Two important things to do when using carbide bits to cut enclosed
track on a speargun:
Cut a shank track first 1/4" wide and 1/4" deep. This important
step reduced the amount of material to remove as you cut the
track with the ball cutter.
Use the slowest speed possible if you use a router. The faster
the speed, the more likely it is you'll break the bit. Some
cheaper routers only have one (very high) speed and it will
probably break the bit quickly. We use a mill at moderate
speed and the bits last for many tracks. Carbide bits
should last a long time if you treat them well.
You can use an undersized wood dowel or a shaft with sand paper
wrapped around it (like a taco) to sand a track if necessary.
