Making Gear cutters

[ozzie34231]

Let me start by saying I'm a hobby machinist.
I recently bought a little book, "Gears and Gear Cutting". Toward the end it talks about making one's own gear cutters, and points out the need for a multi-tooth cutter to be relieved and further includes plans for a tool that will do that relieving operation on a lathe. The tool oscillates and ratchets the gear cutter blank so that one tooth is worked on with each revolution. The parts of the tool seem well within the ability of the home machinist, there are about a dozen parts. The invention was in a Victorian tool catalog and called the Eureka.
The book also points out that a perfect profile of a gear tooth cannot be easily be cut with home type machinery, but that the profile can be very closely approximated by the arc of a circle and therefore a forming tool using two circular cutters can be used to create a gear cutter.
So it occurs to me that with a home CNC lathe and the Eureka, one could make rather exact gear cutters.
I'm interested in hearing any and all comments from those who might add some insight.
Ozzie

[flick]

This sounds basically like how a piston lathe makes non-cylindrical pistons, by plunging/withdrawing the cutter twice per revolution. Is this type of action basically what you are talking about?

If so, I think this will require a great deal more aggressive plunging than what is seen on a piston lathe, and will therefore require a highly relieved turning bit to do the cutting, which will weaken it. Weak cutter + interrupted cut = no bueno, so that's one challenge to overcome.

What are you referring to by ratchetting the blank?

Also, how is the oscillation of the tool to be synchronized with the rotation of the spindle? Is that what the ratchetting is about?

[RICHARD ZASTROW]

Ozzie, you might consider having the form relief "back-off" done on a wire EDM.

Gear hobs and other gear cutting tools are very "fussy" in thier design and manufacture.

Dick Z

[ozzie34231]

After posting I did some further searching and found that the subject has been discussed, fairly extensively, in this forum.
CNCzone.com-The Ultimate Machinist Community > MetalWorking > General Machining Discussion > gear hobbing....
Some of the Eureka design info looks quite different from the plans I have, though the principal is the same. The plans I have I think were reverse engineered by Dr. Chaddock of Quorn fame.
There's not much discussion of combining CNC with the relieving device.
I think that a G-code designed to cut the cutter form, could do that first and then be used with the device to do the relieving. The lathe cutting tool could be just a square carbide tool, (a cutoff tool), one corner for each side of the profile. Given an accurate profile drawing, a good CAM program would make a very respectable G-code.
Ozzie

[ozzie34231]

Originally Posted by RICHARD ZASTROW Ozzie, you might consider having the form relief "back-off" done on a wire EDM. Gear hobs and other gear cutting tools are very "fussy" in thier design and manufacture. Dick Z

Thanks Dick,
Though I have a rough idea of what an EDM is and does, it's not clear to me how that is better.
But in any case, as I said I'm only a hobby guy. I don't have an EDM and I hope you don't convince me that I need one because there is only room for a small package of toothpicks left in the garage.
Ozzie

[RICHARD ZASTROW]

Ozzie, actually I meant to "farm out" that operation to ????. After further consideration, I don't think that would accomplish the total form relief back-off. I know the hob manufacturers use EDM in their processing, but I don't know that process.

The hobs are very expensive and their use complicated. You may be better off using individual gear form milling cutters. These are reasonably priced and readily available.

Dick Z

[Mcgyver]

you know you need a universal mill or hobbing machine to use the hobs right? having said that there's been lots of home shop hob relief setups made including Eureka, there’s one nice example here or the home shop bbs site. you'll also have to set up a the leadscrew to cut diametrical pitches. Find and search indexes of Model engineer magazine and model engineer workshop for several home shop builds of hob relievers and hobbing machines

There's nothing about it that isn’t doable, its just that who makes enough gears to make it worthwhile to make all the tooling AND a hobber (if you don’t have a universal mill). if you do do it, one cutter will make all your gears of a give size and you'll be able to do spur and helical gears

If its a small number of light duty gears, there's a cheat using what i call a faceted hob. its really just a profile cutter - Peter Harrison has a great how to on his site.....but keep in mind this is NOT a hob and the tooth form will be a faceted approximation of an involute profile


http://www.helicron.net/workshop/gearcutting/

Theoretically, the whole hob including relieving should be able to be cut on a cnc lathe.

[One of Many]

This is a fantastic learning experience, if it is a non-standard gear. But....several hours to create one gear cutter and a gear blank and then cut teeth will shortly become drudgery. Been there a few times making cheap tool steel gear cutters manually, so I really appreciate the shortest options possible.

If the gear DP and tooth count is known, it may be cheaper to just buy pinion stock or gear blanks from places like Stock Drive Products, Browning or Martin, than making hobs or buy individual involute gear cutters for less than 10/year. Pinion Wire or Pinion Stock is just parted off in the lathe with or without a hub. Add a set screw or keyway in a secondary operation.

This is of course if there is a choice or the time spent ends up as a worthwhile result.

Tons of youtube video's show gear cutting and hobs for both home and commercial operations.

Here is one showing the Eurika in action. It appears you also need to create chasers for to advance the cutter.

YouTube - Charles Lessig's Hob Reliever Eureka Balzer Gear Cutting

YouTube - Charles Lessig's Hob Reliever Eureka Balzer Gear Cutting

DC

[HuFlungDung]

Many years ago, I had the gear cutter bug myself, and came up with a hydraulic relieving attachment for use on a larger, heavier lathe ( a 19" Summit). The setup consisted of an 8 lobed cam wheel, about 8" diameter, which was affixed to a 1" mandrel. The cutter blank was mounted on the other end of this mandrel.

Then the whole thing was set up in the lathe chuck, using the tailstock to support the outer end. On the lathe bed, I had a short stroke hydraulic cylinder beneath the 8 lobed camwheel. From that cylinder I ran a hose to a small slave cylinder that I had built to mount in the toolpost. The slave cylinder simply had a square hole Sturdy sleeve inserted into the end of its shaft, out of which, the toolbit projected.

Now running the lathe at low speed, the 8 lobed cam wheel thumped along over top of the master cylinder beneath, producing 8 strokes per rev. The spring loaded slave cylinder in the toolpost pulsed out in unison with the impulse from the master, and the spring inside the slave forced the master to stay in contact with the cam wheel.

So then the trick was shaping the toothform on the cutter blank, using a template. The profile was truly form relieved. I always worked with a round cutter disk blank, and then cut the tooth faces afterwards, because I could then mill away the most imperfect part of the soft cutter where the tool was doing the retracting.

I would think this could be done very very nicely on a cnc lathe, but I've never seen one with a cycle that would properly automate the slide motion in step with spindle rotation.

[mikemill]

Ossie
Unless you need your gears to perform at high speed, it is possable to make a simple cutter, the shape depends on the No of teeth on gear. Take a square lathe tool and hold it in a tool clamp using the worn edge of your grinding wheel you can shape the flanks, make sure it is slightly angled for relive, use a magnifier to observe equal sides. You will need to make a holder to suit your mill, essentally a square fly cutter, using a dividing head and the cutter on centre line cut the gear in the normal way.
You might need to experiment with shapes, but its supprising simple process, give it a try

Good Luck

Mike

[Les A W Harris]

There is also this type, with a cnc lathe you can get the form dead on. It's mounted eccentric to provide the clearance while cutting.



Regrind face radially to center, to maintain form. Slow but for home use?

Cheers,

[RICHARD ZASTROW]

For use in a home shop, I believe Les has the best solution so far.

Dick Z