Change gear pressure angle

Just a few suggestions:

1) Make a friend at a precision machine shop that has an optical comparator . You'l be able to look at the gear in excruciating detail and make all the measurements you need.

1a) Take a good, sharp close-up photo, making sure that the camera is squar e with the gear. Blow it up to several times actual size and take your meas urements off the print. You can help this along by including a ruler in the photo for scale (or scribing some lines on the gear).

2) send the busted gear to Boston Gear and ask them to match it. Make sure to talk to someone first and to provide a return shipping label.

Thanks - I'll keep those in mind.

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If you have a scanner, put the gear flat in the middle of the scanner's plate, and scan it. That will probably be as good an image as you can get, and will make sure that the "camera" is square with the gear.

Use the highest resolution (maximum DPI setting) on the scanner.

You can possibl even use a sub-program in an image processing software package (e.g. GIMP or PhotoShop -- whichever you have) to get an outline of the gear -- better to work on importing that to a CAD program.

Good Luck, DoN.

That is a good idea! Why didn't I think of that?

I did do a scan, at 1200dpi (not max) and got a great image:

Not exactly straight-on, probably because I had it at one end of the scanner. It blows-up nicely:

Thanks, Bob

Right! That is why I suggested it be in the center of the scanner's plate.

Yes -- enough to read into a CAD program and draw the tangent lines to pick up the pressure angle.

Good Luck, DoN.

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 Machinis t, so I forget much of the technology, but, FWIW, all of the machine tool b uilders I visited in those days hobbed their own gears. As for using differ ent 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 a nd stuck with it.

Some of the gears in a lathe have to handle a decent load, but the change g ears are just timing gears. The load actually is very light. And, as long a s 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 simpl e and standard -- although stub gear-tooth shapes were pretty common.

This was all pre-CNC.