Fastners for 3D Printing: Screw it, we'll use science. Part 1 of n

On a previous episode, I considered the idea of using wood screws to hold together 3D Printed parts. At the time I conjectured that wood screws might work fairly well for holding plastic parts together without having to drill and tap every hole. The scientific method has now been used to determine conclusively that wood screws may work very well for your application.

Setup

For each experiment two plastic parts were printed. One part had a series of square screw bosses and the other part was a plate with a series of counter sink holes. The assembled parts were placed in a vice and a Ryobi cordless drill model P205 with adjustable torque was used to tighten each screw until failure.

The torque settings on the drill produce an unknown torque and have an unknown error and unknown repeatability. However for the purpose of comparing the relative holding strength of each fastening method, these issues should not prevent a rough estimate from being made.

Some screw bosses were treated with Plastruct Plastic Weld to attempt to improve the performance by chemically melting the layers together.

The parts were printed in ABS on a Stratasys FDM Machine with the pilot holes oriented upwards.

Failure Modes

Stripped threads are by far the most common failure mode, as expected.

Stripped screw are generally caused by poor alignment between the tool and the fastener.

Cracked screw bosses seem to be a factor when the holes are too small for the fastener. They are also commonly caused by poor bonding between the printed layers.

Split counter sinks are caused by the screw acting as a wedge and driving apart the material being fastened. Due to this issue some experiments were repeated with an aluminum plate to compensate for this problem.

Split screw bosses occur after the counter sink has split and the screw is driven until total failure. Here the wedge action caused by the screw is a little clearer.

Experimental Results

Each experiment was repeated 3 times and the results have been uploaded to Google Docs. During the experiment two screws were stripped due to an error on the part of the experimentor, who failed to properly insert the tool into the screw.

Conclusions

Wood screws work are able to be tightened to a higher torque than machine screws without stripping the threads. However, since most commonly available wood screws are countersunk they can not be tightened as much as machine screws when used as plastic to plastic fasteners.

Holes for wood screws should be under sized and drilled out. For example when making a hole for a #6 wood screw, print a 1mm diameter hole and drill it out to 2.5mm. The pilot holes will need to be slightly bigger then if you were using wood screws in wood. The primary benefit of wood screws is that tapping is not required.

The application of the plastic solvent did help improve performance marginally however it is probably more useful as a method of repairing stripped threads by filling the stripped hole with plastic bits and applying solvent.

When the plastic thread forming screws finally arrive they will be covered in Part 2.