Re: Does anyone know of some cheap gear designsoftware ?

Vol 25 of Machinery's Handbook says 18 and 25 teeth for 20 and 25
degree pressure angles respectively, are the minimums that can be
cut without undercutting. However, that does not mean or imply that
gears of those pressure angles aren't cut to lesser numbers of
teeth. In fact, it's done every day. The recommended minimum
number of teeth for a 14.5 degree PA spur gear is 12. Below that,
undercutting becomes extreme, but they're produced with 9 or 10
teeth fairly often, with the tradeoff of strength and wear being
accepted as a known factor. The example John gives of 12T/14.5PA is
indeed a real world example of a gear, and the program does not
generate the typical barrel shaped cross-section of a heavily
undercut gear. I would agree the program is for "pictorial"
purposes only, although the page itself would indicate otherwise.

Cliff

John S. said:

> > This page that Doug has linked to will produce a gear for CAD and
> > illustration purposes only. It's in no way accurate as regards

being

> > a true involute. One way to check this is try to draw a gear of

14.5PA

> > and a low number of teeth say 12, and look at the result.
>
> Hi again John:
>
> You could be correct as to the accuracy <or lack of...> of the

above link,

> I don't know.
>
> However regarding a 14.5 PA (pressure angle) gear of 12 teeth,
>
> Machinists handbook 23 (page 1772) says the MINIMUM number of teeth
> for these pressure angles should be:
>
> 20 degree PA - minimum teeth 18
> 25 degree PA - minimum teeth 12
>
> By extrapolation then, a 14.5 PA angle gear should have more than

18

> teeth (ie a 14.5PA gear of 12 teeth is undefined and so using it

for

> an example/counterexample is not relevant). In any case, for

small

> gears you probably want to be hobbing them (a valid cnc

operation!),

> otherwise the required mill bit dia would be exceedingly small!
>
> Anyway to bring it back 'on topic', I found geargen 1.1 at this

site:

>
> http://www.bookcase.com/library/software/msdos . desktop . cad .

html

>
> and I'll give it a try as you suggest.
>
> cheers
> Doug Fortune
> http://www.cncKITS.com