Buyer guide

Knowledge that circuit factory sellers must know

A purchasing checklist for teams that buy machined parts, housings and hardware from circuit factory sellers and their sub-suppliers. It covers the checks that decide whether a quote is usable: tolerance, lead time, MOQ, certifications and how the shop reports inspection data. Read it before you send the next RFQ.

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±0.005 mm toleranceNo minimum order quantity12-hour quote and DFM

Custom auto spare parts machined by circuit factory sellers using 5 axis CNC

On this page

1
Key takeaways for buyers
2
What to check before you place an order
3
Tolerance and surface finish: what a quote should state
4
Lead time, MOQ and the numbers behind them
5
Certifications and inspection: read the scope
6
Matching the part to the machine
7
Step by step: qualifying a supplier

Short version

Key takeaways for buyers

Tolerance is a range, not a promiseAsk which feature carries ±0.005 mm. A blanket claim on a drawing with loose bores wastes money.

Quote speed hides costA 12-hour quote means someone read your drawing. A same-hour price with no DFM notes usually means nobody checked.

MOQ blocks small buildsNo minimum order quantity matters when you need one prototype and a 10,000+ part run from the same process.

Certificates cover only what they nameISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022 cover different scopes. Match each to your part.

Judgement criteria

What to check before you place an order

Use the middle column as your baseline when you compare circuit factory sellers.

Check	Baseline to ask for	Red flag
Tolerance	±0.005 mm on critical features only	Blanket ±0.005 mm on every dimension
Finish	Ra 0.8–1.6 μm on sealing faces	Ra value quoted with no measurement method
Quote turnaround	Quotation and DFM notes within 12 hours	Price with no machining feedback
First article	Inspection report on request	Verbal 'it is fine' only
Minimum order	No MOQ, one part to 10,000+	MOQ pushed up after tooling starts
Certifications	ISO 9001:2015, IATF 16949:2016 on scope	Certificate with no scope statement
Data handling	NDA on request, secure uploads	Drawing sent over plain email
Late delivery	Historical late rate below 2%	No delivery history shared

Tolerance and finish

Tolerance and surface finish: what a quote should state

A tolerance number on its own tells you little. The useful question is which features carry it. On a milled aluminium housing, a bearing bore at ±0.005 mm and a clearance hole at ±0.2 mm cost very different amounts of machine time. When circuit factory sellers quote a single blanket tolerance across the whole drawing, they are either padding the price or they have not read the drawing. Ask them to mark the tight features.

Finish follows the same rule. Ra 0.2–0.8 μm is a fine finish that usually needs a separate operation after milling. Ra 0.8–1.6 μm comes off a good finishing pass on a 5-axis machine and covers most sealing faces and sliding surfaces. Ra 1.6–3.2 μm is as-machined and fine for brackets, covers and internal structure. Paying for a fine finish on a non-functional face is one of the easiest ways to overspend on a machined part.

Material choice moves the tolerance you can hold. Aluminium 6061-T6 and 7075 hold tight bores well because they are stable after stress relief. Stainless 304 and 316 work-harden, so deep pockets and small bores need slower feeds and more tool changes. Titanium TC4 (Ti-6Al-4V) and Inconel hold tolerance but cut slowly, and thin walls below 1 mm will deflect no matter how good the machine is.

1
Send a marked-up drawingHighlight the two or three features that carry the real tolerance.
2
State the finish per faceNot one Ra value for the whole part.
3
Flag thin wallsBelow 1 mm on titanium or stainless, expect a design conversation.

Lead time and MOQ

Lead time, MOQ and the numbers behind them

Lead time claims are easy to make and hard to verify. 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 for most machined work. The historical late-delivery probability is below 2%. Those numbers rest on 127 high-precision CNC machines across three wholly-owned plants, so a job does not sit in a queue behind one bottleneck machine.

Ask how a shop handles a schedule slip before you award the order. A useful answer names the alternate machine or the second shift. A useless answer promises it will not happen. For circuit factory sellers buying machined enclosures on a build schedule, a two-day slip on a housing can stall an assembly line, so the recovery plan matters more than the original date.

Minimum order quantity is the other quiet cost. No minimum order quantity means you can run one prototype and then a 10,000+ part run from the same process, so the parts you validate are the parts you ship. When a shop quotes low on the first part and then raises the price at volume, the prototype was never representative. Ask for the price at one, one hundred and one thousand pieces in the same quote.

1
Ask for the price ladderOne, 100 and 1,000+ pieces in one document.
2
Ask what happens on a slipAlternate machine, second shift, or partial shipment.
3
Check the prototype processSame machine type as production, not a different route.

Certifications

Certifications and inspection: read the scope

Certificates are scope documents, not badges. ISO 9001:2015 covers a quality management system. IATF 16949:2016 adds automotive-specific requirements around traceability and change control. ISO 13485:2016 is the medical device standard. ISO 27001:2022 covers information security, which matters when you send confidential drawings. A shop can hold all four and still not have applied them to your part family, so ask which certificate covers the process that will run your job.

Inspection is where the certificate becomes real. A 100% inspection before shipment, with raw material check, in-process monitoring and final inspection, is the baseline worth paying for. Reports on request are the second half of that sentence. If a supplier will not send dimensional data on the first article, the certificate is decoration. For a housing with a sealing face, one CMM report on the first part prevents a full batch rework.

Confidentiality belongs in the same conversation. Uploads should be secure and confidential, and an NDA should be available on request rather than treated as a special favor. For circuit factory sellers handling customer designs, a supplier who resists an NDA is telling you how they treat drawings in general.

1
Match certificate to part familyAutomotive work needs IATF 16949:2016 in scope.
2
Ask for a first article reportDimensional data, not a pass or fail stamp.
3
Put the NDA in place earlyBefore drawings leave your side.

Capability fit

Matching the part to the machine

Most quoting errors come from a part being sent to the wrong machine class. A single-setup 5-axis job suits a part with features on five faces, angled holes, or a complex contour that would need three fixtures otherwise. A 3-axis job with two setups is often cheaper for a simple plate. A mill-turn center makes sense when a part has turned and milled features and concentricity between them matters.

Size sets the ceiling. GreatLight runs a 4,000 mm maximum processing size, with travels of 4,000 × 400 × 150 mm for long parts, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm for mid-size work, and 500 × 500 × 450 mm plus 500 × 310 × 200 mm for compact parts. A Ø400 mm rotary table covers most round work that needs indexing. If your part is longer than the travel on the machine class being quoted, the shop will split it into setups and the tolerance will suffer.

Volume changes the answer again. Below a few hundred parts, machining is usually the right call. Above that, die casting or vacuum casting can win on unit cost, with machining left for the critical faces. The mistake is choosing a process by unit price alone and ignoring the tooling cost and the lead time it adds.

1
Count the faces with featuresMore than two sides usually favors 5-axis.
2
Check travel against part lengthA split setup costs tolerance.
3
Compare total cost, not unit costInclude tooling and the added lead time.

Before you award

Step by step: qualifying a supplier

1
Send a complete data pack3D model in STEP, 2D drawing with tolerances and finish per face, material grade, quantity at one and at volume. Missing finish callouts are the most common reason a quote comes back vague.
2
Ask for the DFM notesA real shop returns comments within 12 hours: thin walls, tool reach, features that need a tighter tolerance than the function requires. No comments on a complex part is a warning.
3
Confirm which certificate covers your processName your industry. Automotive work goes under IATF 16949:2016, medical under ISO 13485:2016, and confidential designs under ISO 27001:2022.
4
Agree the inspection planState which features get measured, at what frequency, and in what format the report arrives. First article plus in-process checks plus final inspection is the baseline.
5
Lock the price ladder and MOQOne, 100 and 1,000+ pieces in writing. Confirm there is no minimum order quantity and no price step that appears only after tooling starts.
6
Agree the schedule and the recovery ruleConfirm the ship window and ask what happens on a slip. Get the answer in the same email as the price.
7
Place a small first orderRun one or two parts, measure them yourself, then release the volume. A prototype that matches production is the point of the exercise.

FAQs

Questions buyers ask

What tolerance can a machined housing realistically hold?

±0.005 mm is achievable on critical features such as bearing bores and sealing faces, measured on a CMM. It is not realistic as a blanket tolerance across every dimension on the drawing, and asking for it that way raises the price without improving the part.

Finish runs alongside tolerance. Ra 0.8–1.6 μm suits most sealing and sliding faces, while Ra 0.2–0.8 μm needs an extra operation and should be reserved for the faces that need it.

How fast should a quote come back?

A quotation and free DFM analysis within 12 hours is a reasonable benchmark for a machined part, and production can start within 24 hours once the order is released. A price that arrives in minutes with no engineering comments usually means nobody opened the model.

For repeat work, ask for the price ladder in the same document: one piece, 100 pieces and 1,000+ pieces. That makes the volume step visible before you commit.

Is no minimum order quantity realistic?

Yes, when the shop runs the prototype and the production parts on the same machine class. GreatLight quotes from one prototype to 10,000+ part runs with no minimum order quantity, which lets you validate the exact process you will scale.

Watch for the opposite pattern: a cheap single part on one route and a different production route at volume. The prototype then proves nothing about the shipped parts.

Which certification do I actually need?

Match the certificate to your industry and your part. ISO 9001:2015 is the general quality baseline. IATF 16949:2016 covers automotive work. ISO 13485:2016 covers medical devices. ISO 27001:2022 covers information security and matters when drawings are confidential.

Then ask for the scope statement and the first article inspection report. A certificate without a report is a document, not evidence.

How do I keep drawings confidential?

Send them through a secure upload channel, put an NDA in place before the first file moves, and confirm who inside the shop can see the model. Uploads should be secure and confidential, and an NDA should be available on request.

If a supplier treats an NDA as an unusual request, treat that as information about how the drawings will be handled later.

What size part can be machined in one setup?

It depends on the machine class. Long parts run up to 4,000 mm maximum processing size on travels of 4,000 × 400 × 150 mm. Mid-size work sits at 750 × 1,150 × 550 mm or 600 × 600 × 600 mm. Compact parts run on 500 × 500 × 450 mm and 500 × 310 × 200 mm travels, with a Ø400 mm rotary table for indexed round work.

If the part exceeds the travel, the shop splits it into setups. That adds cost and usually loosens the tolerance between features.