Gear fabrication advise needed

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.

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I can remember seeing a unit derived from old Altlas Lathe company plans, it amounted to a double-ended arbor and pivot to which you bolted a change gear on one end of indexing and your gear blank on the other. It had a plunger arrangement to index on the change gear. It was gripped in the vise of the milling attachment. I think it was in an old HSM magazine. Super-cheap and fast to make up, downside is that you can only make gears of the same tooth number that you already have. You can always use the old bandsaw blade and wooden wheel method, make the circumference of the wheel just big enough to hold a blade of however many teeth you need index positions and use the gullets to index on. Angular error is reduced the larger you make your index wheel. This is in one of Guy Lautard's Bedside Readers.
Another method would be to use one of those 5C collet indexers that are cheap and make up a stub arbor. I've used flycutters, they're slow but reliable. Keep the speeds down for best tool life.
Stan
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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
snipped-for-privacy@fpc.cc.tx.us wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Or you could throw together an honest to god indexer. A 40 to 1 worm is easy to make with a tap, say a 1/2-13, in a lathe. It won't be a acme thread, but who cares? This is not the site I used to make mine, but it looks like the same idea: http://www.atmsite.org/contrib/JSAPP/wormgear/wormgear.html The index plates can use anything with spaced holes. A gear, a real index plate, a lawnmower flywheel...you get the idea. The beauty of this is the 40 to 1 worm cuts the error of the index plate by a bunch. You could always use 2 worm gears for super accuracy!
Ron Thompson On the Beautiful Florida Space Coast, right beside the Kennedy Space Center, USA
http://www.plansandprojects.com
The ultimate result of shielding men from the effects of folly is to fill the world with fools. --Herbert Spencer, English Philosopher (1820-1903)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ron Thompson wrote: ...

...
Neat link! That's something even I could do. Bookmarked.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@fpc.cc.tx.us wrote:

Have you considered making Gingery's indexing head? That could be a neat project for the class. Have you done foundry yet? If not, it can be built up from plate and angle, plus assorted pieces of aluminum, brass, and steel.
If the specified worm and worm wheel from Browning aren't affordable, the thread tap idea another poster mentioned is a great idea. I paid $40 USD for the Browning set but that was over 15 yrs ago, it's sure to be higher. Still have it in a drawer, haven't made the index head yet.
You might be able to salvage a worm drive unit, they are general stock drive components with standard mountings used in industrial situations. You don't necessarily need a 40 tooth wheel, but you may have to make special index plates.
Speaking of which, one of Guy Lautard's Machinists' Bedside Readers tells how to make master index plates using the like of toolmaker's buttons screwed to a plate, against the edge of another round plate whose diameter is accurately sized so all the buttons are touching each other as well as the inside plate. When I used this technique my buttons were turned from 5/16" nuts on an arbor, with the toolbit locked to ensure uniform size. The backing plate was plywood, and the alignment plate 12 gage aluminum plate. I had to shave a tiny hair off the alignment plate several times to get all the buttons to finally touch each other. I used hot glue to hold the buttons in place temporarily during that trial and error session, then drove a drywall screw through each button into the plywood.
I would highly recommend Lautard's books if you don't have them already--they are a goldmine of info for what you are doing.
And my hat off to you and your associates for keeping alive what is turning into a lost art of modern society!
Ken Grunke http://www.token.crwoodturner.com/shop /
--
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.