How to cut spur gears?

[sanddrag]

Can spur gears be cut in a 4 axis CNC mill with an involute gear cutter? If so, what exactly is the process involved and how do you know how far in to go with it? I've never seen gears being cut before so I don't know.

[Tristan Lall]

The traditional method of cutting standard spur gears uses a rack cutter which translates across the gear faces, in combination with precise intermittent rotation of the gear.

An extension of that is the gear hob, which is like taking a rack, and rotating it about an axis, and translating it some distance along the same axis—you get a helix. That helix is subdivided into cutting faces, which shape the gear teeth. The motion can be continuous: rotation of the gear, and translation of the hob.

Gear shapers are a form of tooth cutting used for internal gears, where a hob wouldn't fit.

You can also grind teeth, with precision wheels, profiled to match the desired tooth form.

You might be able to make one of these methods work, with the right tooling, but the tooling (and the fixture to hold it) would be messy and expensive. I'd just send it out for wire EDM cutting instead, if that's available.

[sanddrag]

What about an involute gear cutter like this? Can that be used in a mill?

[sanddrag]

Does anyone know how to use an involute gear cutter? I've probably found about a grand total of less than two hundred words about them after extensive searching.

[Travis Covington]

With a 4 axis, yes. The 4th axis would allow you to index the gear blank at each of the teeth you are wanting to cut. It would work just like a hobbing machine, except the part wouldn't neccesarily continue spinning. I'd imagine you would just make a pass for each tooth, having the machine rotate the part to the next tooth and continue cutting.

[TriggsJr]

Its a lost cause to try and cut spur gears on a verticle mill.

Go find a local machine shop that has a hob or shaper and beg for them to make the gears for you. Sponsor what ever.

Mills can cut sprockets slowly but not gears. True sprockets are hobbed as wheel its much much faster.

If you can get an opertunity to go see a hob working.....GO its a really cool machine to watch run. And once you see how it works you'll understand why not to bother with trying to machine it on a verticle mill.

While your there try and watch a shaper work very simple VERY COOL.

[dlavery]

Sandrag-

Cutting gears on a vertical mill is anything but a lost cause. It is done all the time. I have done it (and most assuredly will be doing it again) on my manual vertical mill with a rotary table and hand-ground gear shaper in a fly cutter. It just takes some patience and some care with the machining operations. With a four-axis CNC system, you are making it even easier. The only real question you have to answer before you jump in is: is it worth it for my application?

If you are just knocking out one gear, or you need it very soon, or if the price is a lower-order priority than immediate availability, then just go to a shop and have the gear EDM'd for you. It will be quicker, and you will be able to move on with your project. However, if you want to manufacture your own gears because you want to learn something new, or time is not the most pressing issue, or to save a few bucks, or just because it would be cool to say that you made your own gears, then go for it!

The specific step-by-step instructions are slightly more involved than we want to go in to here, but the quick summary is:

- determine the gear size and shape you want. There are any number of on-line and printed references for doing this. The best resource that I have ever found for a really good introduction to gear design is the technical section in the Handbook of Gears by Stock Drive Products. You can download the whole thing as PDF files from the SDP web site. There is also a lot of good stuff on the Martin site, but they don't get into the background knowledge you really need to understand what is going on and why the formulae work. The Handbook Of Gears is a bit surprising because it is tucked away inside the SDP catalog. But it is a great reference for understanding both introductory gear theory and application considerations.

- Obtain your source stock material and turn the gear blank on the lathe to the outer diameter of the gear (pitch diameter + addendum for the chosen tooth profile).

- Mount the stock material on a vertical rotary table (if being done manually) or indexing head or CNC rotary table. An example of how to do this is shown here (near the bottom of the page).

- Mount your gear cutter in the mill. You will typically use a Thornton gear cutter, a hob, or a single-tooth cutter ground from a toolbit. The cutter should intersect the center line of the workpiece. Position it to just contact the outer diameter of the gear blank, and set your index to that point.

- Start the mill and move the table to cut the first tooth. Depending on the tooth size, cutter type, and materials used, multiple passes will be needed. The depth of the final pass will be dependent on the tooth size and profile you have selected.

- Move the table so the cutter is beyond the gear blank. Rotate the table or indexing head to the next position, and cut the next tooth. Keep repeating until you are done.

There are frequently good discussions on this topic in places like rec.crafts.metalworking (one example is here and here) or on Metal Web News or on the Machinists Workshop BBS. Also, there are many, many books on the subject of both gear design and fabrication. For the one-off type of situation you seem to have here, look through some of the home/hobbyist metalworking materials such as Home Shop Machinist or Machinists Workshop, or any of the books by Rudy Kouhoupt or Phil Duclos. There are also lots of other links to additional information on the Metalworking Web Sites Index.

-dave

[ajlapp]

Here is a cluster of gears that I cut on the Haas Vertical CNC 4-axis at Michigan Tech when I was affilited with Team 857, Superior Roboworks.

It can be done...in fact We built two robots that season, so we made twice as many gears as pictured there. I also made a 16" diameter, 12DP gear which anchored our turret.

We saved tons of cash. The involute cutters we bought were cheap. We used regular old 1018 steel, but later manufactured the final stage out of AR400. Still quite a bit cheaper than off-the-shelf components, plus you can make them any size or style you like.

We cut blanks for these gears on our waterjet.....when it came time to cut, we did them all in one shot, 4-6 gears at a time.