Author Topic: Helical gear cutting question  (Read 1423 times)

Offline Don1966

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Re: Helical gear cutting question
« Reply #15 on: March 19, 2019, 12:26:22 AM »
Hi Don,
Because both gears fell in the mid range of the cutters I split the tooth count and drew up the tooth profile for that count. It's a little shy on the one and heavy on the other but they're so small it's hard to make the cutters exact anyway. To answer your question, yes, I did use your numbers.
gbritnell
George can you post the cad drawings you did I would like to see them?

Don

Offline Doc

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Re: Helical gear cutting question
« Reply #16 on: March 19, 2019, 03:56:51 AM »
Don I have used you spread sheet a few times easier than doing the math myself I like the way you have them set up. Thanks!

Offline jadge

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Re: Helical gear cutting question
« Reply #17 on: March 19, 2019, 11:34:28 AM »
The question is: how does one find the center of the shaft/gear blank at the point of contact with the cutter? Maybe it's right in front of me and I can't see it but try as I might I can't solve this.
Remember the key word is 'ACCURATELY'

An interesting question; and I think the answer is, there isn't one. Or at least I haven't found it.

Before I cut my helical gears using a universal mill and dividing head it was a question that puzzled me. Quite a lot of professional machining books simply say centre the cutter over the blank and then swivel the table. As far as I can see that only works if the table pivot point is below the intersection of the two axes. Some books, like those from B&S, talk about taking a trial cut in a test bar, after sviwelling the table, and checking that the cut is symmetric.

Before I cut my gears I swivelled the table and then set the front face of one tooth on the involute cutter vertical. That was then moved to be midpoint of the gear blank, on the axis of the blank. I then used the trapped rule trick to set the cutter midpoint perpendicular to the gear axis. May not be "accurate", but the finished gears mesh well when placed on their theoretical centres.

Andrew

Offline gbritnell

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Re: Helical gear cutting question
« Reply #18 on: March 19, 2019, 02:05:30 PM »
Andrew,
Thanks for the reply. Those of us that have been into machining for a long time know all the ways to set up jobs, wiggling, bumping, touching off, indicating and in some cases eyeballing. My question was posed more to see if there was something I was missing. There's always that old timer out there that says "oh you just do it like this." I have several books that I downloaded from the internet, one being the Brown and Sharp book and the other the U.S. Navy Machinists book. They both cover cutting of basically all types of gear forms with the attendant mathematics. The one thing they don't cover is how to pick up the center of a shaft that is at an angle to the axes.
In my years of machining I haven't come across anything that I couldn't solve, one way or the other. It might have ended up as some convoluted setup but it got the job done.
Thanks everyone for your replies.
gbritnell
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Offline 90LX_Notch

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Re: Helical gear cutting question
« Reply #19 on: March 19, 2019, 11:26:43 PM »
George-

I believe this will work well (if I described the math right).

If you mount an indicator on a mag base or surface gage and slide it on the mill table or vise, you should be able to zero to the top corner of your blank.  Then without adjusting the indicator use it to zero the top of your cutter.  One this is established, zero your Z dial.  Then it is half the cutter width minus* half the product of the cosine of the helix angle multiplied by the blank diameter.

A back plunger or drop indicator are better in this situation then a test indicator due to flat face of the tip.

-Bob

*Edit- "plus" changed to "minus".  Since the top of the cutter is being used instead of touching off with the bottom, 1/2 the cutter width needs to be subtracted.
« Last Edit: March 20, 2019, 12:05:42 AM by 90LX_Notch »
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