glcncmachining CNC: How the Cut Works and Where It Stops

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  Key takeaways
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  What actually happens at the cutting edge
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  Where tolerance and surface finish hold
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  3-axis, 4-axis, 5-axis, or mill-turn
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  Design choices that change the cut
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  How material changes the process
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  Choosing the right machine setup

glcncmachining CNC is a subtractive process. A computer converts a CAD model into tool paths, and a spindle drives a rotating cutter through the stock. The material does not melt away. It shears off in chips along a shear plane ahead of the cutting edge.

Three things control that shear: surface speed, feed per tooth, and depth of cut. Get them right and chips break cleanly. Get them wrong and you get built-up edge, chatter, or a burnt surface. The machine only follows the numbers you give it.

Heat is the real limit. Most of the heat leaves with the chip, not the part. That is why climb milling and a good coolant flow matter more on stainless and titanium than on aluminium. On 6061 with sharp tooling, you can push aggressively and still hold Ra 1.6–3.2 μm as machined.

The cutter also defines the shape. A Ø10 mm end mill leaves a 5 mm corner radius at best. Sharp internal corners require a smaller tool, EDM, or a designed relief. This is a geometry constraint, not a machine limitation.

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  Shear, not abrasionChips form by plastic deformation ahead of the edge.
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  Heat leaves with the chipCoolant and climb milling keep the part cool.
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  Tool radius sets corner radiusSharp inside corners need a different process.

A tolerance is a budget, not a promise. Machine positioning, fixture rigidity, tool wear, and thermal drift all consume part of that budget. On a stable setup with a rigid fixture and a pre-set tool, ±0.005 mm (±0.0002 in) is repeatable in aluminium and mild steel.

Thin walls and long slender parts change the picture. A 1 mm wall on a 100 mm aluminium plate will deflect under cutting force, no matter how good the machine is. Support it with soft jaws or reduce radial engagement.

Surface finish follows the same logic. Ra 0.2–0.8 μm needs a finishing pass with a sharp tool, light depth of cut, and a rigid setup. Ra 0.8–1.6 μm is a normal production finish. Ra 1.6–3.2 μm is as-machined and often good enough for non-sealing faces.

Materials behave differently. 6061-T6 and 7075 cut cleanly and hold tight tolerances. 316L work-hardens, so a light finishing pass can rub instead of cut. Inconel and Ti-6Al-4V push tool wear hard and need conservative parameters.

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  Rigid setup firstFixture stiffness matters as much as machine accuracy.
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  Wall thickness limitsThin walls deflect. Support or reduce radial load.
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  Finish needs a finishing passLight depth of cut and a sharp tool.

3-axis milling cuts from one direction. It is fast, rigid, and cheap for plates, brackets, and housings with features on one face. If your part can be reached from three orthogonal directions, 3-axis is usually the right call.

4-axis adds a rotary table, so the part rotates around one axis. This suits cylindrical parts with slots, flats, or holes around the circumference. You cut four faces in one setup instead of four.

5-axis moves the tool or table on two extra rotary axes. It reaches undercuts, angled faces, and deep pockets without re-fixturing. That matters for impellers, turbine components, and complex medical housings. It also costs more per hour, so use it where the geometry demands it.

Mill-turn combines turning and milling in one machine. Parts with a turned body and milled features, like shafts with cross-holes, avoid a second setup and the error that comes with it. With 16 simultaneous 5-axis centers and 16 mill-turn centers on our floor, we can match the process to the part rather than forcing the part onto one machine.

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  3-axis for prismatic partsFastest and most rigid for single-face work.
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  4-axis for cylindrical featuresOne rotary axis cuts four faces in one setup.
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  5-axis for complex geometryUndercuts and angled faces without re-fixturing.

The cheapest part is the one that needs fewer setups. Every re-fixturing adds a datum shift and a chance for error. If you can reach all critical features from one direction, you cut cost and improve repeatability at the same time.

Corner radii are the most common issue we see. A pocket with a 0.5 mm internal corner forces a small tool, which means slower feed and more tool breakage. Open the corner to at least one third of the pocket depth, or add a relief, and the part gets easier.

Thread depth matters too. A thread needs about 1.5 times its diameter in engagement to reach full strength. Deeper than 2 times the diameter adds risk without much benefit. Specify the thread class you actually need, not the tightest one available.

Datums should be accessible. If your primary datum is on a face that gets machined last, the inspector cannot measure from it reliably. Put datums on surfaces that are cut early and stay stable.

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  Fewer setups, tighter resultEach re-fixturing adds datum shift.
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  Open internal cornersOne third of pocket depth or more.
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  Thread depth 1.5 × diameterDeeper adds risk, not strength.

Aluminium is the default for prototypes and many production parts. 6061-T6 machines fast, holds ±0.005 mm, and takes anodizing well. 7075 is stronger but less corrosion resistant. 2024 cuts well but needs care if it will see moisture.

Stainless steels split into two groups. 303 and 304 machine cleanly. 316L and 17-4PH work-harden, so keep the tool engaged and avoid dwelling. A rubbing pass on 316L will harden the surface and ruin the next cut.

Titanium and Inconel are where parameters matter most. Ti-6Al-4V has low thermal conductivity, so heat stays in the cut zone. Use slower surface speed, generous coolant, and sharp tools. Inconel is worse: it work-hardens and wears tools quickly. Expect more passes and higher cost.

Plastics behave differently again. POM and PEEK cut cleanly but can melt if the feed is too low. ABS and PC are soft and prone to burrs. Carbon fibre is abrasive and needs diamond-coated tooling. The material choice often decides the finish and the tolerance you can hold.

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  Aluminium holds tight tolerance6061-T6 is the workhorse for ±0.005 mm.
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  Stainless work-hardensKeep the tool engaged, avoid dwelling.
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  Titanium traps heatSlow speed, sharp tool, lots of coolant.