Sunday, March 28, 2010

Making a Conoco Gas Truck, Part III

I started working on the molds for the tank end, which looks like this:

Tank End_300 Tank End Back_300

The back side of the part presents some issues in terms of both making a mold and extracting the part from the mold. Making the mold requires that I cut some fairly deep slots into the aluminum mold halves. These slots need to about about 1/4” deep, and they’re about .047” wide at the bottom.

After I’ve designed a mold, I need to create tool paths that my computer will send to my CNC milling machine. For this I use a program called VisualMill. It takes as input a “solid” model of the mold. I then tell it what size cutter to use, how deep to make each cut, and so on. After a few minutes, it returns a tool path like this:

Tank End Core Toolpath

This particular tool path was for a 1/16” diameter cutter, which I used for the first pass. However, to get all the way to the bottom of the slot, I created another tool path using a long-reach 1/32” diameter cutter. Long reach in this case mean the thin part of the cutter is about .350” long. But that makes the cutter both somewhat fragile, and expensive (about $30 each).

I only had one of these long-reach cutters left, having broken a few others while making mold for the top of the tank. So I put my last long-reach cutter in the mill and started cutting. Unfortunately, I was a little impatient, so I set the cutting speeds too high and broke the cutter. Doohh! I’ve ordered some more cutters, and they should be here on Monday, so I’ll be able to continue working on this mold. But the next time, I’m going to cut slower so I don’t break this cutter.

By speed, I mean how fast the mill moves while it’s cutting. I had it set to move down at 1 inch per minute, and cut at 3 inches per minute. I thought that was slow enough, but apparently not. The other factor here is that I was using a flat-end cutter for removing most of the material, but I think a ball-nose cutter would be a better choice. A flat-end cutter experiences some shock when it first touches and bites into the metal. A ball-nose cutter experiences a less sudden shock because the tip is round.

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