Glossary A–Z

256 shop and airframe terms from the machining reference, each linked to its full plain-language entry. For training-track topics use Search.

# A B C D E F G H I J K L M N O P R S T U V W Z

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3+2 (positional) The two rotary axes tilt the part or the head to a fixed angle and lock. Cutting after that is ordinary 3-axis. This re
3-axis The tool moves in X, Y, and Z. On a typical vertical mill the spindle stays pointing down. The other side of the part i
4-axis (A) Adds rotation about one axis, usually called A (rotation about X). You can index the part to a new side, and on some jo
5-axis Adds a second rotary (often B about Y, or C about Z, depending on the machine). The name alone does not say whether tho
5-axis dovetail vise A narrow, tall vise that grips a small dovetail machined on the bottom of the stock. Holding only a few millimeters let
“TYP” and “nX” “4X” means the callout applies to four identical features. “TYP” (typical) is older usage meaning “and all the ones lik
↗ Circular runout Spin the part about a datum axis and watch one cross-section. The indicator must stay inside the tolerance at that slic
↗↗ Total runout The indicator must stay inside the tolerance everywhere on the surface while the part spins, not just on one circle. On
∠ Angularity Same idea as perpendicularity, but the basic angle on the drawing is not 90°. The frame limits how far the surface or a
∥ Parallelism A surface or axis must not tilt relative to a datum. The two can still be the wrong distance apart; distance is locatio
⊥ Perpendicularity A surface or an axis must be 90° to a datum, inside the tolerance. The basic angle is square; you do not also write 90°
⌒ Profile of a line One 2D slice through a contour must stay inside a uniform band wrapped around the true shape. It is the outline, not th
⌓ Profile of a surface The whole 3D surface must stay inside a zone wrapped around the true surface. This is the general control for a contour
⌖ Position The everyday location control. The axis or center of the feature must lie inside a tolerance zone, usually a cylinder w
⌭ Cylindricity The whole cylindrical surface must lie between two coaxial cylinders. That folds roundness, straightness along the leng
⏤ Straightness A line on a surface, or sometimes an axis, must stay inside a straight zone. Applied to a surface, it does not need a d
⏥ Flatness Every point on a surface must lie between two parallel planes the stated distance apart. No datum. A face can be flat a
Ⓜ MMC Maximum material condition: the size at which the feature has the most material the size tolerance allows. That is the
○ Circularity Each cross-section circle of a round surface must sit between two concentric circles. It checks roundness one slice at

A

A hundred parts, both sides Usually 3-axis with a flip between two fixtures (or two ops), not a 4-axis program. Move to a rotary when side features
Adaptive / HSM / trochoidal Toolpath styles that hold the tool’s engagement steady: a small width of cut with a deep depth of cut, using most of th
Aileron The hinged surface on the outboard trailing edge. One goes up while the other goes down, and that rolls the airplane. T
ASME Y14.100 and the Y14 family Y14.100 covers engineering drawing practice in general: title blocks, notes, and how drawings are organized. Y14.5 cove
ASME Y14.5-2018 The US standard people mean by “current GD&T.” One practical change: it dropped the concentricity and symmetry symbols.
Assembly fixture A shop frame that holds two or more pieces in the right relationship while they are fastened. This is the “jig” people
ATC and tool pocket The automatic tool changer swaps holders between the spindle and the tool magazine, which may be a carousel, an umbrell
Auxiliary view A view projected square to an angled surface, so that surface and its holes show at true size and shape instead of fore
Axis directions on a VMC The plus directions describe the tool moving relative to the part. +Z is away from the work, so up on a VMC. Facing the

B

Ball end mill (ball nose) The end is a half sphere whose radius is half the tool diameter. It is the tool for 3D contoured surfaces and full-radi
Ball screw A screw whose nut rolls on recirculating balls instead of sliding, turning motor rotation into straight motion with ver
Basic dimension A dimension in a box. It is a theoretically exact value with no tolerance of its own; the tolerance comes from a featur
Beyond 5 axes Mill-turn machines, robots, and pallet indexers are often a process choice, not a new geometric axis. They change how t
Bilateral vs unilateral tolerance Bilateral means the size can vary both ways: 1.50 ±.01, or unequally, such as +.01/−.02. Unilateral means it can vary i
Billet A solid chunk of wrought metal a shop cuts the part from, usually sawed from bar or plate. “Billet” names the starting
Bonus tolerance When position is specified at MMC, the position zone grows by however much the feature departs from that MMC size. A ho
Bore Enlarge a hole that already exists, with a boring bar or boring head, so size, roundness, and location are better than
Boring bar and boring head A boring bar is a single-point cutter on a bar that enlarges a hole that is already there. On a mill the bar sits in a
Boss A raised pad left standing above the surrounding face. Often a place for a bolt, a bearing, or a seal to sit.…
Break sharp edges A general note that asks for a small chamfer or radius on every sharp edge, often with a range such as .005–.015. It re
Built-up edge (BUE) Work metal welding onto the cutting edge. It changes the edge’s shape, tears the surface, and can pull chunks off the t
Bull nose (corner radius end mill) A flat end mill with a small radius ground on each corner instead of a sharp point. The radius makes the corner much st

C

C-frame vs bridge vs gantry A C-frame has one column at the back, and the table moves under the spindle. A bridge (double-column) machine carries t
Cabin The pressurized compartment between the pressure bulkheads, whether it is fitted for seats or freight. Floor beams pick
CAGE code Commercial and Government Entity code: a five-character ID for the design activity (the company or site that owns the d
Carbide vs HSS vs cobalt High-speed steel (HSS) is tough and cheap but softens at a lower temperature, so it runs slower. Cobalt (HSS-Co, often
Casting Molten metal is poured into a mold. The grain is not stretched the way it is in rolled bar or a forging, so strength an
Centerline A thin long-dash, short-dash line marking an axis or a hole center, or a crossed center mark on a circle. It is a line,
Chamfer A flat bevel on an edge, often 45°. It knocks off a sharp corner or gives a screw a lead-in.…
Chamfer mill A cutter with angled edges that bevels edges and breaks sharp corners as it follows a toolpath. A 90° tool leaves a 45°
Chatter A vibration between tool and part that makes a loud squeal or buzz and leaves a regular wavy pattern on the surface. Th
Check fixture A shop tool for inspection. The part sits on locators, and gauges or indicators show whether the shape matches. It is n
Chip auger vs chip conveyor A chip auger is a screw in a trough in the base that pushes chips toward the outlet. A chip conveyor lifts chips out of
Chip box The bin at the end of the conveyor. Empty it before it overflows and backs chips up into the conveyor.…
Chipload (feed per tooth) How thick a bite each cutting edge takes, in inches or millimeters per tooth. Too small and the tool rubs and heats up;
Climb vs conventional milling In climb milling each tooth enters the metal thick and leaves thin. In conventional milling it enters thin and leaves t
Coatings (TiAlN, AlTiN, ZrN, uncoated) Thin, hard layers on the cutting edge. TiAlN and AlTiN (titanium aluminum nitride; the order of the letters hints at wh
Column The fixed upright at the back of the base. It carries the Z rails, the Z ball screw, and the spindle head. On a C-frame
Concentricity An old control on how well the midpoints of a round surface are centered on a datum axis. It is hard to inspect, and it
Contour fixture A shop tool with a shaped nest that supports a formed skin or extrusion so it does not spring while you drill or trim i
Control and pendant The CNC control is the computer and drives that run the program. The pendant is the operator panel on its arm: screen,
Coolant nozzles Adjustable segmented or ring nozzles around the spindle that aim flood coolant at the cut. Some machines aim a nozzle a
Coolant tank and pump The tank (sump) beside or under the machine that holds the coolant, with the pump that sends it to the nozzles and scre
Coolant: flood, mist, air, through-spindle Flood pours a stream of water-based coolant on the cut. Mist is a fine spray of coolant or oil in air. Air blast is jus
Counterbore A larger flat-bottomed diameter above a smaller hole, deep enough that a socket-head cap screw can sit below the surfac
Counterbore and countersink tools A counterbore tool has a flat cutting end, often with a pilot that rides in the existing hole, and cuts the flat-bottom
Countersink A cone-shaped recess so a flat-head screw sits flush. The angle matches the screw, often 82° or 90°.…
Cut taps: spiral point vs spiral flute A spiral point (gun) tap has straight flutes with an angled grind at the front that pushes chips ahead of the tap, out

D

Dash number The suffix after the part number, such as EX12345 -1 . It lets one drawing define several related parts (lengths, hole
Datum A theoretically exact plane, axis, or point the drawing measures from. Real surfaces are chosen to stand in for those d
Datum feature symbol vs datum target A datum feature symbol is a boxed letter on a leader that ends in a filled triangle on the surface or feature that beco
Detail view A small area circled and lettered on the main view, then redrawn larger elsewhere on the sheet as, for example, DETAIL
Dial test indicator (DTI) A lever-type indicator: a small stylus swings and a dial shows the movement in thousandths or finer. Used to straighten
Do not scale the drawing Never measure the paper to get a size. If a dimension you need is missing, ask engineering; do not scale it off the pri
Dovetail A slot or rail that is wider at the bottom than at the opening, so the mating slide cannot lift straight out. Cut with
Dovetail cutter A cone-shaped cutter, widest at the bottom, that cuts the angled undercut walls of a dovetail. 45° and 60° are common a
Drawbar The rod inside the spindle that grabs the holder’s pull stud and pulls the holder up into the taper. A stack of disc sp
Drill chuck A three-jaw chuck, tightened with a key or by hand (keyless), that grips round shanks over a wide range. Common on manu
Drill helix angle (H / N / W types) The angle of the flutes to the drill axis. German standards group HSS twist drills into three application types (DIN 18
Drill jig A shop tool that holds the part and also guides the drill, usually with a bushing, so the hole lands in the right place
Drill lengths: jobber, stub, screw machine Twist drills come in length families. Jobber length is the everyday standard. Screw machine length is much shorter and
Drilling A drill opens a hole. That hole is a starting size and location, not a finished precise bore. Drilling can happen on a

E

ECN / ECO Engineering change notice or order: the formal document that authorizes a change to a released drawing. The change show
Edge finder A two-piece probe spun in the spindle. As its tip touches the edge of the part it stops wobbling and then kicks sideway
EDM Electrical discharge machining. Sparks erode conductive metal instead of a cutting edge shearing it. Wire EDM cuts a pa
Electrical cabinet The cabinet, usually at the back, holding the servo and spindle drives, power supplies, relays, and the machine’s logic
Elevator The hinged surface on the horizontal stabilizer that raises or lowers the nose. Its load goes through hinge fittings in
Enclosure and door The sheet-metal housing that keeps chips and coolant in. The front door has a safety window and an interlock; with the
End mill holder (side-lock, Weldon) A plain bore with one or two set screws that bear on a flat ground into the tool shank, called a Weldon flat. The tool
Engine pylon / strut The structural strut that hangs each podded engine below and ahead of the wing. Weight, thrust, and torque go through a
Engraving and deburring tools An engraving tool is a small pointed cutter, often a carbide V tip, that cuts shallow text and lines. A drag engraver i
ER collet and collet chuck An ER collet is a slotted spring sleeve that squeezes a round shank when its nut is tightened. ER is the most common co
Extrusion Metal is pushed through a die so the cross-section stays the same along the length: tube, angle, or a custom profile. G

F

Face A cut that makes a flat surface, usually the top of the stock, so later features have a clean height to start from.…
Face mill A large-diameter cutter body that carries several replaceable carbide inserts around its rim. It flattens the top of a
Fairing A cover whose job is smooth airflow: wing-to-body, a flap-track cover, a gear door in that sense. Take it off and, if t
Feature control frame The rectangular box on the drawing. Read left to right: geometric symbol, the tolerance (a diameter sign means a round
Feed and speed override Knobs or buttons on the control that scale the programmed feed and spindle speed by a percentage while the program runs
Fillet (aerodynamic) A curved blend where two surfaces meet, such as the wing root flowing into the fuselage, used to soften the air in the
Fillet / radius A rounded corner. An inside fillet is what a round end mill leaves in a pocket corner; you cannot get a perfectly sharp
Finish Lighter cuts that bring the surface to final size and the required finish. Often a separate tool or a separate pass aft
Fitting versus fixture An airframe fitting is a lug, bracket, or attach part that flies: a hinge fitting, a pylon fitting, a gear fitting. A s
Fixture / jig A fixture holds the part in a known place and orientation. A jig, strictly, also guides the tool. On a CNC mill people
Fixture plate A flat plate bolted to the table with a grid of tapped holes and precise locating holes. Vises, clamps, and fixtures bo
Flag note A numbered note whose number appears inside a triangle (flag) on the drawing. It applies only to the features or areas
Flap A trailing-edge surface that adds lift and drag for takeoff and landing. The 707’s main flaps are described as Fowler f
Flat end mill (square end mill) An end mill with a flat end and sharp corners. It cuts on its sides and its end, so it slots, pockets, profiles, and fa
Flight deck The crew compartment just behind the nose, inside the pressure shell. Its window frames break the skin the same way a c
Flutes (2, 3, 4 and more) Flutes are the spiral grooves between the cutting edges, and the flute count is the number of edges. Fewer flutes leave
Fly cutter A single cutting tool, often a lathe-style tool bit or one insert, held at the end of an arm or body that swings in a w
Forge vs machine shop A common chain is billet, then forge, then machine: the forge makes the near-net shape, and a machine shop cuts the pre
Forging Hot metal is pressed or hammered into a shape closer to the finished part. Grain is pushed to follow that shape. Forgin
Form tap (roll tap) A tap with no cutting edges. It pushes the metal into thread shape instead of cutting it, so it makes no chips. It need
Frame / bulkhead A hoop in cross-section. Ordinary frames keep the shell round. A bulkhead is a heavier frame that closes a bay or picks
Fuselage The long body: pressure vessel, people or cargo, and the beam that ties the wing to the tail. It is a skin-and-stringer
Fuselage skin The outer aluminum shell of the body. In the cabin it is also the pressure-vessel wall, so a window cutout or a lap joi

G

Gauge line A reference diameter on the holder’s taper that the machine and the presetter measure tool length from. It is a fixed p
Gear beam / fitting The beams and lugs that lead landing-gear loads into the wing or the body. On the 707 the main gears attach at the wing
General notes The numbered notes that apply to the whole drawing: governing standard, material, finish, process specs, edge breaks, m
Grain direction In wrought metal, grain follows the direction the metal was stretched while hot. On bar that is the long axis. On a for
Grinding An abrasive wheel rubs off a little material for a fine surface or a tight size, often after cutting. It is still subtr

H

Hard jaws vs soft jaws Hard jaws are the hardened steel jaw plates that come with the vise, smooth or serrated. Soft jaws are aluminum or mild
Helix angle and variable helix The helix angle is how steeply the flutes twist around the tool. Common angles run from about 30° to 45°. A higher heli
Hidden line A short-dashed line for an edge you cannot see from that view, such as the counterbores in the front view of the exampl
Hinge fitting A lug or bracket that pins a moving surface, such as an aileron, flap, elevator, rudder, or spoiler, to the fixed struc
Holding fixture A shop tool that clamps a part in a known position so it can be machined, drilled, or assembled. It locates the work. I
Hole callout format Callouts are read in order: count (4X), diameter ⌀ and depth (or THRU), then any counterbore ⌴, countersink ⌵ (with inc
Horizontal stabilizer The fixed tail surface that keeps the nose from wandering in pitch. The elevators hinge from its trailing edge, so its
How a billet is made Melt the metal, cast an ingot, hot-work it (roll, extrude, or forge) so the structure closes up and stretches, then cut
Hydraulic holder Turning one screw pressurizes oil behind a thin sleeve, which squeezes the shank evenly. Low runout, good damping, and

I

Indexable insert A small replaceable cutting tip, usually carbide, screwed or clamped into a tool body such as a face mill, boring bar,
Insert grade The grade is the maker’s recipe of carbide and coating. ISO groups grades by the material they cut, with a letter and a
IPM (feed rate) Table feed in inches per minute: IPM = RPM × number of flutes × chipload. It is the F number in a milling program. The
ISO insert code Inserts carry an ISO code such as CNMG 120408. The letters give the shape (C = 80° diamond, D = 55° diamond, R = round,

J

Joggle A short offset step in a flange, sheet, or extrusion so two overlapping pieces can sit flush. It is a real shop word. I

K

Keyseat (Woodruff) cutter A thin disk cutter on a shank. It cuts a straight groove sideways into a wall, or plunges into a shaft to cut the half-
Keyway A slot that holds a key so a shaft and a hub turn together instead of spinning on each other. Example: a groove along a
Krueger flap A panel hinged from the underside of the leading edge that folds out and forward, making a blunter nose for low speed.

L

Landing gear The nose gear under the forward fuselage and the main gears under the inboard wing, all retractable. They carry the air
Leading edge The forward nose of the airfoil, ahead of the front spar. On the 707 it is also where the leading-edge flap lives, so i
Limit dimension The tolerance is stated as the high and low limits instead of a nominal value and a tolerance, with the high limit on t
Linear guides vs box ways Linear guides are hardened rails with bearing blocks (trucks) full of rolling balls or rollers: low friction, fast rapi
Lip / flange A thin rim or ledge standing off a wall or around an opening. Often a seal land, a weld lip, or a bolt flange.…
Lollipop (undercut) mill A ball-shaped cutting head, more than a half sphere, on a thin neck. The ball can reach under a ledge or cut the back s
Longeron A heavy member running lengthwise along the fuselage, bigger than a skin stringer. Longerons carry body bending from fr
Low-profile clamps (Mitee-Bite style) Small clamps that push on the side of the part, often with an eccentric or cam screw that moves a jaw sideways and slig

M

Machine base (casting) The heavy bottom casting, usually cast iron, that everything else sits on. Its mass and stiffness soak up vibration. It
Machine vise (Kurt-style) The default CNC workholding: a fixed jaw and a moving jaw on a screw, bolted square to the table. “Kurt-style” means th
Machining Subtractive work: a tool cuts material away until the part is what’s left. Milling is one kind of machining. Turning, d
Manual knee mill (Bridgeport-style) A column-and-knee layout. The table moves in X on a lead screw turned by a handwheel, and the saddle under it moves in
MBD (model-based definition) The 3D CAD model, with tolerances and notes attached (PMI), is the authority, and any drawing is reduced or left out. M
Milling A spinning cutter moves across a part that is clamped still (or the table moves the part under the cutter). Typical wor
Milling chuck A heavy holder with a thick nose and a large locking ring that collapses a sleeve around a straight shank, often with a
MPG handwheel Manual pulse generator: a knob that jogs the selected axis a fixed step per click, for example 0.0001, 0.001, or 0.01 i

N

Nacelle The housing around the engine itself: inlet, cowlings, and the exhaust area. A lot of it is removable skin and doors. T
Nose The forward tip of the body, ahead of the flight deck, including the radome area. It sits forward of the front pressure

O

O-ring groove A groove sized so an O-ring sits in it and compresses when the joint closes. Width, depth, and surface finish matter mo
Op / setup An op (operation) is one block of work, often one clamping. A setup is getting the part located and the tools ready bef

P

Pallet changer A second pallet, often a second tombstone, sits outside the cutting zone. You load and unload that one while the machin
Parallels Ground steel bars, in matched pairs, set under the part in the vise to raise it to a known height and keep it level. Ta
Part runout The part’s own surface is not centered or not straight about the axis you are spinning it on. That can be how it was ma
Phantom line A thin long-dash, double-short-dash line showing an alternate position, a neighbouring part for reference, or repeated
Plate and bar Stock that already has a simple cross-section: flat plate, round bar, or rectangular bar, cut to a length you can fixtu
Pocket A cavity with a floor, closed or mostly closed on the sides. The tool steps down and clears the area, then finishes the
Pressure bulkhead The wall, often domed, that closes the pressurized cabin at the front and again at the rear. Cabin pressure tries to bl
Probe and tool setter on the machine A spindle probe usually lives in a magazine pocket and is loaded like a tool. A tool setter is bolted to a corner of th
Profile / contour The cutter follows an outline, inside or outside, instead of clearing a whole area. On a drawing, “profile” can also me
Pull stud (retention knob) A threaded knob screwed into the back of the holder’s taper. The spindle’s drawbar grabs it and pulls the holder up tig

R

Ream A reamer follows a hole that is already there and finishes the diameter. It improves size and finish. It does not move
Reamer A multi-flute finishing tool that takes a small amount out of a drilled hole to reach an accurate size and a good finis
Reference dimension A dimension in parentheses, such as (2.25). It is for information only, usually repeating a value that follows from oth
Relief Extra room cut so something else has clearance: a tool, a mating corner, or the end of a thread. Example: a small groov
Revision letter and revision block Each released change bumps the revision, usually A, B, C… (letters that look like numbers, such as I, O, Q, S, X and Z,
RFS Regardless of feature size: the geometric tolerance stays the number in the frame no matter where the size lands inside
Rib A chordwise former that holds the airfoil shape and keeps the spars spaced. Air load on the skin is passed through the
Rib chord The flange along the edge of a rib, running from leading edge toward trailing edge, where skin or stringers fasten. “Ch
Rigid tapping vs tapping head Rigid tapping: the control locks spindle rotation and Z feed together exactly, so the tap can sit in a solid holder. A
Rotary table and trunnion A rotary table (4th axis) bolts to the machine table and turns the part about one axis. A trunnion is a cradle that til
Rough Heavier cuts that get close and leave extra metal on purpose. Speed of removal matters more than the final surface.…
Roughing end mill (corncob, hog mill) An end mill with a wavy or serrated cutting edge that breaks chips into small pieces. That lets it remove a lot of meta
RPM (spindle speed) Revolutions per minute, worked out from SFM and tool diameter: RPM = SFM × 3.82 ÷ diameter in inches (3.82 is 12 ÷ π).
Rudder The hinged surface on the fin that yaws the nose left or right. Like the elevator, it is a moving panel on fittings, no
Runout How much a surface moves in and out as it rotates about an axis. You see it with an indicator on the spinning part, or

S

Saddle The carriage between the base and the table. It rides the base rails front to back (Y) and carries the X rails on top.…
Section view and cutting plane A view of the part as if it were sawn along the cutting plane line. That line is thick, dashed, and has arrows that poi
Servo motor and encoder Each axis has a servo motor driving its ball screw. An encoder on the motor reports exactly how far it has turned, and
Several parts on a face Each face can hold more than one part. One index cycle then machines a batch before the tombstone turns to the next fac
SFM (surface speed) Surface feet per minute: how fast the cutting edge moves across the material. The recommended number comes from the too
Shank, length of cut, and reach The words on a tool box or print: cutter diameter, length of cut (LOC, the fluted length), overall length (OAL), shank
Shell mill A cutter with a center bore and drive slots instead of a shank. It mounts on a shell mill arbor and is held on by a cen
Shell mill arbor A holder with a short pilot, two drive keys, and a center screw that carries a shell mill or face mill.…
Shrink-fit holder A holder whose bore is slightly smaller than the tool shank. A heater expands the bore, the tool drops in, and as it co
Simultaneous 5-axis X, Y, Z and both rotaries move while the cutter is in the metal. That is what sculpted surfaces, some undercuts, and a
Slat A slat is a leading-edge piece that moves forward and opens a slot over the fixed nose. Later jets use them heavily. Th
Slitting saw A thin circular saw blade on an arbor in the spindle. It cuts narrow slots or parts pieces off. Use: splitting a clamp
Slot A long, narrow channel. Often open at one or both ends. A slot is a shape; a keyway is a slot with a job.…
Spar A spanwise beam. Published descriptions of the classic 707 call the wing a two-spar aluminum structure, front and rear.
Spar cap The heavy upper or lower flange of a spar. When the wing bends upward the upper cap is in compression and the lower cap
Spec callouts Notes that point to other documents: material specs (for example SAE AMS or ASTM), process specs (heat treat, anodize,
Spindle (belt, inline, built-in) The rotating shaft that holds the tool holder in its taper. A belt-drive spindle has its motor alongside, driving throu
Spindle / tool runout The cutter or holder orbits instead of spinning on its own center. Holes come out large, walls taper, and tools wear fa
Spindle cartridge and bearings The spindle shaft runs in precision preloaded bearings inside a cartridge (the housing). Those bearings set how true th
Spindle head The housing that carries the spindle and its motor. It rides the column rails up and down (Z). Its weight is often bala
Spindle probe A touch probe that goes in the spindle like a tool. The control moves it until the stylus triggers, records the positio
Spindle taper: CAT40, BT40, HSK The tapered socket in the spindle and the matching cone on every holder. CAT40 and BT40 both use the steep 7:24 taper i
Spoiler A panel on top of the wing that rises to kill lift and add drag. On the ground it puts the weight on the wheels. In the
Spot drill vs center drill A spot drill is short and stiff, with a pointed tip (90°, 120°, and 140° are common), and makes a small cone so the rea
Spotface A shallow flat circle around a hole so a bolt head or washer sits square. Only deep enough to clean the surface, not de
Stepdown (DOC) How far the tool drops in Z between passes. A smaller stepdown is easier on the tool and usually leaves a better wall.
Stepover (WOC) How far the tool steps sideways between passes. A smaller stepover leaves a smoother floor and takes longer. In roughin
Stickout How far the tool hangs out of the holder. Keep it as short as the job allows; a long stickout bends more and chatters.
Stock to leave Metal the toolpath is told to leave behind so a later finish pass, or another setup, can clean it up. Roughing with zer
Strap clamps and step blocks A strap (toe) clamp is a slotted bar pressed onto the part by a stud and nut threaded into a T-nut in the table slot. A
Stringer A small spanwise stiffener fastened to the inside of the skin. It keeps the skin from buckling between ribs when the wi
Surface texture (Ra, µin) The check-mark symbol (ASME Y14.36) sets surface roughness, usually as Ra in microinches on US drawings. As a rough gui
Symmetry The old partner of concentricity, for a midplane instead of an axis. The symbol was dropped in ASME Y14.5-2018 as well.

T

T-slot A slot shaped like an upside-down T. A T-nut slides in and clamps fixtures. Machine tables are full of them. Cut with a
T-slot cutter A disk of teeth on a narrow neck that cuts the wide bottom of a T-slot. An end mill cuts the narrow upright slot first
Table and T-slots The top of the motion stack. It rides the saddle left and right (X) and has T-slots for T-nuts, so vises, fixture plate
Tail cone The taper behind the rear pressure bulkhead. People do not sit there, but the fin and the stabilizer attach in this nei
Taper, plug, and bottoming taps Names for how long the chamfered lead on the tap’s tip is. Taper: long lead, roughly 7 to 10 threads, the easiest to st
Third-angle vs first-angle projection US drawings use third angle: each view is placed on the side it is seen from. The top view goes above the front view an
Thread / tap An internal thread cut by a tap (or a single-point tool on a lathe). The tap’s shape is the thread. If a tap breaks, it
Thread callout For example 1/4-28 UNF-3B: .250 nominal size, 28 threads per inch, Unified Fine series, class 3 (tighter fit), B = inte
Thread mill A spinning cutter that orbits the hole (or a boss) in a helix and mills the thread. One cutter can cover a range of dia
Through-spindle coolant on the machine An extra high-pressure pump and fine filter feed coolant through a rotating seal at the top of the spindle and down thr
Title block The box in the lower right. It holds the drawing (part) number, title, sheet size, scale, sheet x of y, the drawn/check
Tolerance block The “unless otherwise specified” box. It sets the default tolerance for any dimension without its own, usually by numbe
Tombstone A tall fixture block stood on a rotary indexer, with parts clamped on the vertical faces. The indexer turns from face t
Tool deflection The cutter bending away from the cut under load. Walls come out tapered or the wrong size, and the finish suffers. Long
Tool holder The part that joins the cutter to the spindle: a taper on one end, a way of gripping the tool on the other (collet, set
Tool length offset A stored number that tells the control how far each tool’s tip is from the reference point, so the program’s Z zero lan
Tool setter vs presetter A tool setter sits on the machine table, as a touch switch or a laser, and measures each tool’s length (and sometimes d
Trailing edge The aft, thinning end of the wing behind the rear spar. Flaps and the aileron hang here.…
Trim fixture A shop tool that holds a skin, extrusion, or panel so the edge can be routed or trimmed to a line. It sets the outline.
True position Where a feature is supposed to be, exactly, from the datums. The GD&T position tolerance is a zone around that true spo
Turning The workpiece spins and a tool that does not spin feeds into it. Used for diameters, faces, grooves, and threads on rou

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Umbrella vs side-mount (arm) tool changer An umbrella or carousel changer is a ring of tool pockets that swings or slides to the spindle and swaps the tool direc
Undercut Material that hangs over a feature so a straight vertical tool cannot reach it. You need a tilted axis, a special cutte

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Vacuum, tape, and glue Thin plates that would bend under clamps can be held flat by vacuum through a grid table, or, for light cuts, with doub
Vertical stabilizer The fixed upright fin that keeps the nose from wandering left and right. The rudder hinges to it. Early 707s also had a
Vise stop A stop clamped to the vise that each part is pushed against, so every new part lands in the same place along the jaws w
VMC vs HMC A VMC’s spindle points down. A horizontal machining center (HMC) points its spindle sideways at a part that usually sit

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Way covers and bellows Telescoping steel covers, and folding accordion bellows on some axes, that shield the rails and ball screws from chips
Way lube An automatic lubricator that pumps small amounts of oil or grease to the rails and ball screws on a timer. It has its o
What GD&T is A drawing language for form, tilt, location, and shape, usually tied to datums. A plain ± size still matters, but GD&T
Which machine? Three questions Can a vertical tool reach every face if you flip the part? Then 3-axis plus fixtures is enough. Do features wrap around
Window belt The lengthwise band of skin and frames that holds the row of cabin windows. The repeated holes make this strip a fatigu
Wing (left / right) The left and right wings are the lifting surfaces outboard of the body. Each is a swept cantilever that also carries fu
Wing box The closed beam inside the wing: spars, ribs, and skin working together. Bending is carried mainly by the spar caps and
Wing carry-through The piece of wing box that runs through the fuselage and joins the left wing to the right. Bending from both sides meet
Wing skin The aluminum sheet on the outside of the wing. It is not a wrapper laid over the structure: with the spars it closes th
Wing-to-body fairing The smooth cover that blends the wing root into the fuselage. It guides air. It is not the joint that holds the wings o
Work light The light inside the enclosure, usually LED, that lets you see the part and the tool through the window.…
Work offset (G54) and WCS The work coordinate system (WCS) is the X0 Y0 Z0 the program measures from, usually a datum corner or center on the par

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Zero-point system Quick-change workholding: receivers in the table lock onto studs on the bottom of a vise or fixture, locating it repeat
Zone The grid formed by numbers along the top and bottom borders and letters along the sides. A location such as “C6” points