Thread: Bevel Gear Calculations

Are there any forum members that are know their gears and gear calculations - I need a pair of bevel gears for a project that I started, put aside and now want to continue. I read Ivan Law's book on gears and gear cutting and found the section on bevel gears a bit difficult to follow, however I went through the calculations and came up with the following.

0.4 Mod
16 teeth
OD small end 6.97mm (0.274")
OD large end 10.17mm (0.4")
Tooth length 0.089 "
Tooth length factor 3
Cutter selection was for 23 teeth
Offset for second cut 0.3141mm (0.012")
Roll back 0.25 x Num Teeth

I went ahead and machined them using the offset method and while they look ok and seem to mesh ok with one gear mounted on a rod and the other spun with a cordless drill, my main concern is wear factor, I've read comments on other forums that incorrectly machined gears will not mesh properly resulting in premature wear, considering the trouble I had following Ivan Law's text I'm wondering if my calculations are correct. If the calc is correct do I need to polish/ or run them together with Brasso or similar for a while.

I'm not familiar with the calculations, but we check small bevel gears at work with simple fixtures that simulate how they will be mounted in use; if they run smoothly with a little bit of backlash for clearance, they're good. It's really the only way to do it because accurately measuring a bevel gear that small is impractical at best and the calculations only tell you what you should have, not what you do have.

Thanks for the reply - I'd like to think that what I should have I do have - but - life's not perfect so in all probability I don't exactly have what I should have, as they do seem to mesh ok so I'll assume they are ok, the loading is light, they are for a mini petrol engine distributor drive, looking through a jewelers loupe I've noticed a few burrs so I'll at least remove the burrs with a small fine swiss file.

Hi FamilyGuy,

I've found that the small brass wire brush that comes in the Dremal kit of bits is good for deburing small work. Have a look at the "Tangential Lathe Tool" thread pictures. The steel one works as well but tends to leave scratch marks.

I've got some of those wire brushes - came with my Dremel, I've always been a bit hesitant to use wire brushes, as a high school student I was lucky enough to have a BSA A10 twin to ride to school, using a wire brush in a power drill to clean some of the engine components a piece of wire broke off and lodged in the corner of my eye socket, back then safety specs were around but weren't something you would find in the average household. I'll give one of them a go on the gears and be sure to wear safety specs.

The technique for making bevel gears in Ivan Laws' book gives you not a true form throughout the tooth but an 'approximation' of it- can't recall the exact term that is used to describe the result but it won't produce a gearset that is perfectly smooth-running such as you would get from a commercial manufacturer, certainly near enough for most of our purposes though. That book takes a potentially complicated, confusing subject and explains it in complicated, confusing language.

Yes, is it impossible to cut a 'proper' bevel gear on a mill, because the cutter makes parallel flanks while the rotary indexing makes tapered teeth, but, with fine pitch gears, the deviation from the proper tooth profile is generally not enough to worry about; the low tooth count does make it more pronounced though.

To cut bevel gears with the correct tooth geometry you'd need one of these https://www.youtube.com/watch?v=QTlo6bIIieE