Buyer guide

CNC Service Guide: How to Choose a Machining Supplier

This CNC service guide is written for design engineers and sourcing engineers who need metal and plastic parts built to print. It covers the numbers that actually decide a job: tolerance, lead time, order quantity, certification and how a shop quotes. Read it and you can tell in one review call whether a supplier fits your part.

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±0.005 mm toleranceNo MOQ12-hour quoteISO 9001 / IATF 16949

Custom auto spare parts made under a CNC service guide for 5-axis machining

On this page

1
Key takeaways
2
What to compare in a CNC service guide
3
Start with tolerance, not price
4
Lead time: separate the three stages
5
Order quantity, certification and data handling
6
How a CNC service guide reads a quote
7
Where the process does not fit
8
Five steps to qualify a CNC supplier

Quick answers

Key takeaways

Tolerance is a machine question±0.005 mm needs the right spindle, thermal control and in-process checks, not a promise in an email.

Lead time has three partsQuote, first cut and shipping. Ask which one the supplier is quoting.

MOQ tells you who they serveA shop with no minimum can take one prototype and still run 10,000 parts later.

Certificates map to industriesISO 9001 is general, IATF 16949 is automotive, ISO 13485 is medical, ISO 27001 covers data.

Quoting method shows experienceA supplier who flags thin walls and deep pockets before you ask has seen your part type.

Judging criteria

What to compare in a CNC service guide

Use this table as a checklist when you open a supplier's website or walk their floor.

Criterion	What to ask	Good sign	Warning sign
Tolerance	Can you hold ±0.005 mm on this feature?	Names the machine and the check method	Answers with a generic 'high precision'
Lead time	Is that quote, first cut, or shipping?	Splits the three stages clearly	One number with no definition
MOQ	Can you run one piece first?	Prototype to 10,000+ on one line	Minimum order blocks the prototype
Certification	Which certificates cover my industry?	ISO 9001, IATF 16949, ISO 13485, ISO 27001	Shows a logo with no number or scope
Inspection	What is checked before shipment?	100% inspection, reports on request	Sample check only, no report
DFM feedback	Do you review the model before quoting?	Free DFM analysis with the quote	Quote only, print as-is
Finishing	Which finishes are in-house?	Anodizing, plating, powder coat, laser mark	Every finish is subcontracted
Confidentiality	Will you sign an NDA?	NDA available on request	No answer on file handling

The short version

Judge a CNC supplier on tolerance, lead time, MOQ, certification and quote detail, in that order. Price comes last, because it only makes sense once the other five line up.

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Section 1

Start with tolerance, not price

Price is the last number you should compare. Tolerance comes first, because it decides which machine can make the part at all. A 3-axis mill with a vise can hold ±0.05 mm on a simple plate all day. Push toward ±0.005 mm and you need a different setup: a temperature-stable room, a spindle with low runout, and a probe or CMM check on the critical feature.

Ask the supplier to name the machine for your part, not the shop's best machine. A 5-axis center with a Ø400 mm rotary table is the right answer for a part with features on five faces. A 4,000 mm travel machine is the right answer for a long beam. If the answer is a generic 'we have many machines', you have not learned anything.

Surface finish is the second half of the same question. Ra 1.6–3.2 μm is a normal as-machined finish. Ra 0.8–1.6 μm needs a finer step-over and a good cutter. Ra 0.2–0.8 μm usually means a second operation. A supplier who quotes one finish number for the whole part has not read the drawing.

One more point. Tolerance costs money in a curve, not a line. Going from ±0.05 mm to ±0.01 mm might add 20 percent. Going to ±0.005 mm can double the setup time. Know which features truly need the tight number, and let the rest stay loose.

1
Ask per featureA tight tolerance on one bore is normal. A tight tolerance on the whole part is expensive.
2
Match machine to geometry5-axis for multi-face work, long-travel mills for large frames.
3
Check the finish calloutRa 0.8–1.6 μm is a common functional finish; Ra 0.2–0.8 μm often needs extra passes.

Section 2

Lead time: separate the three stages

A single lead time number hides the real risk. Break it into three stages: quote, production start, and shipping. A shop can promise parts in 3–5 days and still lose a week before the first cut if the quote took four days and the material was not in stock.

Good suppliers publish all three. At GreatLight, quotation and free DFM analysis come back within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. That structure lets you plan a build without guessing.

Material is the hidden variable. Aluminum 6061 and 7075 are usually on the shelf. Inconel, titanium TC4 (Ti-6Al-4V) and 17-4PH stainless often need to be ordered. Ask if the quoted lead time includes material purchase or starts after it arrives.

For repeat orders, ask about the historical late-delivery rate. A shop that tracks it can tell you. One that cannot is telling you something too.

1
Quote stage12 hours including DFM analysis is a reasonable benchmark.
2
Start stageProduction start within 24 hours means the material is already planned.
3
Shipping stage3–5 days for most machined parts, longer if finishing is subcontracted.

Section 3

Order quantity, certification and data handling

MOQ is a filter, not a rule. A shop with no minimum order quantity will run one prototype and then scale to 10,000+ parts on the same process. That matters because the prototype and the production part should come off the same machines and the same inspection plan.

Certification is the next filter, and each one points at an industry. ISO 9001:2015 is the general quality baseline. IATF 16949:2016 is what automotive and EV buyers look for. ISO 13485:2016 covers medical devices. ISO 27001:2022 is about information security, which matters when you upload CAD files.

Ask for the certificate scope, not just the logo. A certificate for a trading office does not cover a machine shop. The scope should name the site and the process.

Data handling is part of the same conversation. Your CAD files carry your design. Uploads should be secure and confidential, and a non-disclosure agreement should be available on request. If a supplier hesitates on an NDA, treat it as a real signal.

1
No MOQOne prototype to 10,000+ parts without changing the process.
2
Match cert to industryAutomotive IATF 16949, medical ISO 13485, general ISO 9001.
3
NDA on requestStandard practice for any supplier handling your CAD data.

Section 4

How a CNC service guide reads a quote

A useful quote has more than a price. It should list material grade, machine, tolerance, finish, inspection method and lead time. If the quote is one line with a total, you cannot compare it against another supplier.

Material grade is a common trap. Aluminum 6061-T6 and 7075 look the same on a quote labeled 'aluminum' but behave differently. 7075 is stronger and harder to machine. 6061-T6 welds and anodizes well. The same applies to stainless: 303 machines fast, 316L resists corrosion, 17-4PH can be heat treated.

Finishing should be itemized too. Anodizing (clear, color, hardcoat, conductive), electroless nickel, zinc, silver and gold plating, powder coating and black oxide are not interchangeable. Laser marking has a minimum character height of 1.5 mm, so small text needs a different method.

Finally, ask what happens if a dimension is out. A shop with 100% inspection before shipment and reports on request will catch it before it ships. That is the cheapest place to find a problem.

1
Line items matterMaterial, machine, tolerance, finish, inspection, lead time.
2
Grade changes everything7075 vs 6061, 303 vs 316L, Ti-6Al-4V vs TA2.
3
Inspection before shipment100% inspection with reports on request.

Section 5

Where the process does not fit

CNC machining is not always the right answer, and a good supplier will say so. If you need 50,000 identical small parts with no tight tolerance, die casting or injection molding will beat machining on unit cost. If the part is a thin shell with no critical dimensions, sheet metal fabrication or vacuum casting may be faster.

Machining wins when geometry is complex, tolerance is tight, or the quantity is low to medium. It also wins when the material is hard to cast, such as Inconel or titanium. A prototype in 6061 that becomes a die-cast part later is a normal path.

Ask the supplier what they would not machine. The answer tells you whether they are selling a process or solving your problem.

1
Consider casting or moldingHigh volume, loose tolerance, simple geometry.
2
Machining winsComplex geometry, tight tolerance, low to medium volume, hard materials.
3
Ask what they would not machineA clear answer is a good sign.

Review process

Five steps to qualify a CNC supplier

Run these in order. Stop at the first step that fails.

1
Send the model and the drawingInclude the STEP file, the 2D drawing with tolerance and finish callouts, the material grade, and the quantity. Mark the critical features. Missing callouts force the shop to guess.
2
Read the DFM feedbackCheck whether they flagged thin walls under 0.8 mm, deep pockets over 4× diameter, sharp internal corners, or features that need a second setup. This is where experience shows.
3
Compare the quote line by lineMaterial grade, machine type, tolerance, finish, inspection and lead time should each appear. A total price alone is not comparable.
4
Ask for the certificate scopeGet the certificate number and the site it covers. ISO 9001, IATF 16949, ISO 13485 and ISO 27001 each cover different things.
5
Run one prototype firstOrder a single part with no MOQ. Measure it against the drawing. If the first part is right, the production run usually follows.

FAQs

CNC service guide questions

What tolerance can a CNC shop really hold?

It depends on the feature, the machine and the material. A 5-axis center in a temperature-stable room can hold ±0.005 mm on a critical bore. Large aluminum frames may sit at ±0.05 mm because thermal growth over 4,000 mm is hard to control.

Ask per feature, not per part. The supplier should name the machine and the inspection method for the tight dimensions.

Is a lower price a sign of a worse shop?

Not always. A shop with in-house finishing, its own 5-axis machines and fast quoting can be cheaper because it removes subcontract steps.

A very low price with no line items is a different story. Check what material grade and what inspection are included.

Do I need to pay for DFM analysis?

At GreatLight, DFM analysis comes with the quote at no charge and returns within 12 hours. It is part of the quoting step, not a separate service.

If a supplier charges for DFM before quoting, ask what the fee covers and whether it is credited against the order.

How do I protect my design files?

Ask for a non-disclosure agreement before you upload CAD. A supplier handling confidential work should have a standard NDA available on request.

Check the information-security side too. ISO 27001:2022 covers how files are stored and who can access them.

Which finishes can be done in-house?

Common in-house finishes include anodizing (clear, color, hardcoat, conductive), bead blasting, tumbling, brushing, polishing, laser marking and engraving.

Plating such as electroless nickel, zinc, silver and gold, plus powder coating and black oxide, may be subcontracted. Ask which steps add to the lead time.

Can one shop handle both prototypes and production?

Yes, and it is usually better. Running the prototype and the production parts on the same machines and inspection plan avoids a second qualification round.

A shop with no minimum order quantity and capacity up to 10,000+ part runs can do both without changing suppliers.