Probably the most complicated and time consuming part of the project was making the lobes of the cam. The lobes are super important. If the geometry isn't quite right, they could come sliding out of the rock when you really don't want them too, or they could deform if the load isn't totally in-line as we'd like. The math behind how the lobes of climbing cams work is really cool, and you should read about it here or here. Someone else did some modelling and experimentation here to figure out the stresses in the lobes. After reading the links above, we know that the shape of a cam lobe is a logarithmic spiral, and Black Diamond claims a 'camming angle' of 14.5 degrees. The caliper measurements of the lobe agreed with this 14.5 degree angle, so I went with that.
Now that we can define the edge, how can we actually make this thing? One option would be to use a manual mill. There were a few problems with this though, all stemming from the fact that I would have to move the cutting tool around the part by hand. For one thing, it would be really difficult to get that outside curve on the lobe anything close to exact. Also, I had to make at least 4 lobes, and it would be tough to make very exact copies.
A way around this, and a good excuse to try out some new toys, was to use the CNC machine. It would be able to get the pattern just right, and it would be able to churn out as many copies as we needed.
The first step to using the CNC was to make a computer model of the lobe. I used AutoCAD 2015, installed on some of the school computers, to make the model, based off of the measurements of the real lobe. It took some ancient-greek-style geometry construction to make a logarithmic curve with a camming angle of 14.5 degrees. This is something that I remember from taking geometry class from my mom in 8th grade, so thanks mom! I ignored some of the small details, like the 'scalloping' on the edge of the lobe, and the tiny holes for the springs and trigger wires. These I would add later by hand.
You can view the model here, or download it with the button below.
cam3d_mod.stl |
Now, the CNC machine isn't smart enough to understand this 3D model directly. It only understands 'GCode,' which are a sequence of directions like 'move to position (x,y,z)' or 'make a clockwise circle of radius R centered at (x,y,z).' So, I used a program called MeshCAM Art, that transforms the 3D model into GCode. After this, we can feed this GCode into the CNC, and it moves the cutting tool accordingly.
We had a few issues with this process. First, we had to use a decently large cutting tool (the machining term for drill bit) on the CNC so that we wouldn't break it, since we were going to have to cut a decently large amount of aluminum. That meant the tool was too large in diameter to fit inside the axle hole and the two smallest 'weight saving' holes in the middle of the lobe. We would have to do these later by hand as well. The second issue was that the GCode generated by MeshCAM Art is terribly inefficient, even if it does cut out the model accurately. It has the tool jumping around all over the place, not really following a smooth, fast, or logical path, so it takes about 5 times longer than needed, which would have made each lobe take almost 2 hours to make. I had to go into the GCode manually and move around blocks of instructions so that the thing would finish in a reasonable amount of time.
Below is a video of the CNC working through a lobe. This was one of the earlier runs, and so we had the speed of the tool set really slow, since we weren't sure how fast we could cut through the aluminum with a pretty small cutting tool. Later, we were able to bump the speed up to 4 or 5 times what you see here. Then, each lobe took about 20 minutes to complete. I made 5 altogether, so that I could use the first one, which accidentally got a chunk cut out of the corner, as a test piece for the rest of the process.
Below is a video of the CNC working through a lobe. This was one of the earlier runs, and so we had the speed of the tool set really slow, since we weren't sure how fast we could cut through the aluminum with a pretty small cutting tool. Later, we were able to bump the speed up to 4 or 5 times what you see here. Then, each lobe took about 20 minutes to complete. I made 5 altogether, so that I could use the first one, which accidentally got a chunk cut out of the corner, as a test piece for the rest of the process.
After they were CNC'ed out, I drilled the last 2 'weight-saving' holes and the trigger wire hole by hand, drilled and reamed the axle hole, and drilled and tapped the holes the spring screws would go in. Then I used a rotary table on the manual mill to add the scallop texture around the edge. The result looks pretty awesome, as you can see in the picture at the top of this post.