There are many ways to produce custom metal parts for a prototype.  In most cases the processes are used only for prototyping, as the setup-cost is low, but the piece-cost is high (the opposite of most production manufacturing processes).  This article is meant to be used as a guide to what is available to make the metal parts of your new product.

Machining
This is the most common method of creating metal parts for prototypes.  There are certain geometric restrictions for machining processes, but with current CAM CNC (Computer Aided Machining, Computer Numeric Controlled) systems, very elaborate shapes can be easily created.

Sheet Processes
Sheet metal parts often have elaborate shapes before bending.  In production, often CNC punches, presses, and brakes form these parts.  For prototyping, there are several options for making complex shaped sheet parts.

Water Jet Cutting: Water jet uses a high pressure stream of water with abrasive particles to cut through metal, and just about anything else.  A CNC table can create just about any shape imaginable.  Water jet can also cut very thick metal, even over1″ thick.

Plasma Cutting: Plasma cutting is similar to water jet, but instead uses very high temperature ionized gas (plasma) to melt through the material.  The heat is intensely concentrated, and does not significantly heat the parent metal.  Even thicker material can be cut with plasma, when compared to water jet, however it is less precise.
Laser Cutting: Laser cutting uses a very powerful, focused laser to melt through the metal.  It is more precise than plasma and water jet, and leaves no burr what-so-ever.

Wire EDM (Electrical Discharge Machining): EDM works like a hot-wire cutting through foam.  But instead of heat, the electrode wire makes repeated electric discharge against the part and slowly erodes the nearby material.  This method is a bit expensive, but has extremely high accuracy and can create the smallest kerfs and cut the thickest materials.

Stereo Lithography: SL is not a cutting process, but uses chemical etching to remove the unwanted material of a sheet part.  A masking is applied to the rest of the part too protect it from etching.  Because this masking is photo-sensitive, and cured by a laser, very precise designs can be created.  This is even more precise than EDM.  However, there is a material thickness limit, and also the ‘cuts’ are tapered, so a hole would not truly be ‘straight through’…none-the-less, SL is a very useful prototyping process.

Spin Casting
Spin casting can make irregular part geometries that might not be possible with machining.  It starts with a master model, usually a plastic RP part, and a high-temperature rubber mold is casted around it.  Then, molten metal is poured into the mold while it spins, and centrifugal force helps to fill all of the cavities of the mold.  This method can produce dozens of parts from a single rubber mold.  However, only low-melting temp metals can be used…most commonly zinc alloys.  Aluminum and steel cannot be used with spin casting.

3D Metal Printing
This is a relatively new process that works very similar to SLS with plastics.  Instead of plastic powder, metal powder is used, and fused with a laser.  The cost is relatively low compared to some other processes, at least for small parts.  Material choices are restricted, and are usually stainless steel.  The other drawback is that it is not very precise, and the surface finish typically has pronounced stair-stepping, which can be sanded away and polished in subsequent operations.

Lost Wax Casting
This is the oldest metal casting process, and dates back to antiquity.  It starts with a wax model…which can be 3D printed, or cast from RP molds.  The wax model is covered in a liquid and powdered ceramic slurry, and left to harden.  The ceramic coating is cured in a oven, which melts and/or burns out the wax model.  Then, molten metal is poured into the ceramic mold, and left to solidify.  Virtually any material can be used with the lost wax method, and the structural quality is identical to production parts, because this is often a production process as well.