One of the main places to get stuff for your fighting machine is the scrapyard. The advantage: All kinds of stuff is lingering behind/under every corner or mount of scrap. The disadvantage: Nothing looks like what you want. Here are some things I found in the scrapyard to be used in the robots we are building or planning to build:

• Several hundred kilograms of blocks of 7075 aluminium (Aluminum in the US, or 'Ali' in this discussion). I bought 100kg for about 1.82£/Kg ($1.30US/lb). Not bad for type 7075 which is one of the strongest types of Ali around.
  
• Some pieces of 90mm (3.5") grade 2 Titanium rod for the same price as the Ali.
  
• 10mm (3/8") and 5mm (3/16") Ali sheet.

Some other treasures we found were complete reduction gearboxes and most importantly several pneumatic cylinders. All this for much less money than comparable new products. But one thing is almost certain: The things you find will likely need some work to get them in useable condition for your robot. I will try to explain some things I learned/discovered/experimented with.

McGyverisms, or How To Salvage Scrapheap Stuff.

Pneumatic Cylinders.

New pneumatic cylinders are rather expensive, bulky and heavier than what we want. The difference between a servicable cylinder and useless scrap isn't big, but noticable. Here are some pointers to look for if you find a cylinder on a scrappy.

The type of metal they are made of.

There are 4 basic variations: steel, stainless steel, aluminium, and bronze/brass. The most interesting to us is aluminium since weight is always a consideration. But Ali is easily damaged and should be checked carefully for dents. Steel cylinders should be avoided since they are often rusted inside rendering them useless once the rust has formed. Stainless steel cylinders are most likely to be found on machines used in the food processing/pharmaceutical industry. Brass/bronze cylinders will likely be corroded, especially if they have been in contact with other types of metals in a damp environment. They frequently have non-standard inner wall diameters.

Disassembling the pneumatic cylinder.

Some disassembly of the cylinder will be required in order to determine its condition. Two methods of assembly are commonly used. The first method features a permanently welded assembly which cannot be disassembled or repaired. The second method consists of typically 4 tie rods along the length of the bore with nuts or allen bolts on both ends securing the cylinder end plates. This type of construction allows the cylinder to be disassembled for repair. If the tie rods are heavily corroded or rusted, the cylinder is likely to be damaged by disassembly if excessive force is used. However, you can saw the rods in two while being careful not to cut or damage the cylinder bore underneath. But will need to find replacement rods should you do this.

Once disassembled, perform a visual check on the cylinder bore. Is the bore still in good shape or are there signs of dents, scratches, or corrosion? This is the most vulnerable and most difficult part to be made at home.

[Editor's note: Be extremely careful if you choose to make your own pneumatic cylinder. An improperly made cylinder can explode with tremendous force causing serious injury to anyone nearby. You should also know that the US Battlebots safety rules discourage the use of home-made pneumatic components. Any home-made pneumatic parts for use in Battlebots require engineering data showing that they are safe to use at their rated pressure.]

Next, examine the heads. Are they corroded, broken, or cracked? If so, then the heads are not servicable. Be sure to check the port threads. Are they damaged? You might be able to re-tap or install a thread insert if so. Also make sure that the inlet ports are not obstructed with dirt or debris.

If the cylinder passes visual inspection, it can then be cleaned and reassembled. Once assembled, you can test how well the piston moves inside the bore. Can you move the pushrod up and down the complete length? Does it move smoothly? You should hear air blowing out the inlet ports as the piston moves. You can test the efficiency of the seals by blocking the inlet ports and trying to move the piston again. This should make the piston much more difficult to move. If the piston still moves easily, then you should suspect that the seals are worn. Fortunately seals aren't that difficult to replace or even to 'rejuvenate'.

Finally, push the piston to the halfway position and try wiggling the pushrod from side to side. It should not rattle inside the cylinder head bushing. If it does, the bushing needs to be replaced or else the pushrod may bind when placed under load.