Spur gear sizing

[Kenre]

I'm working on a new project that needs a set of gears, internal tooth gear rotating around a fixed OD gear.

So far I have Mod .7 which gets me close to the required diameter. ID gear has 30 teeth, Od 20. I need 2/3 ratio.

As the diameters are rather small, is there an option on larger, fewer teeth keeping the sizes and ratio the same?

I will be looking for someone to make a few sets of these so would like to keep them standard sizes.

Ken.

Bonus points if someone works out what there for!

 

[Mud]

Doubling the pitch will reduce the number of teeth 50%. For doing what-if on gear sizing, Gearotic can be very handy - gearotic.com I haven't made an internal gear with it, but it has just about everything.
For calculating backlash and such for making the gears, I find Van Keuren's handbook very useful, they are available new, there are also lots of them on Ebay.

 

[Kenre]

Thanks Mud.
I used a gear design add on in Fusion 360, I will have to check if a pitch function is available.

 

[Oldwrench]

With an internal gearset there's a minimum difference in the number of teeth or you can't make it on a gear shaper (the cutter will interfere on entry and exit). For example you can't shape a 20 tooth internal gear using an 18-tooth cutter. The problem is similar to using too large a threadmill; it leaves marks.

However, with a hobbing machine you can make a broach and cut the internal teeth that way. Such a gearset can only be assembled by sliding them together endwise...

 

[Oldwrench]

Kind of an expensive way to do the job but it worked. One inch face width. Push broaching isn't guaranteed to be axially straight so the OD was finished by holding it on an expanding mandrel—basically a duplicate of the gear with a tapered bore, split 4 ways.

 

[Kenre]

Nice. Looks to be an oil pump.
My smaller gear, now 16 tooth, will have a larger boss on one end for mounting.
Is there a minimum clearance for the tool needed?

 

[Oldwrench]

Is there a minimum clearance for the tool needed?

At least a 1/16 relief at the end of the stroke to shed the chip. As for the required difference between the number of teeth in the gear and on the cutter, look that up in a Fellows manual. It varies somewhat with the DP and the pressure angle (a 14-1/2° internal tooth has a bit less separating clearance than a 30° tooth).

 

[Kenre]

Update,
I picked up an internal gear cutter/broach in Mod 1 , suitable for min 22 teeth. I'm doing 24 teeth.
Set up on the dividing head in the knee mill and had a go. Got 24 teeth all in place but tooth shape is horrible under the comparator!
Quill is sloppy and wasn't locked to stop any rotation, but it sorta look like a gear, and meshes with the 16T spur.

Quotes I recieved to outsource them were way out of my budget to make this profitable.

Thinking of either getting an old shaper or setting it up in the Robodrill mounted off the Z housing to save the spindle bearings.

Thoughts??