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Author Topic: Interference below base line  (Read 212 times)
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Boman33
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« on: May 22, 2019, 06:24:15 PM »

Spur gear design problem: 
I am designing a gear box and have been creating spur gears using the installed gear script in Fusion 360 and I am comparing it to Gearotic but ran into a separate problem with clearing old data.
The Fusion 360 is very convenien t and the result looks good.  I created a 12 and a 38 tooth gear M=0.8 and place them on the correct distance (20mm)  and motion linked them.  All looks great until I really zoom in and slowly rotate them.  Just after the tooth leaves the hypoid curve and goes below the base curve, it overlaps the other tooth.  Not good for proper operation!  As can be seen in the screen capture that section is a straight line where there should be an undercut.

I found a magazine article explainin g this problem: EDM Today: Fall 2017 issue.  I have attached part of that article.

How should this be handled correctly?  How is Gearotic handling it?

The tooling for the gears is US$8,000 so I want to make sure it is correct.


* Gear Problem Fusion-00.png (6.76 KB, 839x331 - viewed 21 times.)

* Gear Problem Fusion-01.png (145.03 KB, 1009x577 - viewed 23 times.)
* Gear Problem.pdf (11659.06 KB - downloaded 11 times.)
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ArtF
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« Reply #1 on: May 22, 2019, 08:41:23 PM »

Hi:

    The lack of trochoida l undercuts is often seen in various programs that make gears.
Generally, its felt that under a certain number of teeth trochoids become necessary,
though with larger tooth counts they become less important . In Gearotic I use a
trochoid generator that determine s how much to add, or if its necessary at all.
    Its been so long I cant give you a quick formula on it, I recall it was a fairly
complex formula to properly take them into account. Its a matter of consideri ng how
the top of the mating tooth will swing as it leaves the pocket. I always suggest one
carefully look at meshing for that reason, the complexit y makes it possible a trochoid may
not be enough so checking is always best. Youll notice as the tooth count reduces
in Gearotic the trochoid gets more and more pronounce d.

   

Art


   
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ArtF
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« Reply #2 on: May 22, 2019, 08:54:50 PM »

Bertho:

    You may be worrying too much on the example, depending on the scale your using
it may be your overlap from fusion is not all that much, Ive found in practice that very
small overlaps in such things often come out in in simple tolerance s in building. Measure
the overlap, it may only be a thou or so..

  If your ever wanting to test a particula r gear generator just generate two 5 tooth
gears and see how they mesh. (Example below). Youll notice I overcorre ct when
numbers hit extremes such as 5 teeth, a tooth count far below recommend ed
minimum tooth counts for gears. I do this to ensure I mesh as the exact formula
is a bit unreliabl e when all meshing factors are considere d. The best way to ensure
proper trochoids is by envelope theory ( virtual hobbing) as I do with the
functiona l and elliptica l gears. This can leave a bit of a rough finish though so its
a tradeoff.

Art


* trochs.jpg (77.99 KB, 1165x814 - viewed 25 times.)
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Boman33
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« Reply #3 on: May 22, 2019, 09:31:53 PM »

Thanks Art!
Fast replay as usual Smiley

Any suggestio n how to really get a clean page in Gearotic and not rememberi ng old gears?
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ArtF
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« Reply #4 on: May 23, 2019, 07:52:15 PM »

Bertho:

   Run Gearotic, open task manager in windows, kill the Gearotic process.
Run again, it will ask if you'd like to clear all settings. If you answer yes, it basically
erases the registry entry under the tag "Gearotic Thoughts.", which puts everythin g
back to defaults.

Art
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Boman33
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« Reply #5 on: May 23, 2019, 10:59:54 PM »

Thanks Art,
That is the info I needed.
I have to redesign a small gearbox , max gear diameter is only 35mm, and there are six gears.
The shaft to shaft distance is predefine d by the current enclosure .  Working backwards it comes to M0.8
I have zero documenta tion on the old gears, they were cast in zinc alloy but they are wearing out.
Switching to steel.
Thank again!
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Boman33
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« Reply #6 on: May 23, 2019, 11:06:10 PM »

Oops, that does not work for me.  I tried it several times.
I am on Win-10 64bit on a regular desktop.
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ArtF
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« Reply #7 on: May 24, 2019, 11:31:40 AM »

Wow, I guess when you do a close process in Win10 it must send a signal for a gracefull exit.

  In the interim, use regedit to find the key "Gearotic-Thoughts". Delete the folder,
a new one will be made on next startup. Ill add a reset to the menu.

Art
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Boman33
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« Reply #8 on: May 24, 2019, 01:26:23 PM »

Thanks Art,
I will do that.
Have a great Memorial weekend,
Bertho
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