Hmmmm...

I don't know about the Cen-Tec rotary thingie; but I suppose I
qualify as one of those old farts who once knew a thing or two about
ingenuity with manual machining.

And I'm wondering if you need an indexer at all. Somehow, you're
going to grind the appropriate shape on the bit for your fly-cutter,
right? That may take some work, and some careful setups, and you might
even want to make a dummy shape out of any old chunk of cheap steel,
before you try to finish the tool bit. And, the last version of this
test piece ought to be exactly the same shape as the final form tool.
And it ought to fit perfectly (or nearly so) into any cut you make with
the actual tool bit.

If that's the case, then the test tool itself becomes an ideal
indexing stop that could be used on the actual gear, as the gear is
machined.

Suppose you just set the gear blank on an arbor and cut the first
tooth (or space between teeth, actually) wherever you like. There's no
need for any particular orientation of the first cut. (Or if there is
some need, like alignment with a key slot, then I'm sure you can figure
out how to get the first cut positioned properly.) Now suppose, just
for the sake of example, and that you're doing all the cutting at the
top of the gear - the 12 o'clock position as you look at the gear along
it's centerline. If that dummy test tool bit I mentioned above were
installed on some kind of little slide, and if that little slide was
mounted underneath the gear (at the 6 o'clock position) then, after the
first cut was done, the gear could be rotated 180 degrees, and it's
postion could be accurately established by engaging the dummy tool bit
with the slot that's already been cut in the gear. Then another cut
could be made at 12 o'clock, and the result would be two cuts, perfectly
spaced at 180 degrees.

And if the dummy tool bit were postioned at 3 o'clock, then you
could make four equally spaced cuts with equal ease. And if you put the
dummy tool bit just 30 degrees away from the 12 o'clock postion, then
you could have 12 equally spaced cuts, just by indexing each sucessive
slot that's cut, so that it can be engaged with the dummy tool stop.

If you have a strange number of teeth, or a large enough number that
the dummy tool stop can't be positioned close enough to the top of the
gear, then you'll have to do some arithmetic. Putting the stop at 40
degrees from the top of the gear, for example, will give a kind of
progressive indexing, as follows:

0 degrees (the first cut, of course)
50 degrees
100 degrees
150 degrees
200 degrees
250 degrees
300 degrees
350 degrees
400 degrees

The 400 degree number is actually 40 degrees past 12 o'clock, after
one full rev, even though you haven't actually set up a 40 degree index
increment. Continuing to index in the same way, you'll eventually get a
full set of cuts spaced just 10 degrees apart (36 total) without needing
to put the stop only 10 degrees away from the cutter, near the top of
the gear.

With a little additional ingenuity, and some other angles that don't
divide equally into 360 degrees, you ought to be able to make any number
of cuts that you like, without ever needing an index plate, a master
gear, or anything else of that type.

The actual setup of proper angles for the stop might be done with a
sine bar and an indicator, or with simple little angle blocks made to
serve as masters for each tooth number.

And, of course, this method might serve as the seed for even more
ingenious schemes, if your students are creative enough.

Hope this helps!

KG



wrote:
While this properly belongs in rec.crafts.metalworking, I am
cross-posting to alt.machines.cnc as that is one of the most
knowledgeable groups on the web as their contributors have an extensive
traditional machining backgrounds in addition to cnc.

Info needed

Background:

I am hosting more than teaching a community college class in basic
manual machining to a small group of very dedicated and enthusiastic
students. This class is stressing traditional machining techniques
emphasizing ingenuity rather than attachments. While I do have one
student that is taking the class from a hobby or home shop standpoint,
the others are looking to change careers (gun-smithing) or are involved
in maintenance, generally involving obsolescent equipment for which
parts are no longer available or available only with unacceptably long
lead times.

In an end of semester discussion about what topics were of interest for
the spring session, production of one-off gears came up. I have made a
few simple spur gears using a dividing head and an involute gear cutter
on a Van Dorne horizontal milling machine with both the cutter and gear
blank on arbors.

The class has available two V-10 Emco lathes, one of which has the
vertical milling attachment.

From a review of the literature including Milling Operations in the
Lather by Tubal Cain and Gears and Gear Cutting by I. Law (Work Shop
Practice series) [see http://www.powells.com for US source] and
Gingery's book Delux Accessories [see
http://www.lindsaybks.com/dgjp/djgbk/series/index.html ] it appears
that we can use a fly cutter and hand ground bits to cut the gear teeth
(spaces) to the proper involute profile, so no problem here.

However we do lack a way to index the gear blank.

Money is very tight and we need to stay under the Administration's
(financial) radar.

It appears the least expensive alternative (other than a one-time ebay
buy) is the Cen-Tec 6 inch rotary table with indexing from Harbor
Freight [see http://www.harborfreight.com SKU 47824-0VGA] for $169.00.
I have been unable to get Harbor Freight to tell me what hole
patterns/divisions are on the indexing plate that comes with this unit
so I can tell what gears it can cut. I am therefore asking the
newsgroups the following questions:

(1) If anyone has one of these units, what are the hole patterns on the
index plate.

(2) What has your experience been with this unit?

(3) Can you do compound indexing with this unit? I came across this in
an old edition of the Machinery's handbook. In addition to turning
the crank/worm so many holes, you also turn the index disk so many
holes on another ring of holes in either the same or the opposite
direction. This gives the same effect as differential screw threads.
Effectively you multiply the number of holes in each of the two
patterns you are using.

(4) Has anyone done gear cutting with one of these units using a
flycutter? For the class project we will be cutting additional Emco
V10 change gears (M1 module) from 1/2 inch thick 6061 T6 aluminum
plate.
Thanks for any hints, advice, etc. anyone may care to contribute.