CNC Machines and Workflow

What a CNC machine does

CNC stands for computer numerical control. A controller reads a program of coordinates and commands and drives motors that move a cutting tool, or the workpiece, along exact paths. Because the program is a file, the machine can cut the same part again and again, hold tight tolerances, and make shapes that are hard to cut by hand. Most CNC machines are subtractive: they start with a block, bar or sheet and remove material until the part is left. This guide concentrates on milling, routing and turning; for lasers see Laser Cutting, and for adding material see 3D Printing.

Machine types

Machine	How it cuts	Typical materials	Typical accuracy	Notes
CNC router	Spinning cutter on a gantry, usually 3 axes	Wood, MDF, plastics, foam, composites, soft aluminum	±0.05–0.2 mm	Large work area, fast, flexible frame; dusty; less rigid than a mill
Milling machine (vertical or horizontal)	Spinning cutter on a rigid column; table moves	Aluminum, steel, stainless, titanium, brass, plastics	±0.01–0.05 mm	Rigid and accurate; high torque; coolant; tool changers on bigger machines
Lathe (turning center)	The work spins; a tool moves in X and Z	Round parts in metal and plastic	±0.01–0.025 mm	Fast for shafts, pins, bushings, spacers; the cheapest way to make round parts
Mill-turn / Swiss	Lathe with live (spinning) tools and sub-spindle	Small, complex round parts in metal	±0.005–0.02 mm	Finishes a part in one set-up; used for production
Wire or sinker EDM	Electrical sparks erode the metal	Any conductive metal, including hardened steel	±0.002–0.01 mm	No cutting force; slow; for sharp corners, hard materials
Laser, plasma, waterjet	Beam, plasma or abrasive water cuts sheet	Sheet and plate	varies	Flat profiles; see Laser Cutting for lasers

The accuracies shown are typical for a well-maintained machine and a competent operator; they depend on the setup, the tool and the part. The accuracy a shop will quote is almost always a tolerance of around ±0.1 mm for ordinary work.

Axes and zero points

• 3-axis: X, Y and Z. Cuts everything the tool can reach from the top. The great majority of milling and routing.
• 4-axis: a rotary axis (A) turns the part around X, so the tool can reach several sides, or cut wrapped features such as a helix, in one setup.
• 5-axis: two rotary axes. 3+2 machining tilts the part to a fixed angle, then cuts in 3 axes. Simultaneous 5-axis moves all five together for impellers, blades and sculpted surfaces. Much more complex to program and set up.
• Machine zero and work zero. Machine zero (home) is a fixed point the controller finds with limit switches. Work zero (G54) is a point on the part, such as a corner or the center, which you set so the program is written relative to the part, not to the machine. See G-code and Programming.
• Convention. +Z moves the tool away from the work, so cutting happens at negative Z. On a lathe, X is a diameter on most controls (the tool moves half as far as the number changes), and Z along the spindle axis, negative toward the chuck.

Cutting basics: speed, feed and removal rate

Every cut is set by a few numbers. Cutting speed V_c is how fast the cutting edge moves past the material (m/min); it sets the spindle speed for a given tool diameter. Feed per tooth f_z is how far the part advances while each tooth cuts: it sets the chip thickness. Together they give the feed rate. The depths of cut, axial a_p (depth) and radial a_e (width), set how much material each pass removes.

n = 1 000 × V_c ÷ (π × D)    V_f = n × z × f_z    MRR = a_e × a_p × V_f    P_c = a_e × a_p × V_f × k_c ÷ (60 × 10⁶)

Here n is the spindle speed in rpm, D the tool diameter in mm, z the number of flutes, V_f the feed rate in mm/min, MRR the material removal rate in mm³/min, k_c the specific cutting force of the material in N/mm², and P_c the power at the cutter in kW.

• Start with the tool maker’s data and adjust: see Speeds & Feeds and the CNC Feeds & Speeds Calculator.
• Too little feed rubs instead of cutting. The edge slides, heats and wears. Low feed is a common cause of tool wear and of burn marks on wood and plastic.
• Power is rarely the limit on a small machine; rigidity is. A hobby spindle with a few hundred watts will chatter or stall long before the formula says it should.

From design to finished part

Step	Check	Common error
Design	Inside corners have a radius, features are reachable, walls are thick enough	A design made for a 3D printer: sharp inside corners, deep narrow pockets
Plan	Stock is large enough, the part can be held, set-ups are few	No way to hold the last operation without damaging the part
CAM	Tool, speeds and depths suit the material and the machine	Cutting data copied from a bigger machine
Post	Correct post for the controller, units and arcs	Inch program on a metric machine, or the wrong arc format
Simulate	Tool holder and clamps clear the part	Holder hits a clamp that the simulation did not show
Run	Work zero and tool offsets are right; air-cut first	Wrong Z zero: the most common crash

Choosing a process and a machine

If your part is…	Consider	Why
Round (shafts, pins, bushings, spacers)	Lathe	Turning is faster and more accurate for round shapes than milling them
Prismatic with pockets, holes and flats in metal	Mill	Rigid, accurate, with the right tools for metals
A large sheet part in wood, plastic or soft aluminum	Router	Large work area and fast cutting of soft materials
Flat, thin sheet profile	Laser or waterjet	No cutting force and fast profiles
Complex shape with internal channels or lattices	3D printing	Cannot be machined; see 3D Printing
One or two prototypes of a simple part	Printing, or hand tools	CNC set-up time can exceed the cutting time for single parts
Hundreds or thousands of identical parts	CNC with fixtures	Set-up cost is spread; cycle time and tool life become important

Hobby machines and production machines

	Hobby / desktop	Industrial
Frame	Aluminum extrusion or steel tube; light	Cast iron or welded steel; very stiff
Spindle	Router (0.5–2.2 kW) or small spindle, 10 000–24 000 rpm	Spindle (7–30 kW), torque at low speed; often 8 000–12 000 rpm, geared or direct
Drives	Stepper motors, belts or lead screws	Servos, ball screws, linear guides
Cuts well	Wood, plastics, foam, brass, aluminum with small cutters	All metals, including steel, stainless and titanium
Typical depth of cut in aluminum	0.5–2 mm, small cutters	5–20 mm or more
Limits	Rigidity and spindle torque, not control accuracy	Cost, space, power and skill

• Choose the cut to suit the machine. A hobby machine can cut aluminum well with a small cutter, shallow passes and fast feed; the same machine will chatter or stall in steel.
• Resolution is not accuracy. A step of 2.5 µm on the drive does not mean 2.5 µm parts; the frame, the tool and the setup decide the result. See G-code and Programming for steps per mm and Troubleshooting, Accuracy and Safety.
• Learn on scrap. Wood and plastic are cheap, forgiving and let you learn workholding, zeroing and CAM before you try metal.

Related

• CNC Machining: the overview of this guide.
• Milling: Operations and Tooling and Turning: Lathes and Operations: the two main processes in detail.
• Designing for CNC Milling: drawing parts that machine well and cost less.
• Speeds & Feeds and the Feeds & Speeds Calculator.

FAQ

What is the difference between a mill and a router?

Both spin a cutter, but a mill is built stiff, heavy and slow-moving for metal, with high torque, while a router moves fast over a large, lighter frame and is built for wood, plastics and composites. A router can cut soft aluminum with small cutters, but it cannot take the heavy cuts a mill can.

Can I make parts on a CNC machine without knowing G-code?

Yes. CAM software writes the G-code from your CAD model. Learning to read G-code still helps you check programs and fix problems; see G-code and Programming.

How accurate is a CNC machine?

The controller can position to a few micrometers, but the part is only as accurate as the machine’s stiffness, the tool, the set-up and the temperature. Hobby machines typically hold ±0.1 mm; good mills ±0.01–0.025 mm.

Should I buy a CNC machine or send the parts out?

For a handful of parts, ordering from a shop is cheaper and faster. A machine pays off when you make parts often, iterate designs quickly, or want to learn the process. Allow for tooling, workholding, software, dust and noise control, and the time to learn.

Do I need coolant?

For plastics and wood, no (use air). For aluminum, a mist or air blast clears chips. For steel, stainless and titanium, flood or mist coolant extends tool life significantly. See Machining Metals.

References

• Oberg, E. et al., Machinery’s Handbook, 31st ed., Industrial Press: cutting speeds and feeds, milling and turning formulas, tolerances.
• Sandvik Coromant, Metal Cutting Technology: Formulas and Definitions (spindle speed, feed, removal rate and power formulas).
• NIST, The NIST RS274NGC Interpreter, Version 3 (the reference definition of G-code).
• Kalpakjian, S. and Schmid, S., Manufacturing Engineering and Technology, Pearson.
• Smid, P., CNC Programming Handbook, Industrial Press.

Last reviewed: October 2026. Cutting data are starting points: machines, tools and materials differ, so test on scrap and follow the tool maker’s data.