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rack-milling attachment

Attachment for cutting gear teeth, usually in a straight line, but, when used in conjunction with universal spiral-index centers on a universal mill, it allows the machining of worms.

radial drill

Large drill with an arm that pivots about a column to provide positioning flexibility and great reach and stability. See drilling machine, drill press.

radial rake

Also known as the tool back rake, the angle between the tooth face and the radial plane through the tool point.


Angle of inclination between the face of the cutting tool and the workpiece. If the face of the tool lies in a plane through the axis of the workpiece, the tool is said to have a neutral, or zero, rake. If the inclination of the tool face makes the cutting edge more acute than when the rake angle is zero, the rake is positive. If the inclination of the tool face makes the cutting edge less acute or more blunt than when the rake angle is zero, the rake is negative.

ram EDM

Process using a shaped electrode made from graphite or copper. The electrode is separated by a nonconductive liquid and maintained at a close distance (about 0.001"). A high DC voltage is pulsed to the electrode and jumps to the conductive workpiece. The resulting sparks erode the workpiece and generate a cavity in the reverse shape of the electrode, or a through-hole in the case of a plain electrode. Permits machining shapes to tight accuracies without the internal stresses conventional machining often generates. Also known as “die-sinker” or “sinker” electrical-discharge machining.

ramp milling

Milling process in which the machine tool spindle is feeding simultaneously in an axial and radial direction.


Bacterial and fungal growths in water-miscible fluids that cause unpleasant odors, stained workpieces and diminished fluid life.

rapid traverse

Movement on a CNC mill or lathe that is from point to point at full speed but, usually, without linear interpolation.

reaction injection molding (RIM)

Molding process that allows the rapid molding of liquid materials. The injection-molding process consists of heating and homogenizing plastic granules in a cylinder until they are sufficiently fluid to allow for pressure injection into a relatively cold mold, where they solidify and take the shape of the mold cavity. For thermoplastics, no chemical changes occur within the plastic, and, consequently, the process is repeatable. The major advantages of the injection-molding process are the speed of production; minimal requirements for postmolding operations; and simultaneous, multipart molding.


Rotating cutting tool used to enlarge a drilled hole to size. Normally removes only a small amount of stock. The workpiece supports the multiple-edge cutting tool. Also for contouring an existing hole.


1. Increasing the carbon content of molten cast iron or steel by adding a carbonaceous material, a high-carbon pig iron or a high-carbon alloy. 2. Carburizing a metal part to return surface carbon lost in processing; also known as carbon restoration.


A turning operation in which a groove is produced on the periphery or inside a hole of a workpiece. The grooving tool moves at right angles to the axis of rotation.


Reduction or removal of workhardening effects, without motion of large-angle grain boundaries.


1. Formation of a new, strain-free grain structure from that existing in cold-worked metal, usually accomplished by heating. 2. Change from one crystal structure to another, as occurs on heating or cooling through a critical temperature.

recrystallization temperature

Approximate minimum temperature at which complete recrystallization of a cold-worked metal occurs within a specified time.

red hardness

Ability of a cutting tool material to withstand high temperatures at the point of cut without softening and degrading.


Optical instrument that measures the refractive index of a liquid, such as a water-diluted metalworking fluid mix. The refractive index can by used to determine the concentration of a fresh metalworking fluid mix.

refractory metal

Metal having an extremely high melting point; for example, tungsten, molybdenum, tantalum, niobium (columbium), chromium, vanadium and rhenium. In the broad sense, this term refers to metals having melting points above the range of iron.


Space provided behind the cutting edges to prevent rubbing. Sometimes called primary relief. Secondary relief provides additional space behind primary relief. Relief on end teeth is axial relief; relief on side teeth is peripheral relief.

residual stress

Stress present in a body that is free of external forces or thermal gradients.


Device capable of performing various combinations of movements, manipulations and actions. Normally computerized but may be electromechanical. Computerized units normally consist of three basic components: machinery or mechanical parts, controller/computer system and software. See industrial robot.


Discipline involving self-actuating and self-operating devices. Robots frequently imitate human capabilities, including the ability to manipulate physical objects while evaluating and reacting appropriately to various stimuli. See industrial robot; robot.

Rockwell hardness number (HR)

Number derived from the net increase in the depth of impression as the load on the indenter is increased from a fixed minor load to a major load and then returned to the minor load. The Rockwell hardness number is always quoted with a scale symbol representing the indenter, load and dial used. Rockwell A scale is used in connection with carbide cutting tools. Rockwell B and C scales are used in connection with workpiece materials.

Rockwell standard hardness test

Indentation hardness test uses a calibrated machine that utilizes the depth of indentation, under constant load, as a measure of hardness. There are 15 standard hardness scales. The most common are the A, B and C scales. The A and C scales utilize the 120º spheroconical diamond indenter and different major loads: 60 kgf is for scale A; 150 kgf is for scale C. The B scale utilizes the 1¼16" (1.588mm) ball and the major load of 100 kgf. The result is expressed as the Rockwell hardness number. See Brinell hardness test; Knoop hardness test; Scleroscope hardness test; Vickers hardness test.

Rockwell superficial hardness test

Indentation hardness test uses a calibrated machine that utilizes the depth of indentation, under constant load, as a measure of hardness. There are five superficial hardness scales (N, T, W, X and Y) and three different loads (15, 30 and 45 kgf) that can be applied to the appropriate indenter. The most common scales are N and T. The 120º spheroconical diamond indenter is used with scale N and the 1¼16" (1.588mm) ball is used with scale T. The 15-, 30- or 45-kgf load can be used for both scales. The result is expressed as the Rockwell hardness number.


Rotation about an axis, as with a rotatable wrist.

rotary attachment

Bolts to a milling machine to permit machining such shapes as circular T-slots and cams.

rotary transfer machine

Type of CNC machine tool for high-volume, extended-length production runs of a family of parts. A workpiece is transferred from station to station in a rotary fashion and a tool or tools at each station performs one or more operations until the part is completed.

roughing cutter

Tool for high-volume metal removal; normally followed by finishing passes. See finishing tool.


Irregularities on the surface of a workpiece caused by production process. Includes traverse feed marks within the limits of the instrument cutoff setting (sampling length). See flows; lay; waviness.