Internal Gears???

[itsme]

Hi everyone,

I've been looking for standard, 'off-the-shelf' internal gears on the internet, but just can't find what I'm looking for in any catalogues. I'm looking for something with a 30mm pitch diameter and module 1 (mm) or a 21mm pitch diameter, module 1.

Due to the fact that I can't find them anywhere (and I've got a feeling they'll be expensive if I do find them...), I've decided it would be a good idea to make my own from scratch. The only problem with this idea, is that I have no idea how to go about it . Does anyone have any experience with small internal gears? How are the small commercial internal gears made?

Any help would be appreciated.

Regards
Warren

[KiwiOS]

You can make anything if you have the patience. What's the application it's intended for?. Is it a sliding fit for a splined shaft or will a smaller external gear be running inside it?. You can single point an internal gear or spline if you have a slotter and a rotary table or diving head. You can even do it on a Bridgeport type mill if you don't mind winding the quill up and down a lot! If you go to a workshop be prepared to be surprised... it will be expensive for a one-off. Internal gears are mostly done on a machine called a Hobber. There is considerable set up time and it takes just as long to set up for one piece as it does for one thousand pieces, hence the high price for small batches. Hope this helps... Just ask if you want to know more.

[itsme]

Thanks for the reply, KiwiOS.

The internal gear will have a spur (external) gear running inside it. It is for a model engine that is still in the very early idea stage (hasn't even reached the design stage yet...), but I need to see what is available before I can do anything. In terms of equipment, I have access to an Emco Compact 5 with milling attachment and dividing head and hopefully a small benchtop mill in the near future. I've got a feeling that the Compact 5 may be a bit small, but I'm sure if I thought about it long enough, I would either find a solution to the problem or go insane... Preferably the former

Regards
Warren

[itsme]

Hello again,

I've just been thinking and was wondering if laser cut or wire EDM gears would work? Would they be fairly accurate? Does anyone know if laser cutting costs a fortune for small quantites?

[CNCgr]

I would go with laser cutting.

Usually the high cost of small quantites is caused by the high setup cost and time. In laser cutting setup time is minimal when you give them the 2d drawing in a format they can use.

[ty1295]

Depending on budget and qty. EDM is still viable. Surface finish will exceed any lazer ever could, and tolerances, well my wire EDM I never worry about that and we go out 4 decimal places all the time.

[DareBee]

Laser cutting at best is +/- 0.002".
In the thickness of material you are (most likely) using I would say more like +/-0.005".
Also laser will cut a small tapered kerf as well as give a finish that will need a lot of polishing.
This is all based on the assumption that you are going to USE this gear.
I wouldn't put a lasered gear in any of my transmissions, but if this is for a toy or something that will run for show a couple of minutes a month you could easily get away with a lasered part as long as you are prepared to moniter it's condition and replace it as needed.

It will be costly, but I would definately have it wire cut (and would consider making a spare) then a quick polish (an extrude hone would be superb for this) and you will have perfection.

[Bubba]

Another thought might be to try chemical machining.

Here is a link to some thought process and possibility.

http://www.indoor.flyer.co.uk/ecm.htm

[HuFlungDung]

One very interesting process I witnessed at the IMTS show was a process called "Extrude hone". The machine used an abrasive paste something like Play-Doh. The machine functioned like an injection molder/press, and you'd insert a "die" in the upper holder which would shape the extrusion of the paste. Then, below this, on the table, you'd place the part, which fit in an accurately placed nest. The platens would close and extremely high pressure was applied to the paste, which extruded through the upper die, and passed right through your part and "honed" it bright bright and shiny. It would clean up a stamped or lasercut edge so it looked like it had been EDMed. They showed some internal splines finished this way.

I do not know the particulars of the "die" and what that cost to make. This thread reminded me of that, not that its feasible for the do-it-yourselfer

[DareBee]

Hu - you missed my last post-
Actually the part setup in an extrude hone is really not accurate at all, all you need to do is have a media channel that directs it into (and out of) the through cavity to be honed. The machine just squishes the media back and forth and personal experience (or constantly checking the finish) dictates the length of time to run it.

[HuFlungDung]

My apologies, Darebee, I did miss that

So is there some kind of an accurate die required, or is the part itself, the only "die"? Is there a tendency to round off the top edge of the part in extrude honing? I'm just trying to understand how much high priced tooling you'd need to get set up to hone something this way?

[DareBee]

No need for apologies, I was just pokin fun.
You are right on, the part itself is the die, in most cases all that is needed is a transition ring. The media "pot" is somewhere around 6" dia. if your part to hone is 4" you need a ring that sets over the pot (maybe 8"dia) with a 4 1/8 to 4 1/4 hole. Set transition ring on pot, set part on ring, set other ring on top, run the machine.
A local aerospace bearing company uses some of these machines in production and they do have fancy multi-part tooling assemblies with media ducts in them.

Yes it will smooth a sharp edge, but remember different media to suit proper application. Also the type of tooling or parts that usually benefit from extrude honing dont require a sharp edge anyway.

I couldn't imagine any use for this process in stamping die tooling where you need a sharp edge. Clearances are generally big and there is no need for a mirror finish.

Mostly for powdered metal tooling, things like this gear and the heads on my car.