Best method for creating bevel gears?

jillian_ogle

Not sure how to actually bevel the gear accurately as needed...

isaku_kamada

Adding on to the discussion with some pictures of work I've done in Onshape.

As mentioned above, if you're just trying to make a bevel gear with a simple tooth design, you can make one by:
1. Creating the main geometry/body of the bevel gear without the teeth by making a section sketch and revolving around an axis
2.Drawing a line coincident along the angled face of the gear body which intersects the central axis of the revolve

3. Making a orthogonal plane at the end point on the outer edge of the revolved body and using this face as the sketch plane of the actual gear profile, then lofting this face to the point intersecting the central axis. (Loft option "add" illustrated below to make it easier to visualize, but use "remove" option when actually making the gear). This sketch plane can be made different ways, but the way I did it was drawing a horizontal line tangent to the gear edge and doing a line angle plane. The gear teeth can be also made different ways based on the accuracy you're trying to achieve. In this screenshot here I simply made a teeth with some line and arc entities. (see end of post for details on involutes gear profiles for even more accurate gears).

4. Face pattern the gear faces around the central axis to cut out all your teeth:


**Now, if you want to actually make your gear face involute, the sketch becomes slightly more complicated. Instead of making the gear face with arc and line entities, instead use the following method using construction lines, circles, and the spline sketch tool. A fairly close sketch of an involute profile can be drawn using the spline tool, disclaimer, this method won't make it perfectly involute, but will get you pretty close.

You can define a involute profile by determining the various diameters of your sketch circles and the pressure angles of your gear teeth. Designs of gears become very complicated for making very accurate gear models, but there is documentation and tables and lookup charts out there for referencing dimensions to use for this that correspond to what type of gear you want to make. I won't go into too much detail about gear design and the math because I'm not an expert on this either. I've attached a screenshot of the methodology I've used. In this case I wasn't looking up the accurate numbers for the diameters for the gear geometry, but more of making a sketch to prove a concept. As illustrated here you can define each spline point by drawing tangential construction lines at even interval angles and calculating the distance based on the formula radius x angle in radians. Recommendation would be to calculate the values in a spreadsheet so you can copy and paste values in. 



Good luck!

andrew_troup

Yikes. That's a fairly difficult modelling challenge (and a machining one, too, for those lacking dedicated gearcutting machinery)

Assuming you want the geometry to be suitable for actual working gears:

You would need to bone up on the involute geometry of an individual toothspace, then recreate it on a suitable sketch plane, then loft a solid body from that sketch to a single point at the intersection of the shaft axes. Then circular pattern the body, and subtract the resulting bodies from the solid representing the blank (deselecting "Keep Tools") using a Boolean operation.

If you're after a visual representation only, you can cheat by simplifying the toothspace profile.

isaku_kamada

Adding on to the discussion with some pictures of work I've done in Onshape.

As mentioned above, if you're just trying to make a bevel gear with a simple tooth design, you can make one by:
1. Creating the main geometry/body of the bevel gear without the teeth by making a section sketch and revolving around an axis
2.Drawing a line coincident along the angled face of the gear body which intersects the central axis of the revolve

3. Making a orthogonal plane at the end point on the outer edge of the revolved body and using this face as the sketch plane of the actual gear profile, then lofting this face to the point intersecting the central axis. (Loft option "add" illustrated below to make it easier to visualize, but use "remove" option when actually making the gear). This sketch plane can be made different ways, but the way I did it was drawing a horizontal line tangent to the gear edge and doing a line angle plane. The gear teeth can be also made different ways based on the accuracy you're trying to achieve. In this screenshot here I simply made a teeth with some line and arc entities. (see end of post for details on involutes gear profiles for even more accurate gears).

4. Face pattern the gear faces around the central axis to cut out all your teeth:


**Now, if you want to actually make your gear face involute, the sketch becomes slightly more complicated. Instead of making the gear face with arc and line entities, instead use the following method using construction lines, circles, and the spline sketch tool. A fairly close sketch of an involute profile can be drawn using the spline tool, disclaimer, this method won't make it perfectly involute, but will get you pretty close.

You can define a involute profile by determining the various diameters of your sketch circles and the pressure angles of your gear teeth. Designs of gears become very complicated for making very accurate gear models, but there is documentation and tables and lookup charts out there for referencing dimensions to use for this that correspond to what type of gear you want to make. I won't go into too much detail about gear design and the math because I'm not an expert on this either. I've attached a screenshot of the methodology I've used. In this case I wasn't looking up the accurate numbers for the diameters for the gear geometry, but more of making a sketch to prove a concept. As illustrated here you can define each spline point by drawing tangential construction lines at even interval angles and calculating the distance based on the formula radius x angle in radians. Recommendation would be to calculate the values in a spreadsheet so you can copy and paste values in. 



Good luck!

andrew_troup

Nice exposition, @isaku_kamada - Just one thing for people to be aware of if trying to construct accurate involutes for bevel gears: if using tabulated data, people need to bear in mind that the involute profile lies normal to the pitch cone (a notional cone roughly half way up the teeth). I'm sure isaku took this into account but it may not be obvious to those reading the method.
The person wishing to build an accurate model will have to specify the angle for this tilted plane (mentioned by isaku) taking the pitch cone angle into account.
For meshing with existing bevel gears, or if the ratio of the pair is not 1:1, it is also crucial where that profile is located along the cone (these are the sorts of details which I had in mind when referring to it as "fairly difficult").

jillian_ogle

Wow guys, thanks for the super thorough answer! This is amazing! Totally helpful. I never had someone give me a visual step by step tutorial on a forum question before. A+++

andrew_troup

To clarify my earlier post and eliminate confusion with other recommendations: step 2 of isaku's method should be slightly amended:

the line for the loft should be within the body of the gear blank, on the pitch cone (whose included angle can be calculated from bevel gear formulae).
Then in step 3, make sure the plane he refers to is positioned normal to this amended line.

isaku_kamada

@andrew_troup, thanks for the clarification comments!

jillian_ogle

Is there a good place online anyone knows to get some pre-made gears that can be edited in OnShape?

pete_yodis

jillian_ogle said:

Is there a good place online anyone knows to get some pre-made gears that can be edited in OnShape?

@jillian_ogle Stock Drive Products would be my pick... http://www.sdp-si.com/

http://www.sdp-si.com/eStore/CoverPg/Gears.htm

http://www.sdp-si.com/eStore/Catalog

michael3424

I think that WM Berg also supplies models of the gears that they sell.

http://precisionparts.wmberg.com/gears

andrew_troup

Misumi supply 3D models  of bevel gears -- not a huge range (small sizes range, in particular, but you can easily scale them up)

http://us.misumi-ec.com/vona2/detail/110300429650/?Inch=0&CategorySpec=00000029849::a

They do supply any item from their range on very short timeframes and generally remarkably affordably, all things considered.

_Ðave_

Rush gear has a good selection and you can create you own spur and helical gears.
You can Download the models.

elonas_radzevicius

For standard (and not so very) parts you may simply go to:
www.traceparts.com
www.partcommunity.com

nicolas_7

andrew_troup said:

Nice exposition, @isaku_kamada - Just one thing for people to be aware of if trying to construct accurate involutes for bevel gears: if using tabulated data, people need to bear in mind that the involute profile lies normal to the pitch cone (a notional cone roughly half way up the teeth). I'm sure isaku took this into account but it may not be obvious to those reading the method.
The person wishing to build an accurate model will have to specify the angle for this tilted plane (mentioned by isaku) taking the pitch cone angle into account.
For meshing with existing bevel gears, or if the ratio of the pair is not 1:1, it is also crucial where that profile is located along the cone (these are the sorts of details which I had in mind when referring to it as "fairly difficult").

it's even worse than that, for bevel gear we don't use involute, but an octoid tooth profile: https://en.wikipedia.org/wiki/Bevel_gear#Geometry_of_a_Bevel_Gear