Technical Question - Pressure Angle

You've been greatly anticipated, search on "Fellowes Gear Shaper" Not CNC, though. One sort used a reciprocating rack-shaped cutter, I believe the pressure angle is the angle of those rack teeth, but it's been a long time since I read anything about tooling up one of those machines. You'll probably have to find some really old specialty gear books.

Stan

I'm NOT handy with math ( wasn't in my long lost youth and have forgotten what little I know back then ) and so my question.

I'm plodding along though, and should get it eventually.

Yes I do.

Again, the theory goes something like this.

A rack is a gear of infinite radius.

The "spiral" that is the involute for the tooth of a gear of infinite radius is in fact a straight line.

So a gear can mesh correctly with a rack of straight sides.

Therefore, a tool of straight sides SHOULD be able to create a tooth of the correct shape.

As documented in a followup article, yes, guys are doing it on a shaper.

My plan is to take the mechanics out of it .. remove the need to create drums of just the right size (for each DP) and simply do it under CNC.

And yes, I do plan to CNC my shaper to do this (the job's half done right now anyway), although it's just as easily on a CNC HMill or VMill with appropriate shaped flycutters.

Here's one VERY simplistic method of doing this.

Let's begin with a mechanical shaper - ram motion by arm power alone.

Grind a toolbit a lot like an ACME threading toolbit, whose angles match the pressure angle of the gear you want make.

Put the gear blank in a rotary device. See the pic in the articles I quoted previously.

Line the toolbit up with dead center of blank.

Lower the cutter so it just touches the blank.

Move the cutter (table) left so it's out of the way.

Lower the cutter to the full depth of the tooth you want to cut, (See why you have to move the cutter?)

Move the cutter (table) back right until it JUST touches the blank.

Take a shaper stroke to remove .. darn near nothing.

Rotate the the blank 1/1000 of a degree while moving the table 1/1000 of an inch to right.

Take another shaper stroke. Now a TINY bit of metal is removed.

Repeat the above steps for a LONG time and a tooth with apparently curved sides appears. The curve is in fact composed of zillions of straight line segments, but by making the steps small enough, it is, for all practical purposes, a curve.

And people CLAIM that the tooth so formed IS an involute.

This I can't prove, and I'm not sure I'm smart enough to ever prove it, but I am going to build it and see.

Alan

This IS one way to do it, but you need an enormously strong shaper and a fantastically rigid dividing arrangement and most of all, a cutter ground to precisely the right shape, including that nasty curve.

And that precisely right shape cutter would only work for one DP of gear and one range of teeth. You'd be back to sets of 8 cutters per DP, the conventional way.

The method I am excited about uses a cutter with straight sides, and but a single cutter can create all the gears of all numbers of teeth and DPs.

Oh yes, it's a VERY slow process. That's where the CNC comes in. It's little more than a curiosity if you had to do it manually, methinks.

Alan

I've just started to subscribe, after buying it locally for a few years.

And yes, I love the English !

I also particularly like the different approaches some people take to their projects. There was one recent article with both metric and imperial dimensions on the same _piece_, and used several different kinds of fasteners; Whitworth, BSW (which might be the same as Whitworth), BSF, metric, UNC and UNF.

Man, that's a lot of taps to keep track of !

Alan

You're missing my point. I'm not talking about using a form ground cutter, but about taking small chips with a straight side cutter. If the gear is 1" thick, you set the shaper for a 2" or so stroke. Each stroke of the shaper will cut one facet of one tooth - you then move the dividing head and the table slightly (they are geared together, or use a wire or cable to link them). The number of facets you cut per tooth will be up to you.

A shaper cuts in a straight line. A mill doesn't, unless you feed the table. Each facet of each tooth on the mill will require a table feed and a return.

My shaper cycles at 30/60/120 strokes per minute. How many table feed/return cycles per minute will you get with a mill?

Sorry for my confusion .. we're both talking about the same thing.

Yes, this will take a very long time on both types of machines. And I hadn't done the math on just how long both would take, but now that I think about it, yes the shaper WOULD be faster .. or at least, as fast.

But that's OK, I'd planned to use the shaper anyway, just because the tooling is easier.

I'll have to complete the CNC "addition" before doing so, of course, but that's well along the way. I say addition because it's my plan to keep all the manual capability, just add a couple of motors.

The secret will be to add a sensor to the RAM, and only move things when the ram is not in a cutting area.

The software I use is CNCPro, and a long time ago, Doug Yeager, the writer, added a "pause" feature for me .. pull this pin low and the program pauses. By adding some timed delays in the software with the sensor on the RAM, I should be able to move the table just when I want to.

Thanks again for the thoughts.

Alan

It has been pointed out to me that my process will take a very long time. Kicking around my head has been the idea that with a suitably shaped milling cutter, the job could be done much more quickly.

I dismissed this as a viable option because no suitable cutters could be found in any of the catalogs and it never occured to me that I could make my own.

But now that you mention it, maybe with a bit of thought ...

Thanks !

Alan

On a mill you can make a cutter that has more than one straight sided cutter that looks somewhat like an Acme tap. Unlike the tap, this cutter has a series of grooves not a spiral.

With it you get some cutting on about three teeth for each table feed and can cut full depth on one table feed. I am not saying this is a great way to cut gears, but it works.