Yesterday we finished structural assembly of the prototype leg. It looks awesome, so we’ll start with some pictures and then tell you everything that went in to it.
“But doesn’t this look just like your last blog post? What’s changed?” I’m so glad you asked. In the last blog post we had welded together the plates we got back from the water-jet cutters and tied them together with ratchet straps and clamps to get them in to position. The keen observer will note that in the last blog post, the weight of the leg is supported by a crane.
There is much more to these leg pieces than the waterjet plates. Every rotating axis has a shaft going through it and reinforcing bosses that need to be welded in to the structural plates. A reinforcement boss is a machined part that distributes the load from the shaft to the plate over a wider area. There are three axes of rotation on this leg, each of which has three pins that need to rotate (the joint itself and the two sides of the actuator). Each of those pins needs between 2 and 4 bosses machined and welded in to the structure. That’s dozens of machined parts, which are quite time consuming to make.
Another non-obvious but very difficult and important process is bushing alignment. Every rotating shaft on this leg has to go through a set of several bushings. For the shaft to turn, those bushings all have to line up perfectly. When we install the bushings in our structure, they are not well aligned. To line them back up, we need to ream them all. Reaming is the process of taking a special drill bit just slightly larger than the hole and shaving a few thousandths of an inch off the internal surface of the bushing. Hand-reaming a bushing is a slow process that takes multiple passes, adjusting the size of the reamer with each pass. Directionally loading the reamer is another trick we use to force the bushings in to alignment faster. There were 30-ish holes on this leg requiring a multi-pass hand reaming process. Phew!
We’ve also been doing a lot of experimentation with our weld practices. For our heavier plates (1/4 inch to 3/8 inch steel) we were originally using only a TIG process, which is quite slow. We’ve improved our MIG procedure enough that we have almost entirely phased out the TIG. For those unfamiliar with welding – TIG is a precise manual process where the operator controls the torch in one hand and feeds in metal with the other. MIG is a one handed operation where the torch automatically feeds metal in to the weld. TIG is generally more precise, but MIG is much faster and easier on the operator.
Our order of operations will also be totally different for our next leg. Welding the water-jet plates together before putting in bosses made for some very awkward grinding and welding angles, resulting in some lower-quality welds that we can get away with on this prototype but want to avoid when this machine actually has a person on it.
Next up – we need to put all the actuators in place on this leg and hydraulically plumb them. We also need to improve our hydraulic powerplant – we are investigating running our propane engine inside and routing the exhaust out but if the building management is not OK with that we will have to come up with something else. Thanks for supporting open robotics, over and out!