GT and GM numbers not the same

[philbur]

I have a couple of questions I can't find the answers to:

1) Why are some of the generated parameters not the same in GT and GM. For example the tool diameter and dedendum for a 24DP 60T involute gear?

2) How do I get GT to give numbers in metric?

thanks
Phil :)

[ArtF]

Hi Phil:

  GM will give different numbers from GT in several area's. This is due to a change to a more accurate spec
document from which the formulas are derived. There are several "specs" out there for amny gear types,
many are approximations of formulaic results. GT should be considered the more accurate program.

  To use metric use the options dialog to select metric. All numbers will then be in metric.

Art

[philbur]

Thanks for that Art. I spent at least 30 minutes looking for an inch/metric option, I thought I had looked in every corner of the programme several times, but clearly not. It's one of those "can't see for looking" I guess.

I'm preparing to have a go at shaving a M1.0 60T helical involute gear in brass using a 1mm carbide end-mill :o. Hence my interest in minimum tool diameter.

Phil :)

[ArtF]

Hi Phil:

  I just double checked that combination to make sure youd be OK.. I suspect you will be though
you'll get a bit of a trochoid on the root with a  1mm tool. Im going to do have to redo that math on the
max tool diam though as Im pretty sure its wrong. Looks to me by measurement that a .8 is the correct
size for a mod 1 tooth.. Ill do a check on Monday and likely issue a fix.  (I think 1 1mm will work, but the root
area will be larger than onscreen..I have to consider why its wrong..)


Art

[philbur]

GM gives max tool diameter as 0.8398mm, so maybe the GM math is correct?

Phil :)

[philbur]

Here's the gear profile generated by GT overlaid on the gear profile from another source. The root width meaures 0.800 so the profile looks to be correct but it seems that the tool diameter calc is not correct.



Phil :)

[ArtF]

Phil:

  No question, GM is more correct. GT tries to generalize so it can be used with all types..pulleys, sprockets..etc..

Doesnt work very well for Evolutes though, SO Ill probably swap it to GM's old code just for spur gears..

Art

[philbur]

Hir Art, here's a comparison of GM, GT and another source:

red = another source
green = GT
blue = GM

Contrary to what you say GT seems to me to be more correct than GM, the only significant difference being the root depth, so I'm a bit confused when you say GM is more correct.

[ArtF]

Phil:

GM is only more correct on max tool Diam stats, GT is the more correct program in
other ways. GT's profiles are those used by Yamaha for example, GM's were based
on various documents found on the net and were therefore simple approximations.
  GT uses formulas more complex and that take into account interactions between
the wheel/pinion sizing. GT, except in tool diam specs, is the more accurate program.
(This is specially so in helicals and such..)

Art

[ArtF]

Hi Phil:

  Ive updated GT's tool diameter code so that round gears are a special case in which
all factors that gm took into account are also taken into account. The accuracy
is much higher now in those gear types.  This release will be a little while as Im working on
other features as well..

Thx
Art