GearHeads Forum

Art, drink coffee before reading this,

I looked around and learned that 3d tool comp is pretty much impossible -- the tool comp needs to happen when the geometric data is converted into the g-code, which pretty much means  it is not usable. Once it is g-code, the only way to do 3d tool comp would  be to compare adjacent tool paths, which is equivalent to converting back to geometric data, which would be dumb.

Producing the spherically concentric tool paths for a bevel gear is vaguely comparable to producing them for a 2-1/2 D spur gear tool path, except instead of a different Z for each deeper pass, there is a different spherical radius for each pass. (There is also a headache in thinking about what the spherical version of an involute is--"spherical involute" or "octoid").

There is a kmoddl that is related:
http://kmoddl.library.cornell.edu/model.php?m=242

These spherical shapes are as much like a cycloid as they are like an involute.

Justin:

>>Art, drink coffee before reading this,

  As it turns out, I just did.

>>I looked around and learned that 3d tool comp is pretty much impossible


  Not really, I understand it has been done in some 5 axis systems.. The GCode as its interpreted can create a solid model internally, which is then used as a
collision detector to ensure the toolpath remains out of collision with the object the Gcode describes. Very expensive system mind you..


>> the tool comp needs to happen when the geometric data is converted into the g-code, which pretty much means  it is not usable. Once it is g-code, the only way to do 3d tool comp would  be to compare adjacent tool paths, which is equivalent to convertin g back to geometric data, which would be dumb.

  I agree that tryng to tell an ordinary interpreter to "tool comp" a 3d object is not the best way, the program generating the Gcode has all the information
it needs so its the Gcode generator that shoudl generate all such offsets as it does the generation.

>>Producing the spherically concentric tool paths for a bevel gear is vaguely comparable to producing them for a 2-1/2 D spur gear tool path, except instead of a different Z for each deeper pass, there is a different spherical radius for each pass. (There is also a headache in thinking about what the spherical version of an involute is--"spherical involute" or "octoid").

  Actually, though in your idea the paths are spherical, the tool shape remians involute..or whatever the shape of the original tooth. Since the Z motion is not
straight up and down, but also in a spherical motion, the produced shape of the tooth remians as the original tooth shape, an involute. It would only change the tooth
shape if the path was circular, not spherical. But if your describing the end motion of the tool, then yes, I agree, the motion woudl be strange, Id describe it more
as a tilted involute, tilted to the bevel angle.  Since bevels are not supposed to include undercuts, ( tooth count on a bevel should always be high enough to ensure no troichoid is used generally ), the scheme should work.  Very hard to say though, Ive found the devil is always in the details on such things..

Art

Art,
Perhaps beer would be a better lubricant than coffee for translating Euclidean topology to spherical topology...

--Justin

Something a bit stronger than beer I fear...

  Actually, Im doing a lot of work now on the new "Gearotic Thoughts", its a lot of 3d modeling work, so Im learnign reams,
it will likely help me quite a bit when I go to new Gcode generators for things like bevels..We'll carry this discussion
on when I reach that point..


Thx for the thoughts,
Art