Commercial bend fixtures are built for metals and plastics and will damage a fragile specimen before the test begins. SunThru needed one matched to its own samples, so I designed a fully printable 12 part fixture built around gentle handling.
Renders are blurred here because the fixture is SunThru hardware. The engineering worth describing is the parametric structure and the design for printing, neither of which needs the geometry shown.
Three-point bend testing is a standard way to measure flexural strength, but commercial fixtures assume a specimen that can take being clamped. When the sample cannot, the fixture itself becomes the thing that breaks it.
SunThru needed a fixture sized to its own samples, gentle enough not to pre-damage them, and cheap enough to iterate on, which meant printing it in house.
A 12 part fixture with interlocked dimensions is a maintenance problem: change one dimension by hand and eleven other parts silently stop fitting. Every part sits on a shared coordinate system, and I automated the rebuilds with SolidWorks VBA so a dimension change regenerates the affected parts consistently.
Each part's geometry is checked against its expected volume before it is accepted, which catches a rebuild that succeeded but produced the wrong solid.
The parts are shaped the way the machine builds them: additive layer friendly geometry throughout, and horizontal M3 cross-holes drawn as printable channels so nothing needs support material or post-drilling.
One hardware size across the whole fixture means one driver, one drill, and no chance of an assembly step needing something that is not on the bench.
The decisions that held up were the ones that thought like the manufacturing process. Geometry built up in layers the way the printer builds it, holes shaped for printability instead of for drills, hardware chosen from one standard size.
Designing for the process beat designing around it.