Noob questions re: 4th axis

[JMac]

I've gotten both GM and GT up and running, and am trying to get set up to cut clock gears the way I have traditionally done it, and more recently have done it with hand-written code for my CNC. 

GT writes nice code to cut gears with a epicycloidal saw (Thornton cutter) in several passes, but one tooth at a time.  What I like to do is do the whole gear in the first pass, then repeat the whole thing in a second deeper pass, and then a final much finer pass at full depth.  I don't find a setting to do this. 

Alternatively, if I could set the depth of cut to a much reduced level, I could make several batches of code for different depths, and then paste them together.  This is pretty much what I've been doing with a spreadsheet to write code.

Finally, I can't seem to convince GT that I'd like to cut my full depth to a few thou deeper than it thinks is required.  For example, GT writes code for a 90t M.6 epicycloidal gear to a final depth of .0778 (I think, as I am writing this from memory), and my Thornton cutters seem to need a few thou more to give a fine point at the top. 

Thanks for any insight.

John MacArthur
http://laplaza.org/~jmac/

   

[ArtF]

Hi John:

>>...al much finer pass at full depth.  I don't find a setting to do this.

  I didnt add many setting for the 4th axis as yet, thats one that isnt there. Im not against adding it though at some point.. Im not sure the difference in the end result however, is it a machine specific thing or more of a preference?

>>Alternatively, if I could set the depth of cut to a much reduced level, I could make several batches of code for different depths, and then paste them together.  This is pretty much what I've been doing with a spreadsheet to write code.

  Now that one I could probably add without too much trouble, while the first requires more drastic changes, this one is more of a simple limiter.. Ill consider adding this one very soon.

>>Finally, I can't seem to convince GT that I'd like to cut my full depth to a few thou deeper than it thinks is required.

  This one I think you can cure fairly easily, there should be no downside to zeroing the Z axis properly, then adding .002 or whatever to the Z position in Mach3 or whatever controller is used.
With that type of cutter, there should be no downside... since it always lifts to clearance levels, it'd
be fairly safe in terms of collisions..So if you add .002 to the zeroed position, it will cut .002 deeper
in the end.

  GT does always cut to the full calculated depth from the tooth profile, so zeroing will always affect
the end depth. I will look at limiting this depth with an overall limitation soon or perhaps allowing a stepdown to occur after all teeth are done at one height...though I'm not sure if that's really any different from doing a tooth at a time. The main reason I complete one tooth before doing another is so that I can tell if the job is hosed by error before doing the entire gear. If one tooth looks bad I can stop. I've committed to separating the 2.5D and 4th axis screens to their own modules which would allow for more options without confusing things... Ill see how soon I can do that..hopefully soon, I have a few other little things t clear up first.

Thx
Art

[JMac]

"Hi John:

>>...al much finer pass at full depth.  I don't find a setting to do this.

  I didnt add many setting for the 4th axis as yet, thats one that isnt there. Im not against adding it though at some point.. Im not sure the difference in the end result however, is it a machine specific thing or more of a preference? "


Thanks for the reply, Art -- It's definitely a preference.  Most small mills will give a smoother finish with a thin final cut.  Mine is a Dyna with ballscrew conversion, keeping tolerances to a couple of tenths.  Control is Flashcut.


".So if you add .002 to the zeroed position, it will cut .002 deeper in the end."


I suppose (although not for sure) I would have gotten to that point pretty quickly.  In modern parlance:  "Duh"


 "GT does always cut to the full calculated depth from the tooth profile, so zeroing will always affect
the end depth. I will look at limiting this depth with an overall limitation soon or perhaps allowing a stepdown to occur after all teeth are done at one height...though I'm not sure if that's really any different from doing a tooth at a time. The main reason I complete one tooth before doing another is so that I can tell if the job is hosed by error before doing the entire gear. If one tooth looks bad I can stop."


Again, I'm looking for maximum possible perfection of shape in my teeth, and have found that, if you cut one to full depth, and then cut the adjacent one, it will deflect a tiny bit from the pressure of the cut.  These are fairly fine teeth, and I've had better luck doing partial cuts, then deeper ones, then final cuts.  This is, of course, the easiest way to screw something up, and CNC has helped as I get more prone to absent-mindedness....





thanks again,

John MacArthur