Re: [CAD_CAM_EDM_DRO] 4th axis gear cutting question

Bill Kichman wrote:

>I am nearly finished with my ballscrew conversion, but a question has been
>nagging me....I want to do some gear cutting projects as a first real
>project on the cnc setup, and with little real machining background
>experience, am wondering whether this is a viable project. What I want to
>do specifically is cut a set of change gears for my South Bend toolroom
>lathe to permit cutting metric threads.
>I located a web based utility that permits entering gear pitch information
>and it spits out a really cool dxf you can save to the hard drive and finish
>tailoring to suit your thickness and other particular 3d info before running
>in a cam app (VisualMill in my case). But is this a reasonable project for
>my full size cnc setup and what cutters should be used?
>Also is this a reasonable job to do using only 3 axes? I don;t ahve the
>rotary table converted just yet.
>
>

If you need to make just one special gear, it makes sense to cut the
gear profile
with CNC. It will be VERY slow, however, for a 3/8" thick gear. If you
want to
make several gears, and especially if they are in the range of 100 to
127 teeth,
I think it will take forever. There are two ways to go from here. One
is to
find a gear of the right pitch and approximate tooth count, and use it
to make
a single-tooth form cutter. You just fit the tooth to the sample gear, and
grind it a little at a time until it makes a very close fit to the
sample. You
then mount this tool in a fly-cutter and cut the teeth in one pass (at
least for fine
teeth).

The other way to do it is to buy (hopefully on eBay) a gear cutter for the
right gear pitch and tooth count. You mount this in a stub milling arbor,
and cut the gear very nicely!

If you think you can profile cut a small 100+ tooth gear with it flat on
the table
of a vertical mill, well it may not be practical. To obtain the correct
tooth
form, you'd need a VERY small diameter cutter. Something in the range of
.031" might work, but it probably needs an even smaller cutter to perfectly
form the teeth. (If it was a 3" diameter 10-tooth gear, it would be no
problem.)
Such a small cutter can't cut a tall vertical wall, as you'd get on the
typical
3/8" thick threading gear. So, you have to have the gear mounted on a
horizontal arbor, and either use a tiny tool to cut straight down on the
teeth,
or a disc-like gear cutter that cuts on the side of the gear. This is
the most
productive way to do it.

The complication of this is you need a rotary table or indexing head to turn
the gear.

Jon