Gear Project

Better precision measuring the diameter is called for. Certainly 1/16" steps are not small enough. Don't you have a digital caliper capable of at least 6" measurements? (Or even a dial one or a vernier one). Measure to 0.001", not to 1/16".

"Stock Gear" is a manufacturer of standard gears. It is likely made by moulding, not cutting. But it is also likely bored out by the person/company who sold it to you, and a new hub pressed in to fit the rest of the stuff.

So -- it is 24 DP (Diametrical Pitch) and has 28 teeth (count the teeth to verify this),. I don't know what the pressure angle is. The photo is not directly on axis, is it is difficult to count the teeth from the photo (the side ones sort of merge together), so *you* count them to verify this. The other gears will have the same diametrical pitch (and pressure angle) an you simply need to count the teeth on each to know its size. I come somewhat close to 16 teeth on the smallest gear (not certain because of the fuzz of the lossy JPEG compression, but that makes 24 teeth seem reasonable for the larger of the two smaller gears.

The easiest way to determine whether it is 20 degree PA or

Let's see -- the formula for outside diameter of a gear is:

N = tooth count P = diametrical pitch

(N + 2) / P

So the precise OD of the 28-tooth one should be 1.2500" (which matches your 1-1/4")

The 16-tooth one: 0.750" (3/4" not your 11/16" -- if it is truly 16 teeth.

The 60-tooth ones: 2.5833" (2" and 9.3333/16", not 9/16")

So -- you see how the precision of measurement is important. Throw away that inch scale (or hide it) and use the calipers. Forget about "about" for measurements of anything you make by machining.

The formulas for the gear measurements are in the book you say you have -- or in _Machinery's Handbook_ (where I go). In the 29th edition, the formula which I used are on page 2132, with others on the previous page.

Page 2129 has drawings of three different pressure angles (10 degrees, 20 degrees, and 30 degrees, to show you how the shape of the teeth change.

The pitch diameter of each is simply the tooth count divided by the DP, so you get (from smallest to largest) 0.6667", 1.1667", and

Good Luck, DoN.

O.K. Does the rotary table have a hole plate and arm with sector arms? If not, it will be tricky for some of the gears. 60 tooh is no problem. 16 tooth (if I am right about that gear), also no problem. But the 26 tooth (or was it 28?) will be tricky. For each cut you need to advance it:

With the right hole plate, and the arm and sector arms, it is easy to set it up to avoid problems. And that is what an index head has.

The rotary table is a far better choice than this. The finest rotation you can accomplish with the spin indexer is 1 degree. And even for the 16 tooth gear, you need 22.500 degrees -- that half a degree you can't get on the spin indexer. (Yes, it would work with the 60 tooth gear, which only requires 6 degrees advance for each tooth. Still a little tricky to calculate the position of the pin for each one.

Good Luck, DoN.

You may find this website of particular interest, provided you haven't already found it :)

especially:

You inspired me to go find these web pages again. I lost them awhile back with a computer crash...

Oops! I forgot not to give approximate numbers. :-)

These reproduction gears I measured are made of Delrin. But I can't attest to the degree of quality control. So I'd just as well start from scratch as long as the number of teeth are correct for each gear.

Thanks. Here are some pics of the 60 tooth gear:

As far as "hole plate and arm with sector arms". I have a rotary table with dividing plates that I haven't used yet. In fact, this one:

Thanks a lot.

Darren Harris Staten Island, New York.

That little table will be perfect with the dividing plates to make your little gears. Please post photos of your finished work. Eric

Thanks for the links. :-)

Darren Harris Staten Island, New York.

Not 16-tooth, O.K.

O.K. A little different from my calculations. It sounds as though the crest of the teeth has been shaved a little, No problem as long as the pitch diameter is correct.

O.K. But we knew the count of teeth there. It was the smaller ones which I was wondering about -- and you have now told met hat the smallest one was 14 teeth, not 16, so my calculation for size was wrong.

O.K. That should do it. Did it come with a table of settings for different divisions? If not, you will need to know how many turns of the crank for a full turn of the table. Common ratios are 90:1 (larger rotary tables, at least), 40:1 (my dividing head), and I believe

Good luck, DoN.

Actually Star Wars was released in 1983.

One of my friends has the world record on it.

Darren Harris Staten Island,New York.

I haven't dug out my manual but an internet search says 90:1

The most difficult part will be the gear cutter. (So I'm off to do my reading).

Thanks a lot.

Darren Harris Staten Island, New York.

The gear cutter will be easy. You can make the button cutter as outlined in Law's book and then use that cutter to make a fly cutter to cut the teeth. You can use water hardening or oil hardening tool steel. Both are easily available and pretty easy to machine. And heat treating with a propane torch will work. For that matter, since the cutters are so small, you could use a gas stove or even an electric burner. Eric

O.K. That will be 1-1/2 turns per tooth for the 60-tooth one, Easy to set up with any circle of holes with an even number of holes.

The 20 tooth one is 0.2222 turns per tooth, or 1/4.5, so any circle with a multiple of 9 holes (likely 18 holes) will do. (The chart I found later does it a different way.)

As for the 14 tooth one -- that is 0.1556 turns per tooth, I can't come up with an easy one for that -- check the dividing tables which should have come with our table. If not -- go into _Machinery's Handbook_ and see if they have the tables for the 90:1 ratio.

O.K. Here is a site where you can download the table (and instructions for a similar rotary table).

And looking at that -- it is doing 6 full turns plus 9 holes on a 21-hole circle. Interesting approach. That is over 24 degrees per tooth, so you eventually come around to complete the set with some number of trips around the gear.

Once you find the pressure angle, you have the choice of making your own cutter for each gear, or buying cutters for each gear. (yes, a separate one for each of those three gears. A set of cutters consists of eight total -- each covering a range of teeth. The last of them covers something like 100 to infinity (I would have to look it up), The first of them has a range of only three different adjacent tooth counts IIRC. The shape of the cutter has to be a little different for each number of teeth. You can sometimes find 1/2 numbered cutters to fill in the gaps and make a better fitting gear. There is a remote possibility that your 14 and 20 tooth gears will use the same cutter, but I don't think so. The tooth count is fairly small, so they probably don't span the same cutter.

Good Luck, DoN.

That is a good, simple explanation of the math. If you have trouble with mixed fractions and know how to set up a spreadsheet you can use one to create a table of all the full and fractional turns from say 2 to 100 divisions. The low numbers are useful for cutting wrench flats etc. In OpenOfficeCalc the format code is # ??/??. jsw