Basic Knowledge of CNC Processing

CNC processing is subtractive. A computer numerical control system reads a program, drives servo motors, and moves a rotating cutting tool or a rotating workpiece along programmed paths. The tool bites into the stock and removes material until the remaining shape matches the CAD model. There is no mold and no forming die, so the first part and the ten-thousandth part come off the same program.

Three motions matter on any machine. Feed is how fast the tool or table advances, usually in mm/min. Speed is how fast the spindle turns, in rpm. Depth of cut is how much material each pass removes. These three values, plus the tool geometry, determine chip load and surface finish. Push feed too high and the tool breaks. Push speed too low and the material work-hardens, especially in stainless and titanium.

The chain from drawing to part is short but has hard stops. A CAD file becomes a CAM toolpath, the toolpath becomes G-code, and the G-code runs on a specific machine with a specific work envelope. If the part exceeds that envelope, the job moves to a larger machine or gets split into operations. That is the first thing to check, not the last.

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  Subtractive, not additiveMaterial is cut away, so internal features need tool access.
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  Same program, same partRepeatability comes from the code, not from operator skill.
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  Envelope firstPart size decides which machine can hold it.

A 3-axis mill moves X, Y and Z. The tool always approaches from one direction, so any feature on the side or underside needs a second setup. Two setups mean two datums, and two datums mean stacked tolerance. For a bracket with holes on three faces, that stack can eat the tolerance budget before the machine even starts cutting.

A 4-axis mill adds a rotary table, usually turning around X. This lets the part rotate while the tool stays put, so features on four sides can be cut in one setup. It suits shafts with flats, cylindrical parts with cross-holes, and housings with features indexed at 90°. The rotary table on our 4-axis mills is Ø400 mm, which sets the practical part size.

A 5-axis machine adds two rotary axes, so the tool can tilt relative to the part. This is not just about reaching five sides. Tilting lets a short, stiff tool reach deep pockets that a long 3-axis tool would have to reach with chatter. It also lets the tool flank cut a curved surface instead of point cutting it, which improves finish and extends tool life. Sixteen simultaneous 5-axis machining centers handle our complex work.

Mill-turn centers combine a lathe spindle and a milling spindle in one enclosure. A part that would need a lathe, then a mill, then a second lathe op can often be finished in one cycle. This is common for hydraulic fittings, connectors and small motor housings where concentricity between the turned bore and the milled flange matters.

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  3-axisFlat plates, simple pockets, one-face features.
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  4-axisShafts, cross-holes, four-sided indexing.
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  5-axisComplex contours, deep pockets, contoured surfaces.
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  Mill-turnParts mixing turned bores and milled features.

Tolerance is the allowed deviation from the nominal dimension. A drawing marked ±0.1 mm is loose; ±0.005 mm is tight. Tight tolerance is not free. It requires a capable machine, a stable setup, temperature control and more inspection. On a part with a 200 mm span, thermal expansion alone can move the dimension several micrometres between morning and afternoon. That is why we hold ±0.005 mm on the features that need it, not on every dimension on the sheet.

Surface finish is measured as Ra, the arithmetic average roughness. As-machined finish on aluminum and steel typically lands between Ra 1.6 and 3.2 μm. A finer cut with a smaller stepover and a sharp tool can reach Ra 0.8–1.6 μm. Achieving Ra 0.2–0.8 μm usually means a finishing pass with a small tool, slower feed, or a secondary operation such as grinding or polishing. Specify finish only where it matters, because a blanket Ra 0.4 μm callout on every face can add significant cost.

Cost in CNC processing scales with time on the machine. Time goes up when you add setups, when you specify tight tolerance over long distances, when you call for fine finish on large areas, and when the geometry forces small tools with slow feed rates. A deep pocket narrower than 3 mm needs a long, thin tool that must run slowly to avoid deflection. If you can widen that pocket or reduce its depth, the part gets cheaper without losing function.

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  Tolerance where neededApply tight tolerance to functional features only.
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  Finish where neededSealing faces and bearing surfaces justify fine Ra.
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  Access drives costDeep, narrow features need small tools and slow feeds.

Aluminum is the default for machined prototypes and many production parts. Grades 6061 and 6061-T6 cut fast, hold tolerance well, and take anodizing cleanly. 7075 is stronger and used for aerospace brackets, but it is less corrosion resistant and costs more. 2024 machines well but has poor weldability. The soft grades, such as 5052, tend to gum up on the tool if feeds and speeds are wrong.

Stainless steel is where feeds and speeds get serious. Grades 303 and 304 are common; 316 and 316L add corrosion resistance for medical and marine use. The austenitic grades work-harden, so a light pass with a dull tool can leave a harder skin that ruins the next pass. 17-4PH (SUS630) is a precipitation-hardening grade used for high-strength shafts and valves. It machines in the annealed state, then gets heat treated.

Steel grades 1018 and 1045 are straightforward. 4130, 4140 and 4340 are alloy steels used for shafts, gears and structural parts; they are tougher, so they cut slower. Titanium, especially TC4 (Ti-6Al-4V), has a low thermal conductivity, so heat stays at the cutting edge. That shortens tool life and demands lower speeds, more coolant and rigid setups. Inconel is harder still and is reserved for high-temperature applications where the cost is justified.

Plastics behave differently from metals. POM and ABS machine cleanly. PEEK holds strength at high temperature but is expensive and abrasive on tooling. Carbon fibre reinforced plastic wears tools quickly and produces dust that needs extraction. Acrylic (PMMA) can chip or craze if the tool rubs instead of cutting, so sharp tools and light passes matter.

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  AluminumFast, stable, anodizes well. Default for prototypes.
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  StainlessWork-hardens. Needs sharp tools and steady feed.
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  Titanium and InconelHeat stays at the edge. Slow speeds, rigid setup.
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  PlasticsWatch for chipping, melting and tool wear.

Design for the tool, not just the function. A corner radius equal to the tool radius lets the tool cut the corner in one smooth motion. A sharp internal corner forces a smaller tool, which must run slower and may leave a witness mark. Fillets on pocket floors reduce stress concentration and let a larger tool pass. Threads smaller than M2 are difficult to tap reliably; consider a pressed insert or a clearance hole instead.

Wall thickness matters. A thin floor under a deep pocket can deflect under cutting force, so the finished wall may be bowed or the surface may chatter. A general rule is to keep floor thickness at least 1 mm for aluminum and 1.5 mm for steel when the pocket is deep. If the design needs a thin wall, plan a roughing pass, a stress-relief pause if needed, and a light finishing pass.

Inspection closes the loop. We check raw material on receipt, monitor dimensions in process, and inspect 100% of parts before shipment. Reports are available on request. For first articles, a dimensional report on the drawing's critical features is the fastest way to confirm the process is stable before the run continues. If a feature is hard to measure, say so on the drawing; that conversation is cheaper than a rejected lot.

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  Match corner radii to tool sizeAvoids slow small-tool cuts and witness marks.
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  Keep floors thick enoughThin floors deflect and chatter during cutting.
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  Call out critical dimensionsInspection focuses where it matters.