My first CNC experience: what actually happens inside the machine

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  Key takeaways
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  What the controller does while the tool turns
• 3
  Where the ±0.005 mm actually goes
• 4
  Why part shape decides the machine, not the tolerance
• 5
  Chip load, heat and the finish you get
• 6
  First article inspection before the batch runs
• 7
  Which machine class fits which part

My first CNC experience started with a file, not a machine. A CAM post wrote out a list of coordinates, feeds and spindle speeds. The controller reads those lines and moves a ball screw, which pushes a linear guide, which carries the spindle to a point in space. Every link in that chain adds a small error.

That is why a drawing tolerance is not a promise the machine can keep on its own. The machine repeats its own motion very well, often within a few micrometres when the ball screws and scales are healthy. What it cannot do is ignore the workpiece. A thin wall will deflect under cutting force no matter how rigid the machine is.

On a 5-axis center, two rotary axes tilt the tool or the table. The controller then blends five motions at once, so a curved surface is cut in one continuous pass instead of a series of stepped setups. GreatLight runs 16 simultaneous 5-axis machining centers alongside 12 four-axis mills and 27 three-axis machines, and the choice between them usually comes down to how many faces the part has, not to how tight the tolerance is.

The screen makes all of this look simple. It is not. Underneath, a servo loop is correcting position thousands of times a second, and thermal growth in the spindle is changing the tool tip position as the morning warms up.

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  Repeatability vs accuracyRepeatability is the machine hitting the same point twice; accuracy is it hitting the commanded point at all.
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  Servo loopPosition feedback closes the gap between the commanded coordinate and the real one.
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  Thermal driftA spindle that has run for an hour sits differently than one that just started.

A tolerance band is a budget, and several items draw from it. Raw stock varies. A 6061 plate may arrive 0.2 mm over nominal thickness, so the first face cut has to remove enough to clean up without eating the whole allowance. Clamping adds another draw. A vise that squeezes a thin section will spring it back when the jaws open.

Tool wear is the slow drain. A carbide end mill cutting 17-4PH stainless will lose edge sharpness faster than one cutting 6061 aluminium, and the diameter it leaves drifts with it. A finishing pass that measures on size at 8 a.m. may be 0.01 mm under by mid-afternoon unless the operator offsets the tool.

This is why we hold ±0.005 mm on features that are checked, and why we tell customers which features are worth that band. A mounting hole pattern with a dowel fit needs it. A clearance hole for an M6 screw does not, and spending the band there raises cost with no gain.

Temperature matters too. Aluminium expands about 23 μm per metre per degree Celsius. A 200 mm part measured hot on the machine and cold on the granite plate will not read the same. For tight work, let the part settle before the final measurement.

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  Stock allowanceLeave 0.3–0.5 mm per face for cleanup on castings and plate.
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  Clamping springSupport thin sections or rough and re-clamp before finishing.
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  Measure coldCompare part and gauge at the same temperature when the band is under 0.02 mm.

A part with features on one face is a three-axis job. Flat plate, drilled holes, a pocket, a counterbore. Setup is one vise, one zero point, and the operator can prove the first piece in minutes.

Add features on a second and third face and the story changes. Each new face means a new setup, a new zero, and a new chance to stack error. A four-axis mill with a rotary table handles the common case: a part that needs work around a single axis, like a shaft with cross holes or a housing with ports on four sides.

When the part has compound angles, curved pockets that wrap around a corner, or undercuts the tool cannot reach from any single direction, simultaneous 5-axis starts to pay. The tool stays short and stiff while the table tilts the work into it. That is the real benefit. Not tighter tolerance. Shorter tools, fewer setups, and better surface finish on sculpted forms.

There is a limit. Deep, narrow cavities in hard steel still favor a three-axis machine with a long reach tool and a rigid setup, because tilting a heavy part on a trunnion can introduce more deflection than it removes. Five-axis is a reach and setup solution, not a cure for poor part design.

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  One face, one setupThree-axis with a vise is the fastest route for flat, drilled parts.
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  Work around an axisFour-axis with a Ø400 mm rotary table covers shafts and ported housings.
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  Sculpted and wrapped featuresSimultaneous 5-axis keeps the tool short and avoids re-fixturing.

Surface finish is set by the chip, not by the last pass alone. A finishing cut with too light a chip load rubs the material instead of shearing it, and the tool edge wears a flat. The result is a torn surface and rising temperature. Feed per tooth has to stay above a minimum for the material and the tool radius.

Aluminium likes speed. A 10 mm three-flute carbide end mill in 6061 can run at 8,000 rpm or more with a healthy chip, and the heat leaves with the chip. Stainless 316 behaves the opposite way. It work-hardens if the tool dwells, so the feed must stay firm and the depth of cut must get under any hardened skin from a previous operation.

Titanium TC4 (Ti-6Al-4V) adds another constraint. Its low thermal conductivity keeps heat in the cutting zone, so the tool edge runs hot even at moderate speeds. Flood coolant and a sharp, positive geometry matter more than raw rpm.

For finish, we work to Ra 1.6–3.2 μm as-machined, Ra 0.8–1.6 μm on a controlled finishing pass, and Ra 0.2–0.8 μm where the drawing calls for it. Getting under Ra 0.8 μm on a milled face usually needs a separate finishing strategy or a secondary operation, and that should be a deliberate choice, not an assumption.

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  Do not rubToo light a chip load polishes the tool edge and tears the workpiece.
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  Match coolant to materialFlood for titanium and stainless; air blast often suits aluminium.
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  Plan the finishRa 0.2–0.8 μm needs its own pass, not a tweak to the roughing program.

The most expensive mistake in a first CNC run is letting it continue after the first part is wrong. So the first article is treated as a separate event. The part comes off the machine, gets cleaned, and is measured against the drawing before the second one is cut.

What gets checked depends on the drawing. Critical dimensions go on a CMM. Hole positions get checked with a pin gauge or an optical comparator. A thread gets a go/no-go gauge. Surface finish gets compared against a certified roughness specimen when the callout is tight.

GreatLight inspects 100% of parts before shipment, with a raw material check at goods-in, in-process monitoring during the run, and a final inspection at the end. Reports are available on request. The qualification rate across those checks runs at 99.99%.

For anyone going through my first CNC experience on their own bench, the lesson is the same at any scale. Measure early, measure the feature that matters, and write the number down. A part that was never checked is not a part that passed.

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  Stop at oneDo not release the batch until the first article reads on the drawing.
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  Check the right featureSpend the time on fits and datums, not on free surfaces.
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  Keep the recordA written result lets you compare parts across the run.