Mechanical Letter Scale DIY

Letter scales are used for weighing letters in order to determine the correct amount of postage. Nowadays electronic scales are used but I like the style of these old mechanical measuring tools. So I decided to make one and explain the design process for the students we have here in Fablab Moscow.

Step 1: Tools and Material

Tools

• Laser cutter
• Pliers

Material

• Plywood 3 mm
• Iron wire 1mm (Paper clips)
• White glue
• 1 M10 x 18 bolt, nut and washer
• 2 M3 x 12 screw and bolt

Step 2: Cut the Parts

Download the parts and cut them.

If you want to have accurate results you need recalculate the numbers position. So weigh the weighing pan (mine is 40g) and the counterweigh (M10 screw and bolt)(37g) then run the Octave script with your own weight parameters. Finally place the number position at the right angle before engraving the scale.

Octave script:

m=40:10:200; % array of masses. From 40 to 200 each 10 grams

l=20; %length in milimeters

M=38; %counterweight

L=80;

alpha=atand(m*l./(sqrt(2)*M*L-m.*l)); %calculated angle

plot(m,alpha,'o') %draw the graph

grid on

xlabel("mass")

ylabel("angle in degrees")

Step 3: Assembly

• Cut four 9mm length axles in the iron wire
• Make the parallelepiped and insert the axles
• Place the second side over it (check that is moves freely)
• Glue the mechanism on the base and the spacers
• Glue the weighing pan and screw the scale and the counterweigh

Step 4: Test and Calibration

Test the accuracy with objects with known weight. You can make small adjustments by adding or removing washers.

Weigh your love letters :)