How to Order CNC Machined Parts

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  Key takeaways
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  Prepare the file package so the first quote sticks
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  Set tolerances and surface finish you can actually inspect
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  Run a DFM pass on the model before you order
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  Read the quote before you release the order
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  Step by step: how to order CNC machined parts
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  Which process route fits your part

Most delays in ordering CNC machined parts start before the RFQ is sent. The shop receives a model with no tolerances, then a drawing arrives three emails later with different geometry. That forces a requote and costs a day. Build one folder first: a STEP or IGES model, a 2D PDF with revision block, and a short text note on function.

Export the model in the units you actually use and state it in the email. Mixed-unit files are a common source of wrong-scale parts. If the drawing says millimeters and the STEP is in inches, the first operation will scrap the blank.

Name the material and temper, not just the alloy family. 6061-T6 and 6061-O machine differently, and 17-4PH in the H900 condition behaves nothing like the annealed bar. If hardness matters for wear, say so on the drawing.

Finally, list any special treatment: anodizing color, heat treatment, plating, laser marking with minimum character height 1.5 mm. Each added process changes lead time and inspection points.

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  STEP AP214 or IGESOne solid, no surfaces-only files, no hidden construction geometry.
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  2D PDF with revisionDatum scheme, GD&T, threads, and surface finish callouts.
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  Function noteTwo lines on what the part does and which fit is critical.
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  Quantity and target datePrototype, pilot, or production — the answer changes the process plan.

A general tolerance block keeps the drawing readable. Something like ±0.1 mm on unmarked dimensions is standard for machined aluminum and steel, with tighter callouts only where a fit demands it. When every dimension is ±0.005 mm, the shop has to slow the machine, add setups, and inspect more, and the price reflects that.

Surface finish follows the same logic. Ra 3.2 μm is fine for brackets and covers. Ra 1.6 μm suits sealing faces and sliding contacts. Ra 0.8 μm or better usually means a finishing pass, a smaller stepover, or a secondary operation, so reserve it for the face that seals or wears.

Be careful with tolerances that depend on a datum you have not defined. If a hole pattern controls position but no datum is called out, the machinist picks one, and it may not match the assembly. Define the mating face or bore as the primary datum and build the rest from there.

Threads are another quiet trap. Call out the standard — M6 × 1, 1/4-20 UNC — and the thread depth or tap drill depth. A note like "thread M6" with no depth leaves the operator to guess, and deep tapped holes in stainless need a different cycle than in aluminum.

Design for manufacturability is not a philosophy, it is a list of geometry checks. Look for internal corners with zero radius. A cutter leaves its own radius, so a square internal corner requires EDM or a file. Add a corner radius at least one third of the pocket depth for milling.

Check wall thickness against the material. Aluminum can hold a thin wall better than stainless or titanium. Walls under 1 mm in metal tend to deflect during milling and chatter during turning. If the design needs a thin web, expect the shop to use light passes and possibly a support fixture.

Look at hole depth. A standard twist drill reaches about four to five times its diameter reliably. A Ø3 mm hole 40 mm deep is a gun-drill job, not a routine one. Reduce the depth, drill from both ends, or accept a higher price.

Finally, check tool access. A deep pocket with a small entry limits the cutter shank, and a part with features on five sides may need multiple setups or a 5-axis machine. Both are fine, but they belong in the quote discussion, not in a surprise email after the first article is cut.

A quote is a scope document. It should say which material, which finish, which tolerance, and which inspection are included. If the finish line says "as machined" and the drawing says anodized, the two do not match, and the shop will come back for a change order.

Lead time is another line worth reading carefully. A 3–5 day ship date usually assumes material in stock and no new fixturing. If the part needs a custom fixture or a special cutter, add time. Ask what the clock starts from: PO date, file approval, or material receipt.

For prototypes, ask what happens if the first article is out of tolerance. A good shop checks the critical dimension before shipping and contacts you rather than sending a part that will not fit. This is where a 100 percent inspection before shipment saves a week.

Finally, keep the paper trail. Quote number, PO number, revision, and inspection report belong in one thread. When a second order comes six months later, that thread is the fastest way to get the same part again.