Gallery, Projects and General > How do I??
Cutting internal gear teeth
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.
--- Quote ---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.
--- End quote ---
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. :med:
John S.
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