Typical Material Analysis for Fabrications
Stainless steel
is defined as a ferrous alloy with a minimum of 10.5% chromium content. The name originates from the fact that stainless steel does not stain, corrode or rust as easily as ordinary steel. This material is also called corrosion resistant.
Stainless steels have higher resistance to oxidation (rust) and corrosion in many natural and man made environments.
Stainless steel's resistance to corrosion and staining, low maintenance, relative inexpensive and familiar luster make it an ideal base material for a host of commercial applications.
Stainless steels will be more expensive that standard steel.
Aluminum
is found primarily in bauxite ore and is remarkable for its resistance to corrosion (due to the phenomenon of passivation) and its light weight. The metal is used in many industries to manufacture a large variety of products.
Aluminum is a soft, lightweight metal with normally a dull silvery appearance caused by a thin layer of oxidation that forms quickly when the metal is exposed to air. Aluminum is nontoxic (as the metal), nonmagnetic, and nonsparking. Aluminum is about one-third as dense as steel or copper; it is malleable, ductile, and easily machined and cast. It has excellent corrosion resistance and durability because of the protective oxide layer.
Improper use of aluminum may result in problems, particularly in contrast to iron or steel, which appear "better behaved" to the intuitive designer, mechanic, or technician. The reduction by two thirds of the weight of an aluminum part compared to a similarly sized iron or steel part seems enormously attractive, but it must be noted that this replacement is accompanied by a reduction by two thirds in the stiffness of the part. Therefore, although direct replacement of an iron or steel part with a duplicate made from aluminum may still give acceptable strength to withstand peak loads, the increased flexibility will cause three times more deflection in the part.
Aluminum will be more expensive than standard steels and approximately the same cost as stainless steel.
Carbon Steel
is an alloy primarily composed of iron, with carbon content between .02% and 1.7%. depending on grade. Carbon is the most cost effective alloying element for iron. Commercial grades of steel used in the production of brackets and fabrications are typically supplied in sheets with thicknesses described as gauges. Typical gauges would be 12 (.10”), 14 (.075”), 16 (.060”), 20 (.035”)gauge. Plate steel would be described in thickness of 3/16 (7 ga), ¼” ,3/8” and ½”.
Light gauge sheet steel (12 gauge to 20 gauge) would normally be cold rolled and have a smooth, clean surface. Heavy gauge steels (3/16” to ½”) are typically hot rolled. The surface is rougher than cold rolled but still acceptable for most applications.
Steel is the most cost effective and durable material available for fabrications. It is not resistant to rust so therefore needs surface coverage. (See Typical Material Coatings for Resistance to Rust)
Typical Material Coatings for Resistance to Rust
Galvanized Steel Sheet
Hot-Dip Galvanized Sheet is carbon steel sheet coated with zinc on two sides by the continuous hot-dip process. The process results in a layer of zinc on each side of the steel sheet that is tightly adhering to the steel through the formation of an iron-zinc alloy bonding layer that is formed by a diffusion process while the heated steel strip is in contact with the molten zinc.
The steel gets coated in layers of zinc because rust won’t attack this protective metal.
If galvanized steel is lasered or cut, the edges will not be protected from rust since the zinc coating is layered on the outside of the sheet.
Hot Dip or Mechanical Galvanized Plating
Galvanizing at its most basic level is dipping steel products into a molten zinc bath. A metallic reaction ensues and a thin coating of zinc is formed, evenly covering all areas of exposed base metal. This simple approach is still used today; called after-fabrication hot dip galvanizing, this process is also commonly referred to as batch or general galvanizing. Products, assemblies, and fabrications coated with zinc by this process are most widely used in exterior applications but are also used in interior designs.
This is done to reduce corrosion (specifically rusting) of the ferrous item; while it is accomplished by non-electrochemical means, it serves an electrochemical purpose.
Galvanized steel is suitable for outdoor use if part is completely coated.
Zinc Plating
Zinc plating involves the electrolytic application of zinc by immersing clean steel parts in a zinc salt solution and applying an electric current. This process applies a layer of pure zinc that ranges from a few microns on cheap hardware components to 15 microns.The finish of this type coating is normally a shiny silver finish. Due To the nature of this process a consistent finish is not always achieved and this type finish would not be considered decorative but effective in resisiting rust. The Zinc deposits on the metal acts as a barrier to oxidation (rust).
Black Oxide
Black oxide is a conversion coating formed by a chemical reaction. The reaction between steel and hot oxide bath results in a magnitite coating being formed on the part. The part surface is normally a dark to matte black color after the process. The finish would not be considered decorative but adds an antique or flat black type look. The finish will not flake and is durable. The process also provides improved lubrication to the part to resist galling. It does not exhibit "white rust" so it is excellent for elecrical cabinet brackets.
Preparation of Holes for Screws or Bolts
Tapped Holes
Tapped holes are formed threads in sheetmetal used to thread a screw or bolt through to hold a component or attached the sheetmetal part to another part. There are a number of specifications you have to keep in mind.
In the American system, a screw thread is designated by two numbers called the screw size and the pitch. For instance, #4-40, #8-32, 1/4-20.
The first number refers to the size (and therefore strength) of the screw shaft; the second to the pitch (number of threads per inch) on the screw. So a #4-40 screw has 40 threads per inch, a 1/4-20 has 20 threads per inch, etc.
A #4 screw is a smaller diameter (and therefore weaker) than a #6, etc. Common sizes are 4, 6, 8, 10, ¼”, 5/16” and 3/8”. For each shaft size, there are only 2 common usage thread numbers. For example, #8-24 & #8-32 are the common sizes for #8 screws.
The two pitches for any size are often referred to as "fine" and "coarse". The fine one will need more turns to tighten up, but will hold more securely in the face of vibration and other factors. The coarse one will be faster to attach but will not hold as well.
An important thing to check, when designing a formed tapped hole in a part, is "Will it be strong enough? How many screw threads am I going to get?" Here's how to figure it out. The pitch is the number of turns per inch. So, if you have a #4-40 screw in 0.055” material, there will be 2.2 turns or threads (40 turns per inch times 0.055 inches). As a rule of thumb, you need 2->2.5 turns to hold well. Depending on your application, the strength of the materials involved, the amount of vibration the part will be subjected to in use, the use of lockwashers, etc, 2.2 may or may not be adequate. It might be wise to try some destructive testing before the design is considered complete.
What if it's not enough? Options to consider:
Insertion Nut – this is what amounts to a nut that is actually pressed into the material with knurled ends so that it forced into the steel or material.
Countersinks
A countersink is a tapered hole drilled with a wide outer portion. A common usage is to allow the head of a countersunk bolt or screw, when placed in the hole, to sit flush with or below the surface of the surrounding material.
Unless otherwise specified, countersink to a diameter 0.10-.015" larger than the basic size of the thread. 80 (or 82) degree and 100 degree counter sinks are most common for flat head screws.
