31-tooth CNC'd Sherline gear

In order to cut some 62TPI threads with my Sherline threading attachment,
the best way is to have a 31-tooth gear in the (backgear?) train, which
Sherline does not supply (and Marcus, I may eventually get to threadmilling
but not quite yet...) So I set out to machine one.

I started searching around on the Net for a program that would at least
draw the right gear profile (31-tooth, 24-pitch, 14.5degree pressure angle)
and ran across Geargen, and old DOS shareware program (do a search for
geargn11.zip)

Not only did this program draw (DXF) gear profiles, it would also output
G-code! It was written by a tooling engineer wanting to EDM electrodes for
burning gear-shaped cavities in injection molds. Further, he assumed the
electrodes would be wire EDM'd themselves, so included a kerf compensation.

Hmmm... If the EDM kerf compensation gave a smaller electrode profile, and
I wanted to machine the *outside* of a real gear, I wonder if I could put
in a negative dimension for the kerf. Voila! A toolpath offset *outside*
the actual gear profile.

Another plus to the program, was that it would calculate the largest fillet
at the root between two adjacent tooth flanks. This number doubled would
indicate the largest diameter endmill that would machine the profile
correctly.

In my case, the endmill needed to be smaller than .052" diameter. I have a
set of miniature endmills, and chose 3/64" (.0469"). The flutes on this
endmill are only a little longer than 1/8", so I made the gear 1/8" thick
(compared to the "real" Sherline gears that are 3/16" thick aluminum.)

For the finishing pass I set the kerf compensation to .0469", and for a
roughing pass, I set the kerf compensation to the full .052", which
resulted in a profile about 2-1/2 thou outside the final profile. I made
roughing passes in .020" depth increments at 1 ipm cutting speed (I had
never used any of the tiny endmills before, and didn't want to take any
chances breaking it.) I made the single finishing pass at full depth,
again at 1 ipm.

The gear turned out beautifully. I had previously drilled and reamed the
bore (I cheated and did that part at lunch at work on a Bridgeport...) of
the blank, rough-bandsawed the OD, and on the Sherline
helically-interpolated a bore for a clamping sleeve to hold the blank in
place. The gear took most of Prarie Home Companion to cut last night
(maybe 1-1/2 hours, after about an hour of making the fixture and clamping
sleeve).

I have not actually used the gear yet, but have no doubt that it will
perform adequately for the 1/2" of 1/16 x 62TPI that I need to cut... :-)

I made an album called "31-tooth CNC'd Sherline gear" in the Photos section
of CAD_CAM_EDM_DRO at Yahoo groups, with a few photos of the gear and some
screen shots walking through the setup process in Geargen.

To sum up, I'll point you to a little recording (Pinky and the Brain fans
will recognize it):

http://www.geocities.com/TelevisionCity/Studio/2975/wav/yes.wav

With a big grin on my face,

Randy


Randy Gordon-Gilmore ,----.___________ ______________ _________________
ProtoTrains // = = === == || == == == = || == == == = == =|
Rio Vista, CA, USA /-O==O------------o==o------------o==o-----------o==o-'
zephyrus@... http://www.rickadee.net/~zephyrus