Buyer guide

Researching a CNC Machining Service: What to Check Before You Send an RFQ

This guide is for engineers and sourcing staff comparing machine shops. It covers the checks that actually separate suppliers: tolerance capability, machine envelope, lead time, minimum order quantity, certifications and how a shop quotes. Read it once and you can shortlist three shops and know which questions to ask.

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±0.005 mm toleranceNo MOQISO 9001 / IATF 16949Quote in 12 hours

On this page

1
Key takeaways
2
Checks that separate one CNC machining service from another
3
Start your researching CNC machining service with the machine list
4
Tolerance, surface finish and where each claim breaks down
5
Lead time, MOQ and how quote speed reveals capacity
6
Certifications, inspection and material traceability
7
A 6-step process for researching a CNC machining service

Key takeaways

Match tolerance to process, not to a wish listAsk what the shop holds on a real part, not the tightest number on the website.

Check the machine envelope firstA 4,000 mm part and a Ø20 mm fitting do not go on the same machine.

MOQ tells you who they serveA shop with no minimum will run one prototype; a mass-production shop will not.

Certifications are filters, not scoresISO 9001 fits general work. IATF 16949 and ISO 13485 only matter if your industry needs them.

Read the quote breakdownUnit price alone hides setup, material, finishing and inspection cost.

Decision table

Checks that separate one CNC machining service from another

Use this as a first-pass filter before you request quotes.

What to check	Why it matters	What to ask for
Tolerance on a real part	A ±0.005 mm claim means little without a part size	Tolerance report from a similar job
Machine envelope	Travel limits decide whether your part fits at all	Max travel and rotary table size
Lead time	Quote speed and ship speed are different things	Hours to quote, days to ship
Minimum order quantity	Reveals whether the shop runs one-offs or long runs	Lowest quantity they will quote
Certifications	Industry approvals are pass or fail, not a score	Certificate number and scope
Inspection method	Determines what data you can hand to your customer	CMM report, first article, 100% check
Quoting format	Lets you compare two shops on the same basis	Line items for setup, material, finish
Finishing options	Controls whether you need a second supplier	In-house vs subcontracted finishing

The shortlist test

If a shop can name the machine, the tolerance it holds on your part size, the ship date and the inspection method, put it on the shortlist. If any of those four answers is vague, keep looking.

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Section 1

Start your researching CNC machining service with the machine list

Most supplier pages lead with tolerance. That is the wrong place to start. The first question is whether the shop owns a machine that can physically hold your part. A hydraulic manifold at 900 mm long and a Ø12 mm sensor housing need completely different setups, and a shop with only compact three-axis mills will quietly outsource the big one or decline it.

Ask for the actual working envelope of each machine group, not a single maximum number. When researching a CNC machining service, three numbers matter: the largest part the shop can cut in one setup, the largest part it can cut with repositioning, and the smallest feature it can reach without a specialty tool. A shop that gives all three is telling you it has run parts like yours before.

Machine count is a rough proxy for capacity, not for skill. A shop with 127 high-precision machines spread across 16 five-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers can move a job between cells when one spindle goes down. That redundancy is what keeps a 3 to 5 day ship window realistic.

Two things to check while you are still in the shortlist stage: tooling inventory and workholding. A shop that stocks carbide for aluminium and has soft jaws, vacuum plates and custom fixtures ready will quote a lower setup charge. A shop that has to buy a special cutter for your pocket will pass that cost to you.

1
Ask for travel in three axes plus rotary table sizeExample: 4,000 × 400 × 150 mm and a Ø400 mm rotary table.
2
Confirm which machines run lights-outUnattended hours are what compress lead time on repeat orders.
3
Check spindle options, not just spindle countHigh-speed spindles for small tools, high-torque for steel.

Section 2

Tolerance, surface finish and where each claim breaks down

A tolerance number on its own is not comparable between shops. ±0.005 mm on a 25 mm aluminium bracket is routine. The same number on a 600 mm steel plate is a different problem, because thermal drift and fixturing error grow with part size. When you compare quotes, compare the tolerance against the part dimensions and the material in the same row.

Surface finish follows the same logic. Ra 0.8–1.6 μm is a normal machined finish on most metals and covers a large share of functional parts. Ra 0.2–0.8 μm means slower passes, sharper tools and more frequent tool changes, so expect a higher unit price. Ra 1.6–3.2 μm is fine for brackets, covers and non-sealing faces and keeps cost down.

Here is the practical test. Send the shop a drawing with one or two truly critical dimensions marked and leave the rest at general tolerance. If the quote comes back with a note asking which features are functional, the shop reads drawings. If it comes back with a flat discount and no questions, the shop is pricing a commodity and the first article will show it.

Do not specify a tight finish on every face. It adds cost, adds inspection time and often has no function. Mark the sealing face, the bearing seat or the sliding surface, and let the rest run as machined.

1
Tie tolerance to part size±0.005 mm is not the same claim on a 25 mm part and a 600 mm part.
2
Pick finish by functionSealing and bearing faces get Ra 0.2–0.8 μm; covers get Ra 1.6–3.2 μm.
3
Mark critical features onlyA drawing with two flagged dimensions is easier to quote and inspect.

Section 3

Lead time, MOQ and how quote speed reveals capacity

Lead time has three parts and shops tend to quote the best one. Ask separately: how fast do they return a quote, how fast can they start, and how fast do parts ship after that. A shop that answers a quotation with a DFM analysis within 12 hours and can start production within 24 hours is running a light backlog. A shop that needs a week just to price the job is either overloaded or not set up for small orders.

Minimum order quantity is the clearest signal of who a shop wants as a customer. No minimum, from one prototype to 10,000+ part runs, means the shop has flexible setup and is willing to absorb the changeover. A high MOQ means the shop has optimized for long runs and your 20-piece order will sit behind a 50,000-piece automotive job. Neither is wrong, but only one matches your program.

Watch for the gap between prototype and production. Some suppliers quote prototypes cheaply and then re-quote at a much higher price once you need 5,000 pieces, or the reverse. Ask for pricing at three quantities in the first quote: one piece, 100 pieces and 1,000 pieces. The shape of that curve tells you where the real cost sits.

Late delivery risk is worth one direct question. A shop tracking its own on-time performance, with a historical late-delivery probability below 2%, will answer with a number. A shop that answers with adjectives has not measured it.

1
Split lead time into three numbersQuote turnaround, production start, ship date.
2
Price at three quantitiesOne, 100 and 1,000 pieces shows where setup cost stops dominating.
3
Ask about the backlogA queue that is too empty is as risky as one that is too full.

Section 4

Certifications, inspection and material traceability

Certifications work as filters. ISO 9001:2015 covers general quality management and is the baseline for most industrial work. IATF 16949:2016 applies to automotive and engine hardware. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you are uploading proprietary CAD files to a supplier portal.

Check the scope on the certificate, not just the logo. A certificate can cover one plant while your parts are cut at another. Ask which site will run the job and whether that site is in scope. If your parts are medical or automotive, also ask whether the shop can supply the documentation package your auditor expects, not just the parts.

Inspection is where a cheap quote becomes expensive. 100% inspection before shipment, with raw material check, in-process monitoring and final inspection, is the minimum for functional parts. Ask what equipment produces the report and whether reports come on request or by default. CMM data, first article inspection reports and material certificates should be listed in the quote, not added later.

Material traceability closes the loop. Aluminium grades such as 6061, 7075 and 6082, stainless 303, 304 and 316L, and titanium TC4 are not interchangeable, and a mill certificate should follow the batch. For regulated industries this is not optional paperwork. It is the difference between a part you can ship and a part you cannot.

1
Match the certificate to the industryISO 9001 for general work, IATF 16949 for automotive, ISO 13485 for medical.
2
Confirm the site in scopeCertificates apply to a plant, not to a brand.
3
Ask for mill certificatesGrade substitution is the most common hidden material risk.

How to run the comparison

A 6-step process for researching a CNC machining service

Work through these in order. Each step narrows the list before you spend time on detailed quotes.

1
Write down the part envelope and the materialNote overall size, the largest single feature, the material grade and any sealing or bearing faces. This alone eliminates shops that cannot reach your part.
2
Set your tolerance and finish per featureMark critical dimensions at ±0.005 mm and sealing faces at Ra 0.2–0.8 μm. Leave everything else at general tolerance and as machined finish.
3
Send the same RFQ package to three shopsUse one drawing revision, one material spec and one quantity list. Ask for pricing at one, 100 and 1,000 pieces.
4
Compare the quote structure, not the totalLine items for setup, material, machining, finishing and inspection let you see where each shop is cheap and where it is hiding cost.
5
Ask two technical questionsWhich machine will run this, and how will you hold the critical dimension. Vague answers are a warning sign.
6
Request a first article before the full runInspect it against the drawing, then release the balance. This costs one part but catches fixturing and datum mistakes.

FAQs

Questions buyers ask when researching a CNC machining service

How many quotes should I collect before deciding?

Three is enough for most parts. One incumbent, one shop you have not used before and one that specializes in your material or industry. More quotes add time without adding information, because the fourth and fifth shops usually land near the middle of the range.

If the three quotes are far apart, the cause is almost always a difference in scope, not a real price gap. Compare the line items before you assume one shop is cheaper.

Is a lower unit price always better?

No. A low unit price with an unstated setup charge, outsourced finishing or inspection billed separately often costs more than a mid-range all-in price. Ask for a quote that names setup, material, machining, finishing and inspection as separate lines so you can compare like for like.

Also check whether the price holds at higher quantities. A shop that is cheap at one piece but high at 5,000 pieces may not have the machining time or tooling to scale.

What tolerance should I actually specify?

Specify the loosest tolerance that still lets the part function. For most machined metal parts that means general tolerance on non-critical features and ±0.005 mm only where a fit, seal or bearing surface demands it. Over-tolerancing raises cost and inspection time without improving the assembly.

If you are unsure, mark the features you think are critical and ask the shop which ones actually drive cost. That conversation usually removes a few tight dimensions from the drawing.

Do I need an NDA before sending CAD files?

It depends on what you are sending. A simple bracket drawing carries little risk. A complete assembly with proprietary geometry deserves an NDA, and most established shops will sign one before you upload anything.

Ask how files are stored and who can open them. Information security certification is a reasonable filter if your parts are unreleased products rather than replacement components.

How do I judge a shop I have never used?

Start with a small, real order rather than a test part with no function. Send one or two pieces of a part you actually need, inspect them fully and watch how the shop handles any deviation. Response to a problem predicts the next 5,000 parts better than any sample.

Ask for the inspection report with the shipment. A shop that produces data by default is easier to work with than one that produces it only when asked.

When should I look at a different process instead?

CNC machining makes sense from one piece to a few thousand, especially when tolerance is tight or the geometry is complex. Below that, or when the shape is organic, 3D printing may be faster. Above a few thousand simple parts, die casting or vacuum casting can beat machining on unit cost.

A supplier that offers several processes and tells you honestly which one fits your volume is more useful than one that quotes every job as machining.