Machinery's handbook confusion

[dhansen]

Machinery's handbook confusion

Today, 10:57 AM




In the 27 and 17 editions it says that the cutting angle on a 16 tooth, 10DP bevel gear is the pitch cone angle (which I have) minus the dedendum angle (which I also have) giving me a cutting angle of 18.72687144 degrees. When you read the section on thinning the gear tooth it says that the cutting angle is the pitch cone angle minus the addendum angle giving me a cutting angle of
19.14342808. I am confused. Do you use a different cutting angle when thinning the teeth?

[J Tiers]

If you are form cutting the gears, with the proper machinery to taper the teeth, etc, then the pitch on the "contact cone" (the pitch diameter cone) would be different than the addendum and dedendum cones. In that case you would be actually cutting them that way.

Since you are doing constant height" teeth, with a fixed form tool, that would not apply. I have always just done the offset, and then had to adjust the teeth a small amount with a file, no fussing with other angles, just straight down the pitch diameter cone, 45 deg for a pair of mitre gears.

[dhansen]

If you are form cutting the gears, with the proper machinery to taper the teeth, etc, then the pitch on the "contact cone" (the pitch diameter cone) would be different than the addendum and dedendum cones. In that case you would be actually cutting them that way.

Since you are doing constant height" teeth, with a fixed form tool, that would not apply. I have always just done the offset, and then had to adjust the teeth a small amount with a file, no fussing with other angles, just straight down the pitch diameter cone, 45 deg for a pair of mitre gears.

Thanks for the response. I believe the cutting angle for the original cut is for the root of the tooth due to the bevel. If I understand you correctly, you are saying that the offset is done at the same cutting angle. I am actually cutting the teeth with a 10dp #6 standard gear tooth cutter as the calculations from the Machinery' Handbook specify. This is not a bevel gear tooth cutter. I made a bevel gear this way in 1967 when I was in machining school, but the memory fades about the offset technique.

[J Tiers]

Thanks for the response. I believe the cutting angle for the original cut is for the root of the tooth due to the bevel. If I understand you correctly, you are saying that the offset is done at the same cutting angle.

That is certainly what I did about the offset. I actually did the cutting angle for the "pitch circle cone" not the dedendum (root of tooth) angle, if those are based on the "proper" form. It is my belief that doing that will ensure that the two gears will properly engage with the pitch circle (cone) of each tangent at the nominal spacing. For a pair to do a 90 degree angle of driven and driver, that would be a 45 degree angle on each.

The cutters make a constant tooth height, and so all the "cones" are "parallel" with a gear cut with a form cutter, and the apex of each cone is at a different point. That is not true for a "proper form" bevel gear, where all the cones come to the same point as apex.

It is good to remember that the cutters cut past the tooth root, to create a clearance. That amount varies with cutter manufacturer, and is not a constant. But the pitch line is a known constant reference. So using the pitch line, and its extension into the "pitch cone", means that the two gears will properly engage. It also makes the theoretical point of intersection of the two shafts at the same point as with a "proper form" bevel gear.

[dhansen]

That is certainly what I did about the offset. I actually did the cutting angle for the "pitch circle cone" not the dedendum (root of tooth) angle, if those are based on the "proper" form. It is my belief that doing that will ensure that the two gears will properly engage with the pitch circle (cone) of each tangent at the nominal spacing. For a pair to do a 90 degree angle of driven and driver, that would be a 45 degree angle on each.

The cutters make a constant tooth height, and so all the "cones" are "parallel" with a gear cut with a form cutter, and the apex of each cone is at a different point. That is not true for a "proper form" bevel gear, where all the cones come to the same point as apex.

It is good to remember that the cutters cut past the tooth root, to create a clearance. That amount varies with cutter manufacturer, and is not a constant. But the pitch line is a known constant reference. So using the pitch line, and its extension into the "pitch cone", means that the two gears will properly engage. It also makes the theoretical point of intersection of the two shafts at the same point as with a "proper form" bevel gear.

Great explanation. Thank you