino just locked up here and crashed. I think the Mac is telling me something.

OK, some very strange behaviour. The file I had been using locked up and crashed. I restarted Rhino and tried to re open it and it locked up Rhino. I checked it from my other Mac running the RhinoWIP and that too locked up. The file had grown to 45MB without much, or so I thought geometry in it.

Not sure what went wrong really.

Bummer file is unusable, lost this afternoons work and have deleted file. 8-( Oh well.

Hello,

Twisting Gear video is up:

https://vimeo.com/169196660

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OSTexo, that’s just what I was after. I managed to do a loft of the gear along a profile line and then use the bend command to achieve the appearance of what I wanted, but the geometry was horrible. I will practice your technique “A number of times” so it sinks in…

Many thanks for the video.

The twist and flow technique is much quicker than lofting multiple sections but you need to be careful on one potential gotcha. Depending upon the intended use this part (looks about right vs. I’m cutting something on a 5 axis CNC) you need to be very careful with the end tangency the twist command leaves. For a true splined gear you need a constant rate of twist with no tangency to the original untwisted end condition. I believe if you build the way OSTexo’s video shows you will have twist with tangency at the ends which will create a discontinuity in the rate of spiral of the tooth profile every 360 where the ends join.

You need to check Infinite=Yes in the Flow Twist command options to avoid this. --Mitch

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Cheers guys, the twist issue I picked up earlier in the thread, thanks Mitch for that note. I tend to 3D print everything now before ever committing to expensive machining.

Hello Bill_Carson,

I’m glad it was helpful. Helvetosaur, thanks for the information about the Infinite constant deformation option for Twist, I’ve learned something new today about Rhino.

http://docs.mcneel.com/rhino/5/help/en-us/commands/twist.htm