WHERE ARE THE SHAPER USERS ?

I'd be interested in that too. Most of my gear books say it isn't as simple as the tooth space being the same as the tooth space for an external gear. But they then don't tell you what it actually is. As I understand it an internal gear is like an external gear turned inside out, but I'm having difficulty imagining the transformation.

Andrew

All the practical examples I have come across say that they made the same shape as an external gear in negative.

So the cutter needs to be shaped like the TOOTH of an external gear not the tooth space, i.e. convex sides, not concave ones. You could generate a cutter using a rack-form hob.

Neil

Last paying job I had was for a company who made high precision epicyclic gear boxes. All the internal gears were cut using standard gear shaping cutters, both dp and module. I know because I cuts lots of them. The cutters were imported from either the States or Switzerland and were mostly Hss with some in carbide, hideously expensive at about £1200 each depending on size.

Michael

Very possibly, but that cannot be exactly correct. The shape is indeed like the tooth of the external gear, but it is not identical to the mating gear tooth, or it would never work due to lack of clearance as the external gear rolled around the internal gear. What interests me is the theoretically correct shape for the cutter. Given that one would be machining the cutter anyway it makes sense to get it right.

Andrew

Machinery's Handbook makes the suggestion that the standard #1 cutter (135 T to rack for standard external teeth) will produce acceptable internal gear teeth of 4 DP and finer, and when there are 60 teeth or more to cut. The finer the pitch and larger the tooth count the better the result. It goes on to say that this approach is considered satisfactory for ordinary jobbing work and it warns that the use falls down badly when the tooth count of the pinion is large in proportion to that of the internal gear.

​The other advice offered is to reverse addendum and dedendum as they are defined for external spur gears.

A 20 degree involute full depth tool form is recommended, but 20 degree stub tooth and 14.5 degree full depth tooth are also used.
My older book on the subject only talks about the reversal of geometry and not the other information, but there were no helpful illustrations provided.

So make what you can from that
Regards
​

Brian

That's pretty much what my Gear Design Simplfied says too. I've never needed to make an internal gear, and I can't see a pressing need at the moment. But there must be a theoretically correct shape and my idle curiosity wants to know what it is; taking into account what happened to the cat of course. I also suspect that it is easier to make ones own arbitrarily shaped cutters now than it used to be; even HSS can be machined with the right tools. So the approximations necessary in days gone by are possibly less critical now.

Andrew

STOP PRESS: A trip round the internet resulted in me ending up on a thread from 'Practical Machinist' about cutting internal gears. A very knowledgeable old school member recommended the Gear Design Manual - Section II by Buckingham for internal gears. So I've taken a punt and ordered a secondhand copy from the US via Abe Books. I went mad and selected the priority delivery service for a total of £11.68p At least I don't have to break the bad news about the spending to a significant other. We'll see what turns in a couple of weeks time.

Andrew

PS: It's almost enough to persuade me to extract a digit and get my shaper repaired and running again.

Seeing this shaper thread is going the way of internal gears i have
added some photos to the album "gears" that may be of interest to some.
The pair of gears here in this photo 1 & 2 are for a model wankel engine
rotor they are 1 mod 20 deg pa 14 tooth for the gear and 21 tooth
for the ring.The engine was from a design published in the
Aeromodeller annual 1971 by Julian Falecki the drawings show
some modification from standard size to avoid undercut on the 14
tooth gear ,the ring gear has the same modification to allow the gears to
mesh and also to obtain the correct centre distance 3.5 mm.
The next photo shows the hobbed broach made from a piece of
steel from the scrap box eventually case hardened.Although there
is no form relief the broach will cut well enough to produce a small
quantity of gears.This perhaps makes the point that the internal form
is the same as the same size spur gear ,however the broach has been
cut a little oversize to allow for some clearance.
Since the broach and gears are both hobbed this is an easy thing to do,
i dont know how this could be done using the usual form type of gear
cutter as you have to stick to correct pcd .

The third photo is from the Mew article Gear hobbing on the mill
and shows this test gear set of 1 module. A silver steel cutter is hobbed
whilst set over at 5 deg giving some clearance ,the cutter is set up
in the Dore westbury and cuts to the initial depth .From there on the
cnc control keeps the gear blank and cutter syncronised and the z axis
planes the gear form ,the cutter does not rotate but rolls around in the
internal gear form photo 4 .The rotary table moves about 2 deg and the
x y table movements move to these positions.
This gear set was only made to see if it could be done , sometimes it is
easier just to try these things to see how it works.

John

This is nice work John, it makes my efforts look rather trivial.

​I think you are right, it seems to be more a case of 'having a go' and learning from that to progress forward to the next version. From what I gleaned from reading, a little more space is needed to allow both gears to mesh nicely over a longer engagement and tooth tip rounding of the pinions can be made quite generous. That might explain Machinery's recommendation in using the #1 cutter to give the broadest base to the internal gear. The tips of the internal teeth are better for some rounding.

​Shapers have to be used for ring gears that are closed at one end, I can't think of another machine that would have the right sort of action for the job

​Andrew
​I am equally curious about the theory and may well order a copy of that book for my own interest. I am a dipper and browser by nature and like to have the information in book form without searching for it electronically. Thank you for sharing the reference. I find Abe Books quite helpful in stocking these older books.

​Brian

John: Thanks for the information and pictures, impressive. With the reciprocating hob do you 'rotate' the blank around a stationary hob or rotate the hob too? If the former I'd have no problem doing something similar. The question is what is the shape of the tooth on the reciprocating hob? For external gears it is a rack form, ie, straight edges. For an internal gear, where the shape to be cut has concave sides, I don't see how that would work? So presumably the hob for an internal gear is the reverse of one for external gears, and has the tooth shape for the smallest gear to be cut? Therein lies the nub, I'd have no problem cutting an internal gear, it's the shape of the cutter I'm not sure about. Of course if I needed to cut an internal gear I'm sure I could make a tool that would work. But since I have no pressing need to make an internal gear at the moment I might as well take the time to understand the design process.

I've found a PDF on the net which shows some of the calculations. It's in module rather than DP which I am more familiar with, but in essence it seems to be pretty simple. The desing process, not surprisingly I suppose, is very similar to that for an external gear. Basically the tooth shape is an involute from the base circle. It gives some inequalties which, if met, ensure no interference of the teeth. It also mentions corrected tooth shapes, but as seems to be common give no details of why that might be needed, or when to use it.

Andrew

My 10 M had it probably 20 years rescued from a scrap yard still scared of it!

My dad who worked at loads of engineering places in the Birmingham area in the 1950s tells me that it was a machine only the toolroom guys ever used

So mainly for repairing bits and making bespoke stuff

That could describe a lot of my work Ady...

Regard,

IanT

Here's a real but small one for the courageous

**LINK**

I have a feeling laughingboy what ever will have to negotiate down from the bid price.

John

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Have you noticed he lives in Hope Valley

Rik

I hadn't even better.

However if some one wanted on that could be worth £100 or so. I have never seen any other make in a toolroom but they have generally been a lot bigger. People get sacked for riding the ram. Every now and again some one or the other just couldn't resist it. Maybe they had found another job.

John

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Well John S. will be please to know I've invested in a floor-holder-down machine. An Adept No. 2 as well!

The seller has an Elliot 10M as well, which is a bit better specced

Now I need to sort out an angle plate or weld something up.

Neil

.

Congratulations

Evidently a man of discerning taste.

MichaelG.

Better spec'd ? If you think an elliot can be a better shaper than a Butler Neil both you and MG have bad taste. It's a bit weird that lathes co haven't a page on them. Might be because they are from what I have seen generally rather large but I wasn't kidding when I mentioned I had never seen any other make in a toolroom.

Couple of differences on the one I linked to. 3 parallel V belts for the drive and a front table support.  Wonder why they did that. Looks like they did try to keep it compact as I would have expected a longer ram

Only problem is that it's likely to be a bit heavy compared with others if it does have a 13" stroke. The vice looks more suitable than some too. That might fetch a fair bit all on it's own.

The craziest offer I have seen recently is probably the one for sale on lathes co uk. It looks to be in very good condition.

One good thing about them all really is they were designed when machines were really made to machine and no worries about shipping weights etc. Even small ones are likely to be pretty well made. A word of warning though. Some have more sophisticated ram drives than others. Breakages in that area could prove difficult to fix. Personally I would feel that the correct box for mounting the work on is also essential.

John

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Edited By Ajohnw on 25/09/2016 20:42:03

Any internal gear has to be be several teeth less in diameter in order to be able to roll. A bit of digging says that the profiles are the same, except (obviously) dedendum and addendum are reversed and "For 14-1/2°PA, the difference in tooth numbers between the gear and pinion should not be less than 15. For 20°PA the difference in tooth numbers should not be less than 12."

I would say the cutter needs to have the same profile as a tooth, not a space, of the equivalent external gear but with the changes to addendum and dendum as mentioned above.

N.

www.chiefdelphi.com/forums/archive/index.php/t-35549.html

"Practical considerations associated with the insertion of desired backlash and preventing the addendums of either gear from "bottoming out" cause slight modifications to the addendum height and dedundum depth for the internal gear in manufacturing (particularly when the pinion is large relative to the internal gear - e.g. the difference in tooth count approaches 15). But the shape of the inverted involute curve is not modified. The shape of the involute curve itself is designed to provide clearance for the external tooth "n" to pass by internal tooth "n-1" while establishing rolling contact with internal tooth "n+1"."

My emphasis.

Edited By Neil Wyatt on 25/09/2016 20:51:55