Sawppy Issue: 8mm Hole Precision

The old mechanical adage is that anything that moves is a potential point of failure, and Sawppy is not immune. Every point of rotational motion uses 8mm steel shafts and they have proven to be challenging for myself and other rover builders. They are kept in location by E-clips, and cutting precise slots for them are hard without a metalworking lathe. For pieces that need to grip on those shafts, my designed used M3 set screws that pushes against plastic with the help of heat-set inserts. And getting them right can also be frustrating.

For pieces of plastic that don’t need to grip on the shaft with set screws, they should still have a snug fit. But 3D printing a precise diameter hole is hard to accomplish given variances of 3D printers. Not just variation from printer to printer, but that a printer may have difficulty hitting the same exact XY coordinates on every print layer to line up a precise hole.

Fortunately this is a problem with an existing solution from the world of machining. When a precise diameter is required, rotational cutting tools called reamers are used. Sawppy’s instructions included use of an 8mm reamer to make precise holes that will fit tightly onto 8mm shafts, and a reamer built for metal had no problems chewing through thermoplastic.

But I found that wasn’t the end of the story, because a reamer only helps make the diameter precise. The location is only as precise as the drill turning the reamer. I had naively thought the reamer would be guided to the correct location and alignment by the existing 3D-printed hole. It did to some extent, but not enough to ensure precision. If the drill is positioned off-center or tilted off-axis, my reamer happily reamed out an off-center, off-axis hole of very precise diameter.

What happens when the hole position or tilt is wrong? In case of suspension components, the articulation will change by a few degrees. In case of wheels, it means the center of rotation would be slightly offset from wheel center. When such wheels spin up, they will visibly wobble. Actually neither would be a functional blocker for Sawppy, as the rocker-bogie suspension can tolerate these variations and keep all six wheels on the ground. But it does look a little silly and drive perfectionists mad.

I don’t think there’s any way to solve the off-tilt problem for handheld drills. Every solution I thought of to hold the drill in alignment ends up looking like a drill press. Why reinvent this wheel? Unfortunately that’s a piece of shop equipment not every aspiring rover builder owns.

The off-center problem is harder to solve. Right now my train of thought is heading towards some kind of centering jig. The challenge is to design something that can be 3D-printed yet help deliver higher precision than what 3D-printing can deliver on its own. Can it be done? I don’t know, but I’m thinking about it.

Back on the topic of drill press: there is a second reason why I might want to make them part of the Sawppy builder’s toolkit: a drill press would allow me to drastically reduce the number of fasteners required to build Sawppy.

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