Predicting the future is a crapshoot. When we were writing "Fabricated: The New World of 3D Printing" and interviewing people about 3-D printing, we discovered that a few underlying "rules" kept coming up. People from a broad and diverse array of industries and backgrounds and levels of expertise described similar ways that 3-D printing helped them get past key cost, time and complexity barriers.

We have summarized what we learned. Here are ten principles of 3-D printing we hope will help people and businesses take full advantage of 3-D printing technologies.

Principle one: Manufacturing complexity is free. In traditional manufacturing, the more complicated an object's shape, the more it costs to make. On a 3-D printer, complexity costs the same as simplicity. Fabricating an ornate and complicated shape does not require more time, skill, or cost than printing a simple block. Free complexity will disrupt traditional pricing models and change how we calculate the cost of manufacturing things.

Principle two: Variety is free. A single 3-D printer can make many shapes. Like a human artisan, a 3-D printer can fabricate a different shape each time. Traditional manufacturing machines are much less versatile and can only make things in a limited spectrum of shapes. 3-D printing removes the over- head costs associated with re-training human machinists or re-tooling factory machines. A single 3-D printer needs only a different digital blueprint and a fresh batch of raw material.

Principle three: No assembly required. 3-D printing forms interlocked parts. Mass manufacturing is built on the backbone of the assembly line. In modern factories, machines make identical objects that are later assembled by robots or human workers, sometimes continents away. The more parts a product contains, the longer it takes to assemble and the more expensive it becomes to make. By making objects in layers, a 3-D printer could print a door and attached interlocking hinges at the same time, no assembly required. Less assembly will shorten supply chains, saving money on labor and transportation; shorter supply chains will be less polluting.

Principle four: Zero lead time. A 3-D printer can print on demand when an object is needed. The capacity for on-the-spot manufacturing reduces the need for companies to stockpile physical inventory. New types of business services become possible as 3-D printers enable a business to make specialty -- or custom -- objects on demand in response to customer orders. Zero-lead-time manufacturing could minimize the cost of long-distance shipping if printed goods are made when they are needed and near where they are needed.

Principle five: Unlimited design space. Traditional manufacturing technologies and human artisans can make only a finite repertoire of shapes. Our capacity to form shapes is limited by the tools available to us. For example, a traditional wood lathe can make only round objects. A mill can make only parts that can be accessed with a milling tool. A molding machine can make only shapes that can be poured into and then extracted from a mold. A 3-D printer removes these barriers, opening up vast new design spaces. A printer can fabricate shapes that until now have been possible only in nature.