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S-N diagram

Plot showing the relationship of stress, S, and the number of cycles, N, before fracture in fatigue testing.

salt fog test (salt spray test)

Accelerated corrosion test in which specimens are exposed to a fine mist of a solution usually containing sodium chloride but sometimes modified with other chemicals. Used to determine resistance to, and rates of, corrosion exhibited by various materials.


Machining operation in which a powered machine, usually equipped with a blade having milled or ground teeth, is used to part material (cutoff) or give it a new shape (contour bandsawing, band machining). Four basic types of sawing operations are: hacksawing (power or manual operation in which the blade moves back and forth through the work, cutting on one of the strokes); cold or circular sawing (a rotating, circular, toothed blade parts the material much as a workshop table saw or radial-arm saw cuts wood); bandsawing (a flexible, toothed blade rides on wheels under tension and is guided through the work); and abrasive sawing (abrasive points attached to a fiber or metal backing part stock, could be considered a grinding operation).

sawing machine (saw)

Machine designed to use a serrated-tooth blade to cut metal or other material. Comes in a wide variety of styles but takes one of four basic forms: hacksaw (a simple, rugged machine that uses a reciprocating motion to part metal or other material); cold or circular saw (powers a circular blade that cuts structural materials); bandsaw (runs an endless band; the two basic types are cutoff and contour band machines, which cut intricate contours and shapes); and abrasive cutoff saw (similar in appearance to the cold saw, but uses an abrasive disc that rotates at high speeds rather than a blade with serrated teeth).


Wavy surface condition caused by deflection, unbalanced tool, loose workpiece or tooling or worn machine.

Scleroscope hardness number (HSc or HSd)

Number related to the height of rebound of a diamond-tipped hammer dropped on the material to be tested. Hardness measured on the Model C Scleroscope is expressed as a 2-digit number followed by the letters HSc, and hardness measured on the Model D Scleroscope is expressed as a 2-digit number followed by the letters HSd.

Scleroscope hardness test

Dynamic hardness test using a calibrated instrument that drops a diamond-tipped hammer from a fixed height onto the surface of the material being tested. The height of rebound of the hammer is a measure of the hardness of the material. The result is expressed as the Scleroscope hardness number. See Brinell hardness test; Knoop hardness test; Rockwell standard hardness test; Rockwell superficial hardness test; Vickers hardness test.

semisynthetic cutting fluid

Water-based chemical solution that contains some oil. See synthetic cutting fluid.

sensitizing heat-treatment

Heat-treatment, whether accidental, intentional or incidental (as during welding), that causes precipitation of constituents at grain boundaries, often causing the alloy to become susceptible to intergranular corrosion or intergranular stress-corrosion cracking.


Refers to control of motion. Applied to the industrial robot, it describes automatic feedback, or “closed-loop,” operation in which sensing devices monitor movement and report any deviation between commands as issued and movement as monitored. Deviations trigger corrective action. Robots without servocontrol may be “open-loop” but most often are controlled by preset mechanical or electric stop-switches. Because they are not self-correcting, they have limited capabilities.


Control mechanism in a machine tool to feed the cutting tool into the part and slow or speed up the drive motors as required.


Main body of a tool; the portion of a drill or similar end-held tool that fits into a collet, chuck or similar mounting device.

shaper tool

Single-point tool that traverses the workpiece in a reciprocating fashion to machine a desired shape.


Using a shaper primarily to produce flat surfaces in horizontal, vertical or angular planes. It can also include the machining of curved surfaces, helixes, serrations and special work involving odd and irregular shapes. Often used for prototype or short-run manufacturing to eliminate the need for expensive special tooling or processes.

shear lip

Narrow, slanting ridge along the edge of a fracture surface. Term sometimes also denotes a narrow, often crescent-shaped, fibrous region at the edge of a fracture that is otherwise of the cleavage type, even though this fibrous region is in the same plane as the rest of the fracture surface.

shear plane

Plane along which the chip parts from the workpiece. In orthogonal cutting, most of the energy is used to create the shear plane.

shear strength

Stress required to produce fracture in the plane of cross section, the conditions of loading being such that the directions of force and of resistance are parallel and opposite although their paths are offset a specified minimum amount. The maximum load divided by the original cross-sectional area of a section separated by shear.

shock loading

Tool is subjected to sudden, heavy loads and/or impacts, as in interrupted cutting. See interrupted cut.

shop air

Pressurized air system that cools the workpiece and tool when machining dry. Also refers to central pneumatic system.

shot peening

Cold working a metal’s surface by metal-shot impingement.

shrink-fit toolholding

Method of holding a round-shank cutting tool in a toolholder. To shrink-fit, the toolholder is heated in order to expand its bore, allowing a tool to be inserted. As the holder cools, the bore contracts around the shank to firmly hold the tool in place.

shuttle mechanism

Mechanism typically used to feed parts into an assembly machine in a back-and-forth motion, inserting a part at the end of each stroke.

single-crystal diamond

Industrial-grade, natural diamond. Not recommended for cutting ferrous materials because it tends to react chemically with them and break down. Also not recommended for interrupted cuts in hard materials. Replaced by polycrystalline diamond in many applications. See diamond; PCD, polycrystalline diamond; superabrasive tools.


Bonding of adjacent surfaces in a mass of particles by molecular or atomic attraction on heating at high temperatures below the melting temperature of any constituent in the material. Sintering strengthens and increases the density of a powder mass and recrystallizes powder metals.

slack quenching

Incomplete hardening of steel due to quenching at a rate slower than the critical cooling rate for the particular steel, resulting in the formation of one or more transformation products in addition to martensite.


Machining, normally milling, that creates slots, grooves and similar recesses in workpieces, including T-slots and dovetails.

slotting attachment

Converts a milling machine’s rotary spindle motion into a reciprocating motion for machining keyways and slots.

slotting machine (shaper)

Vertical or horizontal machine that accommodates single-point, reciprocating cutting tools to shape or slot a workpiece. Normally used for special (unusual/intricate shapes), low-volume runs typically performed by broaching or milling machines. See broaching machine; mill, milling machine.


Prolonged holding of a metal at a selected temperature to homogenize its structure or composition.

solid model

3-D model created using “building blocks.” This is the most accurate way of representing real-world objects in CAD.

soluble-oil cutting fluid

Fluid in which oil is suspended in water. Because water is a superior heat-removal agent, this fluid is primarily used when lubrication is desirable but cooling is the key consideration. The ratio of oils and other additives to water varies with the application. For milling, the ratio of water to oil/additives runs 20:1 to 25:1. For sawing and other work, where a more confined tool/chip/workpiece condition is normal, a 10:1 ratio is used to improve lubricity. Additives include emulsifying agents that help keep the oil in suspension and substances that promote wetting, enhance lubricity, prevent chipwelding and inhibit rusting. Also known as emulsified oil. See cutting fluid.

spade drill

Flat end-cutting tool used to produce holes ranging from about 1" to 6" in diameter. Spade drills consist of an interchangeable cutting blade and a toolholder that has a slot into which the blade fits. In horizontal applications, universal spade drills can achieve extreme depth-to-diameter ratios, but, in vertical applications, the tools are limited by poor chip evacuation.

spade drilling

Drilling operation in which a machine powers a cutting tool consisting of a holder and flat, interchangeable end-cutting blades. Spade drilling takes over where twist drilling leaves off; requires more power and a larger machine but offers lower cost and greater rigidity. Large-diameter spade drills are used when trepanning is impractical or impossible. See drilling; trepanning.

spark-out (sparking out)

Grinding of a workpiece at the end of a grind cycle without engaging any further down feed. The grinding forces are allowed to subside with time, ensuring a precision surface.

specific power consumption

It is also called the unit power, or power constant, which is equal to the power required to cut a material at the rate of one cubic inch per minute or one cubic centimeter per minute. The units of measure are: hp/in.3/min. (customary U.S. system) and kW/cm3/min. (metric system).

spindle adapter

Bushing or toolholder that permits affixing a variety of taper- and straight-shank tools to a machine spindle.

spindle finishing

Mass finishing process in which workpieces are individually mounted on spindles then lowered into a rotating tub containing the finishing media. In most applications, the spindles rotate at 10 rpm to 3,000 rpm, but, in some cases, the spindles oscillate up and down instead of rotating. The process is sometimes automated for robotic loading and unloading. See finishing.

spiral milling

Milling while simultaneously rotating and feeding the workpiece to create a spiral form. Often used to mill flutes on endmill and twist-drill blanks.


Form of metal characterized by a porous condition that is the result of the decomposition or reduction of a compound without fusion. The term is applied to forms of iron, titanium, zirconium, uranium, plutonium and the platinum group metals.


Tool, guided by a pilot, used to machine a recess around a hole.


Similar to counterboring except that, in spotfacing, material around the original hole is cut. Application example: the recessed area into which a washer fits. See counterboring; countersinking.

stainless steels

Stainless steels possess high strength, heat resistance, excellent workability and erosion resistance. Four general classes have been developed to cover a range of mechanical and physical properties for particular applications. The four classes are: the austenitic types of the chromium-nickel-manganese 200 series and the chromium-nickel 300 series; the martensitic types of the chromium, hardenable 400 series; the chromium, nonhardenable 400-series ferritic types; and the precipitation-hardening type of chromium-nickel alloys with additional elements that are hardenable by solution treating and aging.

static stiffness

Relates to the machine tool and is measured in pounds per inch. Static stiffness indicates how many pounds of force it takes to deflect the spindle a linear distance of 1" in a given direction. See dynamic stiffness; stiffness.

statistical process control (SPC)

Statistical techniques to measure and analyze the extent to which a process deviates from a set standard.

statistical quality control (SQC)

Statistical techniques to measure and improve the quality of a given process.

steady rest

Supports long, thin or flexible work being turned on a lathe. Mounts on the bed’s ways and, unlike a follower rest, remains at the point where mounted. See follower rest.


Basically, iron in combination with carbon and other elements. There are five major groups of steels: carbon steel, alloy steel, stainless steel, tool steel and maraging steel.

steel-specification number

System of numbers developed by the American Iron and Steel Institute and Society of Automotive Engineers to identify steel. The first two digits in the code indicate the family and basic alloying elements. The final two digits indicate the approximate carbon content in hundredths of a percent. For steels with a carbon content above 1.00 percent, five digits are used. Numbers with L or S added indicate alloys incorporating lead or sulfur for improved machinability. A number of steels and alloys are identified under different codes, including tool steel, carbon tool steel, high-speed steel, die steel, stainless steel, strain-hardenable or workhardening steel and nickel-base superalloys.

steep-taper toolholder

Common method of securing the cutting tool body to the spindle in a machine tool. Comes in various styles, including CAT V-flange, British Taper (BT) and ISO.


Distance between the passes of the toolpath; the path spacing. The distance the tool will move horizontally when making the next pass. Too great of a step-over will cause difficulty machining because there will be too much pressure on the tool as it is trying to cut with too much of its surface area.


1. Ability of a material or part to resist elastic deflection. 2. The rate of stress with respect to strain; the greater the stress required to produce a given strain, the stiffer the material is said to be. See dynamic stiffness; static stiffness.

straight oil

Cutting fluid that contains no water. Produced from mineral, vegetable, marine or petroleum oils, or combinations of these oils.

straight-cut system

NC system wherein tools move at either 45° or 90° angles to the coordinate axes. Used in turning shoulders or milling rectangular shapes; normally is combined with point-to-point system for greater efficiency and flexibility.


Measure of the relative change in the size or shape of a body. Linear strain is the change per unit length of a linear dimension. True strain (or natural strain) is the natural logarithm of the ratio of the length at the moment of observation to the original gage length. Conventional strain is the linear strain over the original gage length. Shearing strain (or shear strain) is the change in angle (expressed in radians) between two lines originally at right angles. When the term “strain” is used alone it usually refers to the linear strain in the direction of applied stress.

strain hardening

Increase in hardness and strength caused by plastic deformation at temperatures below the recrystallization range.


Force per unit area, often thought of as force acting through a small area within a plane. It can be divided into components, normal and parallel to the plane, called normal stress and shear stress, respectively. True stress denotes the stress where force and area are measured at the same time. Conventional stress, as applied to tension and compression tests, is force divided by original area. Nominal stress is the stress computed by simple elasticity formulas, ignoring stress raisers and disregarding plastic flow; in a notch bend test, for example, it is bending moment divided by minimum section modulus.

stress relieving

Annealing designed to relieve internal stresses caused by machining, welding, casting, cold working, quenching or normalizing.

stress-rupture test, creep-rupture test

Method of evaluating elevated-temperature durability in which a tension-test specimen is stressed under constant load until it breaks. Data recorded commonly include initial stress, time to rupture, initial extension, creep extension and reduction of area at fracture.


Fatigue fracture feature, often observed in electron micrographs, that indicates the position of the crack front after each succeeding cycle of stress. The distance between striations indicates the advance of the crack front across that crystal during one stress cycle, and a line normal to the striations indicates the direction of local crack propagation.

superabrasive tools

Abrasive tools made from diamond or cubic boron nitride, the hardest materials known. See CBN, cubic boron nitride; diamond; PCD, polycrystalline diamond; single-crystal diamond.


Tough, difficult-to-machine alloys; includes Hastelloy, Inconel and Monel. Many are nickel-base metals.


Cooling below the temperature at which an equilibrium phase transformation can take place, without actually obtaining the transformation.


Heating above the temperature at which an equilibrium phase transformation should occur, without actually obtaining the transformation.


Ability of certain metals to undergo unusually large amounts of plastic deformation before local necking occurs.

surface grinding

Machining of a flat, angled or contoured surface by passing a workpiece beneath a grinding wheel in a plane parallel to the grinding wheel spindle. See grinding.

surface model

3-D model defined by surfaces. The surface consists of polygons.

surface texture

Repetitive or random deviations from the nominal surface, which form 3-D topography of the surface. See flows; lay; roughness; waviness.


Metal fines and grinding wheel particles generated during grinding.

Swiss-type machine tool

Type of turning center designed to turn small, complex, precision parts. It has an automatic lathe that has a sliding headstock and a guide bushing. The sliding headstock is the part of the machine that holds the bar stock and rotates it. The cutting tools move in and out of the material to create required diameters while the headstock moves the material forward to create required lengths. Swiss machines generate the features of the part by moving the material and the tool at the same time.

synchronous transfer

Part-transfer system in which all parts are progressively moved to the next workstation or tooling station at the same time.

synthetic cutting fluid

Water-based chemical solution that contains no oil. Normally contains additives that improve lubricity and prevent corrosion and rancidity. See semisynthetic cutting fluid.