Build Your Own Combat Robot

Stainless Steel
Chapter 9: Robot Material and Construction Techniques 187
What you’ll pay for the job will cost you far less than what you might pay for a
cheap TIG or MIG welder, and you won’t have to go through a learning curve and
deal with joints that may fail. Welding is covered more extensively later in this
chapter in the section “Welding, Joining, and Fastening.”
Aluminum is certainly not the only material available for robot construction, and
nobody can say it is the best structural material for all applications. Stainless steel
is popular for many applications with robot construction, especially for tough robot
skin uses. Alloy 304 is one of the most popular forms of these alloys and is
used in many applications where formed sheet steel is best, such as for sinks (and
robot shells). It typically comes in 36-by-36-inch sheets from 0.024 inch to several
inches in thickness. It welds well, providing you have a good TIG welding system.
Again, we recommend that you have your welding done by an expert who deals
with stainless steel, such as professional welders who make food-processing
equipment.
Stainless steel sheet metal is usually recognized by someone who does not know
metals as a “steel-like” metal that is weakly magnetic or totally non-magnetic,
though some high nickel steel alloys are magnetic. Stainless steel alloys contain
iron as the basic element plus a small amount of carbon. They also contain the element
chromium and are sometimes called chrome steel. At least a dozen alloys can
also contain various amounts of nickel, cobalt, titanium, tantalum, manganese,
molybdenum, silicon, and even sulfur that give the different alloys specific properties
for particular uses. The most desired property of stainless steel is its resistance
to corrosion and rust.
Stainless steels are usually categorized in three groups: austenitic, martensitic,
and precipitating-hardening alloys. Austenitic stainless steel alloys are low-carbon
based with nickel added to enhance workability. They are hardened by cold working
and are slightly magnetic. They have excellent corrosion resistance and are
easily welded. Alloy types 304/304L are some of the most popular alloys and are easily
welded, and these are used extensively in food processing equipment. This alloy
can be purchased as round stock from 1/8 inch to several inches in diameter in 3- to
6-foot lengths. Sheets are available from 0.024 inch to several inches thick, and in
sizes from 12-by-12 inches to 36-by-96 inches. It welds well using a good TIG
welding system and a good welding professional. Another useful alloy in this series,
type 347, has tantalum and cobalt added for greater hardness and is used as machinable
rounds and in pressure vessels.
Martensitic stainless steels are not popular in most robot applications because
of their lower corrosion resistance and poor weldability. Type 440C is a high-carbon
alloy that is used in gears, bearings, and shafting. It is available as round stock and
can be heat treated. (Heat treating is done to change the mechanical properties of
the metal.) It is hard, giving good wear and abrasion resistance.