Cutting internal gear teeth

[bogstandard]

This is one thing I have never attempted.

I went in search on the web to see how it is achieved, and it looks like that it is done with a single tooth cutter, like cutting a keyway slot, but going around the internal diameter to produce the number of teeth required (looks to be a good job for a shaping machine).

To all those more experienced than myself, is it a feasible proposition to do this by hand using a dividing head and a hand powered keyway slotter, or is there an easier way?

Bogs

[John Stevenson]

About the only way John,
If the gear is big enough inside another way is to get a small gear of the same pitch turn it dished, harden it, hollow grind it and use this as a cutter indexing round one tooth at a time.

Advantages are finding a ready made gear of the right profile may be easier than hand grinding an involute. As you also cut one tooth you also part cut the teeth to the side. As you index round the multiple cuts will generate a nearly perfect involute.

This is the way it's done commercially but the blank and gear rotate in sync each straoke of the ram.

John S.

[bogstandard]

No worries on that score John, I had delivered this morning a set of module hobs from Arc Euro.

They managed to get me a full set together, even though it says they are out of stock.

What nice people to deal with.

So not only will I be able to cut normal spur gears, I should be able to make the cutter as you suggested.

It sounds like a bit of a long job, but what you have told me, instills me with confidence that I will be able to do it.

Many thanks.


John

[John Stevenson]

You will be please with them, they are very good quality.
When they got the samples in they passed me a 20 DP hob for testing as I have a gear hobber, well I love testing other peoples gear 

So found a nice laser cut blank still with the outer scale on and dross from cutting, dialled in a reasonable high speed and feed and ploughed  thru.

No problems, nice gear good finish and no damage to the hob, might sound rough but someone is going to do the same and moan if it shags all the teeth off.

I'm still using the same cutter to cut all the 20 DP gears I have to produce and so far it hasn't needed retouching.

How are you going to drive these ?

John S.

[DeereGuy]

John's do either of you have a link to the hob  you are talking about?

[bogstandard]

Bob,


http://www.arceurotrade.co.uk/Catalogue/Cutting-Tools/Gear-Hobs



John,

After seeing that DVD about gear cutting a few months ago, it gave me a good idea to try.

Very similar to using a freewheeling blank on the lathe for cutting a worm wheel. See attached C-o-C.

I am contemplating stacking gear blanks together, or make long blanks and part off to the thickness required.

The gashing doesn't need to be too deep or long, it is only there until the spiral of the hob starts to work like a worm spindle thread and drive the blank while it is being fed along it. Once the intial cut is done, it is a matter of going back to the start, put a little more cut on, re-engage the teeth and take another cut.

I think it has every chance of doing a good job, but if not, I will most probably make a small hobbing machine.

http://www.collegeengineering.co.uk/Castings/582.htm

The cost of small gearing is the only thing holding me up on a lot of projects I already have a desire to do. They are ridiculously expensive. If this works satisfactorily, then the cost of the hobs, and if needs be, the small hobbing machine, will soon be recouped by the savings made. This set also contains the correct module for the change gears, and other gears on my lathe, so then that opens up other areas to be explored.


John

[John Stevenson]

Don't see any reason why that won't work although I'd go for full depth at one pass, light cuts have more chance to bounce about wheras a deep cut into a pre gashed blank should hold tit steady, use a sacrificial gear at the start and end as that's were all the errors getting started and ending will occur.

Personally I wouldn't bother with the CES hobber as once the method is established I'm sure a more robust / universal machine could be fabricated up at 1/4 of the cost.

John S.

[bogstandard]

I will suck it and see John. It will be a few months before this project gets to the front of the queue, so I have a little more time to search for answers before then.

If this works OK, I won't even bother with a dedicated hobbing machine. I am not going into production, just making gears for everything I want to do. I reckon if I can cut eight pairs of gears, maybe less, then the hobs will have paid for themselves.


Bob,

Unlike what most people think, they aren't great big stonking beasts of a cutter, but very genteel and superbly made pieces of engineering wonderment.

This is the largest (1.25) and smallest (0.3), the largest is shown against one of my change wheels off the lathe, the cutter is in fact shown upside down.




John

[ScroungerLee]

About the only way John,
If the gear is big enough inside another way is to get a small gear of the same pitch turn it dished, harden it, hollow grind it and use this as a cutter indexing round one tooth at a time.

Advantages are finding a ready made gear of the right profile may be easier than hand grinding an involute. As you also cut one tooth you also part cut the teeth to the side. As you index round the multiple cuts will generate a nearly perfect involute.

This is the way it's done commercially but the blank and gear rotate in sync each straoke of the ram.

John S.

I have a question about this, using one gear to cut another won't cut deep enough to reach the base circle, will it?

Lee

[John Stevenson]

Technically no, the cutters used in industry have been modified to allow clearance.
In the home shop use you can fudge the calculations by adding the clearance to the pitch circle, it will add a gnats cock of backlash to the gear but it will still be a serviceable gear.

To be honest on small model engine gears like wot John is after you would probably be hard pressed to measure any inaccuracy.

One point about involute gears is that even though centre distance and or diameters can be out the gears will still mesh correctly and run OK even though they may have excessive clearance.

John S.

[bogstandard]

Very interesting info coming out here guys.

Keep it up and I will be starting sooner than you think.

Just joking of course, this is a rather large subject to absorb quickly, but every little snippet of information all helps in seeing the big picture.

Reading technical information is OK to a point, having personal experiences to read is even better. There is a great difference between theory and practice.

John

[John Hill]

John, are you going to be able to cut internal gears with those hobbing cutters?

[bogstandard]

John,

This is what we have been discussing.

The hobs won't cut internal teeth directly, only external.

But with the hobs I have, I will be able to make a cutter fairly easily, and that will then allow me to broach the internal gears to the correct profile using a hand cranked keyway slotter.

John

[John Hill]

Oh right!  Sorry I must have been a little switched off!  I think if you want to cut model size internal gears you should call around here and use Sally Shaper.

[John Stevenson]

John,
Not sure what you mill looks like on the back end but many have a slotter position.
Some years ago a friend of mine, now dead, wanted to do similar but for keyways, I scrounged him a big angle plate, slide off a machine and a motor and gearbox and he built a slotter to suit.






Sorry about the state of the pictures but they were done with an early camera phone and I never got any decent ones but I thing you get the idea.

One of these and a RT and you have a gear planer.

John S.

[bogstandard]

John S,

Mine does have the sliding head, but no machined area at the  back to bolt things to. I think it is designed more for the machine to be used like a radial arm rather than a two headed machine. Besides, even if it would fit, the way the machine is sited, I don't think there is enough room to swing the head around by 180 degs. I have enough trouble swinging the cat.

John H,

Being able to use Sally would be great, but the cost of air fair is a bit offputting.

I have yet to make the slotting tool, but I already have the designs for it in my head. Normally it would be used on the lathe, but to do this job, it would need to be able to work on the mill, so as usual, with my interchangeable tooling idea, I will need to mount the lathe compound slide and toolpost onto the mill, to give me the adjustment for cutting when the part is mounted into the dividing head.

I suppose I could use the mill's own quill as a slotting head, but I don't fancy the idea of loading the quill like that, plus I would have to fix up some method of stopping the spindle turning, and it might be very tight on throat dimensions as well. So that idea hasn't even been considered past the thought stage.

As one problem is solved, another one rears it's ugly head. But what would life be like without it's little problems.

John

[John Hill]

John,  yes air fares can be a challenge at times. Living where we are it is a significant air fare to get to anywhere,  three hours is the minimum trip and that only gets one to Australia, its about 13 to the US and I think 25 or more to the UK.  One trip I did to Europe was 46 hours door to door because of diversions due to weather then aircraft problems.

I suppose with some sort of jig you could remove about 75%, maybe more, of the metal by drilling a small hole at the base of the tooth and a bigger drill above it?  Maybe with a dividing head you could drill the teeth without a very complex jig?

[sorveltaja]

I wonder, what is to be expected. Interest plays main role. Set of planetary gears maybe?

[cedge]

John
Have a look at Evan's solution to the hand "hobber/shaper/slotter".  http://metalshopborealis.ca/hobber/hobber.htm

Steve

[bogstandard]

Sorveltaja,

There are a lot of engines that use complex internal and spur gear setups. By doing it this way, I will be able to expand the range of engines I have the opportunity to build, purely because the cost of ready made gears is so prohibitive. Being able to make my own will put them within financial reach.

This is a simple example of what I am on about.

http://www.billreichart.com/images/epicyclicengine_lrg.jpg

Steve,

That is almost what I had in mind, but because it would be very dificult to set up indexing on my lathe to do all combinations of internal gears, I will be setting up the same sort of operation on the mill using my dividing head to give me the exact tooth spacing, and the compound to put the feed for the cut on.
Many thanks for the link, that will come in handy when I start to build the slotter, it is now in my 'favourites'.


John

[John Stevenson]

Best move the handle to work sideways, not vertical.
Unless you are 7 foot tall it becomes very tiring working over the top as opposed to working at waist level.

John S.

[cedge]

John
You're getting lazy...LOL. I added an indexer to my smaller lathe spindle long ago and it worked like a charm. Now that I've got to know the bolt circle thingy on my DRO, making up custom plates wouldn't be much of a job.

I can envision using the DRO to make the gear blank work as it's own indexer by drilling the required number of holes of the right size at the proper radius. Seems like that would cut down on the hand slotting work too. Trust me.... I'm not trying to teach an old dog to suck eggs.... just tossing out ideas that I too might need for building the same machine. When your box has a hole in it large enough to fall through, you wind up thinking outside of it..... even if you don't want to....(grin) You've gotten my head going off in new directions, so I guess I can't complain too much.

Steve

[John Hill]

John Bogs, I think you have some sort of vertical filer machine?  You have a power hack saw too I think?

Perhaps you could sit down with a cup of tea and design some sort of jig for either rmachine to hold your blank while something like a shaped file or a form tool cut the tooth?

[John Stevenson]

John
You're getting lazy...LOL.
Steve

  Not really Steve, the odd keyway especially into alloy isn't bad but steel is another ball game.
I got back into the workshop game making parts for my racing bikes, at the time close ratio gears for Nortons were like hens teeth so I used to make my own.
Putting 6 splines into gear steel on 3 different gears per box soon teaches you the most comfortable way.
I had my lever horizontal and at elbow hight  so my fore arm was straight, the lever was about 24 " long and any less would have been a right pain.

The £400 I paid for my first slotter was money very well spent.

John S.

[cedge]

Sorry John S.... That one was actually targeted at the Bog-o-lounger....(grin)

Steve