Emergency gear generation on CNC machining centers?
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 pressure angles.
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
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? Comments? Suggestions?
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 again quickly.
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.