Buy CNC Machining Service: What to Check Before You Order

When you buy a CNC machining service, the first decision is which machine the part will run on. A flat bracket with holes on two faces is a three-axis job. A part with pockets on five sides, or a deep cavity that needs a short tool to reach the floor, changes the answer. Ask which spindle will hold the part and how many setups are needed. Each extra setup adds a datum shift that shows up as position error on your drawing.

Five-axis work pays off when a part has compound angles, contoured surfaces, or features that would otherwise need a custom fixture. GreatLight runs 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. Mill-turn matters for parts with turned diameters plus milled flats, because one machine holds both features in a single setup.

Size decides the shortlist. Maximum processing size here is 4,000 mm, with travels of 4,000 × 400 × 150 mm on the large gantry, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm on medium frames, and 500 × 500 × 450 mm or 500 × 310 × 200 mm on compact machines. A Ø400 mm rotary table covers round parts that need indexing.

If a shop quotes a five-axis price for a part that only needs three axes, you are paying for capability you do not use. If they quote three axes for a part with undercut walls, the quote will come back with a deviation request. Send the STEP file and say which faces are functional. That one sentence prevents most quoting arguments.

• 1
  Three-axisPrismatic parts, through holes, open pockets, flat datums.
• 2
  Four-axisRound or indexed parts with features on multiple sides.
• 3
  Five-axisCompound angles, contoured blades, deep cavities, one-setup datum control.
• 4
  Mill-turnTurned diameters plus milled features held in one setup.

A tolerance is a cost curve, not a checkbox. General machining holds ±0.05 mm on most features without extra effort. Tightening to ±0.005 mm across a whole part forces smaller stepovers, more in-process checks, and sometimes a temperature-controlled room. The right question is which dimensions actually carry the fit. Usually it is two or three, not all forty.

Put the tight tolerance on the mating bore and leave the rest at general tolerance. You get the same assembly result for less money. Suppliers who quote a blanket tight tolerance across the print are either padding the price or not reading the drawing carefully.

Surface finish follows the same logic. Ra 1.6–3.2 μm is standard as-machined output. Ra 0.8–1.6 μm needs a finer finishing pass. Ra 0.2–0.8 μm is a precision finish and should be reserved for sealing surfaces, bearing bores, and sliding fits. Mark those faces on the model.

Measurement matters as much as the number. A ±0.005 mm callout requires a CMM or a high-accuracy gauge, not calipers. Ask what instrument will verify the critical dimensions and whether the report ships with the parts. GreatLight inspects 100% of parts before shipment and will include raw material checks, in-process monitoring, and final inspection reports on request.

Aluminum is the default for prototypes and enclosures. 6061-T6 machines cleanly and takes anodizing well. 7075 is stronger but galls and chips differently, so expect a higher price on deep pockets. 2024 and 5052 cover higher-strength and formed parts. If your alloy is not in the shop's stock list, the quote will include a mill buy-in and a longer lead time.

Stainless 303 is the free-machining grade and the cheapest way to get corrosion resistance. 304 and 316L are tougher on tooling and slower to cut. 17-4PH gives you high strength after heat treatment, but the heat treat step moves dimensions, so plan a post-treatment finishing pass on critical features.

Steel grades 1018 and 1045 cover shafts, plates, and general hardware. 4130, 4140, and 4340 are for loaded structural parts. Tool steel appears when wear resistance matters. Titanium and Inconel are available (TA1, TA2, TC4, Inconel) but cutting speeds are low, tool life is short, and the price reflects that. Magnesium AZ31B and AZ91D cut fast but need chip handling attention.

Plastics machine on the same centers. POM and PA hold dimensions well. PEEK survives heat and chemicals at a cost. ABS, PC, and PMMA are common for housings. Carbon fibre machines but eats tooling and needs dust control. Give the exact grade with the temper or condition, not just the family name.

A broker forwards your files and marks up the result. A shop owns spindles, programs the parts, and inspects them. The difference shows up when a dimension goes out and someone has to fix it. Ask two questions: how many machines do you own, and can I see the inspection report format? Vague answers are a signal.

Certifications are worth checking against the scope. ISO 9001:2015 covers general quality systems. IATF 16949:2016 applies to automotive production. ISO 13485:2016 is for medical devices. ISO 27001:2022 covers information security, which matters if your files are controlled drawings. GreatLight holds all four. Ask for the certificate and read the scope statement, not just the logo.

Finish work is where accountability usually breaks. If the shop sends parts out for anodizing and the color is wrong, who owns it? Keeping machining, finishing, and inspection under one roof removes that argument. Anodizing (clear, color, hardcoat, conductive), plating, powder coating, black oxide, bead blasting, polishing, and laser marking are all handled in-house here.

Confidentiality needs a process, not a promise. Uploads should be secure and handled under access control. If your program requires it, ask for an NDA before sending drawings. GreatLight offers one on request.