Glossary
Plain-English meanings of the terms used on this site. Underlined words on any page link here. Switch to Detailed for the longer explanations.
- % of thread (thread engagement)
How much of a full thread height the tapped hole has. 75% is the usual choice.
A smaller tap drill gives more thread engagement but needs much more torque to tap. Above about 75% you gain very little strength. Use 60–65% in stainless and other tough materials.
- Acme thread
A strong square-ish 29° thread used for leadscrews and vices.
An inch-based power-transmission thread with a 29° included angle and flat crests. Used for leadscrews and vices.
- BA (British Association)
Small British instrument threads, numbered 0BA (6 mm) down to 16BA and smaller.
A metric-based series of small threads with a 47.5° angle, used in models, instruments and electrical work.
- Back gear
A gear reduction on older lathes that gives very slow speeds for big work and threading.
A set of reduction gears behind the headstock spindle on belt-driven lathes such as the Myford and Boxford. It gives slow speeds with more torque.
- BSP (British Standard Pipe)
Pipe threads named by the bore of old pipe, not the thread size. G is parallel, R is taper.
Pipe threads to ISO 228 (parallel, G) and ISO 7 (taper, R/Rp). A ½" BSP thread is about 21 mm across.
- BSW / BSF (Whitworth)
Older British 55° threads. BSF is the finer series.
British Standard Whitworth and British Standard Fine. 55° threads with rounded crests and roots.
- Butt weld
Two bevelled ends welded together end to end, with full penetration.
A full-penetration weld between two bevelled ends, such as a weld-neck flange and a pipe.
- Carbide
Very hard tool material, usually as inserts. Runs 3–5 times faster than HSS but needs rigidity.
Tungsten carbide tools, usually replaceable inserts. They run much faster than HSS and handle hard and abrasive materials, but chip if the machine or set-up is not rigid.
- Centre height
Distance from the bed to the spindle centre. UK lathes are often described this way (e.g. 3½").
The height of the spindle axis above the bed. Older British lathes are usually described by centre height (a Myford ML7 is a 3½" lathe); swing is about twice this.
- Change gears (changewheels)
Swappable gears between spindle and leadscrew that set which thread the lathe cuts.
Gears fitted on the end of the lathe between the spindle and the leadscrew. Choosing the right train sets the pitch cut. A 127-tooth gear converts exactly between inch and metric.
- Chip load (feed per tooth, fz)
How thick a bite each cutter tooth takes, in mm per tooth. Too small and it rubs; too big and it breaks.
Feed per tooth (fz) on a milling cutter or drill. Table feed = fz × number of teeth × rpm. Too small a chip load makes the tool rub and work-harden the surface.
- Chip thinning
With a narrow cut the chip is thinner than the feed, so the feed can be raised to compensate.
When a milling cutter engages less than half its diameter, the actual chip is thinner than the programmed feed per tooth. The feed is raised so the tool still cuts rather than rubs.
- Chord
The straight-line distance between two neighbouring holes on a PCD; what you set dividers to.
The straight distance between two points on a circle. For n equally spaced holes on a PCD D, chord = D × sin(180°/n).
- Compound slide (top slide)
The small slide on top of the cross-slide that can be set at an angle.
The top slide. For threading it is often set at 29–30° so the tool cuts mainly on one flank (flank infeed).
- Counterbore (weld end)
A short bore machined inside a pipe end so the root face is the same width all round.
An internal bore machined at a butt-weld end so the land is uniform around the circumference and the bores of the two parts line up.
- Depth of cut (ap)
How deep the tool cuts on each pass. On a lathe the diameter shrinks by twice this.
The depth the tool goes into the work on one pass. On a lathe, a 1 mm depth of cut removes 2 mm from the diameter.
- Derate
Turning the book speed down to suit a lighter, less rigid machine.
A factor below 1 applied to published speeds and depths so that they suit hobby and mini machines, which are less rigid and less powerful than the machines the data was written for.
- Distance between centres
The longest piece you can hold between the headstock and tailstock centres.
The maximum length of work that can be held between centres. It is the usual measure of a lathe’s length.
- Dividing head
A geared head that splits a turn into exact divisions using hole plates.
A 40:1 worm-geared head with hole plates. Turns of the crank = 40 ÷ number of divisions.
- DRO (digital readout)
An electronic display of slide positions, so you can drill hole patterns by coordinates.
Digital readout fitted to the slides of a mill or lathe. Lets you move to X/Y coordinates directly, for example for a PCD.
- EST (estimate)
Not confirmed against a published source. Check before relying on it.
On this site, EST means a figure that could not be checked against a published reference: a rule of thumb, an estimate from a range, or a figure from a single unofficial source.
- Feed
How far the tool moves for each turn of the work (mm/rev), or how fast the table moves (mm/min).
On a lathe, feed is how far the carriage moves per revolution (mm/rev). On a mill, it is usually given per tooth (chip load) and multiplied up to a table feed in mm/min.
- FF (flat face)
A flange faced flat across its full width, used against cast iron and plastic flanges.
A flange with the gasket face in the same plane as the bolting face. Used with full-face gaskets, typically mating to cast-iron or non-metallic equipment.
- Half-nuts (clasp nut)
The lever on the apron that clamps the carriage to the leadscrew for threading.
A split nut in the apron that closes onto the leadscrew. Engaging it makes the carriage follow the leadscrew for screwcutting.
- HSS (high-speed steel)
Tough tool steel you can grind yourself. Slower than carbide but forgiving.
High-speed steel tools can be ground to shape on a bench grinder and cope well with interrupted cuts and low-power machines. HSS-Co (M35/M42) contains cobalt and runs a little hotter.
- Infeed
How far the tool is moved in for each threading pass.
The depth added on each threading pass. Passes get lighter as the thread deepens. Flank (compound) infeed cuts on one side of the tool; plunge (radial) infeed cuts on both.
- Leadscrew
The long screw along the bed that drives the carriage when cutting threads.
The screw that moves the carriage in step with the spindle for screwcutting. Its pitch (e.g. 8 TPI on a Myford) is needed to work out change gears.
- Major diameter
The outside diameter of a bolt thread, the “size” of the thread.
The largest diameter of the thread: the crests of a bolt or the roots of a nut. An M10 thread has a 10 mm major diameter.
- ME (Model Engineer)
Whitworth-form threads in just 32 and 40 TPI, used in model engineering.
Model Engineer threads use the Whitworth 55° form at a constant 32 or 40 TPI for all sizes.
- Metal removal rate (MRR)
How much metal you remove per minute, in cm³/min. A guide to how hard the machine is working.
Volume of metal removed per minute. Turning: Vc × ap × f. Milling: ae × ap × table feed ÷ 1000.
- Minor diameter
The diameter at the bottom of a bolt thread, or the hole size of a nut thread.
The smallest diameter of the thread. For a tapped hole it is close to the tap drill size.
- Module and DP
Gear tooth sizes. Module is metric (mm); DP (diametral pitch) is imperial.
Module = pitch diameter ÷ teeth, in mm. Diametral pitch (DP) = teeth ÷ pitch diameter in inches. Myford change wheels are 20 DP.
- NPT (National Pipe Taper)
American tapered pipe thread: taper 1 in 16 on diameter, 60° form.
American taper pipe thread to ASME B1.20.1. The taper is 1 in 16 on diameter (¾" per foot), a half-angle of 1.7899°.
- PCD (pitch circle diameter)
The diameter of the circle that a ring of bolt holes sits on. Also called the bolt circle.
Pitch circle diameter: the diameter of the circle through the centres of a ring of holes, such as the bolts on a flange or chuck backplate.
- Pitch (thread)
Distance from one thread crest to the next, in mm. A 1.5 mm pitch moves 1.5 mm per turn.
The distance from a point on one thread to the same point on the next thread, measured along the axis. Metric threads are described by pitch (M10 × 1.5 means 1.5 mm pitch). Imperial threads use threads per inch (TPI) instead; pitch = 25.4 ÷ TPI.
- Pitch diameter (effective diameter)
The “middle” diameter of a thread, where it is measured for fit with wires or a micrometer.
An imaginary diameter where the width of the thread and the width of the gap are equal. It controls how well a nut and bolt fit, and it is what thread wires and thread micrometers measure.
- Pressure class
The flange rating (Class 150, 300 … 2500). Higher class, thicker flange and higher pressure.
The pressure–temperature rating designation of a flange. The allowable pressure depends on the class, the material and the temperature, from the applicable standard.
- Quick-change gearbox
A gearbox with levers that select threads and feeds without swapping gears.
A gearbox between the spindle and leadscrew, set with levers from a chart on the machine. Most Colchester, Harrison and larger lathes have one; you only need change gears for unusual pitches.
- Ra (surface roughness)
Average roughness of a machined surface in micrometres (µm). Lower is smoother.
The arithmetic mean roughness. 3.2 µm Ra = 125 µin AARH. Flange finishes are specified as an Ra range.
- RF (raised face)
A flange whose gasket face stands proud of the bolting face, usually 1.6 mm on Class 150/300.
The most common flange facing. The gasket sits on a raised ring inside the bolts, so bolt load is concentrated on the gasket.
- Root face (land)
The small flat left at the bottom of a weld bevel, typically 1.6 mm.
The flat at the root of a weld preparation. It stops the edge burning through and controls penetration. Typically 1.6 mm ±0.8 mm (1/16" ±1/32").
- Rotary table
A turntable with a handwheel and degree scale for drilling holes around a circle.
A geared turntable bolted to the mill table. Set the work’s centre under the spindle, offset by the PCD radius, and turn the table to each hole angle.
- RPM (rev/min)
Spindle speed: how many times the chuck or cutter turns per minute.
Revolutions per minute. Set it from the cutting speed and the diameter: a bigger diameter needs a slower speed.
- RTJ (ring type joint)
A flange with a machined groove for a solid metal sealing ring, for high pressure and temperature.
A flange face with a trapezoidal groove that takes an oval or octagonal metal ring (R), or a pressure-energised RX or BX ring. Used for high pressure, high temperature and API 6A service.
- Serrated (stock) finish
The fine spiral or concentric grooves turned into a flange face so the gasket grips.
The standard flange face finish: a continuous spiral or concentric grooves cut with a round-nosed tool. Commonly 45–55 grooves per inch at 3.2–6.3 µm Ra.
- SFM (surface feet per minute)
Cutting speed in feet per minute, the imperial version of Vc. 1 m/min ≈ 3.28 SFM.
Surface feet per minute, the imperial unit of cutting speed. rpm = 12 × SFM ÷ (π × D in inches), often rounded to 3.82 × SFM ÷ D.
- Single-point threading
Cutting a thread on the lathe with one shaped tool, pass by pass.
Screwcutting with a single tool ground or made to the thread form, rather than with a tap or die.
- Socket weld
Pipe pushed into a socket and fillet-welded. Pull back about 1.6 mm before welding.
A joint where the pipe fits into a socket and is joined with a fillet weld. A gap of about 1/16" (1.6 mm) is left at the bottom of the socket to allow for weld shrinkage.
- Specific cutting force (kc)
How hard a material is to cut, used to estimate the power needed.
The force needed to cut a chip of one square millimetre section, in N/mm². Used with metal removal rate to estimate spindle power.
- Swing
The largest diameter that will turn over the lathe bed: twice the centre height.
The largest diameter that can be turned over the bed. It is roughly twice the centre height. Swing over the cross-slide is smaller.
- Tap drill
The drill you use before tapping. It sets how deep the thread in the hole will be.
The drill size used before tapping a thread. Standard charts give one size per thread (e.g. M12 = 10.2 mm). For metric coarse threads the rule of thumb is drill = diameter − pitch.
- Thread dial (thread indicator)
A small dial on the carriage that shows when to close the half-nuts so each pass lands in the same groove.
A dial driven by the leadscrew. Closing the half-nuts on the right number keeps every pass in step with the first. Metric threads on an imperial leadscrew usually need the half-nuts kept closed and the motor reversed.
- Thread wires (three-wire method)
Three precise pins laid in the thread so you can measure its pitch diameter with a micrometer.
Three hardened wires of known size placed in the thread grooves. The micrometer reading over them is converted to pitch diameter.
- TPI (threads per inch)
How many threads fit in one inch of length. Pitch in mm = 25.4 ÷ TPI.
Threads per inch, used for imperial threads such as BSW, BSF, UNC, UNF and ME. 20 TPI is the same as a 1.27 mm pitch.
- Trapezoidal thread (Tr)
The metric cousin of Acme, with a 30° angle. Written e.g. Tr20×4.
An ISO metric power-transmission thread with a 30° included angle, defined in ISO 2904. Tr20×4 is 20 mm diameter and 4 mm pitch.
- UNC / UNF (Unified)
American and Unified inch threads, 60° form. UNC coarse, UNF fine.
The Unified inch screw threads to ASME B1.1, with a 60° form like ISO metric.
- Vc (cutting speed)
How fast the metal passes the tool tip, in metres per minute. Each material has a comfortable range.
Cutting speed or surface speed, in metres per minute (m/min). It depends on the material and tool, not the diameter. Spindle speed is worked out from it: rpm = 1000 × Vc ÷ (π × D).
- Weld bevel
The angled edge machined on a pipe or flange so a full-penetration weld can be made.
The angled weld preparation on a butt-weld end, typically 37.5° ±2.5° with a small flat root face.
- Weld overlay (cladding)
A corrosion-resistant layer, often Inconel 625, welded onto a face and machined back.
A layer of corrosion-resistant alloy deposited by welding onto a sealing face or bore, then machined to size. The base metal is machined undersize first to allow for it.
- Width of cut (ae)
How much of the cutter’s width is engaged sideways in a milling cut.
Radial engagement of a milling cutter. Light widths of cut allow a higher feed per tooth because of chip thinning.
- Work-hardening
Some metals get harder where the tool rubs. Keep cutting, don’t dwell.
Austenitic and duplex stainless steels and nickel alloys harden where they are deformed. If a tool rubs instead of cutting, the next pass meets harder metal. Keep the feed up and get under the hardened layer.