CNC Processing and Delivery Art: A Guide

• 1
  What happens inside CNC processing and delivery
• 2
  Datums, workholding and the first cut
• 3
  Where ±0.005 mm is needed and where it is not
• 4
  Material choice and what finishing adds
• 5
  Moving parts from the machine to your dock
• 6
  When CNC is the wrong answer
• 7
  Which process fits the part

CNC processing is subtractive. A rotating cutter removes material along a path that a CAM program has already calculated. The machine does not decide anything on its own. Every number in the program came from a decision about stock size, workholding, cutter diameter and stepover.

That is why the same part can be machined two ways and cost different money. A pocket with a 6 mm internal corner cannot be cut by a 12 mm end mill. The CAM engineer either switches to a smaller tool and accepts a slower pass, or the designer opens the corner to 12.5 mm and the cycle time drops by a third.

The chain is short: model, stock, setup, toolpath, cut, inspect, ship. Delivery is the last link, and it fails most often because of a decision made at the first one. A datum that nobody agrees on will pass the machine and fail the gauge.

So the useful question is never "can this be machined?" Almost anything can. The real question is which features drive the cost and which tolerance the function actually needs. Those two answers decide the whole job.

A datum is the reference the inspector will use to check the part. In a 3-axis setup, the operator typically touches off X, Y and Z on a vise stop or a machined face. If the drawing calls for position tolerance from a bore, that bore should be the datum, not the outside edge.

Workholding decides how much of the part you can reach in one setup. Every additional setup adds a re-clamp, a re-zero and a stack of error. For a bracket with features on four sides, a 5-axis machine that tilts the part can hold better position than three separate 3-axis operations.

Thin walls are the classic failure. A 1 mm wall in aluminium will deflect as the cutter passes and spring back undersize. Climb milling with light radial engagement, or leaving 0.3 mm and finishing both sides in a second pass, usually holds the wall better than one heavy cut.

Roughing should leave 0.3–0.5 mm of stock on faces you care about. Anyone who machines to final size in one pass is trading a few minutes of cycle time for a scrapped part.

We hold ±0.005 mm (±0.0002 in) on features that need it. That number is a capability, not a default. Applying it to every dimension on a drawing raises inspection time, slows the cycle and adds cost with no functional gain.

Ask what the feature does. A bearing bore or a dowel pin hole usually needs the tight number. A clearance hole for an M6 bolt needs ±0.1 mm at most. A cosmetic outside profile rarely needs better than ±0.05 mm. Write those numbers on the drawing instead of a general block tolerance.

Temperature matters at this level. Aluminium expands about 23 μm per metre per degree Celsius. A 300 mm part measured at 25 °C and again at 32 °C moves roughly 0.05 mm. That is ten times the tolerance you asked for, and it is not a machining error.

Surface finish behaves the same way. Ra 1.6–3.2 μm is fine as machined and suits most brackets. Ra 0.8–1.6 μm needs a finishing pass with a sharp insert. Ra 0.2–0.8 μm usually means a dedicated finishing operation or a secondary process.

Inspection follows the drawing. We check raw material on arrival, monitor the first article and the running part, and inspect 100% before shipment. Reports are available on request, but the report is only as good as the datum it was measured from.

Aluminium 6061-T6 is the default for prototypes and brackets: easy to cut, stable, cheap to anodize. 7075 machines well but is less weldable and costs more. 2024 has better fatigue behaviour and poorer corrosion resistance, so it usually gets a coating.

Stainless 303 is free-machining and suits shafts and fittings. 304 and 316 are tougher on tooling but hold up in wet or medical environments. 17-4PH can be heat treated after machining to reach high strength without distorting the finished geometry.

Titanium TC4 (Ti-6Al-4V) and Inconel are slow. Tool life drops, cycle time can be three to five times that of aluminium, and thin sections are prone to chatter. Use them when the temperature or strength requirement rules everything else out.

Finishing is not decoration. Anodizing adds a hard oxide layer and closes up small burrs. Electroless nickel gives uniform coverage on complex geometry. Laser marking needs a minimum character height of 1.5 mm to stay legible after coating.

Pick the finish before the last cut. A part that will be hardcoat anodized grows a few micrometres, which matters on a press fit. Bead blasting rounds edges slightly, so do not call for a sharp edge and a blasted surface in the same note.

Processing and delivery are one workflow, not two. If the drawing is ambiguous on Friday, the parts will not ship on Tuesday. That is why the file review happens before the quote is final, not after the order is placed.

We return a quotation and a free DFM analysis within 12 hours, and production can start within 24 hours of approval. Standard parts ship in 3–5 days. The historical late-delivery probability across our work is below 2%, which is a record, not a promise for your specific job.

Three plants and 7,600 m² of floor space give us somewhere to put a job when one machine is busy. 127 high-precision CNC machines, including 16 simultaneous 5-axis centers and 16 mill-turn centers, mean a rush order does not have to wait for a single spindle.

Packaging is part of delivery. A 4,000 mm part needs a crate and a lift plan. A batch of 500 small parts needs trays that survive a courier. Tell us how the parts will be handled at your end and we pack for that.

Uploads stay confidential and an NDA is available on request. Files are not shared outside the job.

CNC makes sense from one piece to a few thousand. Below that range it is the only sensible route; above it, the per-part cost stops falling while the cycle time stays fixed.

If you need 50,000 identical housings a year, die casting or forging plus a light machining pass will beat milling the whole shape. The tooling cost is real, but it amortizes across the volume.

If the part has internal channels that no cutter can reach, metal 3D printing or vacuum casting may be the better first step. We run both, and the decision usually comes down to whether the channel is for fluid or for weight.

A simple sheet metal bracket should not be milled from solid. Cutting and forming a flat blank uses a fraction of the material and time. Bending is not less precise than milling when the tolerance is ±0.2 mm.

The honest answer is that CNC is a finishing tool for geometry, not a volume tool for simple shapes. Knowing which one you have saves the most money of anything on this page.