Of Edge Finders and Rotary Tables

Congratulations.

O.K. for bolt circles and for milling circles the rotary table is the preferred tool.

The spin indexer is limited to 1 degree increments, which is fine for some numbers of teeth, but useless for others. For gears, you want a dividing head with a matching tailstock, and appropriate arbors. The index head typically has a 40:1 ratio between the crank and the spindle, though some have 20:1 and some have 60:1. Those, combined with index disks (bunch of concentric circles of holes, each a different integer number) allow you to get many (most) common gear tooth numbers, but if you want to make a metric to inch transposing gear set for a lathe, you are stuck with the 127 tooth one. The dividing head has a pair of arms which can be set to a selected angle to help hitting the right next hole after N crank turns, and then you shift it so the other arm is contacting the indexing pin, and you are set for the next. You

To get the 127 tooth gear, you will need an indexing head with a differential gear setup, which I have only seen in catalog photos, not in real life. Or go for a CNC rotary table controlled by your computer.

Hmm ... as I understand it, hobs need to have the gear blank and the hob coupled by a carefully selected set of gears so the two rotate in the proper relationship. Most hobbing machines are rather large and heavy. Again -- this could be done with a CNC controlled arbor for the gear coupled to the spindle rotating the hob -- and they have to be at just the right angle, unless you want spiral gears.

Good Luck, DoN.

Thank you ! You've answered several questions . The gear hobs I was thinking about were straight cut gears , like one would find in a lathe leadscrew gear train . Or in a back gear on a Logan lathe ... I read somewhere on the net that you can cut the teeth by advancing the blank one tooth at a time . His hob had I think three or five cutters , so it also cut on the flanks of the teeeh adjacent to the one under the center cutter . You describe using a dividing head for cutting gears . From your description I see the cutter's rotational axis perpendicular to the gear's , and the cutter shaped like the space between the teeth . This might be the process I'm calling "hobbing" . Can this be done with a verticle rotary table equipped with index discs (and a tailstock?) ? I need to visit the guy at the machine shop next door to the cabinet shop where I work . He's also a 'cycle rider , and loves to share his knowledge . I want to learn . Saw an article in my Dr.'s office the other day , people who continue to educate themselves (content/field unimportant , it's the act of learning!) have fewer health problems , and a smaller chance of mental (Alzheimers , etc.) problems in their later years . There is SO much I don't know !

I don't see why not. And it might cut larger gears than I can do with my index head.

And the gear tooth cutters (not hobs) used for this approach are sold in sets -- marked "#1" - "#8" for a given gear pitch and pressure angle, and each is used for cutting teeth within a fairly narrow range of tooth counts, because the tooth shape has to change as the count changes. IIRC, the #8 covers from rack gears (such as you will find on the underside of the bed lip of your lathe) down to some number of teeth like perhaps 58 or so. (I could go downstairs and check my set of the pitch for the gears in my Clausing lathe, but I'm comfortable up here, and the number is not critical. The other end of the range covers only about two tooth counts (6-7 or something like that) because the shape of the tooth changes much more quickly with the smaller sizes.

There are add-on sets for the intermediate cutters (1-1/2,

A true hob starts out with something like an Acme thread being turned in a piece of hardenable steel, then flutes are cut into it, it is hardened, and then finish ground. This, of course, produces a slight angle to the teeth if the shafts are at right angles to each other. Anyway -- yes, the adjacent turns of the thread cut the other side of the teeth at the same time -- though not to full depth, and the multiple cuts as it rotates produce a gear tooth shape which will clear any other tooth of the same spec engaged with it, so you don't need different cutters for each tooth count.

There are ways to do this with a single cutter (either a horizontal milling style shaped to the rack tooth shape, or a tool in a metal shaper ground to the same shape), by moving the index head sideways and rotating the gear blank just a little to progressively cut the proper tooth shape -- but a hob on a gear hobber is a lot quicker to use for production. :-)

Good sort to have around. Be good to him.

Well ... that maximizes your protection as you age. :-) Always glad to learn to do something else.

Enjoy, DoN.

I see that Don has given you good information, but to clear up a point about hobbing: As he said, a hob starts out as a shape very much like an Acme screw. The flanks of the "threads" are straight, just like a screw thread. The way they achieve an involute gear-tooth shape in the workpiece is by simultaneously rotating the cutter and the workpiece, with their axes at an angle to each other.

Hobbing is one of several metalcutting processes called "generating." You generate a shape through some combination of simultaneous motion. Although there have been accessories made to do this on a lathe (by hobbyists, as far as I know), it's mostly a stunt. It requires a dedicated hobbing machine to do it productively, and with enough versatility to be useful. Cutting gears on conventional machine tools is more commonly done with the kinds of cutters that Don describes, and some kind of indexing head or table. The cutters have involute-shaped teeth, and they cut like a conventional milling cutter. In other words, there's no simultaneous motion. They do this in job shops when they need just one or two gears. It's too slow and tedious for production, which is done mostly with hobbers.

One other point that you should know about, if you get into gears: You can cut gears on a shaper, as Don mentioned, but that is not what is called a "gear shaper." A gear shaper is another specialized gear-cutting machine that, like a hobber, cuts involute forms with straight-sided cutters by simultaneously rotating the cutter and the work through an angle, as the shaper goes through its stroke. In other words, it's another type of generating machine. They're used in industry for various gear-cutting jobs but the big one is cutting internal-tooth ring gears for planetary gear sets.

This is much easier to explain in pictures, and easier still in animations or videos.

-- Ed Huntress

This is based entirely on reading old books, I've never cut a gear this way. I buy them.

The teeth of a hob are cut to the shape of a rack, meaning that the tooth flanks are straight at the pressure angle. Machinery's Handbook shows the dimensions.

First you rough out most of the tooth spaces in the gear blank with a normal milling cutter on a rotary table. Then you put the hob on the spindle and set the blank up beside it on an arbor that can turn freely without play. The arbor has to be angled so it's parallel to the hob's spiral teeth. Slowly feed the gear blank into the hob, which will rotate the gear and automatically generate the proper involute tooth form as the hob teeth roll in and out of mesh. Whether or not the tooth spacing stays accurate depends on friction and how much metal the hob has to remove. If possible the spindle should drive the arbor through gears.

Although you have to harden and grind the hob and may have to try a few times to get a good gear, hobbing lets you make gears of any tooth count with correct involute teeth starting from a lathe bit ground to a simple straight-sided shape.

OK , this is closer to what I had in mind . The website I got my info from is :

============= To avoid rediscovering fire and reinventing the wheel, buy the book "Gears and Gear Cutting" by Ivan Law. Specifically written for the home/hobby machinist and covers making your own cutters, single tooth and b&s style. several sources in the us but one example is

Also on generating a 127 tooth gear. For best accuracy from the typical rotary table / dividing head, the best best seems to be to use the table to make as big a plate as will fit your head with 127 holes using the degree markings and vernier and use this to generate the gear. The 127 hole plate is not generally available and the other methods are (a) a pain, and (b) not generally possible with the less expensive dividing heads available for home/hobby shop use.

Greetings Terry, What George said about the book by Ivan Law is correct. This book is a treasure trove of information about gears that is geared (like the pun?) toward the hobbiest. Ivan Law has written this book in a fashion that makes it very understandable for the average novice who has never cut a gear. He also writes about indexing and how to achieve indexing with lathe change gears with the result being more accurate that needed. There is one table of numbers in the book that has some numbers mis-labeled. I don't have the book in front of me right now so I can't tell you which one it is. This table is not needed for all types of gear cutting so you may not even use it. But if you do it will become obvious which numbers are mis-labeled and you can then write the corrections in your book. If you want to cut gears with a straight sided tool then you can generate the tooth form in discrete steps. You can use a fly cutter ground to the proper pressure angle. First make a cut to the proper depth. Then raise the cutter a little and rotate the gear blank a little. Then make another cut. The amount the cutter is raised and the blank is rotated depends on how many passes you want to make with more passes making a profile closer to ideal. If you search the group uk.rec.models.engineering you will find posts there that explain the process in more detail. ERS