On Sun, 21 Dec 2003 04:18:14 GMT, Glenn Lyford
wrote:

Hi all,

Not having immediate success (but by no means having tapped out
all available vendors or sources yet, either) I have not yet found
suitable gears to feed the quick change gear box on the Hendey.

I did in the course of poking through some of my boxes come
across a 3"OD x 6" long hunk of 6061-T6 aluminum (aluminium for
those across the pond).

This is where the insanity comes in:

I have two 72T 14DP gears. If I had two 36T 14DP gears, I should
in theory be able to generate all the speeds I need for my gearbox.
If I had 3 or 4, I'd even have extras for idlers. These are 14.5 deg.
pressure angle gears. I also have a 90T gear to also use for idlers.

I've studied John Stevenson's document at
http://www.metalwebnews.com/howto/gear/gear1.html

and see he never posted data there for 14.5 deg. PA gears, though a
wander through Google Groups shows that he's supplied odd numbers
here and there for individual responses. Does anyone know of a
source for similar data to his "table 2" for 14.5 PA gears?

But his technique also got me thinking:

All I really need to create approximate gears is a form tool, it
doesn't have to be one for horizontal milling, be it disk type or
fly cutter type. Why not a single lip milling cutter, aka, form
ground D-bit? Glenn Ashmore just made a simple taper one for his
grinding fixture with apparent success. Probably grind one up out
of a broken tap or drill bit, and use the ultimate reference for
tooth form, one of the tooth spaces of the existing gears. Chuck
the bit of rod in a hand-held drill, and rotate it against the
grinding wheel. Once happy with the form as compared with the
master gear, grind off half to form the cutter.

Next, drill and bore my blank for 1", and turn to 2.571 OD. Not
likely I'll hit either exact, but hopefully well enough for this
exercise. And by doing them both in one set-up, I'll not be relying
on my decrepit 3-jaw for accuracy.

Once removed, mount the blank on a 1" arbor. Will probably use one
or both of a filed keyway and/or a tapped hole for a set screw (or
even 2?). Mount 1" arbor in boring bar holder, on center line, of
my 200 series tool post. On the other end, mount keyed 72T gear to
use as an index with an improvised locating pin. Use the boring bar
clamp to resist rotation and not put too much load on the pin.
Either buy or improvise an end mill holder (4MT), and mount the cutter
in the spindle. Set the blank axis at the proper final gear depth.
Set a feed stop. Feed the blank past the cutter with the crossfeed,
increasing infeed in several passes until I reach the stop for each
tooth space, then re-index.

The big advantage of this setup would be that it does not require
either a milling machine or a milling adapter for the lathe, just the
boring bar holder, and other pieces I already have on hand.

Possible pitfalls: Running out of crossfeed distance to finish across
the gearblank faces. Not having enough room to use the 72T gear
without it hitting the saddle as I infeed. Losing index and messing
up gear spacing, either by bending the locating pin or poor clamping
or rotating the index gear or the gear blank on the arbor. Mangling
or breaking the cutter with poor selection of milling feeds or speeds.
Trying to cut without noticing the cutter has become dull (how do you
resharpen a D-bit form cutter, anyway?). Mangling my cutter beyond
repair before I get all the blanks cut. Before I get any cut. Before
I get all the tooth spaces on one gear cut. Better to risk everything
and cut all the blanks at once? Or better to risk the cutting edge
dulling and cutting one blank at a time? Maybe one to learn on, then
the rest as a group? I should at least have enough stock for two or
three tries, depending on how many gears I want to end up with.
Given that I'll be cutting aluminum and not steel or cast iron, am I
likely to have sufficient rigidity and cutter life to complete this?
Any other obvious or not-so-obvious pitfalls I'm overlooking?

And last but not least, am I likely to run into problems running
6061 gears with cast iron gears in the gearbox feed train?

So give it to me straight, guys, how insane am I to even consider
this? I note that broken drill bits are essentially free, and the
chunk of 6061 cost me maybe $3 at a flea market. Have I mentioned
yet that this would be my first project on the lathe?

Thanks in advance,


The D bit method of cutting gears works fine. I've cut quite a few by
this method - mostly in hard light alloy and and in tufnol (resin
impregnated stacked sheets of linen cloth) together with a few in cast
iron. The method also works well for cutting timing belt sprockets.

I see no problem in using light alloy change gears in a home
workshop lathe. A quick check using a handwheel to traverse the
carriage by the leadscrew shows clearly that the loads on
changewheels are extremely light - even when taking heavy cuts!

The choice of changewheel DP is mainly controlled, not by load
carrying capacity, but by the desire to bridge the large gap between
spindle and lead screw without using wheels with excessively large
numbers of teeth. I've happily utilised a pair of brass 32DP wheels as
part of a non standard change wheel train without problem!

Don't mess about trying to grind HSS for the bits - it's so much
easier to turn carbon steel drill rod to the right profile in its soft
state in the lathe and then flame harden it. Heat to bright red,
quench and then temper to light straw (just beginning to develop a
yellow oxide film) and it will be fine for the fairly small number of
gears that you intend to cut.

Life is much easier if you first hog out most of the tooth
spaces with a roughing cutter and save your carefully profiled D bit
for the finishing cut. The roughing cutter can be an end mill or a
slitting saw no wider than the width of the bottom of the tooth space.
If these aren't to hand make up a second D bit - don't bother to
profile this cutter just make it as a simple taper cutter because it's
so much easier to sharpen. 29 deg (30 deg is near enough!) included
angle for 14 1/2 deg pressure angle gears.

If you go the taper cutter route the small end should taper to
slighly less diameter than the width of the bottom of the tooth space.
The end should not be square to the cutter axis but:-

Tilted in about 5 deg so that only the outer tip cuts

Tilted back about 10 deg so that there is positive cutting clearance
for the rounded part of the end so that itdoes not rub against the
workpiece.

These two angles are simultaneously achieved by holding the cutter
against the grinding wheel at the correct angle - first tilt it in a
bit and then down a bit. The angles are are not critical even half or
twice as much is OK. The important thing is that neither should be
zero.

The remaining problem is cutting clearance for the side of the
D bits. For the roughing cutter, cautiously grind in just below the
cutting edge at about 10 deg clearance angle until the ground face
just reaches the cutting edge. Then heavily grind the remaining
untouched surface so that no part is as high as the cutting edge.

For fine cuts it is possible to use the finishing cutter without
any side clearance - this is the way that D bit taper reamers work.
However the cutting geometry is extremely poor - it's pushing the
metal away rather than cutting. On the other hand it's pretty
difficult to grind cutting clearance right to the cutting edge of form
tools without accidentally damaging the form shape. The usual
compromise which works pretty well is to grind in the same way as for
the roughing tool but stop grinding just before the grind reaches the
cutting edge - ideally leaving an untouched land about 10 thou wide
below the cutting edge.

Good Luck!

Jim