Open CNC Service Guide

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  Key takeaways
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  What to check on an open CNC service quote
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  What open CNC service actually means
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  Tolerance, finish and the features that decide the machine
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  Machine capacity and part size limits
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  Materials and finishing capability
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  Lead time, MOQ and what the quote should contain
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  Common mistakes when buying open CNC service
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  Step by step: qualifying an open CNC service

An open CNC service is a project-based machining arrangement. You send a model and a drawing; the supplier programs, machines, inspects and ships the parts. You do not buy a machine, hire a programmer or maintain a tool crib. The capacity stays with the supplier and is booked against your order.

This model differs from captive machining inside a large company, where the machine shop only serves internal programs. In an open CNC service the same 5-axis centers, mill-turn lathes and CMM equipment are shared across many customers in different industries. That sharing is why a small team can get access to equipment it could not justify buying.

The trade-off is control. You give up direct sight of the machine and the setup sheet. What you get back is a first article, inspection data and a part. The whole selection problem comes down to how much evidence the supplier is willing to put in writing before and after the cut.

For buyers, the practical question is not whether the shop is large. It is whether the shop can hold your tolerance, hit your date and document both. Everything below is a way to test that before you release a purchase order.

Start with the tightest feature on the drawing, not the title block tolerance. A ±0.005 mm bore in a 6061 housing is a different job from a ±0.1 mm bracket outline. The first needs a controlled temperature, a warm-up cycle and a probe check. The second runs on any 3-axis mill.

GreatLight quotes ±0.005 mm on qualifying features. Surface finish is quoted in ranges: Ra 0.2–0.8 μm for sealing faces and bearing bores, Ra 0.8–1.6 μm for general mating surfaces, Ra 1.6–3.2 μm as-machined for non-critical faces. If your drawing calls Ra 0.4 μm across a deep pocket, expect a second operation and a longer cycle.

Feature access drives machine choice more than size does. Undercuts, compound angles and ports on five faces usually need a simultaneous 5-axis center so the part stays in one setup. Deep small-diameter holes need a mill-turn or a dedicated drilling cycle. A shop that only quotes 3-axis work will ask you to split the part or add a fixture.

Ask one direct question: which machine will run this, and how will you verify it? If the answer names a machine class and an inspection method, the quote is grounded. If it repeats the tolerance value, it is not.

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  5-axisCompound angles, undercuts, five-face work in one setup.
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  Mill-turnShafts, bushings and parts with turned and milled features.
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  3-axisPrismatic parts with features reachable from one direction.

Part size sets the floor on what a shop can accept. GreatLight runs 127 high-precision CNC machines across three wholly-owned plants covering 7,600 m², including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Maximum processing size is 4,000 mm.

Travel envelopes vary by machine. Large work runs to 4,000 × 400 × 150 mm. Mid-size work fits 750 × 1,150 × 550 mm or 600 × 600 × 600 mm. Compact cells cover 500 × 500 × 450 mm and 500 × 310 × 200 mm. A Ø400 mm rotary table handles round parts that need indexing around a bore.

Size alone is a weak filter. The stronger question is whether the shop can fixture your part stiffly at that size. A 3,000 mm frame with thin walls will chatter long before it runs out of travel. When you request a quote, note the wall thickness and any unsupported span so the process engineer can plan supports.

If your part sits near a travel limit, say so in the RFQ. Shops will often re-orient the setup or split the operation rather than decline, but only if they see the number early.

An open CNC service is only open if the shop stocks and machines your alloy. Typical coverage includes aluminium 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12; stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH; steel 1018, 1045, 4130, 4140, 4340, A36 and tool steel; copper and brass C101, C103, C110, beryllium copper, C27400, C28000 and C36000; titanium TA1, TA2, TC4, plus Inconel and magnesium AZ31B / AZ91D. Plastics run from ABS and PC to POM, PEEK, PA, PP, HDPE and carbon fibre.

Material choice changes the cutting strategy. 7075 aluminium machines cleanly but moves after stress relief. 316L work-hardens, so light radial cuts and constant feed matter more than spindle speed. Titanium TC4 needs low surface speed, high coolant pressure and sharp tools. Inconel punishes any interruption in the cut.

Finishing is where quotes diverge most. Anodizing in clear, colour, hardcoat or conductive grades, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing are all standard lines. Laser marking has a practical floor: minimum character height 1.5 mm. Below that, the mark may not read after anodizing.

Ask whether finishing is done in-house or subcontracted, and who inspects the part after it comes back. A masked anodize on a ±0.005 mm bore can add 5–10 μm of build-up on the diameter. If that matters, call it out on the drawing.

GreatLight returns a quotation and free DFM analysis within 12 hours, starts production within 24 hours of approval, and ships parts in 3–5 days. Historical late-delivery probability is below 2%. Treat those as the supplier's stated numbers, then ask how they are measured. A date tied to a released drawing and a confirmed PO is worth more than a general promise.

MOQ is the clearest signal of who the shop is built for. No minimum order quantity, from one prototype to 10,000+ part runs, means the same process and inspection route is used at both ends. That matters during NPI, when you may need one part today and 500 next month without re-qualifying a supplier.

A usable quote lists material grade and stock form, machine class, tolerance per feature, finish, inspection method, packing, and the assumptions that would change the price. If a line item is missing, the gap will surface later as a change order.

Certifications belong in this section, not in a logo strip. ISO 9001:2015 covers general quality systems. IATF 16949:2016 applies to automotive and EV programs. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which is what protects your CAD files. Match the certificate to your industry before you match the price.

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  Quote in 12 hoursIncludes free DFM analysis on the model.
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  Production in 24 hoursClock starts after drawing and PO are confirmed.
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  Ship in 3–5 daysStandard parts; finishing may add a step.
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  No MOQOne prototype or 10,000+ part runs.

The most expensive mistake is sending a STEP file with no drawing. The machinist then guesses datum, tolerance and finish. The part arrives, and it is wrong in a way nobody can call a defect because nothing was specified.

The second is treating the cheapest quote as the same scope. One shop quotes as-machined Ra 3.2 μm on all faces; another quotes Ra 0.8 μm on two sealing faces and includes hardcoat anodizing. The numbers differ because the work differs. Compare line by line, not total to total.

The third is skipping the confidentiality step. Uploads should be secure and confidential, with an NDA available on request before CAD leaves your network. This is routine for defense, medical and automotive work, and it should not slow anything down.

The fourth is ignoring inspection. 100% inspection before shipment, with raw material check, in-process monitoring and final inspection, plus reports on request, is the minimum for a part you cannot easily rework. If the supplier cannot describe the inspection route, the tolerance claim is unverified.