CNC machining services in Detroit: a sourcing checklist for engineers

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  Key takeaways
• 2
  What to compare before you award a Detroit CNC job
• 3
  Tolerance and finish: the two numbers that decide the shop
• 4
  Machine envelope and axis count: reading a capacity sheet
• 5
  Materials and finishes: what a Detroit job actually needs
• 6
  Certifications, inspection and paperwork
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  Lead time, MOQ and quote quality: the commercial checks
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  A 6-step supplier check before you send the PO

Most RFQ rejections trace back to one line on the drawing. Either the shop cannot hold it, or it can hold it but cannot measure it. Ask both questions in the same breath. A supplier who quotes ±0.005 mm should also tell you what CMM or optical comparator verifies that number. If the answer is vague, the tolerance is marketing.

Tolerance and finish travel together. A bore held at ±0.005 mm with a Ra 3.2 μm wall may still fail a seal or bearing fit. For sealing surfaces and sliding fits, aim for Ra 0.2–0.8 μm. For general mating faces, Ra 0.8–1.6 μm is normal. As-machined Ra 1.6–3.2 μm is fine for brackets and covers where nothing rubs.

Geometry matters more than the tolerance block. A thin wall at 1 mm will move after clamping no matter what the title block says. Deep pockets, long slender bores, and unsupported overhangs all push the real achievable tolerance looser. Send a STEP file, not just a PDF, so the shop can check wall thickness and tool reach before quoting.

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  Sealing and bearing fitsSpecify Ra 0.2–0.8 μm and a roundness callout, not just a diameter.
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  General mating facesRa 0.8–1.6 μm covers most bolted joints and dowel interfaces.
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  Covers and bracketsRa 1.6–3.2 μm as-machined keeps cost down with no functional loss.
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  Thin walls under 1.5 mmExpect to loosen tolerance or add a stress-relief step between operations.

A capacity sheet that lists 127 machines tells you the shop is busy. It does not tell you whether your part fits. Ask for travel dimensions on the specific machine that will run the job. Three-axis machines often sit in the 500 × 500 × 450 mm and 500 × 310 × 200 mm range. Mid-size mills run 750 × 1,150 × 550 mm or 600 × 600 × 600 mm. Large gantry work reaches 4,000 × 400 × 150 mm.

Five-axis changes the conversation. A 16-machine simultaneous 5-axis fleet can cut compound angles, impellers, and port geometry in one setup. That removes re-fixturing error, which is where most true-position failures come from. If your part has features on four or five faces, ask for the setup count. Two setups on a 3-axis mill will not match one 5-axis setup on position tolerance.

Watch for the mill-turn option. Parts with turned diameters and milled flats normally need two machines and two fixtures. A mill-turn center with a Ø400 mm rotary table does both in one cycle. That is usually the cheaper path for valve bodies, manifolds, and sensor housings.

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  Ask for the setup countEvery extra setup adds stack-up error, not just labor cost.
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  Match travel to the part, not the shopA 4,000 mm frame needs a machine that can reach both ends without repositioning.
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  Check the rotary table sizeØ400 mm covers most round parts under 350 mm diameter.

Material availability drives lead time more than machining speed. Aluminum 6061, 6061-T6, 7075, and 2024 are stock items at most shops. Stainless 303, 304, 316L, and 17-4PH are common. Tool steel, Inconel, and titanium TC4 (Ti-6Al-4V) are not. If your part is Inconel, ask whether the shop keeps bar stock or orders per job. That single answer can add days.

Finishes are where drawings get loose. Anodizing comes in clear, color, hardcoat, and conductive types, and they are not interchangeable. Hardcoat adds 0.025–0.05 mm per surface, which changes a press fit. Conductive anodize is for grounding paths. Electroless nickel, zinc, silver, and gold plating all have different thickness ranges and masking requirements.

Cosmetic parts need a finish callout plus a handling note. Bead blasting, tumbling, brushing, and polishing produce different textures on the same alloy. Laser marking needs a minimum character height of 1.5 mm to stay legible after anodize. Put that on the drawing, not in an email thread.

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  Anodize type changes dimensionsHardcoat builds 0.025–0.05 mm per surface; account for it in the fit.
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  Exotic alloys need a stock checkInconel and Ti-6Al-4V often ship as mill order, not from the shelf.
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  Marking after finishingLaser mark after anodize, not before, or the text washes out.

Certificates are not decoration. They define which industries a supplier can legally serve and what records they keep. ISO 9001:2015 is the baseline. IATF 16949:2016 is required for automotive production parts. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when your CAD files leave your network.

Inspection scope is the second half of that question. A 100% inspection before shipment, backed by raw material checks, in-process monitoring, and a final report, is different from a spot check on the first article. Ask what data ships with the box. For aerospace and medical work, a dimensional report and material cert are standard. For industrial brackets, a first-article report may be enough.

Confidentiality deserves a direct question. Uploads should be encrypted and access should be limited to the programming and machining team. If the shop cannot sign an NDA before you send files, that is a signal about how they treat customer drawings in general.

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  Match cert to industryIATF 16949 for auto, ISO 13485 for medical, ISO 9001 as the floor.
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  Ask what ships with the partsDimensional report, material cert, and finish cert should be named.
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  NDA before files, not afterA secure upload link and a signed NDA are the minimum.

Lead time is not one number. It breaks into quote turnaround, production start, and shipping. A shop that returns a quotation and free DFM analysis within 12 hours, starts production within 24 hours, and ships parts in 3–5 days is running a tight process. Historical late-delivery probability below 2% is the kind of figure you can ask for and verify against past orders.

MOQ tells you what kind of customer the shop is built for. No minimum order quantity means one prototype and a 10,000+ part run can come from the same supplier, with the same fixture strategy carried forward. That continuity matters. Re-quoting a new shop at production volume often means re-proving the process from scratch.

Finally, read the quote itself. A good quote names the material grade, the finish, the tolerance band, the inspection level, and the shipping terms. A one-line price with a lead time is a placeholder, not a quote. If the DFM feedback arrives with the price, the shop has already done engineering work on your part.

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  Split the lead timeQuote in 12 h, start in 24 h, ship in 3–5 days. Ask for each.
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  No MOQ keeps options openOne prototype to 10,000+ parts on the same process.
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  Quote should name the processMaterial grade, finish, tolerance band, inspection level, shipping terms.