On Nov 17, 3:45 pm, "DoN. Nichols" wrote:
On 2012-11-17, Searcher7 wrote:
On Nov 16, 10:43 pm, "DoN. Nichols" wrote:
On 2012-11-16, Searcher7 wrote:
I want to undertake a gear making project for practice if not nothing
else. I figure I'll try something I'll likely need in the future.
[ ... ]
They are about 11/16", 1-1/4" and 2-9/16" respectively.
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".
Oops! I forgot not to give approximate numbers. :-)
60 tooth = 2.575"
28 tooth = 1.245"
14 tooth = 0.655
I'll first have to learn how to measure the gears. The larger 60 tooth
gear has no writing on it, bu the 28 tooth gear says 24P, 28T, and
"Stock Gear".(I assume that means it is an easily obtainable
standard). The writing on the 14 tooth gear is too small to read.
"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.
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.
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
14-1/2 degree PA is with a gear tooth gauge. The gauge is likely 20 PA,
so it if is a clean fit to the gear, it is the same. If there is a lot
of light in the mesh, it is likely 14-1/2 degree PA.
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
2.500") This is the diameter about half way from the crest of the tooth
to the root -- and is best measured with a three-wire technique. It is
mostly good for determining the spacing of the centers of two gears, 1/2
the pitch diameter of one added to 1/2 the pitch diameter of the other.
Thanks. Here are some pics of the 60 tooth gear:
http://i290.photobucket.com/albums/l...jects/60Tb.jpg
http://i290.photobucket.com/albums/l...jects/60Ta.jpg
http://i290.photobucket.com/albums/l...jects/60Tc.jpg
On Nov 17, 3:56*pm, "DoN. Nichols" wrote:
On 2012-11-17, Searcher7 wrote:
Oops! Yes I have an HF Mini-Mill. :-) (Along with a rotary table).
* * * * 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:
26 * * *13.8462 degrees (13 degrees 50 minutes 46 seconds)
28 * * *12.8571 degrees (12 degrees 51 minutes 26 seconds)
* * * * 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.
But all I have is an over sized spin index fixture. (www.ebay.com/itm/
350468227596)
* * * * 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.
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:
www.ebay.com/itm/321022783132.
Thanks a lot.
Darren Harris
Staten Island, New York.