Medium Part CNC Machining Services: A Buyer's Checklist

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  Key takeaways
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  What separates a capable medium part supplier from a cheap one
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  Where medium parts sit in a machine shop
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  Tolerance claims and how to verify them
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  Material range tells you how flexible the shop really is
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  Post-processing and the hidden schedule risk
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  Lead time, quantity and what a quote should contain
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  How to qualify a medium part supplier in 6 steps

Medium is not a formal category. In practice it means a part too big for a bar-fed lathe and too small to justify a large gantry mill. Think transmission housings, pump bodies, robot arm links, manifold blocks, medical instrument frames and EV battery brackets.

That size band is awkward. The part is heavy enough that fixturing changes the result, but small enough that buyers expect short lead times and prototype-friendly quantities. A shop set up for watch-sized components will struggle. A shop set up for 4,000 mm weldments will overprice the job.

At GreatLight, medium parts run on machines with 750 × 1,150 × 550 mm and 600 × 600 × 600 mm travels. Those envelopes cover most housing and bracket work in one setup, which is the main driver of both accuracy and cycle time.

If your part needs more than 1,150 mm in X, it moves into the large-part category and the discussion changes. If it fits under 500 mm, a compact 3-axis or mill-turn cell is usually faster and cheaper. Knowing which side of the line you are on saves weeks.

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  Typical medium partFits inside a 600 mm cube, weighs 2–40 kg, needs 2 to 5 machined faces.
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  Setup count drives costEvery extra re-fixture adds stack-up error and queue time.
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  Prototype volume mattersOne to fifty pieces is normal at this stage.

A tolerance number on a website means nothing without a measurement method. ±0.005 mm on a 600 mm aluminium housing is achievable, but only on specific features, on a warm machine, with a probe or CMM to confirm it. It is not a blanket promise across every dimension on the drawing.

Ask three questions. Which features carry the tight tolerance? What instrument checks them? How many parts from the run get measured? A supplier who answers all three without hesitating has a real process. One who answers with a brochure has a sales process.

Temperature matters more as part size grows. Aluminium expands about 23 μm per metre per degree Celsius. A 600 mm part measured in a cold inspection room can read 15–20 μm off the value taken on the machine. Good shops normalize parts before final inspection.

For medium part CNC machining services, we quote ±0.005 mm where the geometry supports it and say so plainly when it does not. A part with a long unsupported bore or a thin wall may need ±0.02 mm, or a redesign. Telling you early is cheaper than a rejected lot.

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  General machining±0.05 mm is routine on most features and keeps cost low.
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  Tight features±0.005 mm on bores, journals and mating faces.
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  Thin wallsBelow 1.5 mm thickness, expect more scatter and more scrap risk.

Many shops advertise CNC work but only cut aluminium well. That is fine until your housing needs a 17-4PH shaft pressed into it, or the cover has to be PEEK for chemical resistance. Switching vendors mid-project costs more than paying a slightly higher hourly rate up front.

The material list also predicts cutting parameters. 6061-T6 runs fast with generous feed. 316L work-hardens and needs lower surface speed and a rigid setup. Inconel and TC4 need carbide tooling, coolant discipline and patience. A shop that treats them all the same will burn tools and miss finish.

Mixed-material assemblies are common in medium parts. An aluminium body with a stainless insert and a brass fitting has three thermal expansion rates. Machining them in one shop means one team owns the fit, rather than two suppliers blaming each other.

Ask for the exact grades, not families. "Stainless steel" is not an answer. 303, 304, 316L, 17-4PH and 440C behave differently and price differently. The same applies to aluminium: 6061, 7075 and ADC12 are not interchangeable.

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  Aluminium6061-T6, 7075, 2024, 5083, 6082, ADC12.
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  Stainless and steel303, 304, 316L, 17-4PH, 4140, 4340.
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  Titanium and superalloysTC4 (Ti-6Al-4V), Inconel, magnesium AZ31B.
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  PlasticsPOM, PEEK, PC, PA, ABS, carbon fibre.

Machining a medium part is often the fastest step. Anodizing, heat treatment, plating and final assembly can take longer than the cut itself, and they are where delivery dates slip. If the supplier outsources finishing, you inherit their queue.

Hardcoat anodizing adds 20–50 μm per surface and changes the fit. A bore that measured in tolerance before coating may not assemble after. Good shops compensate before anodizing and tell you the allowance they used.

Heat treatment moves dimensions. 4140 or 4340 parts that are rough machined, hardened and then finished need a second setup, and the shop must plan for distortion between the two. Quoting the part as a single operation is a red flag.

Engraving is the small detail. Laser marking with a minimum character height of 1.5 mm stays readable after coating. Below that, letters fill in and the part fails incoming inspection at your end.

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  Finishes availableAnodizing, electroless nickel, zinc, powder coat, black oxide.
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  Mechanical finishingBead blasting, tumbling, brushing, polishing.
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  Coating allowanceConfirm bore and thread compensation before hardcoat.

A useful quote arrives with more than a price. It states the machining envelope, the tolerance band, the finish, the material grade, the inspection plan and the lead time from drawing release. Anything missing becomes a change order later.

Lead time has three parts: quoting, machining and finishing. At GreatLight, quotation and DFM feedback come back within 12 hours, production can start within 24 hours of approval, and parts ship in 3–5 days for standard work. Post-processing adds to that window.

Quantity flexibility matters at the design stage. A supplier with no minimum order quantity lets you run one prototype, adjust the design, then scale to 10,000+ parts on the same process. If the shop forces a 500-piece first order, you are paying to validate their setup, not your design.

Confidentiality is part of the commercial package for defence, medical and unreleased consumer work. Uploads stay secure, and an NDA can be signed before drawings are shared.

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  Quote contentsPrice, tolerance, finish, material, inspection, lead time.
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  Order rangeOne prototype to 10,000+ part runs.
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  DFM feedbackFree analysis with the quote, within 12 hours.