Servo - August 2010

(Figure 6) and then rough-cut 0.2” in a single pass with a big 3/4” end mill from one end of the blade. I then moved the blade in the vise and repeated this on the other end (Figure 7). Ideally, one would not move the part but my vise is too small to adequately support the whole cut at once. However, with the knee of the table locked (the Z axis), the part can be moved and the cut depth remains constant. Note I also left about 0.050” around the boss, as well. I repeated this again for the remaining three blades.

The 0.050” finish cut is made with a 1/2” end mill that has a 0.060” radius on the cutter (Figure 8). This tool leaves a nice 0.060” chamfer on any internal corner and thus reduces stress risers that might allow a crack later.

The blades are then trimmed to length after carefully measuring and scribing the correct distance from the center hole and machining off the excess (Figure 9).

To reduce weight and improve the aerodynamics of the blades, I beveled the edges using a 1/2” 45° chamfer mill (Figure 10). I could have tipped the head of my mill to do the same thing, but this method is quicker and easier.

The teeth fit into pockets in the end of the blade and these were carefully measured, scribed, and then machined using the same 0.060” radius mill used earlier (Figure 11). The teeth had arrived by this time and after ensuring they fit okay, I machined the pockets on the other blades. Next, the two holes for the screws were added and countersunk (Figure 12).

I recommend transfer punching and then drilling one hole first, fitting a tooth, and then using it as a guide for positioning the second hole to ensure that they line up correctly. If you try to just transfer punch and drill both at the same time, it is too easy for the drill to wander, and you’ll end up with the holes incorrectly spaced. I didn’t fully countersink the holes so I could leave more metal around them

FIGURE 12

FIGURE 13

which will make the mounting stronger. The tooth mounting screw heads remain slightly proud of the surrounding metal (Figure 13). You can see that the teeth also have 0.060” radii on all internal corners and are symmetrical (allowing them to be reversed to give a fresh sharp edge if required).

A completed blade can be seen in Figure 14. I need to order slightly longer screws for securing the teeth and a 12-24 tap to clean out a few of the threads them, FIGURE 14