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strokersix
12-21-2010, 01:43 PM
I have a straight bevel gear design, 13/45 tooth 2 module using Dudley's "Handbook of Practical Gear Design". Standard calculations are based on large end of the gear.

I wish to produce approximate gears using the parallel depth method. I have altered the face width so the small end matches 1.5 module. With reference to Ivan Law page 106, I calculate the cutters to use are for 121 teeth #2 cutter for the gear and 10 teeth for the pinion. Standard cutter #8 is for 12-13 teeth. Should I use a #8 cutter for the pinion? Perhaps tip relief might allow this to work? I'll be deburring these by hand anyway so a little hand fitting is acceptable to me.

It seems 13 tooth pinion in a 13/45 ratio may be outside the range of parallel depth method.

These power transmission gears will be grease lubricated nylon and as such will be a little more tolerant of error than steel gears.

strokersix
12-21-2010, 06:31 PM
I must add that the pressure angle is 20 degrees.

Not sure if cutting a 10 tooth spur gear with a #8 cutter (12-13 tooth) is the same as what I'm asking or not.

I do recall reading that sometimes cutter choice is based on neither the large end nor the small end pitch diameter but somewhere in the middle of the tooth. If that's true then the #8 cutter may work. But that might be wishful memory.

Anyone done what I am asking or know what will happen? I might just have to try it and see.

Rich Carlstedt
12-21-2010, 10:31 PM
Anytime you go less than 12 teeth, you will undercut.
Doing a parrallel tooth form on Bevel gears is a handfit anyway.
I would use the #8 for the pinion and the #2 for the spur gear.
If you can layout the gears and cutters on CAD, you may want to look at stepover ( double pass/offset ) on the 10 tooth gear
The Spur gear should be fine, but I would try a aluminum blank pinion first and see how that works
Rich

strokersix
12-22-2010, 07:59 AM
Rich, thanks for the comment.

My bevel pinion is 13 teeth. I have this on CAD, both the proper conical design and the parallel depth design (I don't have the tooth form, just the layout). When you say "double pass offset" I presume you are referring to the method presented in Ivan Law's text where you cut down the middle, then offset and rotate to trim each side of the large end of the teeth.

Ivan Law says I should use a cutter with involute form for 10 teeth to produce this 13 tooth bevel in a 13/45 ratio. And a cutter for 10 teeth isn't readily available.

I'm going to have to try it and see what happens. I understand that this is an approximate solution and accept some errors and hand fitting.

Dunc
12-22-2010, 09:00 AM
I recall a previous post that the above author had a lengthy how-to in one of his build a steam locomotive books - don't recall which one.

Rich Carlstedt
12-22-2010, 10:29 PM
My bevel pinion is 13 teeth. I have this on CAD, both the proper conical design and the parallel depth design (I don't have the tooth form, just the layout). When you say "double pass offset" I presume you are referring to the method presented in Ivan Law's text where you cut down the middle, then offset and rotate to trim each side of the large end of the teeth.


Yes, Thats what I mean
With CAD, layout the cutter tooth profile, and compare it to the circumferences of both the inside and outside peripheral measurement (divided by 13 of course) and you will see the error.( difference )
I haven't read Ivan, but I do it in two passes versus three as you mentioned




Ivan Law says I should use a cutter with involute form for 10 teeth to produce this 13 tooth bevel in a 13/45 ratio. And a cutter for 10 teeth isn't readily available.

I'm going to have to try it and see what happens. I understand that this is an approximate solution and accept some errors and hand fitting.

Yeah , error will abound.
Consider this, I don't know your application , nor the precision requirement, but you may want to conside this.
Cut the large gear as is, and that will make the tooth form wider on the outside, which is normal.
If you can measure the tooth width, that gives you the gullet to shoot for on the pinion, and offset will or should address that.
This method is not Kosher by gear standards, since we are modifing only one gear, but it sure makes adjustments and calculations easier.

Also, by only reworking the smaller gear, you minimise your efforts and time

Rich

strokersix
12-23-2010, 12:54 PM
Rich, thank you.

I will post my results.

strokersix
01-02-2011, 08:48 PM
As promised:

http://i965.photobucket.com/albums/ae131/strokersix/DSCN3757.jpg

I'm very pleased. They seem to roll smoothly by hand. Next step is to run them in and see how they do under load.

Question: If I wish to trade some tooth thickness from gear to pinion what would I need? I can thin the gear teeth easily enough with the offset cuts. But I can't make the teeth any thicker on the pinion. I've seen reference to a bevel gear milling cutter with thinner section. This sounds like it might be what I need. Source for such a thing? Or do I need to make a single flute cutter myself? Maybe wire EDM a HSS bit?

strokersix
01-03-2011, 12:50 PM
How about I get another #8 cutter and alter it? Seems I could remove one side of the profile for clearance on the first cut then flip it over for the second cut.

strokersix
01-26-2011, 02:48 PM
I made several more sets of these gears. I found that I only need to take two cuts instead of three just as Rich suggested above. This means I only have to reset the offset once per gear. Take one cut, adjust offset, take second cut, swap blank, cut, offset, cut, swap blank, cut, offset, cut, swap blank, etc.

I got the cycle time down to about 30 minutes to cut the 45 tooth gear and worked up a sweat doing it.