Machining: Machined Parts

What actually counts as a "CNC machined part"?

A machined part is any component cut from solid stock by a programmed cutting tool, rather than cast, forged, or printed into shape. Because it is cut, the part is fully dense metal all the way through — no porosity, no layer lines — and it can be held to tolerances a casting or a print cannot reach. That is the whole reason you pays for machining: real material properties, tight geometry, and a surface finish that needs no post-processing to function.

Under that umbrella sit four process families, and a part belongs to one of them by its shape. The skill of a manufacturing partner is recognizing which family a drawing belongs to before quoting it, because choosing wrong adds setups, cost, and error — never function.

How does part geometry decide between turning and milling?

The first question on any drawing is simple: which part spins? If the part is fundamentally round — a shaft, pin, bushing, or anything with a centerline — it belongs on a lathe, where the workpiece rotates and the tool feeds along it. If the part is prismatic — a bracket, housing, plate, or pocketed block — it belongs on a mill, where the tool rotates and the workpiece is held.

The distinction matters to cost more than most buyers expect. A round part made on a lathe is cheap and fast; the same part milled from a block is slower and dearer. A drawing that is clearly one family or the other can be quoted in minutes; a drawing that mixes them is where a DFM review earns its keep.

When is a mill-turn or ground part the right call?

Two situations pull a part beyond plain turning or milling. Mill-turn is for parts that have both round and prismatic features that must stay concentric — a housing with a precision bore and mounting faces, a shaft with a hex and a cross-hole. Doing both operations in one setup, on one machine, removes the re-fixturing error that creeps in when a part moves between a lathe and a mill, so the concentricity you specify is the concentricity you get.

Grinding is the finishing word on a critical surface. Where turning or milling holds ±0.003 mm (milling) / ±0.002 mm (turning), grinding reaches ±0.002 mm — the band for bearing seats, sealing faces, and anything whose function turns on a few microns. It is not a replacement for cutting but a second, finer pass on the surfaces that earn it.

Which materials does this shop run, and how do they change the quote?

Material is not neutral on cost. The same geometry in 6061 aluminum and in 316 stainless can differ severalfold in machining cost, purely because of how fast each cuts and how quickly each wears a tool. The cheapest material that still meets the functional requirement is the one to specify.

Every incoming lot is checked by XRF against its mill certificate before it reaches a machine, so the alloy you specify is the alloy you receive — a mixed or substituted lot is caught at the door, not discovered on your receiving dock.

What tolerances can a machined part actually hold?

A tolerance is a promise about how far a measured dimension may drift from nominal, and it is only worth the measurement behind it. Our tolerances are tied to the process, and every critical dimension is verified on a CMM rather than assumed.

We hold these to process capability, not to a best-case claim. On a representative length characteristic (76.1–76.5 mm), our SPC shows Cpk = 1.851 (one characteristic on one part, not a blanket capability claim) against an initial-study target of Ppk ≥ 1.67. If a print calls for a dimension we cannot reliably hit, we tell you during the DFM review — before cutting, not after.

Read the tolerance band before you read the index. Cpk is a ratio of the tolerance band to the process spread, not an absolute measure of accuracy. The figure above was recorded on a length characteristic with a 0.4 mm band (76.1–76.5 mm) — wide by design. Hold the same process spread against a ±0.005 mm feature and it fails outright. Ask for the band, the subgroup count and the sampling window on every capability number a supplier quotes.

What should a drawing include before you ask for a quote?

The fastest quote is the one that answers every question up front. A complete request is not just a file — it is a file plus the decisions that change the price.

• The model or drawing: a STEP or IGES for 3D, or a 2D drawing with every dimension and tolerance.
• Material and grade: name the alloy, not just "aluminum" or "steel," and any heat-treatment or plating requirement.
• Quantity: prototypes and production runs quote differently, so state the range you expect.
• Critical tolerances: mark which dimensions control function; leave cosmetic surfaces loose.
• Target date: tell us when you need it, and we will tell you honestly whether it holds.

How are parts verified before they ship?

Machined parts are useless if they measure wrong. Our inspection bench is the same class of equipment a buyer would use to audit us, and we document the results rather than the pass/fail alone.

• Hexagon GLOBAL S CMMs (2) for dimensional verification, plus vision measuring machines and a gear measuring center.
• KEYENCE VK-X3000 laser microscope and SURFCOM 1800D for surface finish and profile.
• XRF (iCHEQ) spectrometer for incoming material composition checks against the mill certificate.
• HVS-1000A micro-Vickers hardness tester, Fischer coating thickness gauge, salt spray and tensile testers.
• SPC control charts on key characteristics, with Ppk ≥ 1.67 initial critical and ≥ 1.33 general.

What are the MOQ and lead times?

There is no production minimum. Our MOQ is 1 piece, so you can prototype, validate, and iterate before committing to volume. Lead time is set by part complexity and the number of process steps, not by a blanket promise.

Complex parts can run 10–20 process steps, and any surface finishing pushes a part into the longer tier. State your target date and we will tell you honestly whether it holds, not the other way around.

What makes a machined part cheap or expensive?

The price of a machined part is set by a handful of levers, and every one of them is visible on the drawing before a single chip is cut. Understanding them is how you spend less without sacrificing function.

The levers interact. A part in 316 stainless with every dimension at ±0.003 mm (milling) / ±0.002 mm (turning) and a surface finish requirement on a hidden face is paying the premium three times over — once for the material, once for the tolerance, and once for the finish. The cheapest part is not the one that skips quality; it is the one where the material, tolerance, and finish each sit exactly at the level the function requires and no further. That is what a DFM review optimizes, and it is why the review is the cheapest step in the whole chain.

What should you check when the parts arrive?

Machined parts are a deliverable, and the receiving dock is the last place you gets to verify them before they enter an assembly. A short check, run the same way every time, turns an invoice into evidence.

• Count and identity. Confirm the quantity and that the part numbers match the drawing and the purchase order.
• Dimensional spot check. Measure a sample of the critical dimensions — the ones you flagged as controlling function — against the drawing.
• Material certificate. Match the heat number and alloy on the certificate to what you ordered, and keep the certificate with the lot.
• Inspection reports. Confirm the CMM report and SPC chart arrived, and that their results are within the tolerance you specified.
• Finish and cleanliness. Check the surface finish on sealing faces, confirm deburring, and look for chips or residue that should have been cleaned off.

None of this replaces the factory's own inspection — it confirms it. A supplier that ships documented parts makes the receiving check fast and unambiguous; a supplier that ships bare parts leaves you to re-prove what should already be proven. The paper trail that follows the part is the difference between receiving evidence and receiving a gamble.

Frequently Asked Questions