Waterjet Cutting and Actuator Simulation

Individual Progress


After cutting parts out on the waterjet (Figure 1), I:

  • drilled and tapped holes in the bearing holders and bushing holders
  • drilled and reamed shaft holes in the knee joint and motor mounts
  • drilled and reamed bushing holes in the bushing holders, ballnut plates, and spring module bottom & top plates
  • bored bearing holes in the bearing holders
  • milled part surfaces as necessary to adjust thickness of parts
  • sanded and prepared the parts for assembly

All of these operations were carried out in the RI Machine Shop.

Figure 1: Actuator stack components, cut using waterjet

At its present state, the actuator stacks for knee and ankle are complete. The knee joint is also complete (Figure 2). I still need to make the shank, ankle bearing holder, knee bearing holder, ankle joint bar, knee actuator distal shaft mount, and ankle actuator proximal shaft mount. I am considering making the shank with square tubing instead of round because fastening to it is much easier, even though square cross sections do not resist bending as well as round ones.

Figure 2: Knee joint and knee actuator installed in iWalk knee brace

Shank Moment Simulation

In order to determine whether square tubing will be sufficiently strong under bending, I am simulating the forces and moments on the current leg design using positions and forces measured during non-amputee human walking (Figure 3). I read the data from a spreadsheet of data obtained from Nitish Thatte in Professor Geyer's group. Using this data, I calculate the length of and force in each actuator over time. Using this information, I will calculate the moment on the shaft, and thereby determine the required bending strength of the shank.

Figure 3: Conversion between required joint torque/position and actuator force/length


Taper of Waterjet Cut

Parts that are cut on the waterjet have an approximately 1 degree taper on the cut edges. This makes clamping and aligning the parts difficult. This time, we ended up sanding the parts flat on the disk and belt sanders in the RI shop. For positioning, we placed conical edgefinders in waterjet-cut holes and moved the mill table until the edgefinder was true. From there, we extrapolated to the zero point that we wanted based on the dimensions of the hole pattern. Next time, I will do the waterjet cut step last. This will allow clamping of the stock plate in the CNC mill or regular mill, providing easy positioning and clamping. The tricky part will be aligning the plate in the waterjet and zeroing the cutting head. However, I think this extra difficulty will be outweighed by the time saved sanding and positioning parts.

Source: https://sites.google.com/site/mrsdproject2...