Why This Glossary Exists
If you buy custom steel components, you have probably received a quotation that lists processes you only half understand — or sent a drawing that a supplier interpreted differently from what you meant. Most of the cost, delay and rework in custom steel fabrication comes from terminology, not from machining capability.
This glossary covers the terms that come up most often when industrial equipment manufacturers source heavy steel fabrication and CNC machining. Each entry explains what the term means, and why it matters when you specify or buy a component.
Cutting and Plate Processing
Heavy Steel Fabrication
The manufacturing of large steel components — frames, bases, structures and weldments — by combining cutting, forming, machining and welding. It is distinguished from light sheet metal work by material thickness (often 20 mm and above), component weight, and the fact that distortion control becomes a primary engineering concern. A heavy fabrication supplier is judged on whether it can hold dimensions after welding, not only before.
Steel Plate Cutting
The first process in most fabricated components: cutting a profile from steel plate before any machining or welding. The three common methods are flame cutting, plasma cutting and laser cutting. The choice depends mainly on plate thickness, required edge quality and the tolerance that downstream processes need.
Flame Cutting (Oxy-Fuel Cutting)
A thermal cutting process that burns steel with a fuel gas and oxygen jet. Flame cutting is the most economical method for thick plate — typically from around 25 mm upward — and is the standard choice for heavy plate where a machining allowance will be removed afterwards. It leaves a heat-affected edge, so the cut face is normally not treated as a finished surface.
Plasma Cutting
A thermal cutting process that melts metal with a constricted electric arc and blows it clear with a high-velocity gas jet. Plasma cutting is faster than flame cutting on mid-range thicknesses and handles a wider range of materials, including stainless steel and aluminium. Cut edges are cleaner than flame cutting but still usually require machining if the edge is a functional surface.
Laser Cutting
A thermal cutting process using a focused, high-energy laser beam. Laser cutting gives the tightest kerf and the cleanest edge of the three methods, which makes it the preferred choice for plate where cut edges will remain as finished surfaces. It is most economical at thinner gauges; for very thick plate, flame or plasma cutting is usually more practical.
Kerf
The width of material removed by the cutting process. Kerf matters because it consumes material and shifts the effective edge position. When a drawing specifies an outer dimension with a tight tolerance, the cut path must be offset for kerf — which is one reason a supplier will often ask for a machining allowance rather than cutting directly to a finished size.
Nesting
The arrangement of part profiles on a plate to minimise waste. Good nesting directly affects material cost, especially on large or irregular parts. For thick plate, nesting is also constrained by the minimum distance required between cuts to avoid heat build-up and distortion.
Section Thickness
The thickness of the steel being cut or welded. Section thickness drives nearly every downstream decision: cutting method, preheat requirements, welding procedure, distortion risk, and whether post-weld machining will be needed. A supplier who asks for section thickness early is usually one who understands what it changes.
CNC Machining
CNC Machining
Machining in which a computer-controlled machine tool follows a program to cut metal to a required shape, size and surface finish. In custom steel component manufacturing, CNC machining is typically used to produce accurate holes, bores, mounting surfaces and interfaces — the features that determine whether the component will assemble and function correctly.
CNC Milling
A machining process in which a rotating multi-point cutter removes material from a stationary workpiece. Milling produces flat surfaces, slots, pockets, shoulders and drilled or tapped hole patterns. For large steel components, milling capacity is limited by the machine’s working envelope, which is why the maximum component size a supplier can machine is worth confirming before you specify.
CNC Turning
A machining process in which the workpiece rotates while a single-point tool removes material. Turning produces cylindrical features: diameters, shoulders, grooves, threads and bores. Shafts, pins, sleeves and similar rotationally symmetric components are normally turned.
Boring
The enlarging and finishing of an existing hole to a precise diameter, usually with a single-point tool on a boring machine or boring mill. Boring is how large, high-accuracy bores are produced in fabricated components — for example bearing seats and pin bores in equipment bases. Because it can be performed on a large workpiece without repositioning, horizontal boring is often the process that makes post-weld machining of a heavy frame possible.
Vertical Machining Center (VMC)
A CNC milling machine with a vertically oriented spindle, typically enclosed and equipped with an automatic tool changer. VMCs are used for a wide range of milled features and hole patterns. When a component is large or heavy, the practical limit is not only the table size but the machine’s ability to hold the part and reach the features without re-clamping.
Machining Allowance (Machining Stock)
Extra material left on a surface so that it can be removed later by machining. Machining allowance is essential in heavy fabrication: cutting and welding both move material, so a surface that must end up accurate is normally left oversize. If a drawing omits the allowance, the supplier has to either add it from experience or ask — and an omitted allowance is a common cause of arguments about who is responsible for a dimension.
Datum
A reference from which dimensions are measured. A tolerance has no meaning without a datum: “flat within 0.2 mm” only means something if it is clear what it is flat relative to. In fabricated components, the datum structure matters because welding can move features relative to each other — so a sensible drawing states which surfaces are the references, and the machining plan establishes them after welding.
Setup
A single clamping and alignment of a workpiece on a machine. Every additional setup introduces a new opportunity for positioning error, so reducing setups is a core machining-planning concern. For large fabricated parts, “can all the critical features be reached in one setup?” is often a more useful question than any individual tolerance value.
Surface Finish (Ra)
A measure of surface roughness, usually expressed as Ra — the arithmetic average deviation of the surface profile from its mean line, in micrometres. A lower Ra value means a smoother surface. Surface finish is specified when it affects function, such as a sealing face or a bearing seat, and costs more the finer it is specified. Specifying a fine finish on a non-functional surface is one of the most common sources of unnecessary cost.
Welding and Fabrication
Welded Fabrication
The assembly of cut and formed steel parts by welding to create a single component. A welded fabrication may be a frame, a base, a housing or a structural assembly. The engineering challenge is not the weld itself but the dimensional consequence of welding: heat input moves material, and that movement has to be either controlled or removed afterwards.
Weldment
A component produced by welding two or more parts together. A weldment is distinct from a casting or a machined-from-solid part, and its tolerance behaviour is different: the final accuracy depends on the sequence of cutting, fitting, welding and — where required — machining. When a drawing states a tolerance on a weldment without stating where machining sits in the sequence, the requirement is ambiguous.
Welded Frame
A load-bearing structure built from cut and welded steel members, typically used as a machine frame, support structure or equipment chassis. Frames are usually specified for stiffness and flatness at the mounting interfaces rather than for overall dimensional precision. The practical design issue is that surfaces which must be flat and parallel usually need to be machined after welding, which is why frame drawings should state which faces are machined and which are as-welded.
Equipment Base (Machine Base)
A fabricated and machined steel base that carries machinery, transmits its loads to the floor, and provides the mounting interfaces the machine is aligned against. Bases are a good example of a component where welding and machining must be planned together: the base is welded for strength, then the mounting faces and any alignment bores are machined for accuracy.
Heat-Affected Zone (HAZ)
The region of base metal next to a weld whose microstructure has been altered by welding heat. The HAZ is typically harder and more brittle than the surrounding material and can affect machinability and service behaviour. For thick sections, the HAZ is one reason a weldment may need controlled preheat, controlled cooling, or a heat-treatment step before final machining.
Distortion (Weld Distortion)
Movement or deformation of a component caused by the heating and cooling cycle of welding. Distortion is not a defect in itself; it is a predictable consequence of the process. It is managed by weld sequencing, balanced weld placement, jigs and fixtures, and by leaving machining allowance so the accuracy can be recovered afterwards. A supplier who only quotes cutting and welding — with no provision for correction — is likely to deliver parts that do not meet the drawing.
Fit-Up
The accuracy with which parts are positioned relative to each other before welding. Fit-up quality directly determines the final dimensions of the weldment, and it is largely a labour and fixturing question rather than a machine-capability question. On heavy fabricated components, poor fit-up can consume more time than the welding itself.
Post-Weld Machining
Machining carried out after welding to restore accuracy on features that matter. Welding moves material, so features that control fit, alignment or mounting are often machined in their final position rather than before welding. Post-weld machining requires that the component be designed with machining allowance, and that the supplier has machines large enough to handle the finished weldment. It is often the difference between a component that assembles and one that does not.
Tolerances and Quality
Fabrication Tolerance
An allowable deviation from a specified dimension on a fabricated (as opposed to machined) component. Fabrication tolerances are generally looser than machining tolerances because they accommodate cutting, fit-up and weld distortion. Standards exist for welded structures, and quoting a machining-grade tolerance on an as-welded dimension is a common and expensive specification error.
General Tolerance
A tolerance that applies to all dimensions on a drawing that do not carry an individual tolerance. A general tolerance note prevents the drawing from being cluttered with repeated values, but it should be chosen to match the process: the same general tolerance is rarely appropriate for both a cut edge and a machined bore.
Flatness
The condition of a surface having all its elements in one plane. Flatness is one of the most commonly specified requirements on fabricated machine components, and one of the most commonly misunderstood — because flatness on a welded structure can usually only be achieved by machining after welding.
Parallelism
The condition of two surfaces or axes being equidistant from each other throughout their length. Parallelism between mounting faces is a typical requirement on equipment bases and machine frames, where it directly affects how a machine sits and aligns.
Squareness (Perpendicularity)
The condition of two surfaces or axes meeting at a right angle within a specified deviation. Squareness is a key requirement where frames must stack or where a structure must sit true to a reference. Like flatness and parallelism, it is usually established by machining after welding.
Dimensional Inspection
Verification that a component’s actual dimensions match the drawing. On custom steel components, inspection is normally planned around the features that matter functionally rather than a blanket check of every dimension. The most useful thing a buyer can do is state clearly, at the quotation stage, which dimensions and features are critical.
CMM (Coordinate Measuring Machine)
An instrument that measures the geometry of a component by probing points in three-dimensional space. A CMM produces numeric dimensional reports rather than a simple pass/fail, which is why inspection reports on critical features are often requested as part of the delivery documentation.
Material Test Certificate (MTC) / Mill Certificate
A document from the steel mill certifying the chemical composition and mechanical properties of the material supplied. A “3.1” certificate under EN 10204 includes specific test results traceable to the heat number of the material, which is what most industrial buyers need for traceability. If material traceability matters for your application, say so in the RFQ — the requirement affects how material is purchased, not just how it is documented.
First Article Inspection (FAI)
A full dimensional and documentation check of the first component produced in a new production run, before the rest of the batch is released. FAI is the standard protection against a setup error being repeated across an entire order, and it is worth requesting explicitly on any component with tight or functionally critical features.
Inspection Report
A record of the measurements taken on a component, typically including the specified dimension, the actual measured value, and the instrument used. An inspection report is what allows a buyer to accept a component on the basis of evidence rather than trust. Ask which features will be reported before the order is placed.
Sourcing and Delivery
RFQ (Request for Quotation)
The formal request sent to a supplier asking for price and lead time on a defined scope. For custom steel components, the quality of the RFQ largely determines the quality of the quotation: an RFQ that states material, quantity, required processes, tolerances, inspection requirements and delivery stage will get a usable quotation, while a drawing sent alone often will not.
Drawing Formats (PDF, DWG, DXF, STEP)
PDF drawings are the standard for conveying dimensions and tolerances, and are sufficient for quotation. DWG and DXF files carry 2D geometry that can be used directly for cutting programmes. STEP files carry 3D solid geometry, which is useful for complex machined forms and for generating cutting paths. Sending a PDF plus a native format where available generally speeds up both quotation and production.
One-Piece and Batch Production
The order quantity pattern. Custom steel fabrication is frequently ordered as single components for prototypes or spares, small batches for machine builds, or repeat batches for ongoing production. The economics differ substantially between these: one-piece work is dominated by setup and programming, while repeat batches allow that work to be amortised. Stating the expected repeat volume — even if the first order is one piece — helps a supplier quote realistically.
MOQ (Minimum Order Quantity)
The smallest order a supplier will accept. For custom fabrication, MOQ is usually governed by setup cost rather than by any technical limit, and it varies with component size and process mix. Where a supplier can accept one-piece orders, that flexibility is itself a selection criterion worth confirming.
Incoterms (DDP, DAP, FOB)
Standard international commercial terms defining who arranges and pays for transport, insurance and import duties. FOB places the goods on board at the origin port, leaving onward freight and import to the buyer. DAP delivers to a named destination with the buyer clearing import. DDP delivers to the destination with the seller responsible for import clearance and duties. Because import duties on steel components can be substantial, asking for a DDP quotation is one way to see the true delivered cost in a single number rather than estimating it yourself.
Landed Cost
The total cost of a delivered component: not only the unit price, but freight, insurance, import duties, brokerage, and any inspection or certification cost. Comparing suppliers on unit price alone is misleading — particularly for imported steel components, where duty treatment can change the ranking of suppliers entirely. Landed cost is the number that should drive a sourcing decision.
Import Duties and Product Classification
Steel components entering a country are subject to import duties that depend on how the product is classified under the destination country’s tariff schedule. Classification is determined by the product’s characteristics and end use, and different classifications can carry materially different rates. Because classification is a technical question with significant cost consequences, it is worth confirming the applicable treatment for your specific component with a customs broker before finalising a purchase decision — and worth asking your supplier for a delivered (DDP) price so the duty is visible rather than assumed.
How VHOFAB Fits
VHOFAB is a steel fabrication and CNC machining supplier based in Wuxi, China, working from an 8,800 m² workshop with 60+ employees and a monthly capacity of 2,000 tonnes. Maximum steel plate thickness is 300 mm.
We provide flame, plasma and laser cutting, CNC machining and boring, welding and fabrication, and assembly and finishing, coordinated in one production route. Components are manufactured to customer drawings, and critical dimensions and machined features are checked during production.
If you are preparing an RFQ for a custom steel component, see our capabilities for the processes we run, or send your drawings and requirements for a manufacturing review.