This is an instructable on making your own drafting machine. This is a drawing aid that mounts to any desk, and will give you a moving set of rulers that will keep to a right angle in relation to your board. It's not hard to make one, and it's a huge help for hand drafting. These can be made from any material, and since I'm going to add CNC capability to mine I used metal and I have space on the elbow piece for servo motors. I hope to post another instructable soon on how to make one of these draw or cut on its own from vector files. I made this all at the TechShop, and so can you.
Get the dimensions of the board you plan to mount this to.
The length of each of your four arm bars should be an inch longer than half the diagonal length of the board. The shape of your pieces makes no difference so long as your joint holes are exactly as shown. Also, keeping the arms straight gives a wider range of motion.
Decide beforehand whether you want your arms to be perpendicular to one another or angled. You can make the angle of the top two bars different so long as you also make the shoulder joints at the corresponding angle in relation to the board. Either design will still allow your square to remain perpendicular to the top of the board itself. The reason one might choose diagonal is to give the arm a greater range of motion.
Cut your arms, elbow, hand and shoulder parts. Precision is very important here, and remember to have each pair of holes the same distance apart. Secure each piece tightly, but with some ability to move. I recommend lock washers, or the short two-piece wide head screws I used on the shoulder and elbow. Of critical importance is eliminating any slippage from your joints. If they have any room to move, the angle of your square will not be 90 degrees.