Build Thread- Molding a gear - looking for critique

[Sil_x]

Hi everyone! I have been building molds for a short while and am looking to those with more experience for pointers and critique. I will post my build steps so tell me what you think.

1. Build a gear with your favorite cad/cam tool. Nice pretty tool paths

2. Cut the mold. I like to use wax since its easy to machine and does not need any release agent. I pack the edge with clay

3. Mix up some silicone and pour

4. Into the pressure tank for curing. I use about 50 psi.

[Sil_x]

5. Cured silicone. Pretty good cast, not too many bubbles at the top.

6. De-clay the mold. easy enough.

7. Mix up your favorite variety of quick curing urethane and pour.

8. After the urethane is cured, I drill a small hole into the wax and pop out the new mold with compressed air.

[Sil_x]

9. Make a hub. I drill dimples for extra grip.

10. Mix up some epoxy and some secret fillers

11. Pour the epoxy and back into the pressure tank to cure over night.

That's all for tonight. I will post how the casting comes out tomorrow.

Thanks

[mcphill]

Thanks for sharing! Very interested in your results, as well as your proprietary "fillers"...

[posix]

I'm more interested in what do you actually do with cast gears? wouldn't tolerances be way out for them to be usable for anything "precise"? why not just simply cut them from a block of your secret recepie expoxy mix?

[Sil_x]

Originally Posted by mcphill Thanks for sharing! Very interested in your results, as well as your proprietary "fillers"...

Alumina hydrate, strontium carbonate, grog, kyanite, and blue chalk line for coloring. Most of it is left over refractory components from building a forge. Most significant is kyanite. It has needle shaped grains that should provide structural stability. I spent too much time reading the EG thread.

[Sil_x]

Originally Posted by posix I'm more interested in what do you actually do with cast gears? wouldn't tolerances be way out for them to be usable for anything "precise"? why not just simply cut them from a block of your secret recepie expoxy mix?

Good question. I suppose i could cut it out of my epoxy mix or aluminum although the epoxy is a little abrasive. The idea is to make something that is easily repeatable and make a bunch with minimal work. Who knows if I get the accuracy good enough I might sell a few on ebay. As for precise, I have a 2:1 set on my z axis that are way out of tolerance and I am still about .001 accurate.

[posix]

good then. so what does the finished item look like?

[Sil_x]

12. Its done cooking and time to de-mold

13. Looks nice, right?

14. Into the lathe for a bit of boring and cleaning up the back

[Sil_x]

15. The accuracy from cog to cog looks pretty good. My biggest issue with accuracy is the shaft being concentric. Right now I am using really soft silicone. I think it is shore 10A and changing to a much harder shore 40A or 50A should give me my best results.

16. rinse and repeat.

[Janos]

The only thing I would probably do differently is cure it under a vacuum. I have had some exposure (Enough to be dangerous) and used a vacuum chamber to remove any air that had gotten into the part during mixing and pouring of the media. You have to be careful doing this because if you just flip the switch and hit it with the full available vacuum any air bubbles that is in the part will get EXTREMELY large REALLY FAST and boil out of the mold. To counter this, place it under the glass bell and slowly bring the air pressure down. A few small bumps of the vac. switch and watching it carefully will take every tiny void out. Then when released the part will actually be under pressure at zero atmospheres. This makes a very nice consistent part.

You can buy a bell from most any thrift store. They sell them as covers for bread or cheese. No point of spending for one if you can get one on the cheap. Then a little work with a lathe to face off a piece of 1/2" aluminum with a gasket groove and your on your way. (Not to mention the pump) Here is a vid of what I was talking about. http://207.234.149.99/preparing/VacDegas3e-500.wmv I snagged it from a DeGassing thread posted here. I believe that on a part like yours where the top is open enough to allow the air out without having to pressurize the material into a void would work well.

[ynneb]

A great thread here. I have been talking with a friend for years about doing just this.
Maybe if you cut the teeth deeper than they need to be, and turn the outside in a lathe to ensure the centre is concentric with the outer.