CNC Toolpaths, Motor Cooling, and Leg Stump Simulator

Individual Progress

CNC Toolpaths

Figure 1: CNC toolpath using 3/8” endmill for ankle ballnut holder

In preparation for the CNC milling operations, I have been following the HSMWorks tutorials on generating toolpaths (Figure 1) for 2.5D milling. I plan to only do the pockets, bushing holders, bearing holders, and hard-to-remember drill patterns using CNC. I will drill the common mounting holes, such as the holes on the corners that have square symmetry, using a manual mill. This will be easy to do because the part can just be rotated/flipped and re-clamped while keeping the table locked (using the mill as a drill press).

Motor Cooling Solutions

Figure 2: Team Integy cooling fan and heat sink for 40mm outer diameter motors

Momentary loads on our actuation system exceed the RMS rated capacity of our motors. However, the RMS load is less than the rated capacity. To help disperse the heat from these spikes in the load, I found combination heat sink-cooling fans (Figure 2) that match the diameter of our motors. These devices are inexpensive and lightweight.

Leg Stump Simulator

I am in contact with Rohan Krishnan in the lab of Professor Collins about the leg stump simulator. Their device uses a commercially available iWALK crutch. This device sits under a user’s shin (leg bent at the knee), and straps to their calf and thigh. In the modified version, a standard pyramid adapter for prosthetic components sits below the user’s knee.


Redesign for Simpler Machining

As I plan toolpaths for CNC, I am realizing that some of the parts I designed are unnecessarily complex. For example, the bushings are securely seated in their mounting holes and don’t need to have lips on each end to retain them. This modification means that the parts do not need to be flipped and re-milled in the CNC, saving time.

Also, I am modifying the design of the leg to use the same parts for the ankle and knee actuators as much as possible. I rechecked the calculations of the required diameter for the linear slides, and the ankle actuator linear slide can be downsized while still maintaining a large safety factor.

Leg Stump Simulator Length

Because the pyramid adapter sits below the user’s knee, rather than laterally, the prosthetic-leg side of the user’s body will be taller than the unmodified side. The current solution to this is for the user to wear a platform shoe on the unmodified leg to make up for the height difference.

After we have a system for testing the leg with a non-amputee user, then I may explore shortening the leg to allow use without the platform shoe. This is not as high a priority as getting the weight-bearing parts fabricated.