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3D printed binding post covers

Problem

Binding posts are a useful component of various test equipment. They however tend to have metal parts exposed to casual touch and contact with tools and loose wires. As such, they have a potential for causing surprises.

Solution

A cover. The best thing to inhibit unintended touch of live parts is an insulating cover, easily both removable and replaceable.

The binding posts often come with standard spacing of 19 mm, matching the spacing of certain European mains sockets. This allows insertion of some two-pin mains cables.

3D printing was leveraged. A cover box was designed in OpenSCAD, as a hollowed out brick with a pair of properly spaced pins. These pins insert to the banana plug holes in the posts, elastically deform, and hold in place by friction.

Drawing

The draft was done in OpenSCAD, in a parametric way. All the dimensions were specified as variables, so adjusting the dimensions for any different size of the cover needed in the future is easy.

The pins are made as hollow tubes, with a narrow hole through the center. This turned out to be mechanically stronger than solid pins. They have a ball widening on the end, for closer fit. They are also widened at the base, to relieve mechanical stresses.

A version with additional layer of material just around the inner perimeter of the box was added to another version of the part, to strengthen the wall-lid interface and prevent cracks there.


Rendered model

Rendered model, transparent

Faulty version

First printout did not come out correctly. The wall thickness was too high, the slicer injected a filler layer in between and this caused back pressure on the nozzle (due to slightly poor configuration) and frequent filament skipping/grinding. Not only that, the printing time was unnecessarily long and filament consumption was higher than needed.

Worse problem, the pins were fragile and broke off easily.

Corrections

The wall thickness remedy was easy; it was lowered to 0.8 mm, which yielded only two tightly touching layers. The file was rendered and exported to STL, and checked with the slicer until the filament layout was correct.

An attempt to remedy the pin breaking issue was made by inserting a coaxial 1mm hole through the pin length, with the intention to melt or glue in a length of metal wire for strengthening. However, it showed up that the hole caused changes in the slicing, so the pins were now without infill and were made only from two concentric layers of filament. This variant was strong enough for use and resisted the necessary degree of bending.

Material

Transparent PLA was used, for the reasons of cost, availability, and having some degree of translucency to facilitate at least partial visibility of the posts.

Postprocessing

The pins may come out printed "dirty", with droplets of plastic hanging on them. They are somewhat poorly accessible. A short, about 1-second burst of flame from a small torch heats them enough to soften the plastic. The pins then can be shaped easily with tweezers, squeezing the hanging-on plastic pieces into the base material.

The softened pins can also be easily bent a little, to achieve just the right amount of friction to come on and off smoothly but not fall off too easily.


Fresh printout

Fresh printout

Fresh printout

Results

The printouts were made in two sizes; one for smaller cheaper binding posts, one for the bigger kind. They differ in diameter and height, therefore different internal dimensions and pin length/clearance are necessary.

The smaller version, with internal box dimensions of 31x12x21 mm, took about 12 minutes to print and consumed about a meter of filament.

The larger version, with internal dimensions of 35x15x25 mm, took about 15 minutes. In retrospect, it could be made about a millimeter smaller along the first two dimensions.


Both versions

Both versions

Both versions

In use

In use

In use

In use

In use

In use

In use

In use

Round version

Sometimes the square version is too square. Round version, with the base shape based on a block and a pair of cylinders, can be desired at times.

The edgeless nature of the shape is much more vulnerable to the path joining artefacts. This makes the oozing effects more visible. It turned out that the hot end temperature is maybe unnecessarily high.


Rendered model, transparent

Rendered model, transparent

Rendered model, transparent

Round covers, different temperatures

Round covers, different temperatures

Possible improvements


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