I am trying to create some bevel gears. I can create the gear profile given by the standard calculations in Ivan Law's book on Gear cutting. However I then need to extrude the profile on a taper.
If the calculations gave the large end (outer) profile it would be simple - I would just extrude from that profile to the apex point and cut off at the inner profile location. However the gear calculations work on the basis of the small end ( inner) profile and I need to extrude outwards (getting larger) based on the apex point which is at the centre.
Is there any way to use Extrude where I set the extrusion from point then expand the profile outwards? Extrude taper doesn't work either as the end profile is smaller than the starting one.
Ideas?

Hi wisty,

Do you have a .3dm, or picture, to assist with understanding?

- Brian

Hi wisty, the tapered option in Extrude is the only thing that sounds similar to what you are describing.

I'm not sure that I understand the details though, some sketches or files with some example curves and desired result would help a lot.

Thanks, - Michael

Hi wisty,

Re:
> I would just extrude from that profile to the apex point and cut off at
> the inner profile location.

The way I read Law's book p42 it sounds like this can't be done by extruding a planar profile curve because "the teeth are not in a plane parallel to the base of the cone but are, in fact, perpendicular to the to conical pitch surface."

I'd draw one tooth of the involute profile, orient that planar curve to be perpendicular to the pitch cone. Then make a scaled copy that is farther from the apex. Loft between these 2 profiles to get the tooth surface. Then array around the circle. But, I'm not a gear guy, so not sure if this is accurate, or if there needs to be some correction for wrapping the flat involute profile around a circle.

What calculations are you referring to in Law's book? I'm not seeing them.

It might be worth using Gearotic instead. No personal experience, but have heard good things.

- Peer

Thanks,
I attach the basics of what I am trying to do. The calculations I am using are on Fig 80 page 104 of my revised edition of the book.

Peer - you are correct that the profile is perpendicular to the pitch cone. MOI works fine extruding inwards if you set direction to point to the pitch cone apex, and then extrude to a point.

The math in the book generates the inner ( small end profile) and I want to extrude that profile outwards on the pitch cone. I was hoping that setting the extrusion distance negative having set direction to the apex and and extrude to a point would do what I wanted, and it almost does. As you can see in the attachment (where I have set direction and extrude to a point). It will also let me set extrusion distance negative and set direction which extends the profile outwards - but not on the cone.

It won't let me do all three things (set direction, extrude to point and set extrusion distance negative). I can do two (any two) but then extrude closes the object. Interestingly if I set the view to 3D it closes the dialogue after one choice, in Right view it will let me do any two.

Any ideas

Hi Wisty,

Is the attached what you want? I got this result by drawing helper lines in blue, copying your profile curve so that the first corner matched up with the first blue line, and then scaling the profile to get the second corner to match the second helper. Finally, I selected both profiles and did Construct > Loft.

- Peer

I'm thinking that the profile should be appropriately scaled, and placed at the big end target??? And re-extrude to point?
Maybe a cPlane, or at lease direction set?

- B

Thank you Peer , I will use that technique again. It was the fact that extrude was so close that was throwing me.

Why not create the small end profile and scale it to the size of the large end? Then you could extrude this profile to a point.

I made some videos of me making a bevel gear in MoI a long while ago, Maybe they will help.

https://vimeo.com/1047669344

https://vimeo.com/1047670578


Cheers, Martin.