CNC machining work: how metal actually gets removed
This page explains what happens inside the machine, how to pick an axis count, and where the process stops being the right answer. It is written for design engineers and buyers who need to judge a part before it goes to quote.
What CNC machining work really is
CNC machining work is subtractive. A computer reads a toolpath, a spindle spins a cutter, and material leaves the block as chips. Nothing is formed or added. The final geometry is whatever the cutter could reach minus whatever the fixturing allowed.
That last sentence explains most quality problems. A part that looks simple on screen can be hard to hold, hard to reach, or hard to measure. The machine does not care about the drawing. It only follows the code and the setup.
The work splits into three jobs that happen at different times: programming, setup, and cutting. Programming decides the strategy. Setup decides whether the part repeats. Cutting is the shortest of the three on a stable job.
For buyers, the practical question is not which machine runs the part. It is whether the shop can hold the same result on part one and part five hundred. That is a setup and inspection question more than a spindle question.
How a cutter removes material
Every cut is a controlled fracture. The cutting edge presses into the workpiece until the material shears away along a plane. Soft aluminium shears cleanly at high surface speed. Titanium and Inconel resist, and the heat has nowhere to go but into the tool.
Chip load is the number that keeps a cut stable. Feed per tooth times number of teeth times spindle speed gives the advance rate. Too light a chip rubs instead of cutting, which work-hardens stainless and burns the edge.
Heat splits between chip, tool, and part. A generous chip carries most of it away. That is why roughing passes often run harder than finishing passes, and why coolant choice matters more in deep pockets than on open faces.
Rigidity sets the ceiling. A long tool in a deep cavity flexes, and the wall it leaves is neither straight nor parallel. If a feature needs a long reach, expect to slow down or add a support setup.
3-axis, 4-axis, or 5-axis for your part
Three-axis work covers parts with features open to one direction. Prismatic housings, plates, and brackets fall here. Each new face means a new setup, and each setup adds a chance for position error.
A fourth axis rotates the part about one axis, usually A or B. Shafts, cylinders, and parts with holes on a bolt circle become one-setup jobs. The rotary table also lets the tool cut continuously around a diameter instead of indexing face by face.
Five simultaneous axes tilt both the part and the tool. Undercuts, deep cavities, and contoured surfaces that would need four separate setups can be reached in two. On a Ø400 mm rotary table, a single 5-axis setup often replaces three fixtures.
The trade-off is not free. Five-axis toolpaths take longer to program and verify, and setup skill matters more. Use it where the geometry demands it, not as a default.
Where tolerance and finish come from
Tolerance is a system result, not a machine spec. Spindle runout, thermal growth, fixture stiffness, and tool wear all move the same direction on a bad day. A shop that holds ±0.005 mm does it with process control, not with a single good machine.
Finishing passes set surface texture. A light radial stepover with a sharp carbide tool reaches Ra 0.8–1.6 μm on aluminium and steel. Getting below Ra 0.8 μm usually means a dedicated finishing tool, higher spindle speed, and a slower feed.
As-machined surfaces sit around Ra 1.6–3.2 μm and are fine for brackets, covers, and non-sealing faces. Sealing faces, bearing bores, and sliding surfaces need better, and they also need a measurement plan.
Ask what the inspection report will show. Datum choice, probe access, and the measurement temperature change the number more than most engineers expect.
Choosing the right setup for the part
| Part feature | Recommended setup | Why |
|---|---|---|
| Open pockets and flat faces | 3-axis | No rotation needed, fastest cycle |
| Holes on a bolt circle | 4-axis | One index instead of four setups |
| Deep undercuts and contoured walls | 5-axis simultaneous | Tool reaches in without re-fixturing |
| Long shafts with a turned diameter | Mill-turn center | Turning and milling in one program |
| Thin walls under 1 mm | 3-axis with light passes | Rotation flexes the part |
| Large weldments to 4,000 mm | 3-axis gantry travel | Fits the 4,000 × 400 × 150 mm envelope |
When to stop and pick another process
If the part is a simple open shape in one orientation, 3-axis CNC machining work is cheaper and faster. If it needs undercuts, five faces, or a contoured surface, pay for 5-axis. If it is a thin shell with no load, molding or printing wins.
Questions engineers ask before quoting
How tight a tolerance can I actually specify?
GreatLight holds ±0.005 mm (±0.0002 in) on features the machine can reach and the inspection can verify. That number applies to a defined datum, not to every dimension on the drawing.
If a hole is 200 mm from the datum, stack-up and thermal drift eat into the budget. Call out the critical dimensions only, and let the rest run to general tolerance.
Which materials are hard to machine and why?
Titanium TC4 (Ti-6Al-4V) and Inconel keep their strength at cutting temperature, so the edge wears fast and feeds must drop. Magnesium AZ31B machines easily but the chips are flammable.
Plastics like PEEK and POM move with heat. A roughing pass that is fine in 6061 will leave a melted burr in POM unless the feed and coolant are adjusted.
How many setups will my part need?
Count the directions the tool must approach from. One direction is one setup. A part with features on four sides typically takes two to four setups on 3-axis, or one on a 5-axis center.
Every extra setup adds a datum transfer. That is where position error creeps in, and it is the main reason a 5-axis quote can beat a 3-axis quote on a complex part.
What surface finish should I put on the drawing?
Specify Ra only where it matters. Ra 1.6–3.2 μm covers most non-critical faces. Ra 0.8–1.6 μm suits bearing fits and sealing faces. Ra 0.2–0.8 μm needs a dedicated finishing operation and adds cost.
A blanket Ra callout across the whole part is a common reason quotes come back high.
Do I need a prototype before production?
For a new geometry, yes. One machined prototype proves the toolpath, the fixturing, and the drawing before tooling or volume commitments. GreatLight runs from a single part with no minimum order quantity.
If the design is already proven in a similar material, a first article inspection on the production run often replaces the prototype step.
How do you keep my design confidential?
Uploads are handled as confidential, and a non-disclosure agreement is available on request. Files stay inside the project team.
If your program requires it, we sign before the drawing is released for quote.
Send the drawing, get a manufacturability answer
Quotation and free DFM analysis within 12 hours, production can start within 24 hours, and parts ship in 3–5 days.
12-hour quote100% inspectionNo MOQ
More from the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
