On Sat, 21 Jan 2017 13:53:29 -0500, Bob Engelhardt
wrote:

I have a Jet 1024 lathe with a QC gear box. The box has a broken gear
that is really oddball in that its dimensions are inconsistent with
published standards. Such that I cannot find a replacement in any of
the usual places.

Anyhow, I intend to make a new gear on my horizontal mill and will have
to buy a gear cutter. That means that I need to know the pressure
angle. I found a website with a procedure to do that, but it requires
CAD and before I get into that, I'm trying here.
(http://mikesworkshop.weebly.com/chan...ure-angle.html)

Here is a photo of a couple of teeth on my gear and profiles of the same
DP from Boston Gear. The Boston Gear profiles are a different diameter,
so I can't compare by overlaying. I can't tell which angle is closest
to mine. Anybody know gears well enough to tell?

http://imgur.com/a/7O4O3

Thanks,
Bob

BTW - the lathe is from 1980 or so - if that helps.
I assume you tried Jet already to see if a new gear is available and
found out it is not. Since you are only making one gear you could
grind a fly cutter to fit the existing gear. I have done this and it
works well if only one gear is being made. Do you have any radius
gauges? It may be that one will fit your gear teeth exaclty and if so
you can use it as a gauge to grind a cutter. I know, the teeth have an
involute form but only the beginning of the curve is used and it is so
close to a true circle that a true circle shape is within the
manufacturing toleranceof the original gear. As to the picture shown
all the teeth of a particular DP are the same size so the gear
diameter shouldn't matter, you just need to see if your tooth matches
the shape of a sample tooth. You may be able to determine the pressure
angle by tracing your gear teeth carefully on a piece of paper. Now,
find the center of the tracing as close as you can and use this with a
compass to draw a circle that is midway between the O.D. of the gear
and the bottom of the space between the teeth, and another circle that
is the same diameter as the bottom of the space between the teeth.
Next, draw a line the bisects a tooth and pases through the center of
the gear tracing. Where this line intersects the larger of the
circles you drew make a point. From this point draw a line that is
tangent to the smaller of the two circles you drew. Make another point
at the tangent point. Finally, draw a line from this tangent point
through the center. The angle between the two lines you drew that pass
through the center of the tracing is the pressure angle. You should
get an angle that is close to either 14.5 degrees or 20 degrees if
everything is drawn right and the gear is made to one of the two most
common pressure angles. It will be off a little because the smaller
circle is a little smaller than the base circle where the pressure
angle is measured from but I don't think it will make much of a
difference, especially since you are working from a tracing.
Eric