CNC parts processingglcncmachining: How a Print Becomes a Part

• 1
  What CNC parts processingglcncmachining removes
• 2
  Datums, workholding and the 6 faces problem
• 3
  Tolerance stack and what ±0.005 mm really means
• 4
  What drives the price of a machined part
• 5
  How material changes the cut
• 6
  Inspection, traceability and what ships with the part
• 7
  Which machining route fits which part

CNC parts processingglcncmachining is subtractive. A tool with a defined edge travels a controlled path and shears material away until the blank matches the print. Nothing is added, so the starting stock must already contain every finished surface plus whatever the fixture needs to grip.

Cutting happens through two motions. The spindle spins the tool at surface speed, and the axes feed the workpiece or the tool along the path. Chip load per tooth sets the real removal rate. Push it too high and the tool chatters or breaks; too low and the edge rubs and work-hardens stainless.

Heat leaves with the chip. That is why coolant choice and chip evacuation matter more than spindle rpm alone. In deep pockets or long bores, a recut chip dulls the edge in minutes. Air blast, through-tool coolant or a peck cycle fixes most of it.

The practical limit is stiffness. A long thin tool in a deep cavity deflects, and the wall it leaves is tapered. If the print calls for a straight wall at 5:1 depth-to-diameter, plan a shorter tool, a wider pocket, or a different process.

Every machined feature is located from a datum. If the drawing calls out datum A on a face that the first setup cannot reach, the shop has to re-fixture, and each re-fixture adds error. Choose datums that a single setup can touch whenever the tolerance is tight.

A 3-axis machine reaches one direction at a time. A part with features on five sides needs either multiple setups or a 4-axis and 5-axis machine. Each extra setup re-clamps the part, and re-clamping is where positional error creeps in.

A 5-axis center tilts the tool or the table so the cut stays normal to the surface. That removes most re-fixturing and keeps bores and pockets concentric. It also lets a short, stiff tool reach a wall that a 3-axis setup would need a long tool for.

Workholding sets the ceiling on accuracy. Thin walls spring under clamp pressure and relax after unclamping. Soft jaws, vacuum plates, or a sacrificial tab often hold a part better than a hard vise. If a wall is under 1 mm, expect to leave a roughing allowance and finish it in a light pass.

A tolerance on a drawing is a zone, not a target. If two features are each ±0.05 mm from a shared datum, the gap between them can swing 0.1 mm. That is fine for a bracket and fatal for a bearing fit. Stack the tolerances before you release the print.

Tighter is not free. Going from ±0.05 mm to ±0.005 mm usually means a finishing pass, a temperature-stable room, and a CMM check. The cut itself may take the same time; the measurement and the scrap risk do not.

GreatLight holds ±0.005 mm (±0.0002 in) on critical features and inspects 100% before shipment. That number applies to the features we agree on, not to every surface on the part. Tell us which dimensions carry the function.

Surface finish follows the same logic. Ra 1.6–3.2 μm is a normal as-machined face. Ra 0.8–1.6 μm needs a controlled finishing pass. Ra 0.2–0.8 μm usually means a finer tool, lower feed, or a secondary operation, so reserve it for sealing faces and bearing bores.

Machining time is the biggest line. It depends on how much metal leaves, how hard the material is, and how many tools the feature needs. A pocket that needs a Ø6 mm tool takes far longer than the same volume cleared with a Ø20 mm tool.

Setup count is the second line. Every new orientation means a new fixture, a new zero, and a first-article check. One extra setup on a 20-piece order is a small cost. On a 2,000-piece order it is absorbed quickly, so volume and setup trade against each other.

Material choice moves the price too. Aluminium 6061 and 6082 cut fast and hold good finish. Stainless 316 and 17-4PH work-harden and need slower feeds. Titanium Ti-6Al-4V and Inconel are stiffer to cut, so tool life drops and cycle time rises.

The cheapest change is usually on the drawing. Open a non-critical tolerance, deepen a corner radius so a bigger tool fits, or move a hole off a curved surface. Those edits cut cycle time without touching function.

Aluminium is the default for prototypes and housings. 6061-T6 gives a good strength-to-machining ratio, and 7075 machines cleanly at high speed when you want higher strength. Thin walls hold better in aluminium than in steel because the cutting force is lower.

Stainless and steel bring work-hardening and heat. 303 is free-machining and keeps a good finish. 304 and 316 need sharp edges and steady feed to avoid a hardened skin. 17-4PH (SUS630) machines well in the annealed state and can be aged later for strength.

Titanium and Inconel sit at the hard end. Heat stays in the cut, so coolant delivery and tool coating matter more than rpm. Expect longer cycle times and shorter tool life. If the part does not need their properties, another alloy will cost less.

Plastics behave differently again. POM and ABS cut fast but can melt and burr. PEEK holds dimension at temperature but is expensive. Carbon fibre wears edges quickly, so plan for more tool changes on a long run.

Inspection starts before the first cut. Incoming bar or plate is checked against the material certificate. A wrong alloy found at final inspection is already a scrapped part, so the check happens at the door.

During the run, the operator measures the features that drift. Tool wear moves a bore diameter slowly. Catching it at 0.01 mm lets you offset the tool before the part is out of tolerance, which is cheaper than sorting good parts from bad.

Final inspection covers the drawing's critical dimensions. Reports are available on request. The qualification rate on our lines is 99.99%, which comes from in-process offsets rather than from inspecting harder at the end.

Traceability matters in regulated work. We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The last one covers how your drawings and CAD files are stored and shared.