Casting makes a shape by solidifying metal inside a mold

Three casting routes and their trade-offs

Route	How it works	Typical products and cost trade-off
Sand casting	A pattern makes a cavity in bonded sand. Remove the pattern, assemble the mold, pour metal, then break away the mold after solidification.	Pump housings and large machine bases. Relatively flexible tooling can suit small batches; making a new mold for every part adds work.
Investment casting	A disposable pattern, often wax, receives a ceramic shell. Remove the pattern and fire the shell before pouring metal. Break away the shell afterward.	Detailed brackets, valve parts and turbine components. Fine detail can reduce later machining, but pattern and shell preparation add time and cost.
Pressure die casting	Inject molten metal into a reusable metal die, let it solidify, then open the die and eject the part.	Repeated aluminum or zinc housings. Expensive dies and equipment can pay off over long runs; alloy, size and geometry must suit the process.

There is no universal quantity at which die casting becomes cheapest. Obtain estimates for the actual geometry, alloy, tool life and required secondary operations.

Supporting detail: Addendum 1 — Sources for the casting routes.

A casting needs room to release, shrink and feed

The cope is the upper mold half; the drag is the lower half. The sprue leads downward from the pouring opening, and a runner and gate lead into the cavity. A riser supplies liquid metal as the casting solidifies and contracts. A core occupies space that must remain hollow.

Draft is a slight taper that helps a pattern or casting leave its tooling. A shrinkage allowance changes tooling dimensions to account for contraction. Neither is a universal number: alloy, geometry and process matter. Critical faces or holes may need machining afterward.

Supporting detail: Addendum 2 — Why a larger pattern does not replace a riser.

SFSA explains riser feeding and shrinkage voids.

Rolling squeezes, forging compresses and extrusion pushes through a die

Solid-metal forming routes

Route	Shape and examples	Tooling and quantity
Rolling	Rotating rolls reduce thickness or produce a section. Sheet, plate and structural shapes are common products.	Large equipment investment suits substantial throughput; a designer can often buy standard rolled stock.
Open-die forging	Dies compress a solid workpiece without enclosing its whole outline. Examples include large shafts and rings.	Relatively simple dies accommodate varied sizes; repeated positioning and later machining may be needed.
Closed-die forging	Shaped die cavities guide metal flow into repeated parts such as connecting rods and wrenches.	Dedicated tooling favors repeated production. Excess metal may form flash that is trimmed away.
Extrusion	A ram pushes a billet through a die opening. Long channels, rails and heat-sink profiles have a substantially constant cross-section.	A dedicated profile die can serve many lengths. Cut ends, cross holes and local pockets require later operations.
Wire drawing	Pull wire through a reducing die to make it thinner and longer. This is different from pushing stock through an extrusion die.	Die sets and handling equipment suit repeated lengths; several reductions may be needed.

Hot working occurs at temperatures and deformation rates that allow recrystallization, the formation of new grains, during deformation. Cold working occurs where that process does not remove the work hardening as deformation proceeds. The boundary depends on the metal and processing conditions; it is not one temperature for every alloy. Hot rolling permits large shape changes, while cold rolling commonly improves dimensional control and surface quality and can increase strength through work hardening.

Supporting detail: Addendum 3 — Cold forming does not guarantee an unchanged dimension.

Supporting detail: Addendum 4 — Sources for solid-metal deformation.

Sheet forming changes shape; blanking and shearing separate material

Bending changes the angle of sheet, as in a bracket or enclosure. A press brake uses a punch and die to make bends; adaptable tooling suits prototypes and short runs. Roll forming progressively bends strip through roll stations into a long profile, such as a channel. Its dedicated roll tooling favors repeated lengths. It is different from rolling sheet thinner.

Deep drawing pulls sheet from a flat blank into a die cavity to make a cup or shell. Tooling must control material flow to limit tearing and wrinkling. Dedicated tools suit repeat production. This is different from pulling wire through a reducing die.

Stamping names a press-based production route that can combine several operations. Bending and drawing are Forming; shearing and blanking are Separating. In blanking, the cut-out piece is the wanted blank; in punching a hole, the removed slug is usually scrap. A stamping line can therefore perform more than one process category.

Bend allowance accounts for the material length within a bend when developing a flat pattern. The sheet stretches on the outside and compresses on the inside. Use a validated bend table or supplier data for the alloy, thickness, radius and tooling; adding outside leg dimensions alone does not give the correct blank length.

Practice selecting a sheet process

Supporting detail: Addendum 5 — Sources for sheet forming and flat patterns.

Powder metallurgy presses particles, then bonds them by sintering

In conventional press-and-sinter powder metallurgy, powder fills a die and is compressed into a green compact: a shaped part that is not yet fully bonded. Heating then bonds the particles during sintering, generally without melting the main metal throughout. Some systems include a liquid phase.

Small gears and porous bearings are typical products. Dedicated dies and presses favor repeat quantities and can produce a near-net shape, close to the finished dimensions. Tool access, ejection and density variation limit geometry; critical features may still require sizing or machining.

Supporting detail: Addendum 6 — Porosity can be useful or unwanted.

MPIF explains conventional compaction, sintering and secondary operations. This section describes that route, rather than every process that begins with metal powder.

Plan the separating operations that finish the geometry

Follow Milling Operations for machined faces and pockets, Lathe Operations for turned features, and Feeds and Speeds for cutting-data concepts. Use Tolerances and Fits to decide which dimensions require close control and Surface Texture and Finish to distinguish a surface requirement from a process name.

Forming is often only the beginning of a route. A casting can be machined, joined and coated. These pages compare processes for design; operating procedures belong in Equipment Operating Procedure and Machine Hazards and Housekeeping.

Choose a joint around its load, access and need for disassembly

Joining parts is Fabricating. First identify the metals, thicknesses, loading, service environment and whether the assembly must come apart. Then check access for the joining equipment and for inspection.

A welded frame can avoid a large casting tool, but fit-up, fixtures and inspection still cost money. A bolt-on cover allows service access, but needs clearance for the fasteners and tools. See Fasteners and Galvanic Corrosion for mechanical assembly.

This is a process-selection overview. For safety requirements, use Personal Protective Equipment, Fire Safety, Chemicals and Safety Data Sheets and Equipment Operating Procedure.

Welding joins the base metal; melting is not required in every process

Fusion welding melts base metal at the joint, with or without added filler metal. The pool solidifies to form the joint. The common arc, oxy-fuel and resistance spot processes below are fusion processes.

Solid-state welding forms a bond without bulk melting of the base metals. Friction welding is one example. A solid-state process can still heat and alter nearby metal; “not melted” does not mean “unchanged.”

The heat-affected zone is nearby base metal whose structure or properties change because of the thermal cycle, without itself becoming weld metal. Selecting a process includes checking those changes and distortion, not merely whether a bead looks smooth.

Supporting detail: Addendum 7 — Sources for fusion welding, solid-state welding and the heat-affected zone.

Compare the heat source, filler and access of five welding processes

Starting candidates for process selection

Process	How it works and where it fits	Trade-off
Oxy-fuel welding	A fuel-and-oxygen flame heats the joint; a separate filler rod may be added. Oxyacetylene can suit selected thin-steel work and repair.	Flexible equipment, but slower and with more broadly distributed heating than many production arc processes.
SMAW / stick	An arc melts a flux-coated consumable rod. The coating supplies protection for the weld pool. Common for steel repair and construction work.	Portable equipment and flexibility; rods need replacement and slag needs removal between appropriate stages.
GMAW (MIG/MAG)	An arc melts continuously fed wire under external shielding gas. Common for repeated steel or aluminum fabrication with suitable wire, gas and settings.	Continuous wire supports production and automation; shielding and equipment access must suit the job.
GTAW / TIG	An arc from a non-consumable tungsten electrode heats the joint under inert gas. Filler, when needed, is supplied separately. Useful for controlled joints in thin stainless steel and aluminum.	Good control of heat and filler, but often slower than wire-fed production welding.
Resistance spot welding	Opposed electrodes press overlapping sheets together and pass current through them, forming a local fused nugget. Common in repeated sheet assemblies.	Rapid repeat joints with suitable equipment and fixtures; conventional arrangements require access to both sides. Separate spots do not make a sealed seam.

Supporting detail: Addendum 8 — Why MIG is a familiar name but GMAW is the broader term.

Supporting detail: Addendum 9 — Sources for the five process mechanisms.

Practice choosing a joining method

Brazing and soldering melt filler while the base parts stay solid

In brazing and soldering, a molten filler wets the solid base parts. In a suitable close-fitting joint, capillary action draws filler into the gap. The usual dividing temperature is 450°C: the American Welding Society describes brazing fillers as having a liquidus above 450°C and soldering fillers below it. Liquidus is the temperature above which the filler is fully liquid. This distinction concerns the filler, not the flame temperature or the machine's setting. In both processes the base parts remain solid. Neither is simply a cooler version of fusion welding.

Brazing can join suitable dissimilar metals or attach a carbide tip to a steel tool body. Soldering is common in electrical connections. Choose filler and joint design for compatibility, temperature and loads; a soldered electrical connection is not automatically suitable for a structural load.

Joint clearance, surface condition and heating method matter. Brazing can be manual or performed in production equipment such as a furnace. Mechanization changes throughput and tooling costs; the process name alone does not identify the cheapest route.

Supporting detail: Addendum 10 — Sources for the filler-temperature distinction and capillary joint.

Match the fastener to the assembly and its service access

Common mechanical joining components

Component	Purpose	Design check
Bolt and nut	Provide a removable threaded connection that can clamp parts together.	Provide tool access, suitable thread engagement and a compatible strength and material specification.
Washer	Provides a bearing surface or distributes load where the design requires it.	A plain washer does not by itself prevent loosening or electrically isolate an entire joint.
Rivet	Forms a retaining head through permanent deformation.	Suitable for repeated sheet assembly; removal normally destroys the rivet. Blind rivets permit installation from one side.
Threaded insert	Provides an internal thread in a host part.	Select an insert designed for the host material and thickness. Check installation access, pull-out and resistance to rotation.

Standard fasteners often reduce dedicated tooling for short runs, but every fastener adds parts and assembly time. Riveting or automated fastening may suit long runs. Removability, inspection and repair can justify a threaded joint even when another method is faster.

A thread size alone does not specify a complete fastener

The mating threads must agree in diameter, spacing and thread system. Similar-looking inch and metric parts are not interchangeable. Pitch describes axial spacing between adjacent thread forms; threads per inch describes a count over a unit length.

A purchase or drawing specification also needs the relevant length, head style, material or strength grade, coating and thread-fit requirements. The wrench size is not the thread diameter. Use the applicable thread reference and supplier drawing to decode a designation before choosing mating parts.

Read diameter and thread spacing separately

Example	Diameter	Spacing and system
1/4-20 UNC	Nominal thread diameter: 1/4 inch.	20 threads per inch; UNC is the Unified coarse series. The pitch is 1/20 inch, not 20 inches.
M6 × 1	Nominal metric thread diameter: 6 mm.	Pitch: 1 mm between corresponding points on adjacent threads. The 1 is not a count of threads per inch.

These are thread-size examples, not complete fastener specifications. Nominal diameter identifies the size; tolerances govern the actual mating dimensions. A 1/4-20 thread and an M6 × 1 thread differ in both diameter and spacing.

Supporting detail: Addendum 11 — Sources for thread names and size examples.

Practice decoding a thread designation

For dimensional fit concepts, see Tolerances and Fits. For a permanently joined alternative, see Joining Metal.

Galvanic corrosion needs a metal pair, electrical contact and an electrolyte

Dissimilar metals can form a corrosion cell when they are electrically connected and share a conductive liquid, called an electrolyte. Moisture containing salts can provide that liquid. The more active member of the pair acts as the anode and may corrode faster; the other acts as the cathode.

A galvanic series ranks metals by their behavior in a specified environment. It helps screen combinations; it does not predict a service life. Alloy condition, surface films, wetting and exposed areas matter. A dry pair is not automatically a working galvanic cell.

Supporting detail: Addendum 12 — Why the wetted areas matter.

Supporting detail: Addendum 13 — Sources for galvanic contact, coatings and area effects.