Interference below base line

[Boman33]

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 convenient 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 explaining 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.

[ArtF]

Hi:

    The lack of trochoidal 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 determines 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 considering 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 complexity 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 pronounced.

 

Art


 

[ArtF]

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 tolerances in building. Measure
the overlap, it may only be a thou or so..

  If your ever wanting to test a particular gear generator just generate two 5 tooth
gears and see how they mesh. (Example below). Youll notice I overcorrect when
numbers hit extremes such as 5 teeth, a tooth count far below recommended
minimum tooth counts for gears. I do this to ensure I mesh as the exact formula
is a bit unreliable when all meshing factors are considered. The best way to ensure
proper trochoids is by envelope theory ( virtual hobbing) as I do with the
functional and elliptical gears. This can leave a bit of a rough finish though so its
a tradeoff.

Art

[Boman33]

Thanks Art!
Fast replay as usual :)

Any suggestion how to really get a clean page in Gearotic and not remembering old gears?

[ArtF]

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 everything
back to defaults.

Art

[Boman33]

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 predefined by the current enclosure.  Working backwards it comes to M0.8
I have zero documentation on the old gears, they were cast in zinc alloy but they are wearing out.
Switching to steel.
Thank again!

[Boman33]

Oops, that does not work for me.  I tried it several times.
I am on Win-10 64bit on a regular desktop.

[ArtF]

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

[Boman33]

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