The OEDK recently purchased a new 4-axis CNC Mill, and I wanted to work out a way to perform the ENGI 210 Final project using that machine. The machine, a fully enclosed Tormach PCNC 770 with a 10 tool automatic tool changer uses a flood coolant to increase work speed. The drawback however, is that you really should not use wood in the machine because it can get impossible to clean. The only solution I could see then was to make my mold out of aluminum.
The final involved first milling a block to make a positive –half mold, then using that half to create two negatives. The two negatives are then pressed together to make a cavity for the final cast part. I first began by designing my two mold halves. I wanted to have two different halves that could fit together using alignment pins so that my two negatives would always line up for the cast. I also needed a way to pour in the casting material and an air outlet hole to let the pour happen quickly. These components were all designed in SolidWorks on two separate halves of a chess piece.
I purchased aluminum on McMaster and then began playing around Fusion 360 to create the code for my mold components. I had some experience with the program, but I wanted to use this project as a way to improve my skills in Computer Aided Manufacturing (CAM) as well. In order to have a highly detailed mold, I would need to use a small end mill, no more than 1/8” thick. However, when using a flat end mill in aluminum it is unwise to step down into the material more than half of the diameter of the cutting tool. So if I used a 1/8” end mill I would need to take 10 passes to clear the 0.75” of material down into my part. That would take way to long. I decided to use two different tools. First, a ¼” end mill would remove the majority of the material in larger cuts, just 5 passes, and then the 1/8” end mill would go back and “clean-up” the part to higher detail. The pattern I used for the tool motion formed small step-like ridges up the side of my mold, but I substantially decreased the step size so they would be less visible. After the two end mills, the machine would use a ¼” drill bit to drill two holes for the alignment pegs into the block as well. I named both tools T4, T3, and T26 respectively to match their designations in the Tormach computer.
For feeds and speeds, I used the following website to calculate the appropriate values for aluminum 6061 for the three tools I was going to use. http://www.daycounter.com/Calculators/GCode/Feed-Rate-Calculator.phtml
Left to Right: Tool path for ¼” end mill, tool path for 1/8” end mill, tool path for drill
I used the Tormach post process option to create my g-code in a format understood by the mill. I loaded in my program and loaded my piece of aluminum into the machine. The zero point was set to the proper corner of the piece to match my code and then all I had to do was hit “Cycle Start”. The Tormach automatically grabs the first tool from the ones loaded in the machine and begins carving. After about 30 minutes, the machine swapped to my 1/8” end mill to clean up the details. The final step was to drill the two small holes. The final product is pictured below.
Notice that despite having a fine stepdown of 0.02mm, the detailed steps are still clearly visible to the naked eye. These steps were actually more pronounced than they had been on my wooden version of the part because the aluminum is not easily smoothened under simple pressure. This part was an accurate and successful milled component, but I wanted to make it better.
My solution to the step-like look and feel of my part was to add an additional tool pass over the aluminum to smooth off the edges. I chose to use a ball end mill running in a parallel pattern back and forth across the chess piece to create the smooth arched shape. The passes of the ball end mill would run up and down the steps and eliminate them from the part. I chose a 1/8” ball end mill stepping over 0.005” per pass to create an incredibly detailed part. I added this code to my Fusion 360 file and ran the part once again, this time on the reversed side of my chess piece.
The machining for this second half too substantially longer with the parallel ball mill section, but the results were worth the wait.
Next came the process of actually molding and casting my chess piece. I began by constructing cardboard walls around each of my aluminum molds and sealing them up with masking tape. I used two separate molding options, Smooth On and Mold Star to create my two negatives. These negative molds each took about 5 hours to cure, but once they did they were very easy to peel off my aluminum parts without even using mold release. The two negatives (pictured below) each represent the volume of one half of my desired chess piece and fit together using the alignment pegs.
The next step was to cast my two chess pieces. I sprayed both negatives thoroughly with mold release agent and then put the two halves together with their alignment pegs. I used two flat pieces of wood and a clamp to apply an even pressure on the backs of both negatives to keep them pressed together. Too much pressure though would deform my chess piece dramatically by compressing the cavity. On my first attempt pouring, the resin cured much faster than I could pour it into the mold. Turns out I made the hole at the top of my hold way too small to allow the casting material to flow in quickly. I ended up with a well aligned half of a chess piece. I tried again with another casting material, this one with a lower viscosity, and I managed to fill the whole mold on my first try. The finished product when I opened up the mold is pictured below on the left.
The chess piece on the right was the product of about 10 more attempts with the first material. The best part of my incredibly precise aluminum molds is that my final cast chess pieces required little to no post processing. They came out of the mold smoother than I would probably get with sanding.
From this project I learned a few really important points about molding. The first thing is that the orientation of your tool paths can have a huge influence on the final product. Adding the third pass with the ball end mill made my mold incredibly smooth and saved me lots of sanding later in the project. I also learned just how helpful alignment pegs can be in casting the chess piece. I was able to use my pegs to make sure the two halves of my mold were always lined up nearly perfectly. Third, I learned that the pour hole needs to be of a significant size to allow the casing fluid to pour in quickly enough. I had a lot of trouble with the getting the mold filled before the cast hardened midway in. And my final point, is that it can be helpful to tilt/ shake the mold a bit while pouring. This made it possible to get caught air bubbles out of the cavity before the cast hardened.
I had a lot of fun redoing this project with the aluminum mold, and now I would really like to try to make another shape besides a chess piece using the new tormach mill.
