Step 1: Getting the details
Step 2: Making a digital model
Step 3: Trial and error (Foam)
About my CNC setup: It is a 7"x7" Zen Toolworks DIY kit (http://www.zentoolworks.com/product_info.php?products_id=74) controlled by EMC2 Linux software (http://www.linuxcnc.org/) and it is fairly obvious from the below pictures that it is desperate need of a vacuum attachment for clean up.
The foam model was slightly disappointing. I think that the roughing pass was not close enough to the finishing pass so the finish bit had to remove too much to leave behind a smooth finish. This is readily apparent on the outside edge where the finish bit traveled down to the outside bottom and made contact with the edge. This will have to be remedied before the wood cuts are performed, otherwise broken bits and machine could result. (Lesson learned, watch your paths and use foam first)
Step 4: In the "woods"
The results were much more impressive than the foam test, thought the pictures don't really tell the tale as well as the physical models.
Here is a video of the cut out pass, which didn't go deep enough as shown in the pictures:
Step 5: Finished! (Not really)
Next, we experiment with molds...
Step 6: Testing a mold
This first trial involved taking the previously created foam model, placing it face up on a piece of wood and covering it up with a complete layer of drywall joint compound. Once that was dried, I puzzle over how to remove the foam model. After watching a few videos, I thought that a "lost foam" cast might be the best bet. With that in mind I performed a test pour. I placed a wedge of the foam in the sand of my sand fire pit (created based on the awesome instructable by jonsarriugarte http://www.instructables.com/id/Sand-Fire-Garden/) and poured some melted pewter into the foam wedge. The result was sadly disappointing. The metal is not hot enough to melt this type of foam in a quick and efficient manner, leaving behind a solid plastic-like residue that needs to be chipped off.
Here is a video of me pouring melted pewter into the foam wedge. The melting device is a Lyman Big Dipper and comes in a handy kit. (http://www.lymanproducts.com/lyman/bullet-casting/Big-Dipper-kit.php)
With lost-foam out of the question, the next test was to see how to effectively remove the foam. Using a heat-gun, the end result was again that same residue mentioned above. In the end I discovered that a blowtorch was the required item to completely burn away the foam. Surprisingly, the detail of the foam was definitely still there despite the scorching left over form the blowtorch's heat.
Here you can see the heat gun melting the foam in the mold. I did not film the blowtorch removing any remaining residue because I had both hands busy (and a fire extinguisher nearby).
Next. we pour some pewter into this mold.
Step 7: Mold Results
After some work with a Dremel, the surface texture smoothed out somewhat but the line details are still rather poor.
After some research I discovered that some people have successfully cast pewter in MDF wood molds, so I inverted the digital model , created new g-code files and began the process of carving a mold in which to pour the pewter.
Step 8: Getting 'Mold'ier
Step 9: Hot Metal!
As you can see in the below pictures, the process is fairly simple. Melt metal, pour metal, let cool.
This first video shows how I failed to let the ladle heat up enough to carry the metal to the mold. That was rather embarrassing, but even more embarrassing is my lack of full safety gear. I had appropriate eye protection on but neglected to put on the leather gloves appropriate for this operation. Fortunately I did not have to learn a difficult lesson.
After allowing the ladle to heat up, I poured again. This time I learned another lesson, the ladle should be of appropriate size to fill your mold. The result of this pour had great definition on one side but the other side was full of gaps around the model. This was likely due to both smoking and re-pouring metal on already cooling metal. Lesson: One smooth pour.
Step 10: End Result
I've learned lots of lessons during this experiment and I hope that they have been found informative. Not everything goes as expected but every chance to Learn and Do should be explored fully. Please enjoy the below pictures of the several pours of this cast.
Be Safer than you expect to need
The mold is fine, leave it alone
Heat your pouring container
Have enough material to fill your mold in one smooth pour
Don't expect perfection on your first try.
It's OK to fail at something when you learn something (and, boy, have I learned a LOT!)
Thanks for reading! Let me know if you have any questions of suggestions! I hope to be able to do more of these. I plan on using soapstone for 2 part molds in the near future.