Gear cutting advice needed.

[Rik ShawParticipant @rikshaw]

[Rik ShawParticipant @rikshaw]

I need to make a couple of pairs of small 3/8" pitch diameter brass governor bevel gears for my latest project but I have no gear cutting experience at all so any hints / tips would be appreciated.

For now I just need to know if |I have the right kit to do the job. I do have a horizontal / vertical rotary table with dividing attachment which I can use on my milling machine and thought that I could use this in the vertical mode (chuck fitted) mounted on the milling table at 45 degrees in order to cut the 12 teeth. Does that sound OK?

What sort of cutter would I need?

Rik

[Bob Brown 1Participant @bobbrown1]

May be easier to buy in the gears and modify to suit as cutting bevel gears is not any where as straight forward as a spur gear.

[JasonBModerator @jasonb]

The gears for a governor don't usually have to be too exact if its just a stationary engine that will be used for display, you could make a single point cutter quite easily or just use an off the shelf involute one. What dia are teh actual gears. You probably want something like a number 8  32DP cutter, rest of the setup is like this.

Edited By JasonB on 26/05/2014 11:34:22

Edited By JasonB on 26/05/2014 11:35:27

[Neil WyattModerator @neilwyatt]

I find making bevel gears the most rewarding of all the parts of this hobby, for some reason.

It's worth getting Ivan Law's Workshop Practice series book 'Gear and Gear Cutting'.

The bevel gears below were made using his techniques for making cutters and actually doing the cutting, so aside from my time they cost pence each. Note there is a minor error tin the book that only effects pairs of gears not at 90-degrees to each other.

Neil

[Bob Brown 1Participant @bobbrown1]

The problem as I see it is you can not correctly machine bevel gears on a mill or shaper, OK you can sort of make them and they may function. The problem stems from the tooth pitch which changes from outside to inside, hence when you look at the pictures above you start off with a flat on the top of the tooth and end up with a point as you get to the centre.

If they are small then these are cheap enough **LINK**

[Anonymous]

Before I get covered in detritus cleaning out the milling head on my Bridgeport, prior to replacing the broken drive belt here's my summary on the design and manufacture of bevel gears. It's based on my article in the current issue of MEW (216). Was that a trumpet I heard?

Bob is correct in that for a straight tooth bevel gear the tooth size changes from a maximum at the outer edge to a point where the two axes intersect. The shape does not change, only the size.

Bevel gears are normally designed using the diametric pitch at other outer face.

Unlike a spur gear the involute tooth form for a bevel gear is not directly related to the number of teeth in the gear, but to the number of teeth divided by the cosine of the pitch cone – Tregold's approximation.

Bob is also correct that a true bevel gear cannot be cut on a milling machine. However they can be cut on a shaper, with the aid of a Bilgram bevel gear generator.

There are two methods for cutting an approximation to a bevel gear on a milling machine using involute cutters. Both methods use three passes, one through the middle of the space between teeth and two with the blank offset and rotated so that the outer space can be enlarged without affecting the inner edge space. For smaller diameter gears thw first pass can be omitted.

The first method takes the standard design of bevel gear, ie, using the DP at the outer edge. An involute cutter of the appropriate DP and number of teeth is used based on the calculations at the outer edge. However, these are not standard involute cutters, they are narrower than standard, so that while they have the curve associated with the outer tooth shape they are narrow enough to fit through the gap at the small end. These cutters are normally stamped 'BEVEL'. They do not seem to be available now. This method results in teeth that do not have enough curvature at the small end, requiring the use of a file. I have made bevel gears using this method, but I did have the resources of the main Royal Aircraft Establishment machine shop behind me, so was able to borrow the required cutter.

The second method is called the parallel depth method. Here the depth of the tooth is constant, not varying as in a true bevel gear. There is one other crucial difference, the design is done using the diametric pitch at the small end, so a standard involute cutter can be used. This is fine, except that it can be problematic if the gears are to be used in an existing design where the outer diameter will almost certainly be the controlling dimension. Nice round numbers at the inner edge are unlikely to result in nice round numbers at the outer edge.

I cut the gears for my traction engine differentials on a 4-axis CNC mill, so they are true bevel gears. However this method will not work with the bevel gears needed for the governor, as the required ballnose cutter will be too small. I have looked at replacing the gears with those designed using the parallel depth method, but it's a PITA due to the afore mentioned calculation issues. I plan to buy a copy of Gearotic and use the CNC mill to machine the governor bevel gears, but with the crucial difference that the rotary table will be tilted over, thus obviating the need for a ball nose cutter.

All this information is in the book by Ivan Law if you read it carefully.

Regards,

Andrew

[Rik ShawParticipant @rikshaw]

Much help here and thanks to you all. Bob Brown 1, I believe your link might well solve my problem and Andrew, I'll maybe have a read of Ivan Law when the headache I got reading your post has dissipated. (Yes I know, I did ask didn't I?)

[Neil WyattModerator @neilwyatt]

Rapid have a new definition of 30-tooth bevel gears – fifteen teeth on each of the pair!

Neil

[John Stevenson 1Participant @johnstevenson1]

Bit like the weather forecast then when they say it will be 20 degrees but they mean 10 in the morning and 10 in the afternoon.

[JasonBModerator @jasonb]

Click the expand button, you are only seeing the top of a list, full spec for all on the main list. Though I did have hopes that the 15T twin set may mesh with that 31.5T gear I cut a while ago

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