CNC Machining
Also: machining, precision machining, CNC, computer numerical control machining, subtractive manufacturing
CNC machining cuts parts from solid bar, plate or billet on computer-controlled mills and lathes: any machinable material, tight tolerances, from one piece to production.
5,104 CNC machining companies in the Noramark directory
CNC machining removes material from a solid blank with rotating cutters (milling), a rotating workpiece against a fixed tool (turning), or both on a mill-turn center. A program generated from the CAD model drives the machine, so the same part can be cut again months later from the same file. It is the most flexible way to make a metal or plastic part, and the process most prototypes start with.
Job shops hold ±0.13 mm (±0.005") without special effort and ±0.025 mm (±0.001") on features they are told matter. The parts come off the machine with a finish of 1.6-3.2 µm Ra (63-125 µin), which most functional parts use as is; bearing fits and seal surfaces are ground, honed or polished afterwards.
The trade-off is that every feature costs machine time: deep pockets, thin walls, small internal radii, undercuts and tight tolerances all add setups or slow the cut. Above a few thousand pieces a year, casting, forging, stamping or molding usually makes the same part for less, with machining kept for the critical surfaces.
At a glance
| Typical tolerances | Standard ±0.13 mm (±0.005"); ±0.025 mm (±0.001") on called-out features with care; ±0.005 mm (±0.0002") on ground or lapped features. Position of ±0.05 mm (±0.002") between features cut in one setup; add tolerance across setups. |
|---|---|
| Size limits | Vertical mills commonly take 1,000 x 500 x 500 mm (40 x 20 x 20"); gantry mills reach several meters. Lathes commonly swing 500 mm (20") over 1,500 mm (60") between centers. Small parts are limited by fixturing, not the machine: features under 0.5 mm (0.020") need micro-machining shops. |
| Surface finish | As-machined 1.6-3.2 µm Ra (63-125 µin); 0.8 µm Ra (32 µin) with finishing passes; 0.2-0.4 µm Ra (8-16 µin) after grinding, honing or polishing. |
| Lead time | Prototypes in 3-10 business days; production runs in 2-6 weeks depending on quantity and material lead time. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Almost anything: aluminum (6061, 7075, 2024), carbon and alloy steels (1018, 1045, 4140, 4340), stainless (303, 304, 316, 17-4 PH), tool steels, brass and copper, titanium, nickel superalloys, and engineering plastics (Delrin, nylon, PEEK, Ultem, PTFE). Free-machining grades (12L14, 303, C360 brass, 2011 aluminum) cut fastest; work-hardening stainless, titanium and Inconel cost two to five times the machine time of aluminum.
What drives the cost
- Machine time: material removal volume, number of setups, and how slowly the material cuts
- Tolerances tighter than ±0.05 mm (±0.002") and fine surface finishes
- Small internal corner radii, deep pockets and thin walls that force small, slow tools
- Quantity: setup and programming are amortized across the run
- Material cost and availability, especially titanium, superalloys and PEEK
- Secondary operations: heat treatment, plating, anodizing, inspection reports
- Documentation: first article inspection, material certs, PPAP
When to use it
- Prototypes and low to medium volumes, from one piece to a few thousand
- Tolerances tighter than casting or molding can hold
- Parts in metals that cannot be cast or formed economically, such as machined 7075 or 17-4 PH
- Finishing the critical surfaces of cast, forged or printed blanks
- Any geometry that fixtures well and can be reached by a rotating tool
When not to
- High volumes of a simple shape: stamping, die casting or injection molding will be cheaper per part
- Deep, narrow internal cavities and undercuts a tool cannot reach: consider EDM, casting or 3D printing
- Sheet-metal parts: laser cutting and bending waste far less material
- Parts where most of the blank becomes chips (high buy-to-fly ratio) in expensive alloys: a near-net forging or casting saves material
Design tips
- Give internal corners a radius at least a third of the pocket depth, and larger than 1 mm (0.040"), so a standard end mill can cut them.
- Keep walls thicker than 0.8 mm (0.030") in metal and 1.5 mm (0.060") in plastic, and pockets shallower than four times the tool diameter.
- Tolerance only the features that matter and let ISO 2768-m or a general ±0.13 mm (±0.005") note cover the rest.
- Design so the part is cut in as few setups as possible: features on one or two faces cost less than features on five.
- Send a STEP file and a PDF drawing; the drawing carries tolerances, threads, finishes and the material spec, which the model cannot.
- Standardize thread sizes and hole diameters on common drills and taps.
- Ask for the material by spec (ASTM, AMS) and temper, and say whether a mill cert is required.
CNC Machining by material
Frequently asked questions
- How much does CNC machining cost?
- Shop rates in the U.S. run about $75-150 per hour for 3-axis milling and turning and $125-250 per hour for 5-axis and Swiss work, plus material and setup. A simple aluminum bracket is often $50-150 in singles and under $20 at a few hundred pieces. The Noramark cost estimator gives a ballpark from size, material and quantity.
- What tolerances can CNC machining hold?
- Any competent shop holds ±0.13 mm (±0.005") without being asked and ±0.025 mm (±0.001") on features the drawing calls out. Tighter than that means grinding, honing or temperature-controlled inspection, so tolerance only what the function needs.
- CNC machining or 3D printing?
- Machining gives better tolerances, surface finish and material properties, and it works in any machinable alloy. Printing wins for internal channels, lattices and organic shapes, and for a first look at a plastic part overnight. Many parts are printed first and machined for production.
- What files does a machine shop need?
- A STEP (or native CAD) model for the geometry and a PDF drawing for tolerances, threads, surface finish, material spec and any heat treatment or finish. Quantity, delivery date and inspection requirements go on the RFQ.
