View Full Version : gear turner question

courtjester

07-22-2011, 06:03 PM

Ok I have a question for you whom cut your own gears. I have cut gears before (long time ago) using a single point cutter. I want to buy a involute cutter for making some gears.

The question is they sell cutter and talk about range for each cutter 1 to 8 like #1 is 135 to rack #2 is 55 to 134 and on.

http://www.wttool.com/index/page/product/product_id/29700/category_id/13807/category_chain/20929,19595,13807/product_name/High+Speed+Involute+Gear+Cutters+%28WT%29

It seems like a cutter is for a certain range of numbers of teeth on a gear. And the first number is the best number of teeth for that cutter.

Will some one help me understand so I can buy the right cutter

have fun

Tom

John Stevenson

07-22-2011, 06:20 PM

It seems like a cutter is for a certain range of numbers of teeth on a gear. And the first number is the best number of teeth for that cutter.

Will some one help me understand so I can buy the right cutter

have fun

Tom

Not being funny but you have answered your own question.

That cutter in the link is a 36DP No # 5 cutter which will cut from 21 to 25 teeth.

It's most accurate at 21 teeth but the errors at 25 are minute.

pipeclay

07-22-2011, 06:25 PM

You have answered your own question,each Cutter will cut a range of teeth with the Involute profile being closest to the 1st number given in the tooth range of that particular Cutter.

The Cutter that you would require is the 1 that falls in the range of the teeth you require for your gear.

Black_Moons

07-22-2011, 06:27 PM

You awnsered it yourself. Just note you gotta match Pressure angle (20 degrees or 14 degrees) and pitch to make gears that can mesh.

Zahnrad Kopf

07-22-2011, 08:20 PM

You answered your... ... Oh, never mind... :rolleyes:

courtjester

07-23-2011, 06:04 PM

thanks you all for the reply and that is what I though.

But where I was confused is if I cut a gear with 25 tooth (cutter 5) and cut another gear with 100 teeth ( cutter 2). And expect them to mesh together I would think the same cutter should be used. The outside diameter would be different do to the number of teeth. :o

John Stevenson

07-23-2011, 07:16 PM

The shape of the gear tooth is called an involute.

It is the curve given when an imaganary piece of string is unwound from the base circle of the gear.

http://mathworld.wolfram.com/images/gifs/involute.gif

Because the 25 tooth and the 100 tooth have different diameters of base circle the involute on the 25 tooth has a more pronounced shape than the 100 tooth.

In fact if you were to draw the involute of a gear with a base circle of infinity the 'curve' would be a straight line.

In other words you have drawn a rack.

courtjester

07-24-2011, 08:17 PM

Thanks

Tom

Edwin Dirnbeck

07-24-2011, 10:24 PM

thanks you all for the reply and that is what I though.

But where I was confused is if I cut a gear with 25 tooth (cutter 5) and cut another gear with 100 teeth ( cutter 2). And expect them to mesh together I would think the same cutter should be used. The outside diameter would be different do to the number of teeth. :o

The gear cutter is cutting the SPACE that is between the teeth.The shape of this SPACE between the teeth changes as the number of teeth increases or decreases.This is how I understand things. Good luck,Edwin

courtjester

07-25-2011, 12:21 PM

thanks Edwin

That makes sense to me now. So the difference between the cutters in a set is very small. Its not the size difference in the cutter but the shape.

And cutter #1 for rack cutting is that way because a rack's teeth are straight cut as the interlocking action is different.

have fun

Tom

Thomas Staubo

07-25-2011, 01:18 PM

It seems like a cutter is for a certain range of numbers of teeth on a gear. And the first number is the best number of teeth for that cutter.

I have never cut a gear, so bear with me.

But I have never understood why" the first number is the best number of teeth for that cutter".

E.g. the no.5 cutter cuts gears from 21 to 25 teeth. And supposedly it best cuts a 21 tooth gear.

Why not a 23 tooth gear which is in the middle of the range? :confused:

This means that a 25 tooth gear is not cut to an optimal profile?

Please enlighten me. :)

.

...I have never understood why "the first number is the best number of teeth for that cutter"....This means that a 25 tooth gear is not cut to an optimal profile?

To answer the second question first, technically you are correct. In practice the error is very small.

As for the first question, I speculate that if the error is made in this way, the effect is less serious than if the error is made the other way. Consider the 135-rack cutter: if it was correct for the rack (i.e. straight sides) and you tried to cut a 135 with it, the gear would look very stange.

I posted below on HMEM without an answer.

"I have seen somewhere in a book I have how the range for each of the eight B&S cutters is calculated. But can I remember which book it is? Or do I remember enough of my high school maths to work it out? Unfortunately no to both.

I think it has to do with summing a (geometric? arithmetic?) series from 12 (minimum teeth used for cutter) to infinity and then distributing it somehow eight ways.

This is pure speculation, and I could well be talking nonsense but I wonder if it has anything to do with relationship between the radius of curvature of the involute and the number of teeth? For a small tooth count, it's a small roc and hence its inverse is quite big; for a large tooth count, it's a huge roc and its inverse is very small. If you sum the inverses of the roc for 12 & 13 (two gears), it comes to a fixed quantity. For large tooth numbers, you will need a lot more than two gears to get the same sum."

Hope this gives food for thought.