Gear Cutting

It's taken me some time to visualise why different size gears need different profile cutters. Along the way I notice some folk cutting their gears with a cutter array of 3-5 cutting disc profiles. I guess that while the centre cutter is cutting to full depth (on each iteration of the vertical indexer) then the outer cutters modify the profile by the number of teeth/diameter of gear?????

If so can one therefore just invest in a single bought cutter of the largest gear range and go around the gear more times with that cutter raised and lowered appropriately from centre? Time consuming but might save cash...

Possibly, sounds like a complicated (the calculations will be more complex) route to the rack-form cutter approach.

Neil

I'd have thought the calcs were simple enough...rack tooth distances up and down and cut to the same Y axis number? ..Just means going around again 2 to 4 times depending or whether one thinks a 3 or a 5 rack cutter is best??

the reason is really that I still haven't figured out the form tool shape to cut a rack cutter (lazy) but quite fancy having a go (it's supposed to be 20DP ) and i like backup plans...

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This might help: **LINK**

http://khkgears.net/gear-knowledge/gear-technical-reference/involute-gear-profile/

MichaelG.

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Note: it's all Metric Module stuff on this site, but it's very good.

Edit: I just had to include this litte gem from the'Types of Gears' page in KHK's 'Gear Knowledge'

Edited By Michael Gilligan on 29/12/2016 21:56:42

Well i really wasnt expecting that , i think i just hit the quote tag a shade after you hit the edit tag.

I was just about to say nice link MichealG, ( thats not to say it isnt any more ) its just the timing of the edit really

caught me out

pgk post jogged the memory about gear cutting on the shaper , i had look for it but not found it by using the search engine , cant remember what topic it was under

sorry not much help really................b

For involute gears it's basically a trapezoidal shape like a screw thread. If you wrap it round a cylinder you get something like a worm. With some gashing and form relieving you get a hob. With one hob you can cut all teeth numbers for that DP or module, but you need a hobbing machine that turns the blank synchronously with the cutter. Teeth fully formed by one pass.

That I understand but I did find a link the other day to a chap who (as described) just cut 5 identical profiles and centred the middle one to his blank and cut to that depth which presented the 2 each side to a partial cut on those teeth when the blank was incremented. His comment that it worked as a practical answer albeit slightly noisier.

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Here you go, bodge: **LINK**

http://neme-s.org/Shaper%20Books/Michael_Moore/shaper%20gear%20cut.pdf

I posted the link in the recent thread "Where are all the Shaper Users?"

MichaelG.

PGK is talking about the same method that Neil use to cut the gears on his 'oriible 'orrery

A concentric hob rather than stacked cutters.

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... and are we not permitted to discuss other options ?

MichaelG.

If that's all you have to do then feel free...............................

Of course you are... and it's appreciated. Considering that a set of gears for a 1" minnie is pushing £90 and gear cutters are around £20 each with 4 needed.. Well a DIY alternative is promising and fits the idea of actually building the thing myself. The end result may be crap but at least I'll have learned lots along the way.

PGK although this uses a shaper action you can see the rack form of the cutter, you would jusy need to replicate that profile on your hob, the central tooth will do most of the work and then the outer ones just round over the corners of the next tooth out from center.

Sizes for making the rack can be found on the net, such as this one, Minnie would have used 14.5PA

Edited By JasonB on 30/12/2016 07:57:14

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With apologies to Shakespeare:

... for brave JS has killed the Shaper, and JS is an honourable Man.

MichaelG.

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I would have hoped, John, that you would recognise the geometric similarity between the two processes; and might even encourage such discussion ... but clearly your hatred of Shapers is overwhelming.

pgk pgk,

Getting back to your original question.

If you want to have gears with properly formed teeth using involute cutters then you must use the correct cutter for the number of teeth required. Involute cutters are sold in sets of cutters for cutting the whole range of gears from 12 teeth to rack. If it were possible to cut all numbers of teeth properly using just one involute cutter as you suggest then there would be no need for the set of eight. This would imply that all makers and users of sets of involute cutters are wrong and have been for years. Clearly not so.

A simple solution is often to make a single tooth cutter to the profile of the nearest gear that you have or use Ivan Laws “button method”. I made a 26 tooth change wheel for my Myford lathe using the 25 tooth standard gear as template for the single tooth cutter.

If the application is for a low speed, low load application then a certain amount of licence can be taken in producing a gear tooth form. For example a change wheel for a lathe. However if the gear is to be used at high rpm and transmit power for long periods as in a vehicle gearbox then the forms must be spot on then the gears will run quietly, absorb minimum power and have along life.

Have look at Len Mason’s excellent book Using the Small Lathe which explains simply about cutting gears in chapter 16.

Whatever............

Does PGK even have a shaper.

If he doesn't you might as well suggesting he makes the gears on a Drummond Maxicut shaper as I'm sure he won't have one of these either.

All I was suggesting is proven, simple and as Neil has proved can be done in a small workshop produced more gears this year than all the keyboard warriors put together.

Edited By John Stevenson on 30/12/2016 09:06:36

Michael the shaper link does not have a geometric similarity to the hob method PDK is thinking of using as it shows a single point tool that is moved to several positions and the blank rotated a full turn in each position. It would be similar to the method in his second paragraph where a single profile tool is moved to a new position and the blank rotated again for several positions

The hob only needs the blank rotated once and the tool is not repositioned as it cuts several teeth at onece.

For example the first cut looks like this

Then the next cut starts to shape the previously cut ones as well as start the next

Then when you have gone full circle you get something looking like a gear

This method does give a slightly facetted gear tooth but would probably do on a traction engine.

In theory you could use a rack shaped cutter in a shaper and just rotate the blank once but that is different to the linked method.

Edited By JasonB on 30/12/2016 09:22:14

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Is that question relevant ?

He is an intelligent amateur, looking for ways of making gears reasonably economically.

MichaelG.

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P.S. ... before you start sniping again:

It's not that discussion is 'all I have to do' ... it's what I sometimes choose to do.

Jason,

Lovely pictures, but may I suggest you read the article that I linked.

MichaelG.

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Well it won't be so bloody economic if he has to go out and buy a shaper for 4 gears