Cut internal teeth on ring gear

[altaka]

A clock maker friend called me to ask how to cut internal teeth on a ring gear. The gear OD is about 3 inches and inside 2.75 inches, width is 3/16 and made of brass.
Is this a impossible task on a milling machine with dividing head rotary table?
I figure if it can be done you guys would have an idea.

Thanks
Al Takatsch

 

[LongRat]

Plenty have done this by broaching each tooth one by one. This is most commonly achieved by grinding up a single point tool and cutting the tooth on the lathe by repeatedly running the carriage in and out, taking a tiny bit more off with each pass. This is done with the spindle stopped. Then you index the spindle to the next tooth position and repeat the process. You could do the same thing with the indexer in the mill, as long as you can lock the rotation of the mill spindle so the broach stays pointing in the right direction all the time.

How you grind the correct profile in your tool is the main challenge, one good way is to produce a CAD drawing of the internal gear and use that to create a tool path to CNC cut or grind the tool on your mill, if you have CNC. Most of the guys on this board could probably grind up a tool like that manually to a close enough shape, although I'm not good enough at manual grinding for that to be an option.

 

[Jasonb]

As Said planing it out on the lathe or mill is one way to do it, just like cutting a lot of keyways. Could also be done with a shaper or slotter. You could take a lot of the waste out by drilling a ring of holes and then clean up with the profile tool.

If you did have CNC available to cut a tool profile wouldn't you be more likely to just us ethe CNC to cut the whole gear?

 

[tel]

All hail the mighty shaper! Makes operations like this a breeze!

 

[steamer]

WR Smith does it in his book I believe on the Strutt Epicycle Clock where he constructs a simple cutting frame to be mounted on the lathe and uses a standard wheel cutter.


Check him out.

Dave


http://www.wrsmithclocks.com/

Then go to photogallery

Then go to the next to last picture from the bottom of the page.

It shows a cutter frame cutting internal teeth on a clock wheel while being indexed in a lathe

 

[altaka]

Thanks LongRat and Jason

Could also be done with a shaper or slotter.

Jason,
I looked up Shapers and Slotters and see there's a possibility there. We intend to make the cutting tooth ourselves for the perfect shape.
Maybe a vibrating plunger in a punch block moving the cutter with a 1/4 inch stroke. Only plan on taking off a thousand or less per stroke and use plenty of oil.
The punch block could be secured to the table with the Rotary/Dividing head to maintain accurate positioning.
The Rotary/Indexing head would take care of the ring gear rotation.
Provide some clearance front and back for the cutter movement.

Don't like my vibrating punch idea, maybe a reciprocating motion developed by the chuck with a connecting rod to the plunger.

Thanks Steamer,
WR Smith is quite a machinest, good idea's.
My problem lies in cutting internal teeth on a ring gear, with idea's from this board and friends I've been successful in cutting external teeth on my milling machine and making and profiling my own fly cutters. Its the internal tooth cutting that will be my next challange.

I would sure appreciate all the new idea's !!
Al Takatsch

 

[altaka]

Steamer,
I just looked again at WR Smiths pictures, your right he is cutting internal teeth! Got to study that a little longer. Its the third picture up from the bottom

Thanks Again

 

[tel]

Another approach is to plane the teeth out on the lathe, with a gizmo like this.

 

[altaka]

Tel,
Simple and elegant, looks like it'll get the job done.

Al

 

[Allan in Germany]

WR Smith actually uses the gear cutter of the correct module to cut the teeth for the internal ring gear. As said before, he has builders fabricate a small gear holder, and an attachment which holds the cutter to a small wheel, this is then driven by a belt from the right side of the lathe, and then using the topslide, slowly goes into the ring gear, cutting it all in one pass.

As the gear module is not rotating, but the wheel on which it is fixed is, then only one tooth of the gear cutter actually cuts the teeth. As each tooth is completed, the ring gear is indexed to the next tooth location.

Took me a while to understand it, and I'm trying to build the darn thing! Still waiting for the solder for the dial......

Allan

 

[DTANNER]

I just recently cuta few gears on a machine called and EDM (electrical discharge machining) it holds extremely tight tolerances and cuts beautiful gears. However this is most likely outside the realm of most home machinist. But if your friend has the budget for it he can most likely find a shop that can do it for him/her.

 

[LongRat]

I've seen a couple of home-build EDMs. They aren't that complex, I'm sure there are some pointers on the web if you wanted to build one. They rule for making parts from super hard metals. I saw a home made one cut some nice shapes out of a file at a show once.

 

[Jasonb]

This thread will show you what EDM can do

http://www.homemodelenginemachinist.com/index.php?topic=7498.0

J

 

[airbus]

Re Internal Gear,
If you really get determined, How about buying the right cutter and cut one tooth to get the right profile and mount the tooth in a vertical slotter or shaper machine, getter than trying to grind both sides of a cutter tooth.
The cost of one cutter here in OZ is around $40 but you get a bunch of teeth. Good luck. see photo of cutter.

 

[Allan in Germany]

Airbus is right, although Smith doesn't cut a separate tooth from the cutter, but holds the whole cutter rigid in another rotating wheel, held in a frame which fits into the internal ring centre, and is driven from a small motor outside the frame. One tooth is then aligned to suit the internal gear being cut.

The previous references to Smith work.....I have built the frame and have the cutter, just need the time to get on with the clock. Finished the dial soldering though. Used Carr's 170f solder and it worked well.