Aluminium
Lightweight. Doesn’t have the strength of steels and brasses. Good for a slick look.
Can get good detail.
Rolled Stock: is available in the widest variety of profiles beyond the common, because it extrudes so easily. Has more malleability as rolled than cast and will bend like steel. Good corrosion resistance – can easily be scratched – ends and threads easily broken, easily dented on edges.
Casting: Easy to cast. Cast aluminium is brittle so can shear on fine parts. Not suitable for regularly used threads or detail that will have a constant interactive use.
Good to machine (good surface finish – rapid material removal).
Welding: ACDC TIG only on slighter sections – heavier sections can be done with a TIG welder – however, consider other options of fixing (mechanical – bolting, riveting, etc.) as aluminium welding is more specialised.
Mid-range for cost.
Doesn’t really tarnish, just dulls.
Brasses and bronzes
Often called Yellow Metals. Brass is an alloy of copper and zinc; bronze is an alloy of copper and tin. Due to various combinations, they can be seen as alloys on a sliding scale – some brasses are referred to as bronzes and some bronzes are brasses.
Averages 10% heavier than steel! If you need it lighter, you get aluminium alloys.
Excellent for detail.
Rolled Stock: common profiles available.
Casting: easy to cast. Bronze expands ever so slightly in the mould before setting which picks up fine detail.
Good surface finishes – easy and quick to machine whilst having a good strength.
Welding: with the right set-up you can MIG and TIG; otherwise must be brazed.
Generally more expensive than other metals. Bronzes are much stronger and deteriorate less, but are the most expensive.
Certain amount of protection against corrosion – the metal will tarnish – mostly depending on copper content, it will go green.
Cast Iron
If a steel has more than 2% carbon, it is considered a cast iron.
It is the step between the elements and our newer metals. Has a feel of being from a past age – there’s something nostalgic and raw about it. Filthy to work with.
Can get larger detail.
Rolled Stock: plate or block.
Casting: great dimensional stability. Generally good for large castings that can be used in machinery as well as large bodies of weight that can be precisely machined, cheaply cast and be threaded into, turned, milled. Granular in composition, like aluminium when cast, but with greater strength, so can shear over length and over load, but if dimensions are right, it will take a great load as well as having a great compressive resistance (makes great machinery, bedways). Not for fine bits.
Unlike steels and bronzes, it doesn’t have significant surface tension and thus does not bend or deform when machined.
Welding: modern cast iron can be welded. Older parts will need brazing.
Used to be the cheapest to cast. However, energy prices are whittling this saving down.
Generally resists rusting in normal indoor conditions. Will tarnish slightly – goes browny.
Steel – Black Carbon
Generally has a mottled finish akin to cast iron, although some is rolled to have a flat finish with a slight sheen.
Rolled Stock: common profiles available.
Not for casting.
Poor to machine but can be regularly drilled. Will hold a thread.
Welding: easy to weld – can use all processes, depending on job. Welding will deform (bend) the stock slightly – if you level off the heat on opposing sides, you can rectify this.
The actual material dimensions can vary from the stated/nominal stock dimensions which, as well as normally being bent out of shape, can make precise work unattainable. Best suited for basic structures that are welded together and can be negotiated in situ.
Cheapest, most readily used stock.
Will tarnish (rusty brown) on cut or ground edges if left raw.
Steel – Engineering stock
Stronger than black carbon steel. Has a lovely, slightly pitted, dull shine.
EN3 (commonly referred to as mild steel) is the most commonly used grade, although there are long lists of grades available.
Rolled stock: solid, heavier sections available.
Casting: can be cast.
Better to machine than black carbon steel, but time-consuming to do on larger amounts. Machining only one side of stock will release the surface tension and it will distort along that plane; this can be solved by machining the opposing side. This is not a problem in turning as there is only one distorting surface. Great strength. Drills well. Threads well.
Welding: easy to weld, can use all processes, depending on job.
Strengths will be sufficient for most applications, although you can post-harden components if you are going industrial on it.
Slightly more expensive, but definitely worth it for more precise work.
It does tarnish – goes browny, and will rust under poor conditions.
Steel – Stainless
Rolled Stocks: common profiles available. Better for components that can be easily machined from stocks available – making it cost effective in material and labour.
Casting: can be cast.
Two commonly used types
304: commercial grade. Good corrosion resistance. Tougher to machine than steel but easier than 316. Generally slightly cheaper than 316. Good surface finishes.
Strength is akin to engineering steel.
316: food and marine grade. Excellent corrosion resistance. Expensive. Sexy. Good surface finishes. Good to machine – good to turn, but time consuming. Can easily distort due to surface tension. Great for lasting shine.
Welding: easy to weld, can use all processes, depending on job.
Most expensive stock. Good to consider whether it’s better to use another metal and post-coat.
Won’t tarnish under most conditions. Clean with a soft cloth, soap and water, or with a bleach-free glass cleaner. In more extreme cases use solvent. Always wipe the surface dry to prevent the formation of water stains.