I use a variety of materials to machine my projects out of.  Here are some of them:

ACETAL:  This is an excellent and versatile engineering grade plastic.  I have various diameters of black rod from 10mm to 40mm, out of which I can machine almost anything and I am finding more uses for it on every project.  The technical name is Acetal Co-polymer, it has high mechanical strength, hardness, stiffness and toughness.  It’s a great plastic to machine, giving superb surface finish and can keep close tolerances.  If you need an even tougher plastic the next best is ‘Delrin’ which is a Homopolymer, but I find Acetal is a fantastic all round performer.

Aluminium:  Rod or sheet in various grades can be machined very easily on my small machines and a good finish can be achieved.  I have seen aerospace grade aluminium for sale at shows, and I would like to compare different grades in the future, it all depends on the part I’m making, but mostly I find the everyday grade you can pick up at trade stalls is good enough for small engineering projects.

Brass:  Lovely to machine giving a good surface finish without coolant.  It allows a very high polish to be achieved but will tarnish without a clear coat of laquer.  Not quite as strong as mild steel (see below) but is tough enough and strong enough for lots of parts, and can be made very to look aesthetically pleasing.  Can be a lot more expensive than plastic, aluminium or steel, so large projects made of brass are rare. An alloy of copper and zinc, different grades are available for example free machining brass (CZ121) has a certain amount of added lead.  Other elements such as manganese, aluminium, arsenic can be added to the alloy depending on the application.

Copper:  Used rarely as it is a very soft material, can be used for sealing washers and joints.

Steel:  An alloy of iron and carbon, most commonly appears as mild steel in the workshop with different grades depending on the application.  Free cutting mild steel has lead added to aid machining. Mild steel is relatively easy to machine, nuts, bolts, studs, supports, plinths, frames, spacers etc can be made from this steel making it the most versatile material in the workshop although it will rust readily if not protected by oiling or waxing or painting.

Stainless Steel:  An alloy of iron and carbon also containing chromium.  Common grades are 304 (known also as 18-8 because it has 18% chromium and 8% nickel)  and 306 (known also as 18-10 because it has 18% chromium and 10% nickel).  Contrary to popular belief stainless steel will stain if not cared for by polishing or cleaning.  Can be hard to machine so only small parts are made, also you can never be sure which grade you pick up from exhibition stalls.

Perspex:  An Acrylic plastic also known as ‘Lucite’, ‘Acrylite’ or ‘Plexiglas’.  A very stable but brittle plastic which is easily machined. It can be polished to glass like clarity. See my cube in a cube project for an example.