Seeking Knowlege on the Machining of a Bevel Gear.

[Pipescs]

Long Subject Line.

I am delving into the machining of a number of bevel gears using a milling machine and am seeking info from anyone that has actually accomplished it. (Preferably more than once)

The equipment I have to work with so far is a Bridgeport, and a Brown and Sharp Dividing head.

My research so far has led me to a copy of the U.S. Navy Machinery Repairman 1st Class and Chief Manual (1988 edition) and a 1937 copy of Gear Cutting Practice by Colvin and Stanley. Bot explain the process, almost exactly.

Any contacts greatly appreciated.

[rklopp]

You want to read Ivan Law’s book Gears & Gearcutting. He explains a practical way of making bevel gears on home shop equipment.


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[Rich_Carlstedt]

Charlie, I have cut many bevel gears in my shop using my Bridgeport and my 1890 Brown and Sharp Dividing Head
Cutting bevel gears requires more setup and calculations, but it is not that bad.
The gears you cut will not true bevel gears because they do not have a conical form, but will have a straight (Parallel ) depth, however they perform wonderfully for most applications.
Here is the formula , which is also in the Ivan Law Book.
This was reviewed in ME a number of years ago ( 11-1990 ?)
When doing a normal spur gear , you make one pass per tooth ( Gullet really)
For Parallel bevel Gears you make 3 passes per gullet . The first is normal , and then you rotate the gear up and raise the cutter and make a pass.
Then you lower the gear a measured amount and lower the cutter and do a pass
You basically have to multiply the number of teeth ( say 12 for example) by 4 and divide that into 360 degrees
So instead of rotating (360/12) 30 degrees for a spur gear, you do ( 12 x 4 ) every 7 1/2 Degrees
Send me a note

Rich

[Rich_Carlstedt]

[Pipescs]

Thank you for the input.

My one confusion so far has been the mention of Special Cutters made specifically for doing the bevel gears.

In the book, "Gear Cutting Practice" by Colvin and Stanly" there is the statement

"Selection of Cutter for Bevel Gears- The length of the teeth on the face on the bevel gears is not ordinarily more than one-third the apex distance, Ab, Fig 92 and cutters usually carried in stock are suitable for this face. If the face is longer than one-third the apex distance, special thin cutters must be made"

If this is true, the gear I am looking at cutting (40 teeth gear driven by a 20 tooth pinion gear) should be doable with a standard No. 2 cutter. (this according to the chart in the above book)

I will still be looking for a cutter labeled "Bevel" on the thought that I will have more space to work with.

[Rich_Carlstedt]

Charlie, I think you are getting too deep into that book
What they are referring to with the 1/3 rd comment is the face of the gear
In the sketch below, I show the 100 % "Length" they refer to and when they say "one third" or 33 %
When you go more than 33 % , the gullet ( or depth or root -other names) is so shallow that the tooth form changes and the gullet
narrows and needs a thinner cutter.
A 100 % bevel is almost impossible ( But you see them in some coffee grinders )
I don't think i have ever cut more than a 25 % bevel as I recall.
The bottom sketch shows what you get in a Home shop, unless you have a gear shaper (industrial) which makes a true bevel gear
The parallel method works for most applications. I doubt you will find a "bevel Gear cutter" on the market unless it is for s gear generating ( shaping ) machine
Using the sketch (earlier post) to calculate moves produces a gullet that is parallel to the tooth crest and it works very well

Rich

[Pipescs]

So from that I think you concur that I can use a standard cutter?

[Harold_V]

I am far from literate in regards to gear cutting, but it's pretty obvious to me that any attempt at a bevel gear without the use of a gear shaper will result in teeth that make contact in only a narrow portion of the teeth (thus the restricted width). Such contact will restrict the amount of power than can be transmitted without damaging the gear.

Note that I did not say such gears won't work, but you should be able to form an image in the mind's eye of what they are going through as they rotate. Bevel gears, do not have teeth that are parallel at all times in the way spur gears do, have an ever changing angle as the gear rotates. Some compensation is allowed by the angle of the gear face, but that doesn't address fully the ever changing gear form as it relates to the rotation of the gears. A shaper is capable of that, while any fixed form (gear cutter) is not. A true bevel gear tooth is not a consistent form.

H

[Rich_Carlstedt]

So from that I think you concur that I can use a standard cutter?

Absolutely !

But, the Pitch you use ( ie 16 or 20 or ?) depends on the small end of the bevel- not the large end !
Use the small end to determine the Pitch and the number of teeth
All your offsetting is to generate the larger diameter tooth and gullet.
Rich

[Pipescs]

Both the Navy Repairman Manual and the Colvin and Stanley Book explain the fact that a perfect bevel gear cannot be cut on a mill but a close approximation can be produced thru manipulation.

This is what Rich is explaining in the offset and rotation cuts. These are explained in both books but his first drawing he posted helped me figure the amount of rotation and table movement I would need.


There are also diagrams in both books to show you how to file the ends of the teeth to makeup for the mills inability to cut the correct tooth shapes.

This is I believe the answer to Harold's inputs

[Pipescs]

Rich,

This figure helped me understand your formulas

These formula are also in both volume but yours is more inline with my education level.

I am looking at a 5 inch diameter gear with 40 teeth being driven by a pinion with 20 teeth. (Cutter Selection Chart identifies the 40 tooth gear to be cut with a No. 2 and the 20 tooth Pinion to be cut with a No. 5)

This to give a gear ration of 2:1.

In all you and the books say,

First cut is made on all forty teeth with the cutter and table centered.

Second cut made on all forty after moving the table and rotating the indexing head a prescribed amount. This would now mean the cutter is exiting the cut on the inboard side in the same place but cutting the outside of the gear to a wider opening.

Third cut made on all forty after moving the table and rotating the indexing head a prescribe amount the other way past the original center.

All together 120 passes

then blue and file the profiles on the inboard side.

Do you make your teeth cuts to depth in one pass?

[Rich_Carlstedt]

Yes Charlie I go to full depth .
One correction to your comments
You have it by the way !
On the second and third pass, you raise OR lower the cutter as well as rotate ( so it matches the small end exit point)
This means it cuts a taper . Lots on the large end and nothing at the small end.

Here are a couple of pictures to show you some gears I cut
In the first you see that the tooth profile AND gullet at the small end are perfect.
But note the broad tooth top on the large end !
The gullet is straight and constant depth on this first pass.
On the second photo is a finished Quadrant Gear ( gear portion anyway )
Note the tooth form is tapered and the gullet is also tapered
Hope this helps
Rich