Reference / Glossary
Large Part Machining Glossary
67 terms that come up when specifying, quoting or inspecting a heavy machined part. Plain definitions, no marketing, written so both sides of a quote mean the same thing.
A
- Allowance, machining
- Stock left on a blank for the cutter to remove. Usually set twice on a heavy part, once for roughing and again for finishing after stress relief. Read more.
- Angle plate
- A large precision right angle bolted to the table so a part can be mounted on a vertical face. How a tall part gets laid down to reach its end faces.
- As welded
- Condition of a fabrication straight from welding, before any thermal treatment. Carries the full welding stress.
B
- Between centres
- The distance on a horizontal lathe between the headstock and the tailstock, which sets the longest part it can support at both ends.
- Blank
- The raw part before machining. Weldment, casting, forging, plate or solid stock, and the choice changes the machining hours more than the material price does. Read more.
- Boring bar
- The tool that enlarges and finishes a bore. Deflection rises steeply with length, so the usable overhang for a given diameter is limited.
- Boring mill
- A machine with a horizontal spindle built to bore and mill large parts. Table type or floor type. Read more.
- Bridge mill
- See gantry mill.
C
- Centre of gravity
- Marked on heavy parts for rigging and crating. Riggers and crate designers both need it.
- Chatter
- Self excited vibration between tool and part. Common at full spindle extension and on thin walls, and it damages surface and tooling quickly.
- Coaxiality
- The requirement that two bores share an axis. The correct callout when two bores must line up, and far easier to hold and to measure than position from a distant datum. Read more.
- Crane capacity
- The lifting limit in a specific bay, with its hook height. A shop with a 20 t table under a 5 t crane has 5 t of real capacity. Read more.
- Cylindricity
- How closely a bore or shaft matches a true cylinder along its whole length, not just at one section.
D
- Datum
- The reference a drawing measures from. On a large part the datum scheme decides the setup count, because datums that cannot be reached in one clamping force the part to be turned over.
- Deflection
- Elastic bending under load. Applies to the tool, the part, the fixture and the machine table, and all four matter on heavy work.
- Distortion
- Permanent change of shape, usually from released residual stress or from clamping. Distinct from deflection, which recovers.
E
- Envelope, work
- The three dimensional space a machine can cut in, always smaller than the stated axis travel once fixture and tool stack are allowed for. Read more.
- EXW
- Ex works. The buyer takes the part from the shop and carries transport and risk from there. Common on heavy parts because the buyer often has the better freight arrangement.
F
- Facing head
- An attachment that moves a tool radially while the spindle turns, used to face large flanges and to bore diameters beyond the spindle range.
- First article inspection
- Full dimensional verification of the first piece against the drawing, usually before the rest of a batch is released. Read more.
- Fixture
- A structure built to locate and support a part during machining. On heavy parts it is often the only way to hold the part without distorting it and still reach every feature. Read more.
- Flame cut
- Plate cut by oxy-fuel. Leaves a hard, stressed skin that is unkind to the first roughing pass.
- Flatness
- How closely a surface sits between two parallel planes. Called out over an area, and on large parts it should be limited to the area that matters rather than the whole face.
- Floor loading
- The weight per square metre a workshop floor can carry. Becomes the governing limit on floor plate work.
- Floor plate
- A machined plate set into the shop floor that carries parts too heavy for any table, with a floor type machine driven up to the work.
G
- Gantry mill
- A machine where the spindle hangs from a bridge travelling over a fixed table. The part is clamped once and never moves. Read more.
- GD&T
- Geometric dimensioning and tolerancing. On large parts it earns its keep by letting you state which relationships matter instead of tightening every dimension. Read more.
- Grey iron
- Cast iron with flake graphite. Damps vibration well, which is why machine tool bases are still cast from it.
H
- Hook height
- The height a crane hook reaches. Limits how tall a part can be slung, separately from how heavy it is.
I
- In-process probing
- Touching off features with a spindle probe during machining, so the programme works from where the part actually is rather than where it was at setup.
- Indicate in
- Setting a part on the machine so its datums match the machine axes. On a four metre weldment this can take hours and it is chargeable time.
- ISO 3834
- Quality requirements for fusion welding. Relevant when the blank is a fabrication and the buyer needs the welding qualified.
L
- Laser tracker
- A portable instrument that measures position over tens of metres by tracking a reflector. The practical way to prove geometry on a part too big for a coordinate measuring machine.
- Line boring
- Machining two or more bores with a single supported bar, so they are coaxial by construction rather than by alignment. Also done in the field with portable equipment. Read more.
N
- Net shape
- A blank close to the finished dimensions, so little material has to be removed. Castings and forgings get closer to it than fabrications.
- Normalising
- Heating above the transformation range and cooling in air, to refine grain and relieve some stress. Distinct from a stress relief soak below that range.
O
- Outboard support
- Extra tables or jacks carrying a part that overhangs the machine table. Restores rigidity, does not raise the rated table load.
- Oven, stress relief
- The furnace a part is soaked in between roughing and finishing. Its internal size is frequently the real constraint on large fabrications, ahead of any machine.
P
- Parallelism
- How closely one surface or axis runs parallel to a datum. On long parts it is a relationship across distance, which is much harder than a local tolerance.
- Permit, oversized
- Road authority approval for a load beyond the legal envelope. Issued per jurisdiction crossed and often the longest item in an oversized shipment. Read more.
- Portable machining
- Machining performed on installed equipment with equipment brought to the part. Line boring, flange facing, milling and drilling in place.
- Position, true
- How far a feature sits from where the drawing puts it, relative to stated datums. Across metres it is the expensive requirement on a heavy drawing.
- Preservation
- Protection applied to machined surfaces for transit and storage. Specified on the drawing rather than assumed, especially for sea freight.
Q
- Quench and temper
- Hardening then tempering to a target hardness. On alloy steel it is usually done between roughing and finishing, because the part moves in the furnace.
- Quill
- The extending spindle sleeve on a boring mill. Its reach is how the machine gets into deep bores, at the cost of rigidity as it extends.
R
- Rated table load
- The maximum mass a table is built to carry. Careful shops work well below it for tight tolerance work, because the guideways deflect under load.
- Residual stress
- Locked-in internal stress from welding, flame cutting, rolling or casting. Released as material is removed, which is why large parts move on the machine. Read more.
- Rigging
- Slinging, lifting and setting a part safely. On heavy work it is a priced line of its own, not an overhead.
- Right angle head
- An attachment that turns the spindle output through ninety degrees, letting a vertical machine reach a horizontal bore. Less rigid and shorter reach than a dedicated boring spindle.
- Rotary table
- A calibrated turning table that indexes a part to the next face. It is what lets a table type boring mill hold position between faces without re-indicating.
- Roughing
- Bulk material removal before finishing. On a heavy part it is often most of the machine hours, and it is where an over generous blank allowance costs real money.
S
- Setup
- One clamping of the part. Each additional setup means rigging, craning, re-indicating and another restatement of the datum, so setup count drives both price and accuracy.
- Slew ring
- A large diameter bearing that lets a structure rotate, as on a crane or excavator. Typical vertical lathe work.
- Spindle taper
- The tool interface, such as ISO 50 or HSK. On heavy machines it indicates how much torque the spindle can transmit into a cut.
- Squareness
- How closely two surfaces or axes sit at ninety degrees. On a machine base it is what makes the spindle square to the table later.
- Steady rest
- A support that carries a long turned part between the chuck and the tailstock, stopping it whipping under the cut.
- Stock removal
- The volume of material that has to become chips. Drives roughing hours far more than the price per kilo of the material does.
- Stress relief
- A controlled thermal or vibratory cycle that redistributes residual stress, run between roughing and finishing so the part settles before the final cuts. Read more.
- Swing
- The largest diameter a lathe can turn. On a vertical lathe it is the headline capacity figure. Read more.
T
- Thermal growth
- Dimensional change with temperature. Carbon steel moves roughly 11 to 12 micrometres per metre per degree Celsius, which is a real share of the tolerance on a long part. Read more.
- Travel
- How far each machine axis can move. Always larger than the usable part dimension, because fixture, clamps and tool length all take a share.
- Turn over
- Craning a part to present a different face. On heavy work a turn over can take half a shift with two people and a crane.
V
- Vertical turning lathe
- A lathe with a rotating horizontal table, so part weight goes straight down into the machine instead of hanging off a spindle. Suits parts wider than they are tall. Read more.
- Vibratory stress relief
- Exciting a part at resonance to redistribute peak stresses. Fast, no oven size limit, generally treated as less complete than a thermal cycle on heavily welded steel.
W
- W-axis
- The programmable extension of the boring mill quill, treated as an axis in its own right. What reaches into a deep bore.
- Weldment
- A fabricated blank made by welding plate and sections. Cheap at low quantity and the reason stress relief is usually in the sequence. Read more.
- Workholding
- Everything that locates, supports and clamps the part. On heavy work it is often the hardest engineering in the job. Read more.
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