[ngphil]

Total NEWB here with 2 questions and I hope I'm in the right place.

While this isn't necessarily a cnc related question, I plan to use a cnc mill for the project.

I understand that the only way to make "correct" bevel gears is with a special (gleason?) shaping type machine because of the way the teeth on a bevel gear converge in the center of the cone. What I don't quite get is if I were to use an involute cutter or even a bevel cutter for both gears in a 90 degree bevel, would it matter that the profile isn't a true bevel tooth profile? I'm not planning on mating a made gear with a true bevel gear, I'm talking about making the pinion and gear the same way. Wouldn't they mesh fine since they both have the same tooth profile along the face? These aren't going to be high speed/high load gears, they're for a model. The motor output is 10,000 rpm, however it's geared down through a 50:1 gearbox and the motor will be in no way run up to it's full potential. So actual working rpms would be more in the 20rpm range.

The next question is with what the recommendation would be for the best fixture to approach the task with. I've seen videos/photos of people who have cut gears using a lathe and a vise mounted on the toolpost with an indexer on it, so I think anything is possible, but is a dividing head the absolute "must go" route? Could I get away with a rotary table w/ a 4 jaw? How about a "spindex"? I think I saw a video a while back of someone who wrote a program to run the 4th axis rotary table to cut gears (not bevel) and that would be a bit of a luxury.

I'm really looking for what would be an appropriate tooling setup to use to cut some small (I mean micro), somewhat accurate bevel gears using a little sherline mill. The gears I want to make would be brass and the OD of the gear is only 6mm! Probably .35mod/72dp. I'm Crazy right? lol

I have other uses for a 4th axis so I'm interested to see if anyone thinks that will work. As this is hobby stuff, I'm looking at the shars dividing head or the sherline rotary table. Many thanks for any advice/opinions.

Phil

[txcncman]

I don't have any gear specs in front of me, but the tolerances needed are dependent on your application and the power requirements. Gears do not have to be "tight" to work effectively. Maybe a video like this, or others you can search, will help:

https://www.youtube.com/watch?v=RV8Dq6mslnE

[basalt]

Try Ivan Law's book "Gears and Gear Cutting" It includes calculations, set-up, making cutters etc and is fairly simple.
[ame=http://www.amazon.ca/Gears-Gear-Cutting-Ivan-Law/dp/0852429118]Gears and Gear Cutting: Amazon.ca: Ivan R. Law: Books[/ame]
There are 3 ways ( I know of) to cut bevel gears--a fellows gear shaper, using a set of involute "bevel" gear cutters and doing it according to machinist text books (involved), Ivan Law's approximate way.
As for indexing, anything that will fit on your machine will do however a spindex may not cut all the number of teeth you want, for example would it cut 37 teeth? Whereas a rotary table, dividing head will.
The next thing to consider is the cutter, make one or buy (a set of 8), you might not be able to buy a set so small.
And then there is the calculations.......
As for set-up it's all your imagination, and no, your not crazy, we all love a challenge and this is a good one.

[ngphil]

Thanks guys. txcncman, that's one thing I was thinking about as well. I actually bought some tiny end mills and might give it a try. Since they're so small, the tooth profile is somewhat "mushy" anyway so the cnc route may work.

basalt, I'll get the Ivan Law book. I keep seeing it referenced. I'm pretty sure I know the methods explained in the book though. The common process I keep seeing is a 3 pass per tooth setup. 1st to cut the small profile, then the 2nd with the gear offset/rotated for the "left" side of the tooth and the 3rd cut offset/rotated for the "right" side of the tooth.

Way too complicated for such a tiny gear as I don't think the amount of material removed in the 2nd or 3rd cut would be perceivable. I'm really somewhere in the realm of a hybrid between a crown gear and a bevel gear at the size I'm working with. Even tooth profile is almost gone a this size. If I was trying to create an exact replica at scale, then I might try to follow the methods for larger gears, but I'm really just looking for a simulation of a bevel gear.

It's also darn near impossible to find a bevel cutter in the mod I'm looking for. Involutes are hard enough. I might try to form a cutter to save some cash (tiny mod involute cutters are steep) and to see if I can do it. The cnc route might be the easiest way though

[headsmess]

bevel gears are tricky, from this book im reading...


it says that cutters are made in sets just like spur cutters of 8, and have the correct PROFILE for the OD, but are thinner than spur cutters as they have to clear the inner cone diameter. teeth on a bevel gear of course being conical, they converge to a single point.

the blank is gashed, rotated so many degrees AND the cutter OFFSET to finish one face, then so many degrees and offset back to finish the other.

selecting the cutter for the number of teeth is based on the angle of the pitch cone(not circle) diameter. not the actual number of teeth being cut!

im sure one could grind up flycutters... slow but hey.. the real cutters may be impossible to find!




its about ten pages...a lot to scan... but i could?

theres a mention of the gleason generator and the bilgram generator... theyre both just basically indexed shapers if that helps? the spiral bevels and hypoid bevels are a bit trickier... they seem to use a ring of shaped cutters, on a flat spiral path, as the blank is rotated at a certain angle and index.

[ngphil]

Yep, that's the same info I've seen elsewhere. Finding cutters small enough for what I want to do is one of the issues. Actually cutting them via the Ivan Law method or any other more accurate method is another issue. The degree of offset to try to get gears as small as I want would be insane.

Though I wonder if this is where a 4th axis and cnc might come in handy? If I tried with a dividing head, it would be a lot of manual rotating and offsetting of the head. With cnc and a rotary table, I guess I could theoretically write a program that would take care of the rotation and offset?

[rowbare]

The size of the gears you want to make definitely poses a problem. But you might want to check out "Gearotic Motion Gear design Software". It does bevels but I don't know what size end mill it would want for such a small gear.

bob

[txcncman]

Just food for thought: Clock makers of old used to make clock gears by filing by hand and still make clocks that were accurate to within a few seconds per day.

[ngphil]

I forgot about that gearotic site. Thanks! I remember seeing it a while back.

Yeah, if you look at what George Daniels started out with and what he was able to do by hand, you gotta believe anything is possible! I've seen gears exactly like what I want to make already so I know it's possible. Can't buy em anywhere but the japanese knew how to do it in the 60's so I gotta believe there's a solution.

[basalt]

Phil,
My friend cuts Module 1mm bevel gears all the time using spur gear involute cutters from China (ebay), he goes thru the cut in one pass, no filing of the teeth and it works for his Automata which are only hand crank speed. Just choose a cutter that is thinner than the small end of the tooth cone. Also, by reducing the face width of the gear gives you more room for error. I found somewhere that you could buy Involute gear cutters down to .5 module. Since the gear cutter set consists of 8 cutters, if you decide on one or two ranges of number of teeth you could buy just one or two cutters.
Getting into all this can be overwhelming so what I did was ask myself a few basic questions--
1) Gear blank dimensions, face angle (45 degrees), diameter, face width.
2) Spur gear Involute cutter number.
3) Angle to set the work in relation to the cutter ( possibly 45 degrees).
4) Depth of cut as measured from the large end of the cone.
5) Indexing, I calculated my ratios to be evenly divisible into 360 degrees eg. 36 teeth is every 10 degrees whereas 37 is not.
All this can easily be done on a manual machine.
In the attachments the calculations include offset and roll, this is for taking 3 passes and doing it properly but I don't think it is absolutely necessary.
Good luck and happy gear cutting.

[fizzissist]

Jay Leno on gears.... and machinists...

Calling All Machinists

[headsmess]

pulling apart the diffs in RC cars provides one with 3 sets of bevels... and at least one heli has a bevel tail rotor...i used it as a "tight access" screwdriver for motorcycle tuning!


theyre just die cast and the tooth profiles are almost just half rounds... brand being a large factor on quality of course.

the tooth profile accuracy required is dependant on the use...do you require constant velocity power transmission as provided by involutes, or are you happy with the slight jerkiness of teeth that are off kilter?

for most general purposes... i think that avoiding the endmill is the way to go.

mill with 4 axis... mount a LATHE tool on the head solidly, ground to as close to the involute you need. maybe its time to drill and tap some clamp holes in it? bit of pythagoras to get the X and Z travel correct to achieve the bevel angle, and the other calculations to achieve the Y offset and A rotation, subroutine that whole process, index, repeat ad nauseum. assuming you mounted the tool vertical and the rotary spins on the X...

[ame=http://www.youtube.com/watch?v=295Phu8GnjE]4th Axis production model. Makes same test part twice as fast! - YouTube[/ame]

mount the tool like this but parallel to the x good bit of coding and you could pop em off real quick. use the spline wizard as a basis to start on if you run mach3.

(that jay leno rants worth reading... here in au its worse...there werent many machines in the first place! always sad to see an old turret at the scraps, noone knows wtf it is or what it can do...)