Tuesday, August 25, 2015

Techy Tuesday - Gears 101

Gears are fundamental components in mechanical systems, but they have many design considerations.  For example, you might not be able to get a gear ratio you want in the type of gear you want to use.  This handy primer to gears showed up in my mail today and I thought it might be something readers could use.

What’s the Difference Between Spur, Helical, Bevel, and Worm Gears?

These are spur gears, probably the most commonly thought of shape.  Except they're not shaped correctly in this picture (from that link).  

Gears are available in practically limitless sizes and shapes from specialty businesses (such as), but machinists should learn to cut them, too.  The shape of a gear tooth isn't really a triangle like the one closest to the front in this picture, and anything with the very asymmetrical tooth shape and spacing of that one in the top rear is custom - if it's anything other than a computer artist's idea.  Spur gears are cut in a mathematical curve called an involute.  The edges of the gear that make contact are curved, not straight like these pictures.   It reduces wear, reduces noise, and improves power transfer.  What's not to like?  
How do you cut a complex shape like this?  Today, you go to one of your favorite tool stores and buy an involute cutter.  They're kind of pricey, but you don't need tons of them.  I'm pretty sure the involute cutters are ground, not cut.

There has to be weeks worth of videos to watch on gear cutting on YouTube.  One method I've seen uses the milling machine with a fourth (rotary) axis.  The process is to put the blank on the fourth axis (indexed or full rotary), sticking out so the gear blank is perpendicular to the table.  Put the cutter on the motor spindle, and position it so it will cut at the point on the gear blank so the flat of the cutter (parallel to the table) is perpendicular to the gear blank - the 9:00 position.  Make a cut; you'll usually have to smoothly feed the work into the cutter as it's spinning until the cut is fully formed; then retract the cutter, index to where the next tooth will be, make the next cut, and so on.  There are really a few basic ways of making gears - this one is pretty simple for the home shop, but won't cut helical gears.   

There are literally books written on this subject.  Many books.  This is the barest of introductions.  Sound like something you'd like to do? 


  1. Check out this software: http://www.gearotic.com/

    The guy who wrote it was the originator of the MACH3 CNC control software.

  2. It used to be called "Hobbing" in Ye Dayse of Olde.....

  3. A good excuse to buy a dividing head! Some of the older proprietary ones like Van Norman had a gear train to the table traverse leadscrew to cut helical gears.

  4. I'd like to send that book to the guy who came up with gear driven timing gears for Chevy small blocks.
    Excellent idea but have you ever heard how much noise they make?
    Criminy, you can hear them coming two blocks away!

  5. Not so much a comment on cutting gears, but when I worked in the Philly Shipyard I had several chances to see aircraft carrier main reduction gears opened up for inspection and repair.

    Yes, the gear cutting or hobbing principles remain the same, but those are some really big gears.

    Did I mention they were really really big?

    A discussion of blending the high rpm power output from two separate steam turbines into a reduction gear set so the output can drive a large non variable pitch propeller is complex. And discussing all the associated machinery to keep the gearing oiled and cleaned, and getting that power from the forward main space, through all the other engineering spaces to the prop is a little beyond the scope of this comment.

  6. DrJim - Hobbing is one of the (four?) basic ways to make a gear, I'll do something on this Real Soon Now. I swear.

  7. I worked as a machinist making gears in my younger days. Gear hobbling machines are specialized for this purpose, and with an array of hobs and driving gears we could produce most anything. The downside is that you need special measuring tools and a machine to sharpen the hobs in order to produce a quality product.

  8. In the Philippines just about every machine shop has a shaper set up to make gears. Even the ones with dirt floors and a chicken tied in the corner has one.

  9. Ahhhh.....shapers!

    The first time I used one was in college, in the early 1970's.

    My Dad, a Tool and Die maker, shook his head and laughed, and then told me shapers had been obsolete for quite some time.

    Still, if it works, and it aint broke.......