CNC Precision Machining Services

How do you match a GD&T callout to a machining process?

The first decision in any precision job is which process makes which feature, because tolerance lives in the process, not in the drawing. A flatness callout on a sealing face, a circularity callout on a bearing bore, and a position callout on a bolt pattern each point to a different machine, and a shop that cannot explain the mapping is guessing.

The mapping works from the tolerance down. Grinding holds the tightest band and the best form; turning holds roundness and concentricity on rotating parts; milling holds prismatic geometry and position across a housing; mill-turn keeps turned and milled features aligned in one clamping. The sections below walk through each one, so you can read a quote and see whether the process matches the print.

Which tolerances does CNC milling hold?

Milling is the process for prismatic parts — housings, plates, brackets, and pockets — where flatness, parallelism, perpendicularity, and position between features are the working callouts. We run 3-axis, 4-axis, and 5-axis centers, so a part can be held once and machined from multiple sides without the re-fixturing error that stacks position tolerance.

Capability	Detail
Held tolerance	±0.003 mm (milling) / ±0.002 mm (turning)
Axis range	3-axis (50+), 4-axis (20+), 5-axis (20+)
Max workpiece	1300 × 700 × 600 mm
Typical GD&T callouts	Flatness, parallelism, perpendicularity, position of holes and pockets

The 5-axis machines matter most when a position callout links features on different faces. Because the part does not leave the fixture between faces, the datum is preserved and the position tolerance is not consumed by re-clamping. For a housing with a bore pattern on three sides, that is the difference between a part that assembles and one that almost does.

Which tolerances does CNC turning hold?

Turning is the process for round parts — shafts, pins, bushings, and sleeves — where the callouts that matter are diameter, roundness, runout, and concentricity. Because the part rotates and the tool moves along its axis, a lathe naturally produces features that share the same centerline, which is exactly what concentric fits require.

Capability	Detail
Held tolerance	±0.002 mm
Machine count	25+ lathes, plus mill-turn and Swiss-type machines
Max workpiece	Ø720 × 1280 mm
Typical GD&T callouts	Circularity, cylindricity, concentricity, circular runout

For a shaft that must spin true in a bore, turning holds the roundness and the runout in the same pass that cuts the diameter. When a bearing seat demands more than turning can hold, the feature is ground, but the turning pass still sets the geometry that grinding finishes.

Which tolerances does mill-turn hold in one clamping?

Mill-turn combines a turning spindle with a milling spindle in a single machine, so a part that needs both a turned body and milled flats, slots, or holes is finished without moving between machines. The payoff is positional: a cross-hole that must sit at a true position relative to a turned diameter stays where the drawing put it, because the two features share one setup.

This is the process to request when a concentricity or position callout links a round feature to a prismatic one. Machining them in separate setups lets the re-fixturing error eat into the tolerance; machining them in one clamping removes that error at the source. The result is a tighter, more repeatable relationship between features, and fewer scrapped parts on a fit that only works when both features are right at once.

Which tolerances does precision grinding hold?

Grinding is the finishing process, and it holds the tightest band on the floor. We run surface, centerless, and cylindrical grinding, and it is the process for the features that carry the load: bearing seats, sealing faces, and any surface where flatness, roundness, or surface finish is the function.

Capability	Detail
Held tolerance	±0.002 mm
Machine count	7+ (surface / centerless / cylindrical)
Typical GD&T callouts	Flatness, circularity, cylindricity, tight diametral fits

The rule of thumb is simple: if a surface must be flat or round to within a couple of microns, or if a bore and shaft must share a controlled clearance, grinding is the process. A shop that quotes a ground tolerance on a milling-only process is a shop that will miss it, which is why asking which process holds each callout is your cheapest quality check.

Which secondary processes complete the fit?

Most parts are not finished when they leave the machine. Heat treatment changes hardness and, through it, how the part wears; surface finishing changes final dimensions, which matters when the finish is part of the fit. Both are handled in the same workflow so the finished part — not the machined blank — is what assembles.

• Heat treatment: vacuum, bright, carburizing, nitriding, and stress-relief aging. In-house ovens reach 500 °C; titanium stress relief above that goes to a qualified partner.
• Surface finishing: standard and hard anodizing, zinc / nickel / silver / gold / chrome plating, powder coating, painting, brushing, and mirror polishing.
• Deburring, tapping, laser marking, and sub-assembly as the print requires.

An anodized bore is a different final size than a machined bore. If a coating sits on a mating surface, the shop must machine the substrate to a size that leaves room for the coating, and the DFM review should call that out. Send the finish you intend, not just the machined size.

How does the shop verify the fit across processes?

A part that passes through milling, turning, and grinding accumulates the errors of each step, which is why verification is not a final check but a chain. We measure at the machine and after it, and the instruments are the same class a buyer would bring to an audit.

• Hexagon GLOBAL S CMMs verify position, profile, and form against the CAD model.
• Vision measuring machines capture fine two-dimensional geometry.
• SURFCOM 1800D and Mitutoyo roughness testers measure the surface finish a fit may depend on.
• SPC control charts track key characteristics across the run, with Ppk ≥ 1.67 at the initial study on critical characteristics and Cpk ≥ 1.33 in production on general characteristics.

Every critical dimension is reported, so you receive evidence the part assembled, not a promise that it should.

What does a buyer send to get the right process quoted?

The process you are quoted depends on the callouts you send. A drawing that says only "±0.1 mm" will be quoted as milling and may miss a roundness requirement it never stated. A drawing with GD&T callouts tells the shop exactly which features need grinding, which need mill-turn, and which can stay on a 3-axis mill.

• A toleranced 2D drawing with datums and GD&T callouts — the contract for which process holds which feature.
• A 3D STEP or IGES model for the nominal geometry.
• Material, quantity, and finish, so the process and the secondary operations are scoped together.

Send these to [email protected] and the review comes back with the process mapping spelled out, which machine holds which callout, and what it costs. That is the difference between a quote and a plan.

How do you choose between 3-axis, 4-axis, and 5-axis milling?

The choice between 3-, 4-, and 5-axis milling is a choice about how many times the part leaves the fixture. A 3-axis machine cuts from one direction at a time; a part with features on several faces must be re-clamped, and every re-clamp risks losing the datum and eating position tolerance. A 4-axis machine adds rotation around one axis, so several faces are reached without unclamping. A 5-axis machine orients the tool to the part freely, so a bore pattern on three faces or a compound angle is cut in one setup, datum intact.

The rule of thumb: use the lowest axis count that reaches every feature without re-clamping. Fewer setups means the position callout is held by the machine, not by the operator's re-fixturing skill, which is exactly what a tight print wants.

What does Swiss-type turning add for small parts?

For small, long, slender parts — pins, screws, connector bodies, valve components — Swiss-type turning is the process. A Swiss machine feeds the bar through a guide bushing while the tools cut close to the support point, so a thin part does not deflect under the tool the way it would on a conventional lathe. That support is what lets a long shaft hold its diameter and roundness along the whole length, and it is why Swiss parts can carry tight concentricity between a turned body and a threaded end.

The trade-off is the diameter: Swiss machines run bar stock up to a limit, so they suit small parts in volume. When a drawing calls for a long, thin feature with a concentricity callout, Swiss is the honest answer, and a shop should say so instead of trying to hold it on a standard lathe.

How are tight features sequenced across milling, turning, and grinding?

A part that needs milling, turning, and grinding is a sequence, and the sequence decides whether the tolerance survives. The order is set by where the datum is and which features depend on which. Rough cutting comes first, to remove stock and settle the material; then the reference surfaces — the datums — are finished, because everything after them is located from them; then the tight features are finished last. Earlier operations cannot disturb them. Grinding always comes after machining, because a ground surface is finished from a machined surface, not from raw stock.

Get the sequence wrong and you chase a dimension that an earlier step keeps moving. A DFM review that spells out the sequence is the difference between a shop that knows the tolerance is a chain and one that treats each operation as its own island.

How does CAM programming protect the tolerance?

The toolpath is where the tolerance is won or lost, and it starts in CAM programming, not on the machine. The programmer chooses the cutting strategy — climb versus conventional milling, step-over, entry and exit moves — and those choices decide the surface finish and the size, not just the cycle time. A plunge entry leaves a mark; a ramped or helical entry does not. A climb cut leaves a cleaner wall than a conventional cut. And the inspection program is written from the same model, so the CMM probes the same features the toolpath produced.

A shop that programs and measures from one CAD model closes the loop that a shop using paper drawings leaves open. Ask how the program and the inspection are linked; the answer tells you whether the tolerance is engineered or just attempted.

What this shop actually runs

If you ask for capability evidence, you deserve a specific answer. This facility operates 100+ CNC machines across 6,800 m2 in Dongguan, China, staffed by 100+ people. The machines, listed by category:

• 5-axis machining centres (10+) - Mazak VARIAXIS I-600 (510/910/510 mm travel, 12,000 rpm, 22 kW, 71.6 N·m, 30-tool BT40) and Mazak INTEGREX I-250H S (B-axis 0.0001°, 24 kW, 124 N·m, 38-tool HSK-A63). Intended for compound-angle parts, single-setup multi-face prismatic work, and aerospace fittings where datum transfer loss is unacceptable.
• 3-axis / 4-axis machining centres (50+) - Brother SPEEDIO S500Z2N / S700Z2N (10,000 rpm, 14/21-tool ATC, 0.7 s tool-to-tool, Z-axis 2.2 G) and Mazak VCE-430A L / VCE-570C L (10,000 rpm, 118 N·m, 24-tool ATC). Intended for prismatic parts, plates, brackets, housings.
• Swiss-type turning centres (10+) - Citizen Cincom A20-3F7 (Φ20 mm, 10,000 rpm), TSUGAMI B0206-III / B0205-III (Φ20 mm, 0.1 µm), STAR SR-32JII / SR-32J Type N (Φ32 mm, 8,000 rpm). Intended for long, slender rotational parts at ±0.002 mm.
• CNC turning centres (20+) - HEADMAN T70M/1250 (720 mm swing, 117 mm bore, 3,000 rpm, BMT75 12-station turret). Intended for shafts, bushings, flanges up to ±0.002 mm.
• Wire EDM (5+) - Sodick AQ535L (550/350/300 mm travel, ±20° taper, 800 kg workpiece). Intended for hardened tool steel, internal splines, sharp internal corners that milling cannot reach.
• Inspection: Hexagon CMM + 20+ gauges - Hexagon CMM for coordinate measurement, plus vision system, profilometer, hardness tester, coating thickness gauge, XRF for material verification. The 10:1 rule (gauge 10x more accurate than the band) is applied to every feature.
• Machine highlights - VARIAXIS I-600: one-setup multi-face 5-axis + MAZA-CHECK auto-calibration; INTEGREX I-250H S: DONE-IN-ONE mill-turn, B-axis 0.0001° + Ai thermal shield; SPEEDIO S500Z2N / S700Z2N: Z-axis 2.2 G + 0.7 s ATC; Swiss-type: guide-bush switchable + 0.1 µm resolution.

Tolerance bands: ±0.003 mm milled, ±0.002 mm turned, ±0.002 mm ground or wire-cut. Capability indices: Ppk >= 1.67 at the initial study; Cpk >= 1.33 ongoing in production.

Certifications: ISO 9001:2015 (116024/A/0001/UK/En) · IATF 16949:2016 (131941/A/0001/SM/En, design clause 8.3 excluded) · ISO 14001:2015 (116024/B/0001/UK/En), held by Dongguan Zhuohang Ltd (USCC 91441900MA4UWT6X39, since 2005).

Service area: we supply Austria, Belgium, the Czech Republic, Denmark, France, Germany, Greece, Hungary, Ireland, Italy, the Netherlands, Spain, Sweden and the United Kingdom in Europe; Brazil, Canada, Mexico and the United States in the Americas; China, Israel, Japan, Singapore and Sri Lanka in Asia; Australia and New Zealand in Oceania; and South Africa in Africa, with worldwide delivery from China.

Contact: Felipe Iglesias, [email protected], +86 13126536955.

Frequently Asked Questions

Standards referenced on this page

• ASTM A582/A582M-12 — Standard Specification for Free-Machining Stainless Steel Bars
• ASTM B348/B348M-21 — Standard Specification for Titanium and Titanium Alloy Bars and Billets
• ISO 9001 — Quality management systems — Requirements