Thursday, April 3, 2008

Castings

I’ve been kind of silent lately….and a few mayor things happen since then. First. I failed to deliver the project on time. There where many reasons for that, they run the gamit, designs that didn’t work out, mistakes, supplier delays, even local holidays. I am not used to failing in this way so it was hard to write or even focus on what next. But after some consideration I feel it’s time to focus on what we have learned and what is coming up.

So I will focus the discussion on what we have learned: First lets talk about castings…they are key to a CNC machine, at least in small quantities. Not only do you have to get the molds right you have to take into consideration the time it takes to get every step done and what happens when there are complications. Here are some tips, get your carpentry gear out , sharpen your scraper, measure twice, cut once and start gluing MDF. Mind this is the cheapest way to get it done. We used spray and paint on wood sealer on the MDF, otherwise it will be hard to paint since it will absorbed to much paint and distort itself. The sandable kind was nice, its finish after a light hand of sanding can be almost ideal. We finish off our molds with enable paint. This helps with the de-molding. I also advice reading a little theory about how to designing cast parts –constant wall thickness, draft, material flow and part cooling. There is a good description of this in Slocum’s book “Precion Machine Design” and on his course posting. http://pergatory.mit.edu/2.75/2-75_Lectures.htm.

Keep in mind that when creating tall enclosures you may have porosity problems, the material creates gases that can be trapped if the casting acts like a bell. So make you you leave openings for the piece to breath. Also put some thought into how you want the material to flow. The person doing the casting is probably better at this so make sure you have this discussion if you are making a complicated piece. I will post all the molding instructions for our part, which took two modification to get it right. Also make sure you talk to your caster about none compliant parts, make sure you have a clear understanding of how he will redo pieces that did not come out right the first time.
Motor mountings: Painted MDF mold after releasing - Cast motor monut top view - motor mount next to Y axis servo motor

The other part that is important to note is that this pays off if it’s a local effort. Many time large foundries won’t take your work and overall castings are heavy to shipping so cost can be high. In Colombia we are where being quoted between 4500 to 9000 peso a kilo which is the equivalent of $2.43-5/kg or $1.10-2.20/lb. this may sounds cheap but remember that there are post machining operations. For small parts we paid about $100 for all three motor mounts, for larger pieces it wasn’t so simple, we have gotten quotes for $3500 for squaring and machining the Z axis and it X axis interface. Clearly that is way too much so we are finding ways around this. I will complement this post once I’ve perfected a method for replicating flat/parallel surfaces with $50 of metallic resin.


Making the Zcasting mold. MDF mold views




Modeling the Zcasting in SolidWorks





Lina and Camilo working on the Zcasting Mold


The Making of the Zcasting: Mold released -Mold sealed - Two views of the casting once out and cleaned.

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