CNC Processing Manufacturer Guide
This CNC processing manufacturer guide is written for engineers and sourcing teams who need to pick a supplier without flying to the factory. It covers the checks that actually change part quality, price and schedule. By the end you should know which questions to ask and which answers are red flags.
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What matters before you send a PO
Matching part geometry to machine type
Use this as a first filter before you ask for a quote.
| Part situation | Right machine | Watch out for |
|---|---|---|
| Prismatic part, features on one face | 3-axis mill | Cheapest route; do not over-specify |
| Open part that needs four sides | 4-axis with rotary table | Rotary table up to Ø400 mm |
| Deep pockets, undercuts, angled holes | Simultaneous 5-axis | Higher rate, fewer setups |
| Turned body plus milled flats | Mill-turn center | One setup keeps concentricity |
| Long frame, rail or extrusion | Large-travel machine | Up to 4,000 × 400 × 150 mm |
| Thin wall under 1 mm | 5-axis, light passes | Chatter and spring-back |
What a CNC processing manufacturer guide should cover first
Most supplier problems are not machining problems. They are information problems. A shop that never asks what the part does, or where the datum sits on the assembly, will hit the drawing and stop there. The first thing to check in any CNC processing manufacturer guide is whether the supplier reads your model as a function, not just as a shape.
Send a STEP file and a 2D drawing with tolerances, datums and surface callouts. Then watch what comes back. A useful reply points at a feature and asks a question: this wall is 0.8 mm, will it be supported in the assembly? That corner has a 1 mm internal radius, do you need it or can we open it to 3 mm for a stiffer tool?
That kind of reply tells you more than a certificate PDF. It means someone opened the file, measured a wall, and thought about the cutting tool that would reach it. In low-volume work, this step is where cost is won or lost.
Tolerance, finish and the cost of over-specifying
A blanket ±0.005 mm title block is the most common way engineers pay for precision they do not use. General tolerances on a CNC drawing are usually fine at ±0.05 mm to ±0.1 mm for non-mating features. Reserve tight bands for bearing bores, seal grooves, dowel holes and mating faces.
Surface finish follows the same logic. As-machined at Ra 1.6–3.2 μm suits brackets and housings. Ra 0.8–1.6 μm is a normal target for sliding contact. Ra 0.2–0.8 μm usually means a finishing pass, a smaller stepover and sometimes a secondary operation. Each step adds machine time.
On the drawing, put a tight tolerance only where it is measured. If a bore needs ±0.005 mm to fit a press-fit pin, say so and name the pin. If a pocket edge is cosmetic, loosen it. Suppliers quote the tightest number on the sheet, so a loose sheet is a cheaper sheet.
One more habit: call out which dimensions are critical. A drawing with three marked critical dimensions gets inspected properly. A drawing with ninety equal-looking dimensions gets a sample check and hope.
- 1As-machined, Ra 1.6–3.2 μmBrackets, covers, non-contact faces
- 2Ra 0.8–1.6 μmSliding surfaces, general sealing faces
- 3Ra 0.2–0.8 μmFine seals, optical and mating fits
Capacity checks: axes, travels and machine count
Ask for the machine list, then match it to your part. A shop with 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, can route a job to the right machine instead of forcing it onto the only one free.
Travel limits decide feasibility before price. A 4,000 mm frame needs a machine with 4,000 × 400 × 150 mm travel. A compact housing fits a 500 × 500 × 450 mm or 500 × 310 × 200 mm envelope. Medium work often lands on 750 × 1,150 × 550 mm or 600 × 600 × 600 mm machines. Send the bounding box with the model and you skip a week of back-and-forth.
Five-axis is not automatically better. It pays off when the part has features on several faces, deep pockets with drafted walls, or holes that must stay true to a single datum. If a 3-axis setup with two fixtures holds the same tolerance, the 5-axis rate buys you nothing.
Lead time, MOQ and how the shop plans capacity
Lead time promises are easy. Ask what drives them instead. Quotation and DFM feedback within 12 hours, production start within 24 hours, and parts shipping in 3–5 days describe a shop that keeps capacity open for short runs. That matters for prototype rounds where you need three iterations in a month.
A no-MOQ policy is a useful signal for hardware teams. It means the same shop can cut one prototype and later run 10,000+ parts without you changing supplier and re-qualifying the process. Confirm that the second article comes off the same machine family and inspection plan as the first.
For production, ask about historical on-time performance and how late orders are handled. Numbers below 2% late delivery are a reasonable benchmark to request. Any supplier who will not share a figure is telling you something.
Material availability sits behind all of this. Confirm that your alloy is stocked or has a known mill lead time before you commit to a build date.
Certifications, inspection and data handling
Certifications are scope documents. ISO 9001:2015 covers quality management generally. IATF 16949:2016 is the automotive requirement. ISO 13485:2016 applies to medical device work. ISO 27001:2022 covers information security, which matters when you upload proprietary CAD. Ask which site holds which certificate and check the expiry date.
Inspection is where tolerance claims become real. A workable plan includes incoming raw material check, in-process monitoring, and final inspection before shipment, with reports available on request. For tight features, ask what instrument is used: a micrometer, bore gauge, CMM or optical comparator. The tool must be at least four times more precise than the tolerance it verifies.
First article inspection should be standard on a new part number, not an add-on. Get the report before you accept a production run, and keep it as the baseline for later lots.
On data, request an NDA before you upload. Uploads should be transferred and stored under a defined access policy, not emailed around as loose files.
- 1ISO 9001:2015General quality management baseline
- 2IATF 16949:2016Automotive and EV programs
- 3ISO 13485:2016Medical device components
- 4ISO 27001:2022Information security for your CAD data
Materials, finishes and where they change the plan
Material choice changes tooling, speed and sometimes the machine. Aluminum 6061, 6061-T6, 7075 and 6082 cut fast and hold a good finish. Stainless 303 and 304 machine cleanly; 316L and 17-4PH are tougher and need slower feeds. Titanium TC4 (Ti-6Al-4V) and Inconel demand sharp tools, low cutting speeds and patience.
Plastics behave differently again. POM and PEEK hold dimensions well; ABS and PP move with heat and need lighter passes. Carbon fibre is abrasive and wears tooling, which shows up in the price.
Finishing is often a separate supplier, and that is where schedules slip. A shop that runs anodizing, plating, powder coating, bead blasting and laser marking in-house removes a handoff. Laser marking has a practical floor: minimum character height 1.5 mm. Below that, characters blur.
Ask which finishes are done in-house and which are subcontracted, then ask how the subcontracted ones are inspected on return.
How to vet a CNC processing supplier in seven steps
Run these in order. Each step filters out some suppliers before you spend engineering time.
- 1Send a complete RFQ packageSTEP file, 2D drawing with GD&T, material grade, finish callout, quantity and target date. Missing material grade alone delays most quotes.
- 2Score the DFM replyLook for named features and specific suggestions. A reply that only confirms price and lead time is a warning.
- 3Check axes and travel fitState your bounding box and required faces in one setup. Confirm the machine size covers it, up to 4,000 × 400 × 150 mm.
- 4Review the tolerance planAsk which dimensions are critical and how each is measured. Tight bands should be few and clearly marked.
- 5Confirm MOQ and scalingAsk how prototype and production differ. No MOQ means one part to 10,000+ parts on the same process.
- 6Verify certificates and inspectionRequest current certificates and the inspection plan. Ask for a first article report on the first run.
- 7Sign the NDA before uploadGet it in place before CAD leaves your network. Confirm upload and storage access rules in writing.
Questions engineers ask before choosing
How tight a tolerance can a CNC processing manufacturer hold in production?
On well-fixtured features, ±0.005 mm (±0.0002 in) is achievable and routinely inspected. That number applies to specific features, not to the whole part.
For general dimensions, ±0.05 mm to ±0.1 mm is usually enough and costs much less. Mark only the mating features as critical.
Should I always choose 5-axis machining?
No. Five-axis pays off when a part has features on multiple faces, deep drafted pockets, or holes that must stay true to one datum in a single setup.
If a 3-axis or 4-axis setup with a rotary table holds the same tolerance, the 5-axis rate adds cost without adding value.
What does no minimum order quantity actually include?
It means one prototype and a 10,000+ part run can go through the same supplier and process. That removes a re-qualification step when you scale.
Ask whether the prototype and production parts come off the same machine family and inspection plan.
Which certifications should I ask for?
It depends on the industry. ISO 9001:2015 is the general baseline. IATF 16949:2016 suits automotive and EV work. ISO 13485:2016 applies to medical devices.
If you upload proprietary CAD, ISO 27001:2022 covers information security. Ask which site holds each certificate.
How fast can a quote and first shipment move?
A workable benchmark is quotation and DFM analysis within 12 hours, production start within 24 hours, and parts shipping in 3–5 days.
These hold for standard materials and simple finishes. Titanium, Inconel and subcontract finishing add time.
How do I protect my design files?
Sign an NDA before uploading CAD, and confirm how files are stored and who can access them.
Ask for a written statement that uploads are secure and confidential, and keep the NDA on file for the whole program.
Send your drawing and get a DFM reply
Upload your model and we will return a quote with DFM notes, a tolerance plan and a lead time you can build a schedule on.
12-hour quoteNo MOQ100% inspectionNDA on request
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