How to construct ("What we learned the hard way...")

Working on pib, we collected a lot of experience, what works and what not. Although we learned much more about things that do not work , we want to have a shared reference of our practices and standards.

Screw connections

Our screws are all ISO metric M3 with hexagon socket. On the entering-side of the screw we use a countersink with the following parameters:

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The through-holes are 3.2mm. The countersink has an top-diameter of 6.2mm and an angle of 90 degrees.

Currently there are variables in document linked to these values so you don’t have to do it manually.

The corresponding M3-nuts have a height of 2.5mm. In the CAD the nut-insets are modeled as hexagon with an inner radius of 2.85mm:

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Using ball bearings

This table showcases all bearings used including their dimensions and inset. Inset for ball bearings have a clearance fit with minor difference in diameter, additionally crush ribs are used to provide a better grip on ball bearings, a feature script in the pib CAD document does this automatically

Using small bearings with rods

The default clearance fit value used around Pib is 0.1 mm while interference fit is -0.1 mm, this is used with connecting metal rods that are sometimes used as fixed axis which needs a 1.9mm hole (interference fit) and sometimes used as rotating axis which needs a 2.1mm holes (clearance fit).

Minimum wall thickness = 1mm

Usually, the layer height used in most print jobs are 0.2 or 0.3mms, this should make walls above 0.6mm theoretically possible. However, from some failed experiments any wall thinner than 1mm either falls to print or is very week and breaks by hand