i've been researching how to make small involute spur gears (96 or 120
DP, 60-120 or so teeth) using my equipment: a taig CNC with 4th axis
rotary table + taig lathe (not CNC).
i have a book on making gears on order ("Gears and Gear Cutting" by
Ivan Law) as recommended by people who responded to my previous
post...but i was researching a bit on the net and came across several
manufacturers of hobs. i was wondering if i can purchase a hob (rather
than making one) and use it with my CNC and/or lathe in order to cut
here's a link to one company that sells hobs (not meant to be
promotional, just an example):
i'm not averse to purchasing a bit of equipment...so i was wondering
if i could use something like this to cut gears with my equipment. the
homebrew techniques seems to focus on building the hob...i thought
perhaps i could just save some time and purchase one.
any help would be appreciated.
Hobs are faster, by far, and don't require cnc.
While I have never used an actual hob, I have cut worm wheels to ANS
threads using a tap.
The first time I did it, I was in a shop class and the instructor wanted
to see it cut. I touched off the cutter just to get a good start before
calling him over, and I had cut to depth before I realized it! I had to
set up another blank and show him and the rest of the class. It was a very
On the Beautiful Florida Space Coast, right beside the Kennedy Space Center, USA
Be sitting when you see the price !
On the other hand, you could also use some of that CNC stuff to make a
A CNC co-ordinated 4th axis would be pretty sweet setup for hobbing
gears with a commercial (or shopmade ) hob.
IIRC there was a fair bit in print on this subject, in Model Engineer,
or Model Engineer's Workshop magazines a while back.
You're on the right track, but not quite there yet.
I am contemplating the same action you are, hobbing gears using cnc
Pre-enjoyed hobs are quite reasonable in price for the small sizes; I
seem to recall $US 25 to 75 each, depending on the remaining life
expectancy. Considering that one hob will cut all gear sizes from
minimum teeth pinion to rack, of a given pitch and pressure angle, the
cost is reasonable.
As for cnc equipment, neither the cnc lathe nor a 4 axis mill will do
the job UNLESS the mill has an encoder on the cutter spindle so that
you can specify the "gear ratio" between the cutter spindle and the
rotary / indexing spindle.
Consider what it is you are trying to achieve:
The hob must be driven at an rpm that is related to the number of
teeth you wish to hob; with a single start thread hob, cutting say 57
teeth, the hob needs to make 57 revolutions for one revolution of the
work (gear blank) spindle.
In a mechanical hobbing machine this is achieved through a gear train,
a different gear train is required for each number of teeth to be
hobbed. This is where cnc shines because you don't have to muck about
with changing gear trains.
Consequently using cnc software designed for a screw-cutting cnc lathe
will have the attributes required for hobbing.
Add an encoder to the milling machine spindle, have a stepper motor
driven work spindle, connect everything with the cnc lathe software,
and hardware, and you are ready to hob. You will need to figure out
the parameters to achieve the correct "gear ratio" between the two
Just keep in mind that to hob parallel tooth spur gears requires the
hob spindle to be inclined with respect to the work spindle to an
angle called the helix or lead angle. The hob will have this angle
engraved on it. With small hobs the vertical axis milling machine is
handy because this angle is easily adjusted by swivelling the vertical
head. To hob parallel tooth spur gears on a horizontal mill (required
for big hobs) a universal milling machine is required because it
permits swivelling of the table to set it's axis to the helix angle
with respect to the work spindle.
Anyhow, I encourage you to give it a try and keep us informed on the
method you choose, and how you make out.
Perhaps - wire edm could work -the cost maybe higher than you thought -
but we have been
successfull in short gear type jobs for keyways, teeth, ect.
Just a thought--visit our web site for more info.....