Re: Does anyone know of some cheap gear designsoftware ?

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, "metlmunchr"
><metlmunchr@y...> wrote:
> 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
>

http://homepage.ntlworld.com/stevenson.engineers/lsteve/files/12tgear.
jpg
Shows the difference between the program in black only generating a
correct form above the pitch line and a true 14.5 PA tooth form in
blue.

For a laugh go to the program
http://sinfor.lcs.mit.edu:8180/pcbmill/gears
and follow the instruction at the bottom of the page to draw an
internal gear by reversing the addendum and dedendum values and see
what you get.
Remember it's an internal gear so those square blocks are the teeth
that have to roll in mesh <BG>
Can you say Klunk ?

John S.

> 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