On 2012-03-03, Bob La Londe wrote:
"Karl Townsend" wrote in message
...
On Fri, 2 Mar 2012 20:13:56 -0700, "Bob La Londe"
wrote:
The HF mini lathe hasn't been used since I got the slightly bigger much
older, and much better HF lathe. I wanted to use it the other day though
because it has gear selectable forward and reverse carriage feed. The I
remembered I stripped those little nylon gears. I am sure I can get a
[ ... ]
My next thought was maybe to try and make some metal gears, but I have
never
done that before. Any suggestions on the approach for that? Best alloy
for
reasonable wear?
You don't want it *too* strong, or some other part of the
machine will die when the load gets too high.
You'll need a dividing head,
I don't have a dividing head, but I did recently fit my little rotary table
with a lathe chuck, that is reasonable centered and parallel. Is that "good
enough", or do you think a head with dividing plates is a must.
Before I answer that (assuming that your rotary table does not
have dividing plates), I have to ask *you* -- how many teeth should the
gear have?
If is is something like 30 teeth, no problem. Most rotary tables
have a 90:1 ratio, so three full turns for each tooth and you are fine.
However, if it were say 28 teeth, you need 3.2143 turns per
tooth. that is 4 degrees for the full turn, and 0.8572 degrees left
over. That is 51' 26" added per tooth. Even if you use a spreadsheet
or other program to print out a chart -- how long do you think that you
could go before you made a mistake? Now, if you have a dividing plate
with 14 (or some integer multiple like 28) holes, that works out right.
Set the arms on the plate to allow an extra motion of 3 (for 14), or 6
(for 28) holes. Then reposition the arms after that, so the next
division advances the same number of holes, and keep going until you
reach the starting point again. (And be careful to remember to turn
three full turns before going that extra three or six holes, or you
will still be out of line.)
*This* is why people use dividing heads for making gears.
and involute gear cutter for the
diametrical pitch and pressure angle and the approximate number of
teeth in your gear. eBay is a good place to shop.
Involute gear cutter? Is that one of those "wheel" type cutters I often see
in videos on a horizontal mill being used for gear cutting?
"Involute" is a term for the math defined shape of the gear
tooth, and an involute gear tooth cutter is one which will form close to
the precise shape needed. These shapes are so the teeth roll against
each other, instead of sliding and wearing.
With these two tools, its a piece of cake, cut a tooth to depth,
rotate head, cut next tooth.
You might read "Gear and gear cutting" by Ivan Law to get a good grasp
of the subject. You'll need machinery handbook to calculate your
depths and blank and rotation values, etc.
To replace a plastic gear, I'd use AL.
I could certainly cut aluminum. You wouldn't worry about galling in this
application? I don't really think of aluminum usually when I think of an
interacting mechanical part. As a static or linking part sure, but aluminum
to steel with mechanical interaction and friction?
No friction with a proper involute shape -- just rolling
friction. But this is for *one* gear made of aluminum meshing with the
other original plastic gears. If you have to make two or more gears
which must mesh with each other, you want dissimilar metals (unless you
are using well lubricated steel or cast iron gears) to avoid galling.
Aluminum on steel? Fine! Aluminum on brass? Fine. Brass on steel?
Fine. Aluminum on Aluminum? *No*!
Otherwise, I've made gears out
of 4140 and then hardened. I've had one running in my backhoe for ten
years now.
LOL. On the mini lathe they would outlast the entire rest of the machine.
Almost anything will outlast the rest of the machine. Those
plastic gears are made to be the "fuse" -- the weak point which breaks
before other more expensive parts break.
This is a good basic skill to have. You're sure to be confused at
first, but then it will seem easy. Just like learning to ride a
bicycle.
I have seen some gears cut with a slitting saw, but I wasn't super
comfortable with the process. What do you think? Probably go pretty quick
if I went with aluminum.
A slitting saw? *No*! Not for power transmitting gears. Now,
if you were making a really large gear (say 1" between teeth or larger),
and had the shape laid out with layout die on the flat, you could use a
vertical bandsaw to cut a rough approximation, leaving a little extra
metal, and then using a file to take to to final shape. This is the way
old machinists made large gears. But your gears are not the right size
at all for this.
A slitting saw *might* work for the gears used in desktop
clocks, which don't handle much torque, and which have the flat gear
meshing with a cage of rods one of which goes into a slot at a time.
And even with this, you would probably have to file the ends of the
teeth in the flat to allow the rods to enter properly.
Now -- an involute gear tooth cutter looks a lot like s slitting
saw -- until you examine the shape of the teeth.
Oh yes -- each involute gear tooth cutter has a different shape,
precisely appropriate for a specific number of teeth, and close enough
for a number around that value. The larger the number of teeth, the
wider the range that is close enough. One end of the set is capable of
cutting any tooth count from 135 teeth to a rack gear (infinite number
of teeth). However, when the number of teeth gets small, the range is
also small. The cutters are marked #1 through #8. If you have to cut
two gears of different numbers of teeth, the odds are that you will need
two cutters unless you are quite lucky. These different cutters make up
for the different angles at which the gear teeth mesh.
Good Luck,
DoN.
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