CNC Basics Video Guide: How Metal Becomes a Finished Part

• 1
  What a CNC Basics Video Guide Leaves Out
• 2
  CAD, CAM and G-code: The Three Files Behind Every Cut
• 3
  Roughing, Finishing and Why Setup Counts More Than Speed
• 4
  3-Axis, 4-Axis and 5-Axis: What the Extra Axes Buy You
• 5
  Material Behaviour and Where the Process Stops Working
• 6
  Tolerances, Inspection and What the Numbers Mean
• 7
  Which Machine Class Fits Your Part

Most CNC basics video guide clips start with a spinning tool and a block of aluminium. That is the easy part to film. The part they usually skip is the decision chain before the spindle turns: which stock size, which workholding, which tool reaches the feature, and where the tool has to stop. Those four choices decide cost and lead time far more than spindle speed does.

Subtractive machining removes material with a rotating cutter that follows a programmed path. The cutter diameter sets the smallest internal corner you can get. A Ø6 mm end mill leaves a 3 mm corner radius at best. If your drawing calls for a sharp internal corner, someone has to cut it with a smaller tool, or the corner has to be relieved by another process such as EDM.

That single fact explains a lot of quoting. Deep pockets, thin walls and tight corners all push the tool smaller, and a smaller tool has to run slower and take lighter passes. The machine is not the limit. The cutter is.

A video can show a 4,000 mm travel machine cutting a large frame. It rarely shows that the same part could be split into three pieces and bolted together for less money. That trade-off is where a real shop adds value.

The chain has three stages. CAD defines the geometry. CAM turns that geometry into toolpaths, choosing tools, stepover, feed and speed. The post-processor then writes G-code, the instruction list the controller reads. G-code is not a design file. It is a set of moves, and every move has a cost.

G-code lines do a small number of things: move to a position, set a feed rate, start or stop the spindle, change a tool, turn coolant on or off. A typical finishing pass on a 300 mm aluminium plate can run to tens of thousands of lines. The controller reads them in order, and one wrong coordinate ruins the part.

This is why CAM decisions matter more than most people expect. The same model can be programmed with a continuous contour pass or a series of plunge cuts. The first leaves a smooth wall. The second leaves witness marks and takes longer. Both are technically correct G-code.

Tolerance drives the choice. Holding ±0.005 mm on a bearing bore calls for a different strategy than holding ±0.1 mm on a bracket face. On our 5-axis centers we usually finish critical bores in a single continuous pass, then measure before the part comes off the fixture.

Machining splits into roughing and finishing. Roughing removes the bulk of the stock with a large tool and heavy passes. Finishing brings the surface to size with a smaller stepover. Trying to do both in one pass is a common mistake in beginner programming. It either breaks tools or misses the tolerance.

Roughing leaves around 0.3 to 0.5 mm of stock for finishing on most aluminium work. On stainless and titanium the allowance is smaller, because the material work-hardens and the finishing tool needs less to bite. Feed and speed come from the material and the cutter coating, not from a single chart that fits everything.

Setup is the silent cost. Every time the part is unclamped and rotated, position error creeps in and the operator spends time re-datuming. A part with features on five faces needs either several setups on a 3-axis machine or one setup on a 5-axis machine. That is the real reason 5-axis exists.

Surface finish follows the same logic. A fine finish of Ra 0.2–0.8 μm needs a light stepover and a sharp tool. An as-machined finish of Ra 1.6–3.2 μm can come from a heavier pass. Specify the finish you actually need, because polishing a whole part to a mirror adds cost with no functional gain.

A 3-axis machine moves the tool in X, Y and Z only. The part stays in one orientation. For flat plates, simple brackets and parts with features on one face, that is enough and it is the cheapest route. Most of our 27 three-axis machines run exactly this kind of work.

A 4-axis machine adds rotation around one axis, usually A. This lets the tool cut around a cylindrical part without re-clamping. Shafts, cam profiles and parts with holes on several sides of a prism fit here. We run 12 four-axis mills for that middle ground.

A 5-axis machine moves along three linear axes and two rotary axes at the same time. The tool can approach a surface from almost any direction in one setup. That matters for impellers, turbine blades and any part where a deep undercut or a compound angle would otherwise need three fixtures. We have 16 simultaneous 5-axis centers.

The extra axes are not free. Programmers need more experience, the machine costs more per hour, and verification takes longer. If a part can be made on 3 axes with two setups, that is often the better commercial answer, even if a video makes 5-axis look like the default.

Aluminium alloys such as 6061, 7075 and 6082 cut fast and hold a good finish. Stainless 304 and 316 work-harden, so the tool must keep moving and never rub. Titanium TC4 (Ti-6Al-4V) conducts heat poorly, which means most of the heat goes into the cutter, and tool life drops. Inconel is harder again and needs low speeds and rigid setups.

Plastics behave differently from metals. POM and ABS cut cleanly but can melt if the feed is too slow. PEEK holds dimension well but costs more than aluminium. Carbon fibre is abrasive and eats tool edges, so we plan for more tool changes on those jobs.

There are parts CNC should not touch. A hollow shell with 1 mm walls and complex internal channels is usually better as a casting or an additive part. A run of 50,000 identical simple brackets is often cheaper as a die casting. Knowing when to say no is part of the job.

Where CNC wins is the middle: tight tolerance, moderate volume, and geometry that has to be functional rather than decorative. That covers most aerospace brackets, medical housings, robotics joints and EV components we see.

A tolerance of ±0.005 mm is roughly ±0.0002 in. On a metal part that is a real constraint, not a marketing line. It requires temperature control, sharp tooling and a measuring plan. If your drawing says ±0.05 mm, we can use a faster strategy and the part costs less.

Inspection should match the tolerance. Calipers are fine for ±0.1 mm. For ±0.005 mm you need a coordinate measuring machine and a controlled environment. We inspect 100% of parts before shipment, covering raw material check, in-process monitoring and final inspection, with reports on request.

Certifications tell you which systems are in place. We hold ISO 9001:2015, IATF 16949:2016 for automotive work, ISO 13485:2016 for medical devices and ISO 27001:2022 for information security. Those cover process control and data handling, not a guarantee about any single part.

If a video tells you a machine can hold any tolerance you ask for, it is skipping the measurement side. Tolerance is a paired number: the dimension plus the method used to verify it.