For several years now I have been trying to make my own gear
cutting hobs. The Eureka relieving device shown in Workshop
Practice Series 17 is what I started with. They started with a
picture of the Balzer device and reverse engineered it.

I followed the plans closely because I had no idea how it worked.

My first change was to make it longer so a hob blank would
fit. The hobs for diametral pitch have a lead which is a factor
of pi. I used a 42t and a 44t gear together to make 22/7 pi.

This makes the half nuts unusable like metric conversion
gears do.To do plain turning on hob blanks the tool is pulled
back and the lathe is reversed with the half nuts engaged
for the next pass.

The problem with the Eureka relieving device is that it drives
through a ratchet so it loses place when you reverse it.

I was able to relieve hobs with it but it was very tedious since
the tool had to be adjusted to pick up the thread each pass.

My solution is to use a pattern thread on the reliever and
a half nut connected to the lathe saddle to drive the carriage.

Now the lathe is only reversed while cutting the pattern
thread and half nut. A different pattern and nut are needed
for each thread pitch.

Plain turning the hob and relieving it are all done with the
pattern thread so there is no need to reverse the lathe.
The pattern thread half nut always engages correctly.

A plain arbor that matches the reliever arbor is used.
Both have a keyway to index the pattern thread and hob
blank.

There is a better description of the Balzer Relieving
device at

http://books.google.com/books?id=iu1IAAAAMAAJ&printsec=frontcover&dq=...

pages 26 to 28 which seems to be the origin of this
tool in 1896 or so.

Taylor's 1906 patent 817885 is a paraphrase of Balzer's
design.

Balzer has a 1895 patent 535127 for a reliever with no
ratchets and a trick gear but it was not the design that
he manufactured.

Here is my hob reliever on U-tube. It is not as complicated
as building a backing off lathe but it can't do small
hobs for cutting worm wheels.

http://www.youtube.com/watch?v=kJ8kyC_bpHs

So far I can't get U-tube search to find it.

Best Regards Charlie