CNC Machining Service Instructions: 7 Checks Before You Send a PO

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  Key takeaways
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  CNC machining service instructions by part type
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  Tolerance: what the number actually covers
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  Lead time: build the schedule from the quote stage
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  MOQ, materials, and finishing under one roof
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  Certifications and inspection: read the scope
• 7
  Where a precision shop is the wrong choice
• 8
  How to vet a supplier in seven steps

Every supplier advertises a tolerance. The useful question is which feature that number applies to. A shop that holds ±0.005 mm on a Ø20 mm bore may only hold ±0.05 mm across a 4,000 mm frame, because thermal drift over that length is larger than the servo error.

Ask for the tolerance per feature class: bores, flatness, hole position, and surface finish. On our 16 simultaneous 5-axis centers, ±0.005 mm is realistic on turned and bored features up to Ø400 mm. On a 4,000 × 400 × 150 mm travel, we quote ±0.05 mm and say so up front.

Surface finish travels with tolerance. Ra 0.2–0.8 μm needs a finishing pass and often a separate operation. Ra 1.6–3.2 μm comes straight off the machine on most aluminum and brass parts. If your drawing calls for Ra 0.4 μm on a deep pocket, expect a cost jump and a longer schedule.

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  Ask for tolerance by featureBore, flatness, and position each carry a different number.
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  Check the size range±0.005 mm on a 50 mm part is not ±0.005 mm on a 2,000 mm part.
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  Tie finish to toleranceRa 0.8–1.6 μm is the common as-machined band.

Lead time quoted at the PO stage is too late to negotiate. The number you want is at the quote stage: how fast does the supplier come back with a price and a DFM note? Ours is 12 hours, and production can start within 24 hours once you approve.

Parts then ship in 3–5 days for most jobs. That figure assumes material in stock and no missing information on the drawing. A single undefined thread callout can add a day of email. Write your drawing so a machinist can run it without asking.

Treat the delivery promise as a risk statement. Ask what share of jobs shipped late last year. Our historical late-delivery probability is below 2%, and we will share the basis for that number if you ask. A supplier who cannot describe how they measure it is guessing.

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  Quote stage, not PO stage12-hour quotes and 24-hour production start keep the program moving.
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  Complete drawings cut daysUndefined features are the most common cause of delay.
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  Ask how late delivery is measuredA number without a definition is not a commitment.

Minimum order quantity decides whether one vendor can carry your part from prototype to production. We have no MOQ, so a single prototype and a 10,000+ part run can go to the same shop. That removes a second round of first-article approval when the design is frozen.

Material range matters more than a long list. Aluminum 6061 and 7075, stainless 303 and 17-4PH, titanium Ti-6Al-4V, and PEEK each cut differently. A supplier who stocks the grade you need will quote days faster than one who has to source it.

Finishing is where schedules slip. Anodizing, electroless nickel, powder coating, and laser marking are often sent out. When the machine shop and the finisher are in the same building, the parts do not sit in a truck between operations. Laser marking needs a minimum character height of 1.5 mm to stay readable.

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  No MOQ keeps one vendorPrototype and production share the same process history.
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  Check stocked grades6061, 304, 17-4PH, and Ti-6Al-4V cover most drawings.
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  Keep finishing in-houseFewer handoffs means fewer scratches and fewer lost days.

A certificate on a website says the system exists. It does not say the certificate covers your part. ISO 9001:2015 covers general quality management. IATF 16949:2016 applies to automotive production parts. ISO 13485:2016 applies to medical devices. Match the scope to your industry before you ask for a sample.

Inspection depth is the second question. We inspect 100% of parts before shipment, with raw material check, in-process monitoring, and final inspection. Reports are available on request. If your drawing has a critical-to-function dimension, name it on the PO so it gets measured and recorded, not just machined.

Ask what happens when a dimension is out. A shop that catches it in-process reruns the part. A shop that catches it at final inspection ships late. The difference is where the measurement sits in the process, not how good the CMM is.

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  Match scope to industryISO 9001, IATF 16949, and ISO 13485 cover different products.
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  Name critical dimensionsA dimension not flagged may not be recorded.
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  Measure in-processCatching a drift early saves the whole run.

CNC machining is not always the answer. Above roughly 10,000 parts of the same geometry, die casting or injection molding usually wins on unit cost. CNC stays competitive when the design changes often or the volume is low.

Very thin walls below 0.5 mm on aluminum are a poor fit. Cutting forces deflect the part faster than the tool can follow. If the wall is structural, redesign it or accept a slower process with more passes.

Hard materials above 55 HRC wear cutters quickly. Milling plus EDM is often cheaper than trying to mill the finished form. The same applies to deep, narrow pockets with a high depth-to-diameter ratio: the tool has to be long, and long tools chatter.

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  High volume favors castingAbove about 10,000 identical parts, unit cost shifts.
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  Thin walls deflectUnder 0.5 mm on aluminum, expect to slow down.
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  Hard steel favors EDMAbove 55 HRC, cutter wear changes the cost model.