Internal gear

Gordon · Aug 10, 2024

I built Elmer's geared steam engine several years ago and have since sold it. It is a neat engine but it has an internal gear which he specifies a 72 tooth Boston Gear. It is super expensive. Is there some way to make this gear? The only thing I can think of is to make some kind of a broach and index it on a rotatory table. I am not sure that I have the skill or the knowledge needed to make the broach. Does anyone have a method of making an internal gear?

petertha · Aug 10, 2024

I looked into it when I built my radial. I think the answer is you can broach the shape it but the magic is in making the proper shaped cutter profile which is kind of an exact shape when you get into the details. Depending on the pitch, other tooth compensations enter the relief design especially with smaller sizes. Maybe if its slower rotating, lower load... collectively looser tolerances a home cut may well 'work'. Others have done it, but the devil is in the details.

Initially I was considering an Edwards radial which used Imperial/inch ring gear. It was very expensive landed in Canada. The other engine I contemplated used metric gears & the equivalent (module-1) ring gear was a fraction of the cost. There were a few other pro/con considerations but this ring gear issue was enough to tip the balance for me. If you can find something similar in metric & work out the difference in pitch diameters so it accomplishes the same thing, that might be an option. You can still cut your own external/spur gears the same way (with appropriate cutters) or buy matching module gears.

This place specializes in metric stuff & there is a USA outlet.
https://maedlernorthamerica.com/

Jasonb · Aug 10, 2024

I've done a couple for epicyclic gear boxes. I just printed out a single tooth profile and hand ground a cutter from HSS. On something like Elmers you are not going to be transmitting massive loads so if you don't get the profile right it is not the end of the world.

I just used my mill's quill like a slotting head taking a thou or two at a time and then moving the rotary table round to the next one. Start reading from this post where I did the test cut in plastic and then through to completion in post #138

https://www.modelenginemaker.com/index.php/topic,7440.msg164712.html#msg164712

danallen · Aug 10, 2024

Here is how I made the ring gear for my Edwards radial.
https://www.homemodelenginemachinist.com/threads/ring-gear-for-edwards-radial.35994/

Gordon · Aug 10, 2024

I had not thought about making it in three pieces. I have the Mod cutters so it is possible. Making the external gear is easy and I will have to think about this method. Like most of us hobby folks cost is a consideration but the satisfaction of doing it yourself is a big plus. I could redesign the Elmer's in any scale that worked.

Steamchick · Aug 12, 2024

A friend made an internal gear in Zinc alloy cast around a gear from his lathe change wheel set. Said it too a good bash to remove the cast iron gear from within the zinc casting. but it worked for his model after fettling. He also made the planet gear to run inside it again in zinc alloy - but not sure how?
His epicyclic engine was a treat to watch running!
Can't find my old picture now...
K2

Gordon · Aug 12, 2024

I wonder about making the gear with my 3D printer. I made a gear for my Covel band saw a couple of years ago with ABS and it seems to be holding up to my rather limited use. Obviously the gear on the Elmer's geared engine would not be subject to a heavy load or extended use.

timo_gross · Aug 12, 2024

I posted this a while ago. It is doable, but I think you should have a shaper or slotter of some sort for it. Slotting it with the quill of the mill will be possible but will take forever.

https://www.homemodelenginemachinist.com/threads/how-to-cut-internal-gear.35851/

I would guess a 3d printed gear does the job, more a matter of the looks (to me a brass or steel gear looks just more fancy)
If you make the 3d printed gear make sure you make a proper ring gear.

When I made the ring gear model I modelled an external gear with adendum=1.25 x module and dedendum=1 x module. Then substracted this external gear from the outer disk.
A usual external gear with same tooth count will then fit inside the ring gear and leave the clearances at the tips.

Good luck!

p.s. I guess you can print it or do the 3 piece trick, just remember to make the internal diameter "too big" and cut same depth of cut (3piece method is quite a good idea)
I was surprised how small plastic gears I could print with a normal filament PLA printer. (Module 0.7 and a 6 mm bore)

Module 0.5 with a diy slotting tool. (lots of tooth took about a min per tooth)

a41capt · Aug 13, 2024

Gordon said:
I built Elmer's geared steam engine several years ago and have since sold it. It is a neat engine but it has an internal gear which he specifies a 72 tooth Boston Gear. It is super expensive. Is there some way to make this gear? The only thing I can think of is to make some kind of a broach and index it on a rotatory table. I am not sure that I have the skill or the knowledge needed to make the broach. Does anyone have a method of making an internal gear?

I downloaded the CAD drawing from the website and imported it to CAD, and then exported to my 3D slicer. Printed it in PETG, but I’m going to re-print in PA6.

It’s accurate as all get out and meshes with my printed spur gear perfectly.

Best part? It costs pennies and is VERY easily duplicated if it wears out!

Next step will be redrawing Elmer’s plans so I can 3D print the whole thing.

John W

Gordon · Aug 13, 2024

a41capt said:
I downloaded the CAD drawing from the website and imported it to CAD, and then exported to my 3D slicer. Printed it in PETG, but I’m going to re-print in PA6.

It’s accurate as all get out and meshes with my printed spur gear perfectly.

Best part? It costs pennies and is VERY easily duplicated if it wears out!

Next step will be redrawing Elmer’s plans so I can 3D print the whole thing.

John W

I am working on drawing the gear so that I can 3D print it. I would be interested to see how a completely 3D printed engine would work. I may try that. I had not thought about that.

a41capt · Aug 13, 2024

Gordon said:
I am working on drawing the gear so that I can 3D print it. I would be interested to see how a completely 3D printed engine would work. I may try that. I had not thought about that.

I can send you the STL files if you’d like. I left the mounting holes out and just drilled them according to Elmer’s plans.

Drop me a message with email and I’ll send along both ring and spur.

John W

Gordon · Aug 13, 2024

a41capt said:
I can send you the STL files if you’d like. I left the mounting holes out and just drilled them according to Elmer’s plans.

Drop me a message with email and I’ll send along both ring and spur.

John W

I have the internal gear drawn and I can draw the spur gear with no problem. Have you or anyone else made a complete 3D engine like this? I would think that the cylinder would be the most likely to be a problem. The axle would probably still be made from steel.

a41capt · Aug 13, 2024

Gordon said:
I have the internal gear drawn and I can draw the spur gear with no problem. Have you or anyone else made a complete 3D engine like this? I would think that the cylinder would be the most likely to be a problem. The axle would probably still be made from steel.

I haven’t printed Elmer’s geared engine yet, but I did print and build this double acting engine that is completely plastic except for a couple fasteners. It runs well on as little as my breath, and the video shows it running on 5 psi. I downloaded the STL from Thingiverse and I can post the link if anyone is interested.

This was my test bed to see if I could build the geared engine 100% out of printed components.

John W

rolphill · Aug 14, 2024

I've had sub-par results with ABS gears. It was just too soft to last. However, PLA works great as long as it doesn't get hot, as it's quite hard. I haven't tried PETG yet or more exotic materials. I actually only use PLA+ instead of regular PLA, as it's a bit more temp resistant.

Don't forget, it's actually fairly cheap to get them printed in bronze using an online service like shapeways!

Gordon · Aug 14, 2024

rolphill said:
I've had sub-par results with ABS gears. It was just too soft to last. However, PLA works great as long as it doesn't get hot, as it's quite hard. I haven't tried PETG yet or more exotic materials. I actually only use PLA+ instead of regular PLA, as it's a bit more temp resistant.

Don't forget, it's actually fairly cheap to get them printed in bronze using an online service like shapeways!

I am going to try it. I am not out much if it does not work. I have not looked into having parts printed in metal and perhaps I should.

timo_gross · Aug 15, 2024

Gordon said:
I have the internal gear drawn and I can draw the spur gear with no problem. Have you or anyone else made a complete 3D engine like this? I would think that the cylinder would be the most likely to be a problem. The axle would probably still be made from steel.

I just sayin; as I mentioned before "3d printed gears are evil!"

a41capt said:
I left the mounting holes out and just drilled them according to Elmer’s plans.

John W

Modelling mounting holes has the advantage that there will be a more solid "outside wall".

Gordon said:
I am going to try it. I am not out much if it does not work. I have not looked into having parts printed in metal and perhaps I should.

What can possibly go wrong. Worst case it does not work at all. With a half decent printer there is not much chance of that.
Removing the Elefant foots on a gear is a pain and even very small one can make the gears run much worse.

Ring gear modelling is at the beginning of the video, so no need to watch it in full, but his method produces working models.

Tips from this guy, champfer model to preven elefant foot, or print with raft if model is already finalized.

If I plan to bore, drill or ream a PLA part to size, I (if I remember) increase the wall thickness in the slicer, be sure there is "wall" left not coming to close to the infill.
If it runs only for 20 h or so, that might even be good enough.

rolphill · Aug 15, 2024

timo_gross said:
I just sayin; as I mentioned before "3d printed gears are evil!"

3d printed gears are a godsend. I'm not paying a hundred dollars for two gears.

The trick is to design for the material, just like when working with wood or composites. Wood has just as many design quirks (like grain direction) as printing, but we are so used to it that half of them are subconscious.

People have put some pretty ridiculous loads through printed gears. I've also seen printed gears designed in two sections, where the teeth are printed in nylon for wear resistance, but the core of the gear is printed in a stiffer material because nylon was too flexible.

Of course, if you make the module about 2.5mm, you can laser cut gears out of wood or plastic, or machine them with a 1/8th end mill on a CNC without any special tooling.

mrehmus · Aug 15, 2024

I printed PLA gears for a friend's lathe about 5 years ago. They are still working fine—just black PLA on my 2016 Qidi X-one printer.

a41capt · Aug 15, 2024

mrehmus said:
I printed PLA gears for a friend's lathe about 5 years ago. They are still working fine—just black PLA on my 2016 Qidi X-one printer.

I printed an emergency gear for my Mini Mill in PETG and it not only let me complete the part I was working on during the crash, but continued in use until the new gear arrived. I waffled on the purchase of a belt drive due to price, and once I found I could keep my gear drive and insure continued operability for a couple pennys worth of plastic, I left it alone.

Now that I’ve got a core X-Y printer added to my flock, nylon will be the next iteration in gear printing!

John W

petertha · Aug 16, 2024

rolphill said:
Don't forget, it's actually fairly cheap to get them printed in bronze using an online service like shapeways!

Apparently went kaput just recently

https://www.tctmagazine.com/additiv...s-ceases-operations-and-files-for-bankruptcy/

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