On Thursday, January 26, 2017 at 10:26:26 PM UTC-5, DoN. Nichols wrote:
On 2017-01-25, Bob Engelhardt wrote:
On 1/23/2017 1:19 PM, Bob Engelhardt wrote:
I was looking at a picture of the lathe and the rack for the carriage
power feed caught my eye. That rack won't have the same DP as the
broken gear, but it will _probably_ have the same pressure angle. I
mean, what's the chance that a lathe maker would use different pressure
angles in his gears? The point being, of course, that the rack teeth
have straight sides and easily measured angles.
I took a photo of the rack & measured the angles: 20 degrees
http://imgur.com/pRfDmj8
I haven't yet convinced my self that the rack HAS to have the same
pressure angle.
It only *has* to be the same pressure angle as the pinion in the
carriage -- but it is unlikely that a maker would use different pressure
angles in different parts of a lathe. The main difference is how much
force is generated trying to move the axles of the gears apart. And
probably a difference in how much rubbing is going on between the
surfaces of the teeth. They proably buy all the gears from one vendor,
or just buy the equipment to make the gears from one vendor.
The only time I might think that there would be different PAs
would be in an *old* company, which continues to use some parts from
earlier designs, and uses newer parts for things which have been
re-designed in later years.
Enjoy,
DoN.
It's been almost 40 years since I covered gear making for American Machinist, so I forget much of the technology, but, FWIW, all of the machine tool builders I visited in those days hobbed their own gears. As for using different pressure angles, one hob can cut a variety of gear diameters, but only one pressure angle. So they normally settled on one angle for their gears and stuck with it.
Some of the gears in a lathe have to handle a decent load, but the change gears are just timing gears. The load actually is very light. And, as long as any pair of gears are within a reasonable size range of each other, they don't encounter problems with undercutting or excessive sliding motion. All of the gear trains in a lathe, and in most machine tools, are pretty simple and standard -- although stub gear-tooth shapes were pretty common.
This was all pre-CNC.
--
Ed Huntress