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April 30, 2017, 06:01:50 am


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Author Topic: best tactics to cnc mill an internal gear  (Read 204 times)
fourchette
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« on: April 17, 2017, 19:04:18 pm »

Hello,

I'm almost about to cnc mill my own planetary gearbox.



one question puzzles me.

For milling the outside ring, which is an internal involute gear (teeth are going "inside" the rind, not outside like a regular gear), I am faced with a classic "my endmill wont fit in the corners while later on another part must fit in there anyway" kinda problem (sorry for lacking the proper vocabulary but i'm pretty sure most of you must know what i'm talking about.


In general, when i face such issues, I design the part in CAD so that later on the end mill will always fit whatsoever.

A typical example would be


But for the involute gear ring, i would rather avoid weakning the part or do pretty much anything that might endanger the desired fonctionality.

Here my approach is different. I am making drawing the inside teeth slighly deeper so that the extra clearance is enough so that the part that will remain after the endmill has done its job cannot collide with the planet gears

screenshot below. the ring in green. the circle corresponds to my 1mm endmill. As you can see it should be fine (i hope so)

My question is : would you have better tactics in such a situation ?


Any idea is welcome.

thanks














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lloydsp
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« Reply #1 on: April 17, 2017, 21:06:00 pm »

That works!  Another approach is to do a 'finishing pass' with a much smaller cutter.

Lloyd
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Bubba
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« Reply #2 on: April 17, 2017, 21:29:27 pm »

As Lloyd said. It not only will work but it is common practice to allow for little clearance.
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kvom
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« Reply #3 on: April 18, 2017, 01:30:04 am »

Yes.  See definition of addendum and dedendum wrt gears.
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kiwibrick
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« Reply #4 on: April 21, 2017, 10:40:13 am »

Nice solution!
The thing I had a problem with with machining gears wasn't the tool diameter but the tool length, easy to get a 1mm end mill but not at 6mm doc, never did find any that long
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