Sheet Metal CNC Machining Service: How to Pick a Supplier
This guide is for engineers and sourcing staff comparing a sheet metal cnc machining service before an RFQ goes out. It covers the checks that change the outcome: tolerance and finish, tooling cost, blank size, order quantity, inspection reports and certifications. Read it before you send drawings.
In this article
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Key takeaways
Which process fits your sheet metal part
Match the part to the process before comparing quotes.
| Part condition | Best process | What to watch |
|---|---|---|
| Prototype, 1–50 pieces | Laser cut + press brake | Bend relief and hole-to-bend distance |
| 500–10,000 pieces, fixed design | Hard tooling + stamping | Die cost and die ownership |
| Tight bore or mating face | CNC milling after forming | Datums shift after bending |
| Blank over 4,000 mm | Large-format cutting | Flatness over the full length |
| Enclosure with flanges | Form + mill + hardware insert | Stack-up across three bends |
| Cosmetic outer panel | Form, then anodize or powder coat | Grain direction and touch marks |
What to ask before you award the order
Score each supplier on the same six lines.
| Question | Weak answer | Strong answer |
|---|---|---|
| Can you hold ±0.005 mm on a formed flange? | Yes, we hold everything | Only on machined features; here is the datum plan |
| Who owns the die? | We keep it | Die ownership and transfer terms in writing |
| What is your inspection record? | We check every part | Raw material, in-process, final; reports on request |
| What is the real MOQ? | Depends | No MOQ; one prototype to 10,000+ parts |
| Which certificates apply to this line? | We are certified | ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, ISO 27001:2022 |
| What happens if the design changes? | New quote, new lead time | DFM feedback within 12 hours |
The short version
Pick the process from the part, not from the price list. Machined features need a machining step, formed features need bend control, and the crossover between laser and hard tooling depends on volume and design stability. Send the drawing set with datums and functional features marked, and ask for DFM feedback with the price.
What a sheet metal cnc machining service actually covers
A sheet metal cnc machining service is a mixed process. Sheet is cut flat, formed on a press brake or die, then machined where the formed geometry cannot hold tolerance on its own. The CNC work usually lands on holes, slots, counterbores, tapped bosses, sealing faces and mating edges. Cutting and bending get the shape. Milling and turning get the interface.
This matters at the quoting stage. If a supplier prices your part as pure fabrication, the tolerance callouts on the drawing will be answered by bend angle error, not by a machined datum. Bends typically hold around ±0.5° to ±1° on angle, and hole position after forming drifts with material springback. A ±0.005 mm position callout on a formed flange is not a fabrication tolerance. It is a machining tolerance, and it needs a machining step.
The practical split: anything that touches another part, seals, or carries a bearing gets machined after forming. Everything else can stay as formed. Send the assembly context with the drawing so the shop can tell which holes are functional and which are clearance.
- 1Formed onlyBrackets, covers, brackets, mounting plates with clearance holes.
- 2Formed then milledBores, counterbores, seal faces, flat mating surfaces, tapped bosses.
- 3Mill first, form secondRare. Only when a feature must be machined in the flat to keep a datum.
How to read tolerance and surface finish on a sheet metal drawing
Put the tolerance block where it belongs. General tolerances of ±0.1 mm on a formed part are realistic. Tolerances of ±0.005 mm belong only on machined features, and they should be called out individually, not in the title block. When a drawing says ±0.005 mm across the whole sheet metal part, the shop either quotes a machined blank or it quotes a number it cannot hold.
Finish follows the same logic. An as-machined surface sits around Ra 1.6–3.2 μm. A high-finish machined face is Ra 0.8–1.6 μm, and fine finishing reaches Ra 0.2–0.8 μm. Formed and coated panels rarely need better than the coating texture. Do not put a fine Ra callout on a powder-coated face. The coating defines the surface, not the substrate.
One more trap: datums. A hole dimensioned from a bent edge will move when the bend angle moves. Dimension functional holes from a machined datum or from a pair of tooling holes. That single change removes most of the back-and-forth during first article inspection.
- 1General form tolerance±0.1 mm is a normal starting point for cut and bent sheet.
- 2Machined feature tolerance±0.005 mm is achievable on milled bores and faces.
- 3Finish calloutsMatch Ra to the function, not to habit.
Blank size, thickness and material: the limits that matter
Maximum part size decides which supplier can even bid. GreatLight runs a 4,000 mm maximum processing size, with a large travel of 4,000 × 400 × 150 mm. Medium work sits at 750 × 1,150 × 550 mm or 600 × 600 × 600 mm. Compact work runs on 500 × 500 × 450 mm and 500 × 310 × 200 mm platforms. If your blank is 1,800 mm long, the compact machines are out before price is discussed.
Thickness changes the tooling. Thin sheet under 1 mm deflects during clamping and needs light finishing passes. Sheet above 6 mm starts to behave like plate: cutting speed drops, bend radii grow, and springback needs compensation. Tell the shop the thickness range up front. A quote built on 1.5 mm sheet will not survive a switch to 5 mm.
Material availability is the third limit. Common grades such as 6061, 304, 316L, 5052 and 1018 are routine. Titanium grades like TA2 or TC4, Inconel, and magnesium AZ31B need a callout and often a longer material lead time. If the drawing specifies 17-4PH for a bracket that sees no corrosion load, ask whether 304 does the job at lower cost.
- 1LargeUp to 4,000 mm; travel 4,000 × 400 × 150 mm.
- 2Medium750 × 1,150 × 550 mm and 600 × 600 × 600 mm.
- 3Compact500 × 500 × 450 mm and 500 × 310 × 200 mm.
- 4Rotary workØ400 mm rotary table for round or indexed features.
Order quantity: where sheet metal cnc machining service wins and loses
Below roughly 500 pieces, laser cutting plus press brake forming is usually the cheaper route. There is no die to amortize, design changes are cheap, and the setup cost is absorbed in the first part. This is the zone where a sheet metal cnc machining service makes sense for engineers still iterating on a design.
Above a few thousand pieces with a frozen design, hard tooling starts to pay back. The die cost is real, but the per-part cycle time drops sharply. Ask for the crossover quantity instead of guessing. A supplier that can quote both routes and show where they cross is more useful than one that only sells the process it owns.
GreatLight sets no minimum order quantity, so a single prototype and a 10,000+ part run both go through the same shop. That matters when the prototype and the production part must come from the same process window — otherwise the first article you approve is not the part you will receive.
- 11–50 piecesLaser plus press brake. Expect design changes.
- 2500–10,000 piecesCompare hard tooling against soft tooling.
- 310,000+ piecesHard tooling, dedicated line, tighter cycle time.
Certifications, inspection and what the paperwork really proves
Certificates are scoped documents. ISO 9001:2015 covers the quality system. IATF 16949:2016 applies to automotive production. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters if your drawings are confidential. The right question is not whether the supplier holds them, but whether they cover the site and the process your part will run on.
Inspection is where the paperwork meets the part. A serious shop checks incoming material, monitors during production, and inspects before shipment. Reports should be available on request: material certificates, dimensional reports, first article inspection. Ask for the report format early. If a supplier cannot describe its inspection sequence in one paragraph, the certificate is decoration.
For medical and automotive programs, add traceability. Lot numbers on the material, revision control on the drawing, and a record of which machine ran the part. This is standard practice, but it has to be requested before production, not after.
- 1Quality systemISO 9001:2015
- 2AutomotiveIATF 16949:2016
- 3Medical devicesISO 13485:2016
- 4Information securityISO 27001:2022, NDA available on request
Lead time, quotation and the DFM feedback loop
Ask what happens in the first 24 hours. A useful benchmark: quotation and free DFM analysis within 12 hours, production start within 24 hours, parts shipping in 3–5 days. Those numbers assume the drawing is complete. Missing bend radii, unclear datums, or a thread callout without depth will each add a round trip.
The DFM response is the most valuable part of the quote. A good one flags a hole too close to a bend line, a bend radius smaller than the material allows, a tolerance that needs a second operation, or a finish that will not survive forming. That feedback should arrive before you commit, not after the first article fails.
For high-mix work, the supplier's historical delivery record matters more than the quoted date. GreatLight reports a historical late-delivery probability below 2%. Treat that as a supplier-side figure, and ask for the on-time record for parts similar to yours.
- 1Quote and DFMWithin 12 hours.
- 2Production startWithin 24 hours.
- 3Shipping3–5 days for standard formed and machined parts.
Finishing, marking and assembly: the steps after machining
Finishing is where sheet metal parts are most often damaged. Anodizing, powder coating, electroless nickel, zinc plating and black oxide all change dimensions slightly. Threads and bores that must stay on size should be masked. Ask how masking is handled, and whether the finish thickness is included in the tolerance stack.
Laser marking and engraving have a physical limit: minimum character height 1.5 mm. Any logo, part number, or serial smaller than that will not be legible after coating. Put the marking requirement on the drawing with size and location, and decide whether it goes under or over the finish.
If the part ships as a sub-assembly, say so. Pressed-in hardware, rivet nuts, standoffs and captive screws all add process steps and inspection points. They also change the packing method. A flat stack of panels is easy to ship; a welded frame with hardware is not.
- 1Mask before coatingThreads, bores and sealing faces.
- 2Marking limit1.5 mm minimum character height.
- 3HardwareRivet nuts, standoffs and inserts add inspection points.
Step by step: running the quote properly
Seven steps from drawing to purchase order.
- 1Send the full drawing set3D model plus 2D drawing with the tolerance block, datums and finish callouts. Missing datums cause the most rework.
- 2State quantity and annual volumePrototype count, pilot count and projected annual volume. The crossover between laser and hard tooling depends on all three.
- 3List the functional featuresMark holes, bores and faces that mate with other parts. Everything else can stay as formed.
- 4Request the DFM note with the priceAsk for specific feedback, not a generic statement. Expect a response within 12 hours.
- 5Confirm tooling and ownershipIf a die is required, agree on cost, ownership and where it is stored before the PO is issued.
- 6Agree on inspection and reportsDecide which reports are needed: material certs, dimensional report, first article inspection.
- 7Lock the revisionConfirm the drawing revision and date. Any change after this point resets the first article.
Questions engineers ask before ordering
Can a sheet metal cnc machining service hold ±0.005 mm on a bent part?
Not on the bend itself. Formed geometry typically holds ±0.5° to ±1° on angle, and hole position shifts with springback.
The ±0.005 mm figure applies to machined features — bores, counterbores, sealing faces and flat mating surfaces — that are cut after forming or in a separate operation. Call these out individually on the drawing so the shop can plan the operation sequence.
What is the largest sheet metal part you can machine?
GreatLight handles a 4,000 mm maximum processing size, with a large travel of 4,000 × 400 × 150 mm.
Medium work runs on 750 × 1,150 × 550 mm or 600 × 600 × 600 mm platforms, and compact work on 500 × 500 × 450 mm and 500 × 310 × 200 mm. Check the blank size against these before you request a quote.
Do you have a minimum order quantity?
No minimum order quantity. Runs go from a single prototype up to 10,000+ parts.
Keeping prototype and production in the same shop matters when the first article has to represent the production process.
When is hard tooling cheaper than laser cutting?
It depends on quantity and design stability. Below roughly 500 pieces, laser cut plus press brake usually wins because there is no die to amortize.
Above a few thousand pieces with a frozen design, hard tooling reduces cycle time enough to pay back. Ask the supplier to quote both routes and show the crossover point.
How do you handle confidential drawings?
Uploads are handled as secure and confidential, and an NDA is available on request.
GreatLight also holds ISO 27001:2022 for information security, which covers how design data is stored and accessed.
Which finishes are available after forming?
Anodizing (clear, colour, hardcoat, conductive), electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing.
Laser marking and engraving are available with a minimum character height of 1.5 mm. Mask threads and bores that must stay on size.
Send your sheet metal drawings for review
Quotation and free DFM analysis within 12 hours. No minimum order quantity, from one prototype to 10,000+ parts.
12-hour quote100% inspection before shipmentNDA on request
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