Basics of Building

Most prpol parts attach together with m3 (metric 3mm diameter) machine screws and m3 nuts.
Attachment holes in prpol parts align with a one centimeter grid.
Some parts have pockets for m3 nuts to slot into. If needed, you can customize the exact dimensions of these pockets by setting custom values for nut thickness and width.

Solid parts are rectangular prisms with side lengths that are a whole number of centimeters. In other words, you can think of them as being made of a collection of 1 cm cubes joined together.
Plate parts are divided into flat parts and shell parts.
Shell parts are so called because they form "shells" around a solid part of the same nominal size. For example, a 3x3x3 block part will fit neatly inside a 3x3x3 bin, cee, tube, or any other shell part of that size.
Keep this in mind when building a structure with shell parts: the hole grids of shell parts joined together don't always maintain a global 1 cm grid the same way solid parts do when joined together. This is simply due to the parts' wall thickness being a shell "outside" of an imaginary cluster of 1 cm cubes.

The long side of a right triangle is called the hypotenuse, and the two short sides are called the legs. This is why the longer struts are called hyp struts and the shorter ones are called leg struts.
Hubs can be printed with or without internal pockets for nuts. If you want to use this feature, you need to set up your 3D print to automatically pause at certain layers. See the Hub part page for more details.
In the prpol truss system, the centers of hubs are located on a 10 cm 3D grid.
When assembling a truss structure, make sure that all the hubs are oriented the same way.
To build a truss structure:
1. Use only leg struts and hubs to make squares and cubes. This will create the basic shape of the structure. It's normal for the structure to be wobbly at this stage.
2. Once you are happy with the basic shape of your structure, introduce hyp struts to make the structure completely rigid.

Attachment holes can also be used as bushings for 3mm round steel rods.
The default size of the attachment holes is designed to be just a little too snug for a 3mm round rod to spin freely.
To get a hole to the ideal size (little or no slop, little friction):

Put a 3mm round rod in the chuck of a power drill
Run the round rod through the hole back and forth while spinning.
This will erode excess material while softening and shaping the plastic into the perfect size.
Check the fit by trying to spin the part freely on the round rod.

Use a hand file to cut a flat spot onto the axle where the set screw of the part will be.
Insert a nut into the wheel or gear's nut pocket, and use an m3 screw as a set screw to lock the part onto the axle.

To simplify this discussion, let's imagine the gears are circular wheels that roll against each other.
The diameter of this circle for a particular gear is called its pitch diameter. Its radius, the pitch radius.
In the prpol gear system, gears have whole number standard sizes.
A size 1 gear has a pitch diameter of 1 cm, and thus a pitch radius of 5 mm. A size 2 gear has a pitch diameter of 2 cm, and thus a pitch radius of 1 cm. And so on. There is no hard limit to the size of gears in the prpol system; you can print as large a gear as your printer will allow.
Crucially, if you are using the standard 1 cm prpol grid to mount gear axles, odd-sized gears will only mesh with odd-sized gears, and even-sized gears will only mesh with even-sized gears. This is due to the fact that the pitch radius of odd-sized gears is always half of an odd number of centimeters, i.e. a whole number plus 0.5 cm, and the pitch radius of even-sized gears is always a whole number. So when we mount an odd sized gear on the standard 1 cm prpol grid, its pitch radius lands on a point halfway between grid holes. For even-sized gears, the pitch radius lands exactly over a grid hole.

Where possible, instead of using a custom bracket or transition part, the best overall practice is to use general-purpose, re-usable transition parts like the prpol slot bracket. Doing so will:
- Increase layout flexibility during the current build project.
- Make it so more parts from the current build project can be re-used in future projects.
To use slot brackets:
1. Insert a nut into the hexagonal nut pocket of each bracket. Ideally, the nut should be snug enough to stay in place. Push it all the way until it is against the top surface of the pocket.
2. An m3 screw goes through the slot of a slot bracket and into a hole on a part with a standard prpol hole grid. It can be rotated about this mounting screw and slid along the slot to adjust the position of the captive nut. Position as many brackets as needed and line them up with the mounting holes of the thing you are trying to attach. Do not fully tighten these screws until they are all positioned correctly.