Steel sheeting is easy to find on a scrapyard. It’s also rather cheap to buy new if you use industrial size sheets like 2m*1m which are available in every thickness varying between 0.3mm to several centimeters. But mild steel isn’t the best material we can use. Compared to aluminium, it’s nearly 3 times as heavy and not that much stronger. Titanium also has much better properties than steel given that it's lighter and at least as strong.

The big advantage of mild steel over other materials is that it's rather easy to work with. Steel is weldable with very cheap tools and reasonably easy to cut. Angle grinders, hacksaws and chisels are up to this task. The same isn't true for most advanced plastics or high tech metals which can be very difficult to work with.

Out in the field, steel has a very big advantage in that battle damage can be easily repaired. When dented, steel can be hammered back into shape. Rips and tears can be hammered and rewelded. In extreme cases, completely destroyed sections can be removed with an angle grinder and replaced using a portable welder. This kind of field work is well inside the capabilities of a do-it-yourself (DIY) person. For example, Bigger Brother was repaired from a little pile of buckled steel to a very tough robot within 2 hours. Admittedly, all of the expensive parts in this machine were very well armored with 2mm steel sheet AND several layers of polycarbonate. On the otherhand, it’s not a particularly expensive or special machine. The most expensive part was the drivetrain which used Bosch 750 motors and 4QD speed controllers. As we can see, mild steel offers a number of advantages to the DIY bot builder. But it requires care to use well if we intend to stay within our weight limits.

Stainless steel is a lot more interesting. It's stronger than mild steel, a little harder to work with, and gives a nice finish. It's also very findable on a typical scrapheap. But it’s also a little more expensive, even in a scrapyard. Welding stainless steel isn’t much more difficult than mild steel. It only uses other kinds of welding material. As for the rest, stainless steel has the same advantages and disadvantages as mild steel.

Aluminium is an attractive metal given its light weight. But in its pure form (type 1100) it's very soft and is unlikely to resist much punishment. The type of aluminium available in DIY stores (probably type 1100, 3003 or 5005) is much too soft and very weak for use in a robot. A much better kind of aluminium include types like the 2024-T3/T4, 6061-T6 or 7075 which are not easy to find, and not cheap. To find these on a scrapheap in useable size is a chance in a million. Your best bet is to find a scrapheap near a shop that builds or repairs commercial aircraft. Military air bases will sometimes sell their scrap as well.

Aluminium has other problems as well. Its softness tends to clog up saws, files and grinding discs. It can even dangerous to grind aluminium as it tends to grab cutting discs, causing them to shatter. The clogging can be reduced by using a lubricant like tallow (a type of grease) on the tools, but afterwards using the tools on steel is funny.

Welding aluminium is a more specialized process and is not done with cheap tools. Also the welds tend to form weak spots if not heat treated afterwards.

Threading holes in aluminium is not a good idea either as bolts will tend to pull the threads out if the thread depth isn’t at least 3 times the thread diameter. For example, for a M6 bolt you need 18mm deep tread. In my opinion, aluminium is best used for internal mounts and large pieces like flanges and supports. To attach it, use bolts and nuts with big washers.

Titanium has quite a reputation. It is supposedly indestructible, lightweight, much stronger than steel, very hard, and not workable with normal tools. I can speak from some experience. Titanium isn’t the wonder metal most of us think. It’s weight is halfway between steel and aluminium. As far as being lot stronger than steel, well that depends. Titanium's weight/strength ratio is superior when comparing mild steel to one of the titanium alloys. If you compare high strength steels like “Dokol” or “Hardox” with non-alloyed titanium, then titanium looks like a piece of cardboard. The point is, you need to keep in mind that there are many types of steel and titanium, each with different properties. Titanium's reputation for being indestructible, very hard and not workable with normal tools is also a myth. Like most metals titanium isn’t indestructible. But it’s rather acid resistant and even capable of resisting seawater (a very corrosive substance). But titanium can be a fire hazard since it is combustible under the right conditions. It is used to create the white sparks in fireworks for example. As for the hardness of titanium, that depends on which alloy you have and what kind of heat treatment it got. Grade 1 and grade 2 titanium can be worked with good quality steel working tools. It does require that it be cooled very abundantly, because titanium is a poor heat conductor causing heat buildup which shortens tool life and can cause measurement errors in precision parts.

Welding titanium is a real problem and requires more skill than welding steel. It has to be welded in a completely inert environment. Above 800 degrees, titanium will bind with oxygen/nitrogen/carbon/whatever resulting in very poor welds which are easily broken. Even worse, the whole piece may be destroyed by burning it up, if enough oxygen is available. And not every alloy is weldable with good results even with good technique as Wheely Big Cheese demonstrated in RobotWars: Series 4. In general, welding titanium is even more difficult and expensive than aluminium.

Finally, finding titanium on scrapheap is also a miracle.

The last metal we'll consider useable for frame/supports is Magnesium. Magnesium has poor corrosion resistance and it’s very difficult to find on scrapheaps. Its workability is similar to aluminium, but welding is more difficult. It’s lighter than aluminium, but also weaker. Magnesium is brittle and is often used as an alloy with aluminium and zinc. These alloys have a dull gray in appearance and slight greasy feel. Many cast aluminium pieces are actually magnesium/aluminium alloy. Magnesium alloys are not weldable. The only stuff capable of repairing this material is "Techno-Weld", which is a kind of low temperature brazing compound.

In the next part of the scrapheap tutorial, we'll look at some hints for finding plastics in the scrapheap.

For information on Techno-weld, click here
For information about Titanium, click here.