Start the CNC Milling Course

• 1
  How a CNC mill actually removes metal
• 2
  Why axis count changes what the course can teach
• 3
  Setup decisions that decide the tolerance
• 4
  What a mill needs from a CAD model
• 5
  How material choice reshapes the cut
• 6
  A first course project, step by step
• 7
  Which milling route fits which part

A CNC mill is a subtractive machine. A spindle holds a rotating cutter, and a control system moves either the tool or the table along programmed axes. The cutter enters solid stock and shears away material in chips. Nothing is formed or added. The final geometry is whatever remains after every pass has been subtracted.

The cutting edge does the work, but the control decides where and how fast. G-code tells the machine the tool position, feed rate, spindle speed, and coolant state at each moment. The machine then closes a servo loop on each axis to hold that position. Accuracy comes from the loop, not from the operator's hand.

Three numbers govern every cut: surface speed, feed per tooth, and radial or axial depth of cut. Change one and the other two usually need to change. Push surface speed too high in aluminum and the edge welds. Push it too low in stainless and the tool rubs and work-hardens. The course material is mostly about reading those relationships, not memorizing a single speed chart.

• 1
  Subtractive, not additiveChips leave the cut; nothing is deposited.
• 2
  Servo loop holds positionThe control corrects axis error continuously.
• 3
  Three variables interactSpeed, feed, and depth of cut move together.

A 3-axis mill moves X, Y, and Z. The tool always approaches from one direction, so any feature on the side of a part needs a second setup or a re-fixture. That is fine for plates, brackets, and housings with features on one or two faces. It becomes slow when a part has angled ports or undercuts.

A 4-axis mill adds a rotary table, typically Ø400 mm on our machines. The part turns while the tool cuts. This is the natural fit for shafts, impellers, and parts with features spaced around a bore. One setup replaces three or four, and the angular position stays tied to a single datum.

A 5-axis machine tilts the tool as well as the table. Short tools reach deep pockets without long overhangs, so chatter drops and surface finish improves. It also machines compound angles in one pass. The trade-off is programming time and simulation cost, which only pay back when the geometry is genuinely complex.

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  3-axisFlat plates, single-face features, simple pockets.
• 2
  4-axisShafts, rotary features, features around a bore.
• 3
  5-axisCompound angles, deep cavities, short-tool access.

Most out-of-tolerance parts are not cut wrong. They move. A vise with 2 mm of jaw lift, a thin floor that springs under clamp pressure, or a fixture bolted to a table that has not been indicated will all shift the part between roughing and finishing. The course teaches you to check the setup before the first cut.

For thin walls, reduce radial engagement and take the finish pass in two light cuts. For long parts, support under the cut, not just at the ends. For a second operation, locate off a machined datum rather than the raw stock edge. These choices often matter more than the machine's rated accuracy.

Thermal drift is the quiet one. A spindle that has run for 20 minutes is longer than a cold one. On a ±0.005 mm job, let the machine warm up and keep the finishing pass in the same thermal window as the measurement. We inspect 100% before shipment, but the operator has to give the part a chance first.

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  Indicate the fixtureCheck runout before trusting the setup.
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  Light finish cutsTwo passes beat one heavy pass on thin walls.
• 3
  Warm-up mattersSpindle growth shifts Z on tight jobs.

Programming starts from a 3D model, not a sketch. STEP (.stp), IGES (.igs), Parasolid (.x_t), and SolidWorks (.sldprt) all carry solid geometry that CAM software can offset into toolpaths. A PDF or DWG/DXF drawing still matters because it carries the things a model often does not: critical dimensions, tolerances, surface finish callouts, and material notes.

The drawing and the model must agree. When a model shows a 6 mm fillet but the drawing calls for a sharp corner, the machinist has to ask. That question costs minutes. Guessing costs a scrapped part. Send both whenever a feature is tolerance-critical.

For the first prototype, a single 3D model plus a marked-up drawing is enough for us to return a DFM review within 12 hours. We flag thin floors, deep pockets with small corner radii, and tolerances that force a second setup. Catching those before programming is cheaper than catching them at inspection.

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  Send a solid modelSTEP, IGES, Parasolid, or SolidWorks.
• 2
  Send the drawing tooIt carries tolerances and finish calls.
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  Flag critical featuresMark what must hold and what is free.

Aluminum 6061-T6 cuts fast and holds a good finish, which is why it dominates prototypes. It also moves under heat and can gum on a dull edge. Stainless 304 and 316 work-harden if the tool rubs, so the feed has to stay heavy enough to stay under the hardened layer. Titanium Ti-6Al-4V (TC4) burns tools when coolant and speed are wrong.

Harder and gummier materials force slower surface speeds, lighter depths of cut, and more frequent tool changes. That shows up in cost, not just time. A part in Inconel may need three times the machining hours of the same part in 6061, even with the same toolpath strategy.

Plastics bring a different problem. POM and PEEK cut cleanly but melt if the chip cannot clear. ABS and PC soften and smear. The fix is sharp tooling, high spindle speed, and air blast rather than flood coolant. Material choice is not a footnote in the course. It changes every number on the setup sheet.

• 1
  AluminumFast cutting, watch heat and chip welding.
• 2
  Stainless and titaniumKeep the feed heavy, avoid rubbing.
• 3
  PlasticsSharp edges and air blast, not flood coolant.