Basic CNC Machining Service Guide

A CNC machine follows a program to cut material away from a solid block, bar or casting. The program controls the spindle position, feed rate and tool changes. Nothing about the shape is decided by hand during the cut, which is why one setup can repeat the same geometry across thousands of parts with almost no variation between them.

A basic service usually covers milling, turning, drilling, tapping and boring, plus the inspection and finishing steps around them. Milling uses a rotating multi-point tool against a stationary or slowly rotating workpiece. Turning spins the workpiece against a single-point tool. Many parts need both, and the choice of which operation comes first affects how you hold the part for the second one.

The limits come from tool access, not from the control system. A deep pocket with a small corner radius needs a small tool, and a small tool cannot reach far without deflecting. A hole on a side face needs either a second setup or a machine that can tilt the part. These are the questions worth answering before you send a drawing out for quote.

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  MillingFlat and contoured surfaces, pockets, slots, faces, bosses.
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  TurningCylindrical and conical forms, threads, grooves, bores.
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  Drilling and tappingHoles from a few tenths of a millimeter up to large bores.
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  InspectionCaliper, micrometer, gauge and CMM checks against the drawing.

A 3-axis mill moves in X, Y and Z only. The tool always approaches from one direction, so every face you need to machine must be reachable from that direction. Simple brackets, plates, housings and manifolds often run fine on three axes. If the part needs four or more faces cut, you either add setups or change machine type.

A 4-axis machine adds rotation around one axis, usually A. This is the practical choice for parts with features spaced around a cylinder: shafts with cross holes, couplings, valve bodies, and anything where you would otherwise need a rotary fixture and several re-clamps. One rotation lets you cut several faces in a single program.

A 5-axis machine adds a second rotary axis, so the tool can approach the part from nearly any direction. That removes setups and lets a single operation cut undercuts, drafted walls and compound angles. It also shortens the tool, because you tilt the tool instead of reaching deep. Shorter tools deflect less, which helps hold ±0.005 mm on tall features.

Five axes is not automatically better. For a flat plate with holes, a 3-axis machine is faster to program and cheaper to run. Use five axes when the geometry genuinely needs the reach, for example an impeller, a turbine blade, a medical implant with organic surfaces, or a part that would otherwise need four separate fixtures.

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  Choose 3-axisFlat parts, through features, one dominant face.
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  Choose 4-axisRound parts with features around the diameter.
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  Choose 5-axisCompound angles, undercuts, contoured surfaces.

General machining tolerance is ±0.005 mm on critical dimensions when the drawing calls for it. Not every dimension needs that. A ±0.1 mm tolerance on a clearance hole costs far less to hold than ±0.005 mm, and putting tight limits only where they matter keeps the part affordable.

Surface finish is quoted as Ra. As-machined surfaces typically land between Ra 1.6 and 3.2 μm. A finer pass, slower feed or a finishing tool can reach Ra 0.8–1.6 μm. Below that, Ra 0.2–0.8 μm, usually means a secondary operation such as lapping, polishing or fine turning, not just a different cutter.

Two rules save rework. First, do not stack tight tolerances across a long dimension chain, because each link adds error. Second, give a datum that a machine can actually reach in the first setup. If the datum only exists after the part is flipped, the second operation inherits the first one’s error.

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  General±0.005 mm on critical features, ±0.1 mm on clearance.
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  As-machinedRa 1.6–3.2 μm, normal for most structural parts.
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  Fine finishRa 0.8–1.6 μm, requires a finishing pass.
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  Mirror finishRa 0.2–0.8 μm, needs polishing or lapping.

Aluminum is the default for prototypes and low-volume parts. 6061 and 6061-T6 machine cleanly, hold tolerance well and take anodizing. 7075 is stronger but harder on tooling, so it suits aerospace brackets and structural fittings rather than large thin walls. 2024 is common where fatigue matters, but it needs care because it is less corrosion resistant.

Stainless 303 and 304 are the usual choices for corrosion resistance. 303 machines more freely because of added sulfur, while 304 gives better weldability and food-contact behavior. 316 and 316L are for marine and medical work. 17-4PH can be heat treated after machining to reach high strength, which is useful for shafts and valve parts.

Steel grades 1018 and 1045 cover general shafts and fixtures. 4130, 4140 and 4340 are for loaded parts such as pins and gears. Titanium TC4 (Ti-6Al-4V) and Inconel demand slower speeds, more tool changes and more rigid setups, so they raise cost per part. Plastics such as POM, PEEK and PC machine quickly but move with temperature, so hold them to a realistic tolerance.

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  Light and fast6061-T6 aluminum for housings, brackets and prototypes.
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  Corrosion resistance303, 304, 316L stainless for wet or medical use.
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  High strength4140, 4340, 17-4PH for loaded mechanical parts.
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  High temperatureTC4 titanium and Inconel for hot or aero sections.

Finishing changes both appearance and function. Anodizing adds a hard, corrosion-resistant layer and comes in clear, colored, hardcoat and conductive versions. Electroless nickel gives a uniform coating on complex shapes. Powder coating and black oxide cover steel parts. Bead blasting, tumbling and brushing remove tool marks; polishing goes further. Laser marking needs at least 1.5 mm character height to stay legible.

Inspection has to match the part. A one-off prototype might be checked with calipers and micrometers against the drawing. A production run needs in-process monitoring so a drifting dimension is caught before the whole batch is cut. We run raw material checks, in-process checks and a final inspection, and we inspect 100% of parts before shipment. Reports are available on request.

When you read a quote, look at what is included. Does it list the material grade, the finish, the tolerance class and the inspection method? Does it flag features that will need a second setup or a special tool? A quote that only gives a number tells you little. A quote that names the setup count tells you where the cost sits.