Cutting internal gear teeth

[John Stevenson]

OK no problem, I'm actually thinking of adopting John [ bogs ] so I can legally change his name.
If it pans out I'll call him Bandit and Bandit will become Bogs, what do others think ?

John S.

[cedge]

Man.... are you ever gonna pi$$ Bandit off with that idea....LOL

Steve

[bogstandard]

Hey, you two, pick on Bog's time is it?

Unfortunately, Bandit doesn't want anything to do with me at the moment, that is why I am having trouble coming up with new ideas.


Anyway, back on track to put you straight, but first, this turned up this morning, I had forgotten to order it when I ordered the hobs.

A nice shiny 8mm arbor that fits the hobs perfectly. So part way along with the project.




Back to reality, and thinking INSIDE the box.

As I have already explained to John in private and everyone else now, plus having to come up with reasons for not using what has been suggested. I have a lot of thinking time on my hands at the moment, but not a lot of machine time, so even though your ideas are good, I have come up with my own. The main problem was getting the cutter made, and John has solved that for me.

My way of thinking.

WHY make life difficult for myself when I have most of the items available at my disposal? It also keeps the costs well down.

The first, accurate indexing. I have my dividing head to do that,

Putting an accurate cut on. I have the compound slide off the lathe to do that. Plus if I really wanted super accuracy, I can just plug the compound read head into the mill display, and I could, if I wanted to, be working to the nearest 0.00005" (or even more accurately if I decide I want to swing the compound round a little).

So what else needs to be done. The slotter, which has to be made anyway.

Then a support block for the compound to mount it onto the mill square and at the correct height.
Much easier to make than some sort of indexing bits for the lathe.

Can you think of anything I have forgotten?

Does the well know phrase or saying spring to mind - KISS.



Do I hear any mumblings in the background? Or are we going to extract the urine out of Bogs once again.


Bogs Bandit

[Darren]

Very nice John, very nice indeed ...... 

[Bernd]

Bogs,

Been following this thread silently. Very interesting. The place I worked at, as you are well aware of, was a gear cutting machine manufacture.

So, in one of the departments were they cut gears for our own use on machines, they had several internal hobbing, for lack of better terminology, machines. The arangement is like yours only verticle and all machine powered. If I recall right the cutter also revoled as it was cutting the internal teeth. This brings a question to my nind, does your cutter also have to rotate while you rotate the gear you are cutting?

I would think that if you rotated the gear a few degress you would cut all the metal and not be left with any teeth. Or am I missing something here that I don't see clearly.

Don't bother trying to explain in words to me now. I'll understand better once you have the set up done and are actually cutting gears. I work better with pictures than words.

Bernd

[bogstandard]

Bernd,

As explained to me by John S, the cutter is made to the same profile as a small spur gear.

The central tooth on one side is the one that does the main cutting, and the teeth either side of it (depending how large the cutter is made) partially shapes the other teeth that are due to be cut either side of the main cut.

So basically to reduce the manual loading while cutting, you make a small cutter, so that say only one either side gets shaved, whereas if you made a large cutter with many teeth, you might be shaving say 8 or 9 at the same time (this will put more load on the cutter and of course the arm that is operating it).

I hope that is the way John meant it to happen.


Bogs

[tinkerer]

This is a very interesting topic. I thought the cutter was shaped round like a gear, so it would turn with the piece you are cutting to cut the clearance profiles as it turned. Looking forward to your pictures of the process.

[dsquire]

John

Here is a link on gear cutting that may or may not be of interest to you. It is on a site by a new member Joachim Steinke from Germany. There are lots of other goodies there as well.

http://www.metallmodellbau.de/GEAR-CUTTING.php

Cheers 

Don

[bogstandard]

Don,

The method Achim is using on the mill is what I was describing the other day, only he is cutting teeth one at a time using a straight cutter, whereas I will be cutting the whole blank in one go by freewheeling the blank using a spiral cutter, and how he has explained the cutting of adjacent teeth is what I posted earlier, but mine was a lot less technical.

He is having some very good results there. If mine turn out that well, I will be a very happy chappie.

Unfortunately, that is all concerned with external tooth cutting, of which there are a lot of techniques to achieve the same results.

There is very little on the net for the home machinists about cutting internal teeth, which is what I am trying to solve as easily and cheaply as possible.

It seems that very few people have even tried, and lived to tell the tale.

Bogs

[John Hill]

John,  I have just about scratched out the last few strands of hair on my head trying to imagine how a rotating cutter could be used to cut an internal gear without the axle of the cutter getting in the way, unless the gear were is to be very big.

Maybe if the cutter had a very coarse thread it would be possible to swing the axle enough to clear the gear while keeping the cut parallel to the axis of the new gear.  Then maybe that was what you have in mind anyway?

[tinkerer]

Are you saying you are trying to cut all of the internal teeth full profile in one whack? Wouldn't that take a drophammer? It might be able to be done. he support of the gear would be tough. maybe sandwiched in a holder of some type with only the effected area exposed. I hope you take a video of the process.

[bogstandard]

John,

I am just wondering where you got the information that the cutter rotates.

The cutter is only round because that is the easiest way to make it with the cutting hobs I have, e'g make a spur gear out of high carbon steel (silver steel, drill rod) instead of say brass, then harden the cutter, and grind the cutting edges to shape afterwards. It is then locked onto the end of the ram with one cutting tooth perfectly horizontal.
The rest of the teeth except say two below and two above the main tooth could be ground away as they play no part in the cutting action.

The first tooth is cut by the slotting action of the cutter, then the blank is rotated to the next position and another tooth is cut, and so on until the inside gear is cut. Like cutting multiple keyway slots in a bore.

If you only used a single tooth cutter, then you wouldn't get the involute shape to the tooth form, it is the secondary cutting action of the above and below teeth, being 20 degs out from the major cutting tooth position that forms the involute 'shape'. It won't be a perfect involute because the two shaving cuts are in fact flat rather than curved, but close enough for the gear teeth to operate as though they were true to form.


John

[John Hill]

Thanks John, I guess even Bandit would have understood it but not me!

[tinkerer]

The dog is smarter than I am too.  (now I know why my dog looks at me the way she does) On the referenced site, the cutter is spinning and he is cutting most of one tooth and parts of a tooth on either side. The full profiles are completed as he works around the gear. I see now that you will be doing the same interior cutting, but with the action a shaper makes. I am probably still shagging balls, but it exercises however many cells left between my ears.

[andyf]

Bogs, your idea of making a hardened gearwheel to use as the cutter looks brilliant to me  . The great advantage over a single point one is that if the tooth which is doing most of the work loses its edge, you can rotate the cutter, remesh it with the part-done job, and start off again with a fresh tooth.

BTW, you mentioned at one point that gearwheels at popular prices were hard to find. The cheapest I could find for my dedicated fine feed banjo were here:
http://www.technobots.co.uk/acatalog/Online_Catalogue_Gears_365.html  (usual disclaimer), and they are thick enough (15mm) to cut up and get two approaching 7mm thick for the price of one, as in the first pic here: http://andysmachines.weebly.com/fine-feed-banjo.html

Andy

[bogstandard]

Andy,

I have used Technobots before now for gears, unfortunately the module sizes are a little on the large size for some models I have in mind. I have a very big box of metal gears, but most times I can find one spur gear but not the other in the size I want, and internal cut gears with matching spurs the right size are like rocking horse s**t, impossible to find.

Unless you are willing to have them cut in the materials you want (very expensive) you are usually stuck with either plastic or ferrous gears, and of course, the outside shape, if I make them myself, is only limited by my imagination.

http://www.hpcgears.com/newpdf/internal_gears_1.0mod.pdf

http://www.davall.co.uk/documents/davall_catalogue_048.pdf

Please, no further links like the two I have shown, otherwise the post will end up like a shopping trip, not how to make them yourself.

The great advantage over a single point one is that if the tooth which is doing most of the work loses its edge, you can rotate the cutter, remesh it with the part-done job, and start off again with a fresh tooth.

Andy, I have given this a lot of thought, and when I make the slotting tool, I will make it with a resettable holding nose so that what you have suggested will be easily done.
Also, if I put a little more thought into, and find an optimum diameter for the cutter, I will only need to make one of each module size to do all expected internal sizes.

Of course, this is still many months away, but I do like to have all the information in place before I start anything, and the response from the members on here has been great, and has solved a lot of my envisioned problems without too much difficulty.

Lots of minds are better than one.


Bogs

[Bernd]

Bogs,

I did a bit of research into the cutting of internal gears. I contacted a friend that I used to work with. He was the head of the gear cutting department. I asked him about the cutting of internal gears. He cleared up several questions I had, plus the big one of rotating the cutter while cutting the internal teeth.

I had forgotten that there are two methods of forming gear teeth. One is called "generating" one is called "shaping". The generation of a gear allows a true involute, were as the shaping of the gear the cutter needs to be made very accurately to cut the tooth profile.

So your method of shaping the internal gears on the lathe will work with an accurately made cutter.

Below is a link to some interesting info on shaping gears. Scroll down through and you'll see all sorts of cutters used in gear manufacturing.
http://www.gearshub.com/gear-cutting.html

I've also found some pictures of Fellows Gear cutting machines.

The first pic is similar to the gear shapers we used at work.


This second pic is of a smaller gear shaper. The size a home machinist won't mind having.


The last three are of a medium size gear shaper.




Who ever set the machine up didn't get the gear ratios right between the cutter and the blank. As you can see there are no teeth, just razor sharp edges. Ask me how I know about razor sharp edges on teeth and not getting the gear ratio right between cutter and gear blank.


I hope this info has helped some in the questions always asked about gear cutting.

Regards,
Bernd

[bogstandard]

Many thanks for all that info Bernd. It was an eye opener.

It looks like my settup will end up being a lot more basic and a little less bulky than the machines you have shown.

The way I am looking at this project, is that, in the beginning, someone must have started off making these internal gears, just like I am trying to achieve.

They wouldn't have had the modern sort of machinery available to them. So those original artisans must have started, as I am attempting to do, with the least amount of basic equipment to get the job done. It was from those humble beginnings, that the modern day automated machines evolved. It would only be one step away to fit a windscreen wiper motor fitted with an arm, to push and pull the slotting tool handle, then a ratchet onto the cut feed and you are almost there. But that is getting much too complicated for where I want to be.

I think what I am doing is called reverse engineering. Starting with the finished article, and working out how to make it.


John

[Bernd]

I just hoped it would benefit those who have never seen these machines before. It should give them some idea of what they look like next time somebody mentions interl gear cutting.

A google search on fellows shapers will bring up a history of the man that engineered the Fellows Gear shaper. As you said he started out with very basic machine.

As they say "not very many people can start with a blank sheet of paper".

Bernd

[John Stevenson]

John,
Slight alteration to your C- o - D drawing to save your arms. 



John S.

[bogstandard]

John,

That is definitely a good idea. AFTER the system has been proven to work manually.

It would only be one step away to fit a windscreen wiper motor fitted with an arm, to push and pull the slotting tool handle, then a ratchet onto the cut feed and you are almost there. But that is getting much too complicated for where I want to be.

After a little more brain time I have hit a problem
 
The slotting head would have to have total control of the rotation of the ram, with no play allowed at all, and would be a little difficult to do.

However, I do have a few precision linear slides that could do the job for me. I will have to measure them up, to see which would be best size wise.

John

[John Stevenson]

John,


 
The slotting head would have to have total control of the rotation of the ram, with no play allowed at all, and would be a little difficult to do.


John

I have often wondered why most people follow the same paths ?
Like stepper motor mounting, everyone stands them off on 4 pillars which has to be the most inapt way, was everyone a watchmaker in earlier lives ?

Same with slotting heads, always round rams and bushes and a silly pin to stop rotation, why bushes anyway they don't run at 10,000 rpm.

What about a length of square silver steel in a brass block with vee's machined in and clamp bolts to set preload.
this way the whole of the 4 sides are load bearing AND guide the bar.

1" round bar in a 4" housing has a surface area of 12.56 square inches but a location of minute proportions on the index pin.

A 1" square bar has a surface area of 16 square inches and every one of those locates it.

John S.

[andyf]

Yes, I see it all in my mind's eye   . John S's square ram with an offset mounting on its end for the gear-shaped cutter, leaving some of the end surface exposed when the cutter is in place. To that exposed surface, fit some sort of detent to go between the teeth of the cutter opposite the tooth which is doing the hard work, so the cutter can be rotated, if it is necessary to bring a fresh tooth into play. In effect, the cutter indexes itself. To provide rake and relief, the cutter blank to be dished on the front and slightly tapered in side view. No, that would only help at the tip of the cutter
I'm only posting this because I may want to make an internal gear myself in the not too distant future, so if I'm getting it wrong, please stop me now and prevent  

Andy

[NickG]

The slotting attachments I've seen are usually a dovetailed affair akin to the ram on a shaper. Infact, someone from the club gave me just a slotting attachment when I was thinking about making a vertical head for my horizontal milling machine. I think I gave it back when I bought my vertical one though. I don't like keeping things of other peoples if I'm not really going to use them ... somebody else may have given it a good home by now, or it could be sat under his bench still.

Nick

[sbwhart]

Ok this is just an idea to get you thinking who knows where it will lead too

When I was using the small arbour press to test out the leather hole punches I looked at it and though square ram,  if you turned it on its side, clamp it to a verticle slide put the tool at the other end you could use it to shape out internal gear teeth.

Here's a press this one may be a bit big but you can get smaller ones.

http://www.mscjlindustrial.co.uk/CGI/INSRIT?PMRQIT=BIJT3-33710M&origin=VIRTUAL:VIRTUAL_CAT&PMAKA=BIJT3-33710M&returnurl=&partnerURL=http://catalogs.shoplocal.com/jlindustrial/index.aspx%6Fpagename=listpage%50circularid=15737%50storeid=1039181%50uniqueid=%50fsid=%50jsessionid=%50pagenumber=1%50deptid=%50title=%50keyword=press%50returnurl=

Her's a C-0-C of the idea




You may have to cut the press up a bit to get it fitted but they are dirt cheep, I bet you could fix them to a lathe as a key way slotter.
 


Have fun chewing this one

Stew