15 years ago
One segment of increasing machine shop activity in my area is
machine repair that frequently requires the scratch production or
remanufacturing of non-stocked parts. A recent thread in this
newsgroup indicates this is becoming an increasingly common
activity in other areas.
One area of concern is the production of non-standard gears,
especially those with odd diametrical pitches/modules and/or
I have come across two articles for the home shop or hobby
machinist that shows how to generate the involute curve, using
only a single point tool ground to the shape of the rack
[straight sides] with a slight additional length to provide
clearance at the root. Standard M2 lathe tool blanks appear
adequate, and most any shop with access to a surface grinder
should be able to grind such a tool very accurately, even with
odd pressure angles. Where 14-1/2 degree pressure angles are
used, an ACME thread gage and a carbide style grinder [with a
little care] should prove adequate.
Basically this requires a shaper with the means to rotate the
gear as it is traversed past the reciprocating tool as if it were
rolling on the rack, along with a means to index the gear for the
required number of teeth. In the article from the 1950s UK, a
hand-powered shaper was used.
To review the articles [these have some suggested tool geometries
in addition to the how-to] click on
It occurs to me that if you have a CNC lathe with a fine indexing
spindle, you can simulate the shaper reciprocating action by
using the Z motion, and can traverse the tool in sync with the
rotation of the gear blank to generate the required involute
teeth. Of course the tool would have to be adjusted to the
correct height and clamped. For ease of setting the tool
vertical and the holder parallel to the Z-axis, something
fabricated out of square section CRS should work well. Assuming
that the top of the bar was parallel, I can see setting with gage
or space blocks after touching off on the gear OD. This would
allow accurate roughing and finishing cuts.
While not producing AGMA class 8 gears, this could be a useful
dodge when a non-standard gear is required ASAP and where the
involute B&S style disk cutters are not immediately available.
Basically, if you can turn the blank, you should be able to make
the gear. No clapper box should be necessary as there is no feed
until the tool is clear of the part [with proper programming] or
the part can be back-rotated "just a tad"
Indeed, if the tool-holder/bar was set at the correct helix
angle, it appears this dodge could be used to generate helical
gears as well as well as straight cut gears if
simultaneous/synchronized X, Z and controlled rotation are
allowed, which it appears to be possible on at lest some of the
lathes that produce the "Higby" style thread starts.
Has anyone used this dodge? Does it seem like it should work?
It may be helpful along with that roll of racer tape and coil of
bailing wire in the bottom of your toolbox to get things running
Unka' George [George McDuffee]
Merchants have no country.
The mere spot they stand on
does not constitute so strong an attachment
as that from which they draw their gains.
Thomas Jefferson (1743-1826),
U.S. president. Letter, 17 March 1814.