When designing and building a scale, the mechanical design and the 3D-printing are just one aspect of the problem. In order for the scale to be of practical use, it's at least as important to also provide an easy method for generating a weight dial which is calibrated to that particular scale. This project tries to satisfy both of those needs.
As for the constructional part, the provided files generate a mechanical letter scale using an old-fashioned classic design with a moving quadrant that contains the weight dial and a counterweight.
The SCAD file is fully parametrized and commented, so that the size and strength of the scale can be adapted, if needed. By default, the scale is dimensioned to about the maximum size which still allows printing of the complete scale in on a single 20x20cm printing plate (actually it needs only 19x18cm).
An STL file is provided for printing the complete scale on one single plate, as well as separate STL files for the individual parts.
The maximum measurable weight the letter scale depends on the counter weight. I use as counterweight two M10 nuts, two M10 washers and a little piece of M10 rod, which results in maximum measurable weight of about 130g. Of course, the bigger the counterweight, the bigger will also be the maximum measurable weight. In addition, the levers contain a second set of weight pivot holes, that are 20% shorter and using these will increase the maximum weight accordingly.
As each printed letter scale will be slightly different (type of filament, filling, screws, counterweight ...), it is not possible to use the same dial markings for all scales. Thus, it is very important to have a good and simple way to calibrate the scale. For this reason, a PostScript file is provided with this project, which helps calibrating the scale and also prints the properly calibrated scale dial.
For the calibration, it suffices to measure two angles:
To facilitate measuring these angles, the PostScript file contains a auxiliary dial with a graduation showing 0-90 degrees.
Once the angles b0 and a1 have been measured, the values of b0, m1 and a1 (resp. a1+b0) in the PostScript file are modified accordingly and the file is reprinted, now producing a scale dial which is a properly calibrated for this particular letter scale.
The two scales I have built and calibrated so far using this method are both precise to about the nearest gram.
When mounting the scale, it is very important that the pivot axes have the least possible friction, so that the scale moves as freely as possible. I use small nails cut to the proper lengths as axes and drill the pivot holes to just the right size for the nail to turn freely. In order to keep the nails in place, I stick cut-off ends of cable binders onto the rear ends of the nails.
If you plan to build this letter scale, I recommend you follow the procedure explained in detail on the instruction page...