CNC machining fundamentals

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  CNC machining fundamentals: how a CAD model becomes a part
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  What sets tolerance, finish and repeatability
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  Milling, turning and 5-axis: which one fits the part
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  How material choice changes the cut
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  Design choices that decide whether the part is easy to machine
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  CNC machining fundamentals: matching process to part

CNC machining is subtractive. A rotating cutter or a single-point tool is driven along a path that a control computer calculates from your CAD model, and material that falls outside the final surface is removed as chips. Nothing is molded or grown. The shape exists only because everything around it was cut away.

Three things have to agree: the machine, the tool, and the program. The program is a list of coordinates and feed commands generated by CAM software. The machine moves those axes to within a few microns. The tool determines what geometry is physically reachable. If any one of the three is wrong, the part is wrong, and it is wrong in a way that shows up at final inspection.

The practical consequence is simple. Machining rewards parts whose features can be reached by a stiff tool from a known direction. Deep pockets, thin walls, sharp internal corners and features on five different faces all cost money because they force extra setups, longer tools, or both.

On our floor this runs on 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers, 16 mill-turn centers, 27 three-axis machines and a Ø400 mm rotary table. Maximum processing size reaches 4,000 mm.

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  SubtractiveChips leave; the part is what remains.
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  Program-drivenCAM output decides path, feed and speed.
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  Tool-limitedReach and stiffness set what is possible.

Tolerance is not a single machine number. It is the sum of machine positioning error, thermal drift, tool wear, fixture rigidity and how many times the part is re-clamped. A machine that holds ±0.005 mm on a warm spindle in one setup will not hold it across four setups with hand-loaded fixtures.

Surface finish follows cutting parameters more than the machine. A sharp tool, correct feed per tooth and enough coolant give Ra 0.8–1.6 μm as a matter of routine. Pushing to Ra 0.2–0.8 μm usually means a finishing pass with a smaller stepover, a different tool, or a secondary operation.

Repeatability is the part buyers rarely ask about and always feel. It is the ability to make part 500 as good as part 1. This is where in-process monitoring and a fixed inspection plan matter more than the peak spec sheet number.

We run raw material check, in-process monitoring and final inspection, and we inspect 100% of parts before shipment. Reports are available on request. Historical qualification rate on machined features is 99.99%.

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  Fewer setupsEach re-clamp adds stack-up error.
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  Thermal controlWarm-up and stable coolant cut drift.
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  Tool managementChange on wear, not on failure.
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  Finish passSeparate light cut for the final surface.

Turning suits parts that are mostly round: shafts, bushings, fittings, connectors, pistons. The workpiece spins and a single-point tool follows the profile. Diameter control is strong, and a mill-turn center can add cross-holes and flats without a second machine.

Three-axis milling covers prismatic parts with features on one or two faces. It is the cheapest and fastest option when the geometry cooperates. If a part needs holes on four sides, three-axis work becomes four or five setups, and each setup eats tolerance.

Five-axis machining solves that by tilting the tool or the table so more of the part is reachable in one setup. It also lets a short, stiff tool reach deep features at an angle instead of a long tool bending its way down. For parts with compound angles, deep cavities or tight position between faces, it is often the only route that holds ±0.005 mm.

The trade-off is programming time. Five-axis toolpaths take longer to prepare and verify, so it pays off on complex parts and repeat quantities. For a simple bracket, three-axis is the right answer and we will say so.

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  Round partsTurning or mill-turn.
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  Prismatic, few facesThree-axis milling.
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  Multi-face, deep pockets4-axis or 5-axis.
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  Large envelopesUp to 4,000 × 400 × 150 mm travel.

Aluminium is the default for prototypes and housings. Grades like 6061, 7075, 2024 and 6082 cut fast, hold tight tolerances well and take anodizing cleanly. 6061-T6 is the usual starting point when you have no reason to pick something else.

Stainless steels such as 303, 304, 316L and 17-4PH machine less freely. They work-harden, generate heat and wear tools faster, so feeds and speeds drop and cycle time rises. 303 is the free-machining grade; 316L is chosen for corrosion and medical use, and you pay for it in machining time.

Titanium and nickel alloys are a different category. TC4 (Ti-6Al-4V) and Inconel keep their strength at temperature, which is exactly why they are hard to cut. Tool life is short, rigidity requirements are high, and the process needs to be planned around heat removal rather than speed.

Plastics behave differently again. POM and ABS cut cleanly; PEEK and carbon fibre are abrasive and expensive. Soft plastics can deflect away from the cutter, so fixturing and sharp tooling matter more than spindle power.

We machine aluminium 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12; stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH; steels 1018, 1045, 4130, 4140, 4340 and A36; copper and brass C101, C103, C110, C27400, C28000 and C36000; titanium TA1, TA2 and TC4; plus Inconel, magnesium AZ31B and AZ91D, and plastics including ABS, PC, PMMA, POM, PA, PEEK, PP and HDPE.

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  AluminiumFast, stable, anodizes well.
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  StainlessWork-hardens; slower feeds.
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  TitaniumHeat-driven tool wear.
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  PlasticsDeflection and chip clearance matter.

Internal corners are the first thing to check. A cutter has a radius, so a square internal corner cannot be milled. Specify a corner radius at least equal to the tool radius you are willing to pay for. Small radii mean small tools, slow feeds and broken cutters.

Wall thickness is the second. Thin walls vibrate under cutting load, and vibration shows up as chatter marks and lost tolerance. If a wall is below roughly 1 mm in aluminium or 1.5 mm in steel, expect extra passes and possibly a support fixture.

Threads and holes follow the same logic. Deep holes need long drills that wander; a depth beyond about four times the diameter usually needs peck drilling, and beyond ten times the diameter the cost climbs sharply. Standard metric threads are cheap; unusual pitches mean special tooling.

Finally, decide what actually needs a tight tolerance. Marking a whole drawing ±0.005 mm when only two features are functional doubles the cost for no benefit. Datum the critical features, and let the rest run at general tolerances.

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  Corner radiiMatch the largest tool you can use.
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  Wall thicknessUnder 1 mm aluminium needs care.
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  Hole depthBeyond 4× diameter, cost rises.
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  Tolerance mapTight only where it functions.