CNC Machining 101: Beginner's Guide to How Parts Are Cut

• 1
  What actually happens inside a CNC machine
• 2
  What tolerance numbers mean on a drawing
• 3
  How material choice changes the cut
• 4
  Design rules that save money and rejects
• 5
  Where the cost of a CNC part comes from
• 6
  From CAD file to finished part: the steps
• 7
  When is a part a good fit for CNC machining?

A CNC machine does one simple thing: it moves a spinning cutting tool, or a spinning workpiece, along a path stored as coordinates. The CAD model is converted into toolpaths by CAM software, then posted as G-code. Each block of G-code is a position, a feed rate and a spindle speed. The machine reads it, the servos follow it, and metal comes off.

That is the whole loop. Everything else in CNC Machining 101 is a consequence of it. The tool has a fixed geometry, so it cannot reach into every corner. The tool bends under load, so deep cuts deflect. Heat builds up at the cutting edge, so feeds and speeds have to balance removal rate against tool life.

A three-axis mill moves X, Y and Z. A four-axis machine adds rotation around one axis, usually A. A five-axis machine adds a second rotary axis, which lets the tool tip stay normal to a curved surface. GreatLight runs 16 simultaneous 5-axis centers, 12 four-axis mills and 27 three-axis machines, plus 16 mill-turn centers that turn and mill in one setup.

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  3-axisFlat pockets, plates, brackets, simple profiles. One setup per face.
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  4-axisCylindrical parts with slots or flats, where rotation replaces a second op.
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  5-axisContoured surfaces, undercut features, and fewer setups on complex parts.
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  Mill-turnTurned bodies with milled features, held to one datum through the cycle.

A tolerance is not a wish. It is the window the process must hold, and every tightening step costs time. General machining holds about ±0.1 mm without much effort. At ±0.05 mm you start controlling tool wear and thermal drift. At ±0.005 mm, the machine, the fixture and the inspection method all have to be right, and the part usually needs temperature stability.

The trap for beginners is calling out tight tolerances on features that do not need them. A mounting hole that clears an M4 screw does not need ±0.01 mm. A bore that locates a bearing does. Put tight tolerance only where function demands it, and mark the rest as general.

The same logic applies to surface finish. As-machined finish sits around Ra 1.6–3.2 μm. A high-quality cut reaches Ra 0.8–1.6 μm, and fine finishing gets to Ra 0.2–0.8 μm. Each step down in roughness usually means a slower pass, a sharper tool or a secondary operation.

Geometry matters as much as the number. A deep pocket with a small corner radius needs a small tool, and a small tool deflects. If the drawing says R1 in a 40 mm deep pocket, the shop will either slow down a lot or ask you to open the radius.

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  ±0.1 mmGeneral machining. Brackets, covers, non-critical fits.
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  ±0.05 mmCommon precision work. Bores, slots, mating faces.
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  ±0.005 mmTight fits. Bearing seats, spigots, alignment features.

Aluminum is the default for prototypes. 6061 and 7075 cut fast, hold good finish and take anodizing well. 7075 is stronger but slightly more prone to chipping at the edge, so sharp corners benefit from a small chamfer or radius. If a part is going to be anodized, remember that hardcoat builds thickness and slightly shifts dimensions.

Stainless is where beginners get surprised. 303 machines cleanly and is the friendly choice for turned parts. 304 and 316 work-harden, so the tool has to keep moving; a dwell in the cut turns the surface hard and dulls the insert. 17-4PH adds strength after heat treatment, and it is common in aerospace and medical work.

Steel grades such as 1018, 1045 and 4140 cut predictably but need more spindle power and more rigid fixturing. Titanium, including Ti-6Al-4V, and Inconel sit at the difficult end: low thermal conductivity, high tool wear, and a strong case for finishing passes rather than heavy roughing.

Plastics are not a soft option. POM machines beautifully and holds tolerance. PEEK and carbon fibre are abrasive and expensive, so scrapping one part hurts. ABS and PC tend to melt and smear if feeds are too high, and thin sections can deflect under clamping pressure.

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  Easy6061, 7075, 303 stainless, brass, POM.
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  Moderate304, 316, 1045, 4140, 17-4PH, titanium.
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  DifficultInconel, magnesium, carbon fibre, PEEK.

A machined part has to be reachable. The tool is a cylinder with a finite length-to-diameter ratio, so a pocket 6× deeper than its corner radius is where things get slow and expensive. A good rule is to keep pocket depth under 4× the corner radius unless the feature is genuinely critical.

Internal corners cannot be square. A rotating cutter always leaves the radius of the tool, so a pocket floor corner will carry that radius. If a mating part needs a square corner, add a relief or change the corner to a radius and match it on the other part.

Threads need enough wall around them. A tapped hole close to an edge can bulge or crack, especially in plastics and thin aluminum. Thin floors deflect under cutting force, and thin walls vibrate. Both show up as chatter marks and out-of-tolerance dimensions rather than as a clean failure.

Text and logos should be simple. Small engraved detail fills with chips and reads poorly after finishing. Raised or engraved characters at 1.5 mm and above survive anodizing and plating much better.

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  Pocket depthKeep under 4× the corner radius where possible.
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  Wall thicknessAvoid below 0.8 mm in metal, 1.5 mm in plastic.
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  Corner radiiUse the largest radius the function allows.
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  EngravingMinimum character height 1.5 mm.

Beginners often assume cost scales with part size. It scales with time on the machine. A small part with a deep narrow pocket can take longer than a large flat plate. Setup, fixturing and inspection are fixed costs per order, so the first part carries most of the engineering effort.

Quantity changes the calculation. One prototype is dominated by programming and setup. At a few hundred parts, cycle time takes over. Above that, it is worth asking whether a different process, such as die casting, would be cheaper per unit.

Material is a smaller factor than most people expect for aluminum, and a much larger factor for titanium, Inconel and PEEK. Finish also adds cost: anodizing is routine, hardcoat and selective plating are not.

The cheapest part is the one that does not need a second operation. Designing features to come off in one setup, with a sensible tolerance callout and a finish that matches the function, removes more cost than any negotiation.

• 1
  SetupFixed per order. Fewer setups means lower cost.
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  Cycle timeDriven by material removal volume and tool paths.
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  ToleranceEach tightening step adds inspection and slower cuts.
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  FinishAnodizing is routine; hardcoat and plating add steps.