This hot air balloon was one of the first projects we cut on our four axis CNC router.
We cut it from a block of 30 lbs. Precision Board high density urethane (HDU) measuring 14" x 14" x 20" long. We had previously drilled a hole in the center of each end and driven a steel pipe into it for chucking the block up.
The routing operation had three phases.
The first step was to true the lathe. (We assign a steady feed rate for the y axis and the z feed is done in three passes as a straight line across the piece.)
The second step was to rough out the balloons shape — done in four passes. This was done with a half inch ball nose bit with a 50% overlap.
The final step was the cleanup pass — done with a 3/8" ball nose bit and a 95% overlap.
A higher than typical overlap was necessary because the piece had a large diameter. (The farther from the centreline the bit is, the more overlap it requires.)
As soon as the piece was done, we removed the steel pipes, drilled a hole, and mounted it to the driver's cab.
Our next instalment will show how we designed and routed the nose and tail cones for the balloon.