Just as the home computer grew out of behemoth industrial machines, home manufacturing may bring the abilities of a factory to the home.

TechMan is talking about a desktop machine driven by a computer that can make things. Has TechMan gone off his medicine again?

No, such machines already exist.

It's been obvious for a long time that computers can control mechanical devices -- your desktop printer is a common example. And computers can be used to design things using computer-aided design (CAD).

Linking computer design and control to lathes and other machine tools produces CNC (computer numerical control) machines. CNC machines can be fed computerized designs and can fashion objects out of wood or metal or other materials.

Next came the idea that rather than form the object out of a block of material, why not build it up by "printing" layers of materials. Thus was born 3D printing, a way to do rapid prototyping.

Prototyping is the beginning of the manufacturing process in which a model of the object to be produced is made from drawings. It was time-consuming because the plans had to be sent to craftsmen who made the model or mold.

For example, TechMan has a wooden form from a steel foundry where his grandfather worked. It is for making something that looks like it could have been an artillery shell and it had to be painstakingly handmade out of wood, then pushed into sand to form a mold to receive the molten metal. If the design were changed, a new wooden form had to be made.

But with 3D printing, protoypes can be produced quickly and altered easily, as can molds for casting metals or plastics.

Some 3D printers spray a fine powder such as plaster, cornstarch or resins in layers whose shape is controlled by CAD files. A spray adhesive bonds the layers.

Others use inkjet-type nozzles to spray layers of a liquid photopolymer. The layers are then hardened with ultraviolet light.

To go from using 3D printing to make protoypes to using it in the home is not such a great leap.

Let's say, for example, that your child is building a Lego vehicle and finds that a wheel is missing from the set. Now, you would have to send the Lego company money and an order and it would have to send you the wheel -- a two- or three-week process during which your child would lose interest.

But suppose you could download the computerized design file for that wheel from the Lego Web site, feed it into your home fabber and minutes later the required wheel would be "printed" out, in the correct color.

Futuristic? Not as much as you think. Desktop fabricators (fabs) are already commercially available.

Desktop Factory (www.desktopfactory.com) offers a printer that "prints" plastic objects. It costs about $5,000.

But home computing really started to boom in the 1970s when hobbyists got involved, such as the famed Homebrew Computer Club in the Silicon Valley that counted among its members the founders of Apple Computer.

The same thing is happening in desktop fabrication. At a site called fab@home (fabathome.org), plans for a home fab can be downloaded for free. Hobbyists have been building the machines for about $2,500 then using them to make objects out of Play-Doh, silicone, wax and even chocolate. Or you can buy an assembled one for about a thousand dollars more.

But the revolution won't truly take off until the price of a home fab system becomes affordable. To that end, a team of scientists is working on something called a RapRep (for rapid replicating), a desktop manufacturing machine that can print its own parts, thus replicating itself.

Estimates are that if this project is successful, the price of a home fab system could fall to about $400.

So in the future, when you go into the kitchen to make a cup of coffee, you might make the cup, too.