Gear Cutting for a begginner?

[Owain1602]

Hi, does anyone have any experience in gear cutting on a milling machine or lathe?
Me and another guy from work are starting a machine shop night class in a few weeks, he's got a steam locomotive to be doing and i need things to make really.
I know gear cutting is unbelievably complex so it would take ages to learn this but would it be possible to make my own gearbox for my rally car? Choose the exact ratios i want, make the gears straight cut.
I have started reading about gear cutting and its a very large field, Cyclodic curves, Hypocyclodic, PCDs.
I could sort of have this project in the back of my mind after years of practice i might begin to make it.
The lecturer said he can make absolutley anything on the machines, i think he was just boasting. So if things are to complicated for me to make, he'll make them.
What do you guys recon? The machine shop is amazing, every machine you could ever think of, plus they do heat treatment and tempering and things there.
So, I know it is possible cause anything is possible, but just how difficult and complex is it? and whats the difference between doing it in a lathe or a milling machine?
Im in the process of looking for a milling machine and maybe lathe for my workshop at home so I will be able to practice and play about a LOT. Im not going to just go to the course for 1 term, im going there for years and years to get as good as I can. Everything I do, i want to be the best.
The gearbox project wont get under way for many years probably, because there's no point in starting if my skills aren't good enough

Thanks for your time and patience.

[lakeside]

We have this thread going Questions about Gears and yes depending on form you can cut a gear on a mill

[Owain1602]

is that gear cutting with cnc i take it.
what aspect of gear cutting is so difficult then? i have been told that most engineers avoid it because of its difficulty.
from my very small knowledge about the subject, it seems to me that most of the work is in the designing of the gears and obtaining the right cutter. Such as deciding your cycloid and hypocycloid curves.
In that sortware, i see blocks for entering stress angles or whatever, how are these found then?
Im 21 and a head mechanic in a fairly large workshop and regularly service and re-build car gearboxes, but this time i want to make my own gearbox using the original box casing and bearing housings but changing the design of the gear type from helical to straight cut for less transmission losses and changing the ratios to suit different evens and my engine torque curve.
Any help will be appreciated.
P.S I have a book on order "Manual Gearbox desgin" which might help me a little.
Thanks a lot

[fkaCarel]

Making gears in the traditional way using a dividing table is a piece of cake, using gear cutters. These gears are limited usable, because they have evolving errors. So for better gears you go for hobbing, where the gear is made, like it is contact with another gear. But car transmissions use different gears. All the different set of gears must have the same pitch (centre to centre distance). They must also have prime numbers of tooth, to avoid the same tooth constantly meeting the same tooth of his partner. Then they use techniques of tooth bottom strength heightening, where they enlarge the pitch circle. This is advise and expertise land. After this comes case hardening. Same land.

Conclusion:
For industrial use: Make your own, only in emergency, if not for sale and preferrably not above 1000rpm.(noise).
For automotive: outsource.

You live between open gearboxes, start measuring gears and see how it's done.

[diarmaid]

Hi Owain. Who publishes the book 'Maual Gearbox Design' ? Also if you could look at the thread in post no.2 above and give me any advice on attaching the drive shaft to the gear from your experience it would be much appreciated. There won't be any housing around the gear. Thanks.

[Owain1602]

Why are the teeth made using a dividing head and a cutter so different to other methods then? Is there a way of cutting the gears like this then modifying them to have some properties of the other processes?
They are gonna need to cope with around 150nm and about 8000rpm!
I take it making them straight cut gears will make then A BIT easier to cut than helical plus they wont have to cope with side thrust.
I have been advised by a guy to use N39 to cut the gears from by a historic car builder, any comment on this material?

[fkaCarel]

With the dividing method you use a set of 8 cutters for the range of tooth between approx 15 tooth and straight, somewhere around 135 tooth. They are approximations. The hobbing method uses the a mill with the "straight profile" and is geared to the gear to make. The profile of the gear is in this way automatically corrected. You would see, feel and hear immediately the difference of this two methods.
For "sport" appliances straight is the normal choice, more noise, no side pressure. Keep also in mind, that you will have to broach the internal splines and make the intricate connecting lips on the gears. The lips have an angle to keep the gear "in gear" and radiuses on the bottom that with high power, scratches and unoptimal hardening can come of.

[Owain1602]

You seem to know your stuff about gear cutting.
So do you recon the hobbing method will be doable in the machine shop they have at the engineering college im going to? Its a well equipped shop.
So say i want a 5 speed box, i will need 10 gears plus an idler for reverse. Will i require 11 different cutters for this then?
I know what broaching is but how difficult is this to do? I guess all the gears can have all the same splines on them so id only broach them all the same then?
What dissadvantages would i see from making the gears from a dividing head and cutter over the other? I dont really care about noise from the box anyway, the 4 throttle bodies i have sucking away make quite a bit of noise already.
Thanks a lot

[fkaCarel]

Well, you need a hobbing machine. They are certainly more rare than lathes and mills.
The thing I worry about is the elevated pitch circle. Once I had to outsource primary transmissions for racing motorcycles. I had everything sorted out, according to industrial standards. The manufacturer, who was experienced in this (and also in hardening) told me, that standard sizing, for the small gear, would make the teeth to vulnerable. The correction for low tooth gear weakens the base. An extreme example is the ancient roll dryer, used for drying clothes. The design (roll dryer) allowed only for four extreme corrected and vulnerable teeth. When you see one, look at this detail.

So he designed within the dimensions his own pitch circles and hobbed the special gears with a standard hobber. These gears never broke down and a championship proved he was right. Knowing this, and putting your caliper here and yonder, you see this is widespread. Transmission gears are high-stressed components.

[Owain1602]

Ah i see what a hobber is now, looks like a torture instrument.
Can you use the hobbing bit in a lathe or mill and use a dividing head or is that being stupid? Im sorry in rubbish at this right now as i have no experience.
I dont see that being a consideration really with gear sizes, they all seem to be of a fair size, cant remember diameter of smallest gear right now. So you're saying smaller gears tend to be weaker because of the smaller number of teeth or smaller diameter? Because the teeth can still be same pitch cant they?
Sorry, what do you mean by elevated pitch circle? Do you mean Pitch circle diameter? sorry again

[fkaCarel]

I'd say google a little on "Module" or "Modula", read up a catalogue of industrial gears, find out how a hobbing machine works, reread what I've written and come back next week. It doesn't sink in just on one saturday night.

[Owain1602]

module or modula in conjunction with what then? Modula the programming language?
Sorry, thanks.lol