johnsmachines

Bevel gears seem to me to be rather difficult, even with CNC control of X,Y,Z and A axes.  The bevel gears on the model Armstrong cannon are rather small, being 32mm and 14mm outside diameter.

I read Ivan Law’s book on the subject, and I think that I understand the requirements, and I was prepared to try and cut the gears.  But, first, I decided to try to cast them.

That involved…

1. Using “Gearotic” to design the gears, and save them as an STL file which was able to be imported into the 3D printer.
2. Made PLA gears with the 3D printer.
3. Attached the gears to a wax “tree”.

3 pinions and 3 gears.  I need 2 of each.  1 spare of each.  Plenty of venting sprues.  And a head of about 70mm.

4. Then mixed the investment, poured it into the flask.  At least that was the intent.  The investment makers specify exactly 40:100 by weight of water:powder.  But the bloody scales switched themselves off while I was adding the powder to the water, so I had to guess the quantity of powder.   This was not looking promising.  First bronze casting pour not off to a good start.

5. Dry the mold flask in the potter’s oven for 2 hours, then 2 hours of burning out the PLA and wax, then 2-3 hours of baking at 750ºc.  A few minutes into the burnout phase, the oven died.   ?heating coil failure, ? control box failure?, ?thermocouple failure,  something else?    So I replaced the control unit and thermocouple (I had a spare of each), but problem persisted.  I rang my expert friend for advice.  “sounds like a broken wire” he says.  Suggested 3 or 4 things to try.  And the 4th suggestion worked!  The oven was working again!  Brilliant!   Thanks Stuart Tankard.  So I restarted the oven at the burnout temperature (400ºc) and continued.  Nothing to lose, after all.

6. Melted a couple of bars of LG2 bronze at 1100ºc in the melting furnace.  Added a pinch of Borax.  Let the investment oven cool to 710ºc for 1 hour to let the core of the mold cool to 710ºc.

7.  Without any great expectations of success, considering the various problems, I poured the molten bronze into the mold flask.  It seemed a bit more viscous and thick than I was expecting.  Oh well.  It is experimental.

8.  When the mold flask had cooled to 150ºc, I plunged into cold water, and flushed out the investment.

THE RESULT….

Unbelievable.  No voids.  Hardly any surface bubbles.  ALL teeth intact and complete.  6 good gears!   You can see the head of molten bronze between the funnel and the top gear.  It did not need vacuum or positive pressure.

I will turn the faces, bore the shaft holes, and if necessary file the teeth.

Totally delighted with this result.  Beginner’s Luck.