CNC Machining Derby: How the Process Works and Where It Stops

• 1
  What CNC machining Derby work actually removes
• 2
  Where tolerance sits on the cost curve
• 3
  Material choice and how it changes the cut
• 4
  When five-axis helps and when it does not
• 5
  What to verify before you release a purchase order
• 6
  Which process route fits the part

CNC machining is subtractive. A rotating cutter or a turning tool takes material off a solid block, bar, or casting until the shape left matches the CAD model. Nothing is formed or added, so the starting stock must be bigger than the finished part in every direction the tool has to reach.

That single fact drives most cost decisions. A part that fits inside a 100 × 100 × 50 mm block needs less stock, less cutting time, and less fixturing than one that needs a 600 mm bar turned down to a 40 mm diameter. Stock removal volume, not part complexity, is usually the first number a shop prices.

Three motion types do the work. Milling spins the tool and moves it in X, Y, and Z. Turning spins the workpiece against a fixed tool, which suits anything round. Mill-turn centers combine both on one platform, so a shaft with cross-drilled holes and a milled flat can come off in a single setup instead of three.

Every extra setup adds error. Each re-clamp re-references the part, and the stack-up of fixture error plus tool offset error lands in your dimensions. That is why five-axis work exists: not to look impressive, but to reach five faces with one datum.

A general machining tolerance of ±0.1 mm is routine on most metals and costs nothing extra. Tighten to ±0.025 mm and the shop starts adding semi-finish passes, checking tool wear more often, and sometimes moving the part to a temperature-stable machine. Push to ±0.005 mm and you are paying for the whole chain: climate control, probing, and slower feed rates.

Aluminium 6061-T6 holds ±0.005 mm on a well-fixtured feature. A thin wall of 1 mm on the same part will not, because the cutter pushes the wall away and it springs back after the pass. Deep bores behave the same way. The bore measures right after machining and drifts 0.01 mm overnight.

Surface finish and tolerance are separate purchases. Ra 1.6–3.2 μm is as-machined and normal. Ra 0.8–1.6 μm needs a finer stepover or a finish pass. Ra 0.2–0.8 μm usually means a separate operation such as grinding, lapping, or polishing, and the price step between those three bands is not linear.

Do not tolerance the whole drawing at the tightest number. Put the tight callout on the two features that mate with something else and leave the rest at the block tolerance. Shops quote what the drawing says, so a blanket ±0.005 mm title block can double a price for no functional gain.

Aluminium 6061 and 7075 cut fast and hold good finish, which is why prototypes and brackets land there first. 7075 is stronger and machines reasonably well, but it is less weldable and more prone to stress movement when you remove a lot of stock from one side. 6082 and 6063 behave closer to 6061 and are common for enclosures.

Stainless 303 is the free-machining grade and produces a clean chip. 304 and 316 gum up more, work-harden if the tool rubs instead of cuts, and need slower surface speed and rigid setups. 17-4PH machines well in the solution-treated condition and then ages to high strength, which suits shafts and valve parts.

Titanium Ti-6Al-4V and Inconel sit at the other end. They conduct heat poorly, so the cutting edge takes the temperature instead of the chip. Tool life drops, feeds drop, and cycle time climbs several times over aluminium. Magnesium AZ31B and AZ91D cut easily but the chips are a fire risk and need dedicated handling.

Plastics are not a soft option. POM and PA move with temperature, ABS can chip at the exit edge, and PEEK needs sharp tooling and generous coolant. Carbon fibre eats carbide, so expect shorter tool life and a finish that may need a sealing coat.

Match the material to the function first, then to the process. If a part only needs stiffness and light load, aluminium plus a thicker section is often cheaper than steel plus a thin section.

Five-axis machining pays off when a part has features on multiple faces at compound angles, when a deep pocket needs a short rigid tool, or when one datum has to carry the whole part. A turbine blade, an impeller, or a medical housing with angled ports all fall into that group.

It does not pay off for a flat plate with holes. A three-axis machine with a good fixture will produce that faster and cheaper, and adding rotary axes just adds setup and programming time. If the part is prismatic and all features are reachable from two or three orthogonal directions, stay on three-axis.

Size matters as much as axis count. Our largest travel is 4,000 × 400 × 150 mm, which covers long extrusion profiles and long shafts. Mid-size platforms run 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, and compact cells handle 500 × 500 × 450 mm and 500 × 310 × 200 mm. A Ø400 mm rotary table covers round parts that need milling on the face.

Ask one question before choosing five-axis: can a three-axis machine reach every feature with a tool long enough to cut it? If yes, five-axis is a cost, not a benefit. If the tool would need to be so long it chatters, five-axis is the answer.

Start with the inspection story, not the machine list. Ask how first-article inspection is handled, what reports ship with the parts, and whether in-process checks happen or only a final gate. A shop that inspects 100% before shipment and can send dimensional reports on request is giving you a traceable path if something is wrong.

Then ask about material traceability. For aerospace, automotive, and medical work, the mill certificate matters as much as the part. Certification coverage such as ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022 tells you which quality systems are already in place.

Lead time claims deserve a follow-up question. A quote in 12 hours is useful only if production can start soon after, and our own benchmark is production start within 24 hours and parts shipping in 3–5 days. Ask what happens when a tool breaks mid-run, because that is when schedules actually move.

Finally, check the commercial terms before the technical ones. No minimum order quantity means a single prototype and a 10,000-part run can both be quoted. Confidentiality should be explicit: secure uploads and an NDA on request. If a shop will not sign one, that is your answer.