Are there any simple gear drawing programs available that are not part
of a big CAD program. I'm not a ME, but a sometimes woodworker.
Years ago I built a wooden clock from plans. The clock is great,
works fine, pretty.
The plans were drawings of the gears that were to be cut out and glued
to a wooden blank so the gears could be sawn out with a band saw. The
drawings were not very good, the outline of the teeth were not uniform
size/shape, I don't thing the drawings of the gears were perfectly
round...
I copied the drawings on a photocopier so I could preserve the
originals. That probably didn't help either.
As I am getting near retirement age I may get another woodshop and
would like to make some more clocks, maybe try to sell some. I could
use some higher quality gear drawings. I would like something that
can make gears with correct tooth shape/size/spacing around a gear
that is round and the correct size. Hopefully with a printer output.
Anyone have any ideas?
Thanks,
Richard

How about a _small_ CAD program?
Softkey "Key Cad Complete" for instance. I got mine from a clearance bin at
Office Depot for $7. The list price is $20 or so. It looks like it can accept
what it calls 'XYZ' files, which I assume means just lists of point coordinates
in ASCII.
That sort of list could be generated using a spreadsheet. You can get a good
one for free as part of a (huge) download of Open Office at OpenOffice.org, or
buy the CD pretty cheap.
You'd have to do a fair amount of computing in the spreadsheet to figure out a
tooth profile, array it to make a gear profile, and convert that from polar to
rectangular coordinates. But it's the sort of thing that spreadsheets do well,
and that way you get to pick your tooth profile. I think wooden clocks
traditionally use cycloidal gears because the teeth are strong.
Okay, it's a _lot_ more work than just plugging numbers into a 'gear drawing
program', but it could be a lot more fun, too.
-Mike-

Hi Richard:
Actually you should just lay the gear out the old-fashioned way and skip the
fancy CAD program.
I'm assuming these gears are for mostly ornamental purposes, but still need
to roll on each other properly. Since they probably don't have to mate with
existing gears of standard pitch, you have a lot of leeway to design them.
First, you need to decide what size you want to make your gears. The first
gear is easy because for any desired diameter and number of teeth you can
calculate a pitch. The pitch might be some bizarre number like 12.2359, so
you wouldn't want to use it in a standard machine, but if you only want to
mate it to another gear that you're making, who cares what the pitch is?
Let's say you want to make a 6" dia. gear. Now, you can have any number of
teeth you want (within reason). The more teeth you want, the smaller those
teeth will be. Let's say you want 24 teeth. Your pitch (actually
"diametral pitch") is just the number of teeth divided by the diameter
(actually "pitch diameter"). So, in our case, the DP would be 24 / 6 = 4
[incidentally this is actually a standard pitch].
A gear has to have a "pressure angle". This is the angle of the tangent
point where the gear teeth just kiss each other. The common standard angles
are 14-1/2 deg. and 20 deg., but again you can really use any reasonable
angle you want between about 10 and 30. Let's use 20 deg. since it's a nice
round number.
Now let's draw our gear teeth on the wood. The contours of an involute gear
tooth are generated by a string unwrapping from a "base circle". So the
first thing we need to do is make a base circle. The diameter of this
circle is just our pitch diameter (6 inches) multilplied be the cosine of
our pressure angle. Using a calculator we calculate this to be
D = 6 x cos20 = 5.638 inches
So start by marking such a circle with a compass and cut it out of some
material (preferably something thick that you can wrap a string around).
Lay this disk onto the wood that you are going to make your gear out of and
drive a nail through the center (where you stuck your compass) to attach it
to the wood . Tape the end of a piece of string to the outer rim of the
disk and wrap it around it about one turn. Attach a pencil to the end of
the string (tie it near the tip of the pencil). Now strike an arc outward
from the disk for a couple of inches. Since we want 24 teeth you should
mark one of these lines (an involute) every 15 degrees (that's 360 divided
by 24 teeth). Just rotate your disk 15 degrees each time and draw another
curve. These are the contours of one side of each gear tooth.
Remove the disk temporarily and draw your "pitch" circle with your compass
(a 6-inch circle, remember?). Now we need to draw the other side of the
tooth. The new arc should cross the pitch (6") circle exactly half way
between any two of the arcs you already drew. If your gear is big and your
teeth are many, you can just measure the midpoint with a ruler. Otherwise
you can draw a line at 7-1/2 degrees from where one of the old curves
intersects the 6-inch circle (7-1/2 is half of 15) and this will give you
your midpoint. Nail the disk back on and wind the string around it in the
*opposite* direction. Rotate it until the pencil is right on that point we
just found and the string is taut. Strike your arc. Now rotate the disk 15
degrees and strike the rest of the curves like before.
OK, now all you need is the tops and bottoms of the teeth. The distance out
from the 6-inch circle to the top of the tooth is the "addendum"; the
distance inward to the bottom of a tooth is the "dedendum". In a perfect
world these distances are just the reciprocal of the pitch (1/4 inch). In
reality they need to be a little more to provide a little clearance for the
gears to rotate freely. If you made them exactly 1/4 inch, the tops of the
teeth of one gear would just touch the bottoms of the other. So let's add
about a sixteenth and call the addendum/dedendum about 5/16" So you need to
get your compass again and draw two circles: one will be 6.625 dia. and the
other 5.375 dia.
Now you have drawn your whole gear profile! Just jigsaw it out!
But what about the other gear? Believe it or not, you can make it any size
or number of teeth you want! As long as the other gear has the same DP (4)
and the same pressure angle (20 deg.) they will mate perfectly. So you can
either make another identical gear, or one of a different size. This time
you have to pick either a number of teeth N, or a pitch diameter D. If you
pick one, the other can be calculated
N = D x DP
or
D = N / DP
So if you wanted the second gear to have 30 teeth, the pitch dia. would then
be 30 / 4 = 7.500 inches. Then you just do the same thing over again.
Good luck!
Don
From: "Richard"
Newsgroups: sci.engr.mech
Sent: Monday, November 10, 2003 7:48 AM
Subject: Gear drawing program

This was a very readable tutorial on the basics of gear design.. (something
I've wondered about for a while now...) The Handbooks that I've browsed
haven't made it as easy to understand as you just did. Thanks.
Al...

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