CNC Machining

What CNC Machining / Milling Is

CNC machining is subtractive manufacturing: a solid block, bar, or near-net casting is clamped on the machine and a rotating cutter removes material until what remains is the finished part. The "CNC" — *computer numerical control* — means the toolpath is not steered by hand but by a program, so the same part comes off the machine to the same dimensions every time, run after run.

The form practised at this station is milling: a multi-tooth cutter spins at high speed while the part moves underneath it (or the cutter is fed into the part), each tooth shaving off a chip. Milling is what cuts flat faces, pockets, slots, bores, and contoured surfaces — the geometry of brackets, housings, flanges, and machined fittings. It is the natural complement to CNC turning, which spins the *part* against a single-point tool to make round, concentric shafts and bushings; milling spins the *tool* to make prismatic and free-form features.

At steelhui, the milling station is the complex-part, high-precision capability: a high-speed vertical machining centre that takes stainless and other metals from raw stock to a finished, tolerance-controlled component — including the multi-face and curved-surface work that simpler processes cannot reach. Where laser cutting and press-brake bending shape sheet, milling carves solid stock to dimension.

The Milling Principle & Multi-Axis Motion

In milling, the cutter rotates and its teeth engage the workpiece intermittently — each tooth enters the metal, peels off a chip whose thickness varies through the cut, and exits. The spindle supplies the rotation (its speed sets the cutting speed at the tool edge), while the machine axes feed the tool and table relative to one another to sweep the cutter along the programmed path. The combination of how fast the spindle turns, how fast the table feeds, and how deep the tool is buried in the metal is what defines the cut — covered in the next section.

3-axis milling

A 3-axis machining centre moves the tool and part along three linear axes — X, Y, and Z. The spindle stays vertical and reaches the part from one direction, from above. This is the workhorse arrangement: it cuts flat faces, pockets, holes, and any feature accessible from the top in a single orientation. Features on other faces require the part to be unclamped and re-fixtured for each new side, and every re-clamp introduces a fresh chance for the part to sit slightly differently — a small loss of accuracy between faces.

5-axis milling

A 5-axis machine adds two rotary axes to the three linear ones, so the tool (or the part) can be tilted and swung as well as translated. Two things follow. First, the tool can reach multiple faces of the part in a single setup — the part is clamped once and the machine presents each face to the cutter by rotating, instead of the operator re-fixturing. Eliminating re-clamps removes their cumulative error, so features on different faces hold position relative to one another far better. Second, the tool can be held at a controlled angle to a curved surface throughout the cut, which is what lets a 5-axis machine produce complex free-form surfaces — impeller-like contours, blended fillets, angled bosses — that a vertical 3-axis tool simply cannot reach cleanly.

The practical payoff of the single setup is both accuracy and consistency: a part machined complete in one clamping carries no setup-to-setup error between its features, and it comes off the machine finished rather than passing through several fixtures.

Cutting Parameters & Surface Roughness

Every milling cut is governed by three cutting parameters, and they trade off against one another:

• Cutting speed — how fast the cutting edge moves through the metal, set by spindle speed and cutter diameter. It drives the temperature at the edge and, with it, tool wear; within limits a higher cutting speed actually *lowers* surface roughness by suppressing built-up edge (see below).
• Feed rate — how fast the tool advances per tooth (and per revolution). Feed is the dominant lever on surface roughness: a heavier feed leaves taller cusps between successive tool passes, so Ra rises roughly with feed.
• Depth of cut — how much material the tool removes per pass, radially and axially. Together with feed and speed it sets the metal-removal rate (how fast the job gets cut) and the cutting force the tool and fixture must absorb.

These three set the headline trade-off of all machining: push them up and the part is cut faster but the finish degrades, the forces and heat rise, and tool life shortens; pull them down and the finish improves at the cost of cycle time. The usual strategy is to split the work — a heavy roughing pass at high removal rate to get close to size fast, followed by a light finishing pass at low feed to deliver the surface and the final dimension.

How the parameters set Ra

Surface roughness — the fine peaks and valleys left on the machined face, quantified as Ra — is not random; it follows the parameters. Feed rate is the primary driver: each tooth leaves a scallop, and a higher feed spaces those scallops further apart and deeper, raising Ra. Cutting speed works the other way: at low speed, especially on gummy materials like austenitic stainless, metal welds onto the cutting edge to form a built-up edge (BUE) that smears the surface and worsens Ra; raising the cutting speed past the BUE-forming range gives a cleaner shear and a lower Ra. The third influence is tool condition — a sharp tool cuts cleanly, but as the edge wears the cut degrades and Ra rises, which is why roughness is used as a practical indicator of both surface quality and tool state.^[1]

Rule of thumb: feed up → Ra up (more, deeper scallops); cutting speed up → Ra down (less built-up edge, cleaner shear); tool wear up → Ra up. Achieving a low Ra is a finishing-pass job — light feed, sharp tool, adequate speed.

Dimensional Tolerance

No machined feature is made to an exact dimension — it is made to a dimension plus a permitted band of variation, the tolerance. Tight tolerances cost time (slower finishing passes, more measurement); loose ones are quick. The engineering convention is to call out a tight tolerance only on the few features that need it, and let everything else fall under a general tolerance.

Those general tolerances — the band that applies to any dimension with no individual tolerance written next to it — are standardised by ISO 2768. It defines tolerance classes for linear and angular dimensions, designated f (fine), m (medium), c (coarse), and v (very coarse), so a drawing can specify the whole part's default precision with a single class call-out rather than a tolerance on every dimension.^[2] The class chosen states how precise the *unmarked* features have to be; critical fits then carry their own tighter, individually stated tolerances on top.

For a milled part, holding the band is the combined result of machine accuracy, a rigid setup, sharp tooling, and — on difficult materials — controlling the heat and tool wear that push a dimension off as a cut progresses. The single-setup advantage of 5-axis work matters here too: features cut in one clamping carry no re-fixturing error between them.

General (unmarked-dimension) tolerances are specified by class per ISO 2768 — f / m / c / v. Individually critical features carry their own fit tolerances on the drawing.

Surface Finish (Ra)

Surface finish is the second axis of a machined part's quality, alongside dimension. It matters for function — sealing faces, sliding fits, fatigue life, and the corrosion behaviour of stainless all depend on how smooth the surface is — as well as appearance.

The most widely used measure is Ra, the arithmetic mean deviation of the assessed profile — in plain terms, the average height of the surface's peaks and valleys away from its mean line over a measured length. A lower Ra is a smoother surface. The parameter and its measurement are defined by the current surface-texture standard, ISO 21920-2, the geometrical-product-specification standard that sets out the terms and parameters for profile surface texture and that supersedes the older ISO 4287.^[3] Specifying a surface "to ISO 21920-2, Ra ≤ X" is the modern, unambiguous way to call out a finish requirement.

As the cutting-parameter section showed, Ra on a milled face is set chiefly by feed rate and the condition of the tool. A required Ra is therefore delivered by a dedicated finishing pass: light feed per tooth, a sharp cutter, an adequate cutting speed to avoid built-up edge, and good coolant — exactly the conditions that keep the scallops shallow and the shear clean.^[1]

Surface roughness is specified per ISO 21920-2 (Ra = arithmetic mean deviation of the profile); this standard replaces the older ISO 4287. Ra is delivered by the finishing pass — light feed, sharp tool, sufficient speed.

Difficult-to-Machine Materials: Stainless & Titanium

The metals that make this station valuable are also the ones that are hardest to cut. Austenitic stainless steel and titanium are both classed as difficult-to-machine — for different reasons, but with the same consequence for the operator: accelerated tool wear, and with it a creeping rise in surface roughness.

Why stainless is hard

Austenitic stainless such as 304 and 316 work-hardens rapidly: the metal at the cut surface gets harder as the tool deforms it, so a tool dwelling or rubbing instead of cutting cleanly drives itself into an ever-harder skin. Stainless also conducts heat poorly, so cutting heat concentrates at the tool edge rather than flowing away in the chip, and it is gummy enough to encourage built-up edge at low speed. The combined effect is high tool wear — and research on turning stainless steels documents the direct chain: as the tool wears, the machined surface roughness Ra rises, so Ra serves as a quantitative indicator of both surface quality and tool condition.^[1]

Why titanium is hard

Titanium runs hot and chemically reactive at the cut: it too has low thermal conductivity, so the heat stays in the tool, and at temperature it tends to react with tool materials. The result is, again, rapid tool wear that demands disciplined parameters and cooling.

The tooling & coolant strategy

The response to both materials is the same family of measures. Tooling: use a tool grade and coating matched to the material, kept sharp and changed before wear runs away — because a worn edge is what drives Ra up. Cutting parameters: hold cutting speed above the built-up-edge range while keeping heat in check, and avoid the rubbing that work-hardens stainless. Coolant: flood coolant to carry heat out of the cut and flush chips, since neither material sheds heat through the chip well on its own. Rigidity and setup: a stiff, well-clamped part resists the higher cutting forces and the chatter that wreck a finish. And a single setup — the 5-axis advantage — both improves accuracy and reduces the handling of a part that is expensive in time and material by the time it is roughed out.

From CAD to Finished Part

A machined part follows a fixed sequence from model to inspected component. The part is programmed in CAM (the toolpaths, speeds, and feeds), the stock is clamped in a rigid fixture, material is taken off fast in a roughing pass, the surfaces and final dimensions are brought in with a light finishing pass, and the part is inspected against its drawing before it leaves the station.

Splitting roughing from finishing is the heart of the strategy: roughing buys speed at the cost of finish, finishing recovers the surface and the tolerance. On a 5-axis machine the clamping step is done once and the machine re-orients the part itself, so the whole sequence runs without the operator re-fixturing between faces.

Precision & Tolerances

This is the high-precision capability, and the figures below are the working envelope of the machining centre — the spindle speed that sets the available cutting speed, and the travel that bounds the part it can hold and cut.

A high spindle speed matters because cutting speed — the lever that suppresses built-up edge and lowers Ra — is the product of spindle speed and cutter diameter; the headroom to spin fast is what allows a clean finishing pass on stainless. The achieved tolerance and surface finish on any given part are specified to class per ISO 2768 (general tolerances) and per ISO 21920-2 (surface roughness, Ra) as covered above, with critical features carrying their own tighter limits.

Materials Range

The station is built around stainless steel and other engineering metals, including the difficult-to-machine grades that reward a capable machine and disciplined tooling. Austenitic stainless — 304 and 316 — is the staple, with titanium and other alloys handled under the same wear-aware strategy.

Machinability is itself a selection criterion: where a part allows it, a free-machining or more readily cut grade reduces tool wear and cycle time, while a corrosion- or strength-driven choice of stainless or titanium is machined to its own demanding parameters. The grade and the cutting strategy are chosen together.

Machining Equipment

CNC milling here runs on a vertical machining centre — a high-speed spindle over a moving table, with a large-envelope, multi-axis working range suited to complex stainless components. The high spindle speed is what underwrites clean finishing cuts on difficult materials, and the working travel bounds the size of part the station takes from solid stock to finished component. Full machine specifications are listed below.

Applications by Sector

CNC milling earns its place wherever a part is too complex, too precise, or too three-dimensional to make by cutting and bending sheet — where solid stock has to be carved to dimension.

Brackets & Mounting Components

Machined brackets, mounts, and structural fittings that carry load and must locate other parts precisely — bored holes, faced seats, and tapped features held to tolerance so they assemble without adjustment.

Housings & Enclosures

Machined housings and enclosure bodies with internal pockets, sealing faces, and bores — the multi-face, single-setup work that 5-axis milling does best, where every feature must stay located relative to the others.

Flanges & Sealing Faces

Flanges and mating faces where both the bolt pattern and the surface finish of the sealing face decide whether the joint seals — a controlled Ra on the face is as important as the hole positions.

Precision Fittings & Custom Parts

Machined fittings, adapters, and bespoke stainless or titanium components where corrosion resistance or strength dictates a difficult-to-machine grade, and the part is cut complete to a tight tolerance and specified finish. These often pair with CNC turning for any round, concentric features.