Normal CNC Machining Service: What to Check Before You Order

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  Key takeaways
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  When a normal CNC machining service is enough, and when it is not
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  What a normal CNC machining service actually covers
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  How to judge a supplier for a normal CNC machining service
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  Where the money goes on a standard machining quote
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  Tolerance, finish and size limits to write into the RFQ
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  Common mistakes buyers make with a standard machining order
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  Step by step: from RFQ to first article

A normal CNC machining service is the baseline offer at most job shops: 3-axis milling and 2-axis turning. The workpiece sits on a table or in a chuck, and a cutter moves along X, Y and Z. A lathe spins the part and feeds a single-point tool in X and Z. That is the whole machine envelope in most cases.

This setup handles prismatic parts. Blocks, plates, housings, shafts, bushings, flanges, spacers, brackets. If the part has features on one face or can be flipped once, a normal machine cuts it well. If the part is a turbine blade or a complex manifold with ports on five sides, it does not.

The word normal is not a quality grade. It describes the machine axis count and the number of setups, not the tolerance the shop can hold. A well-run 3-axis mill with ground tooling and a temperature-controlled room can hold ±0.005 mm. A poorly run 5-axis machine cannot. The axis count tells you what shapes are possible, not how accurate the shop is.

So when a buyer asks for a normal CNC machining service, the useful question is not whether the shop is good. It is whether your part geometry matches a 3-axis or 2-axis envelope, and whether the shop has the inspection and process control to hold the tolerance you put on the drawing.

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  3-axis millingOne spindle, three linear axes. Best for plates, pockets and drilled holes.
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  2-axis turningRound parts with diameters and faces. Fast for shafts and bushings.
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  Multiple setupsEach extra face adds a fixture, a re-clamp and tolerance stack-up.
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  Standard stockBar, plate and billet. No near-net forging or casting required.

Start with the tolerance band on your drawing. If the tightest callout is ±0.05 mm, most shops can hit it. If it is ±0.005 mm, ask how they hold it. The answer should mention thermal control, tool wear offsets and in-process probing. A shop that says our machines are accurate is not answering the question.

Next, look at the finish callout. Ra 1.6–3.2 μm is as-machined and comes off the cutter. Ra 0.8–1.6 μm needs a finishing pass with light depth of cut and a sharp insert. Ra 0.2–0.8 μm usually means extra passes plus polishing, and the cost steps up sharply. Put the finish on the drawing only where the part needs it. A whole part at Ra 0.4 μm is rarely necessary.

Then check the setup count. Every time the part is unclamped and rotated, you add fixture error. A part that needs four faces machined on a 3-axis machine has four setups. On a 4-axis or 5-axis machine it may need one. Ask the shop how many setups they plan. If they cannot say, the quote is a guess.

Finally, match the certification to the industry. ISO 9001:2015 is the baseline. IATF 16949:2016 matters for automotive and EV parts. ISO 13485:2016 matters for medical devices. ISO 27001:2022 covers information security, which matters if you send CAD files under NDA. Do not pay for certifications your program does not need, and do not skip the ones your customer audits.

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  ToleranceMatch the tightest callout to the shop's stated process control, not the machine spec sheet.
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  FinishSpecify Ra only on functional surfaces. Cosmetic areas can run rougher.
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  Setup countEach extra face adds cost and error. Ask for the plan before you approve.
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  CertificationISO 9001 baseline; add IATF, ISO 13485 or ISO 27001 by industry.

Machine hour rate is the number buyers compare first, and it is the least useful one on its own. A cheap hourly rate with four setups and a hand-polish step costs more than a higher rate with one setup and a clean finish off the cutter. Ask for the setup count and the operations list, not just the rate.

Material drives a large share of the price on small runs. Aluminium 6061 and 7075 cut fast. Stainless 316 and 17-4PH cut slowly and wear tooling. Titanium TC4 (Ti-6Al-4V) and Inconel cut very slowly and need rigid setups and low feed rates. The same geometry in 6061 and in Inconel can differ by a factor of three or more in cycle time.

Tolerance and finish are the next two levers. Going from ±0.05 mm to ±0.005 mm adds inspection time and may add a finishing pass. Going from Ra 3.2 to Ra 0.8 adds a separate light-cut pass. Both are real costs. Put them on the drawing only where the function requires them.

Quantity changes the picture. On one to ten parts, programming and fixturing dominate. On 1,000 parts, cycle time and material dominate. A normal CNC machining service with no minimum order quantity lets you buy one prototype and a 10,000-part run from the same process, so the first article and the production part come off the same machine.

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  Setup countThe biggest hidden cost. Ask for the operations list.
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  MaterialInconel and titanium can triple cycle time versus 6061.
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  ToleranceTighter bands add inspection and finishing passes.
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  QuantityLow volume pays for programming; high volume pays for cycle time.

Put a tolerance band on the RFQ, not a single number. ±0.005 mm (±0.0002 in) is a practical floor for a well-controlled normal machining process on small features. On a 500 mm part, thermal expansion alone moves the dimension more than that over a 10 °C shop swing. If your callout is tighter than the part size supports, expect a conversation.

Finish should be split by surface. As-machined at Ra 1.6–3.2 μm is fine for brackets, covers and non-sealing faces. Ra 0.8–1.6 μm suits sealing faces and sliding contacts. Ra 0.2–0.8 μm suits bearing bores and optical mounts. Mark these on the drawing so the shop does not polish the whole part.

Size limits matter for the quote. Standard 3-axis machines at GreatLight cover travels of 500 × 500 × 450 mm, 500 × 310 × 200 mm, 600 × 600 × 600 mm and 750 × 1,150 × 550 mm. Larger work goes on a machine with 4,000 × 400 × 150 mm travel, up to 4,000 mm maximum processing size. If your part is over 1,150 mm, say so in the RFQ so the shop routes it correctly.

Add the material grade, the quantity, the finish, the tolerance and the certification requirement to the same document. That set of five items is what lets a shop return a real quote in 12 hours instead of a range.

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  Tolerance±0.005 mm floor on small features; loosen as part size grows.
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  FinishSplit by surface: Ra 3.2 cosmetic, Ra 1.6 sealing, Ra 0.4 bearing.
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  SizeCompact travels start at 500 × 310 × 200 mm; large work up to 4,000 mm.
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  RFQ setGrade, quantity, finish, tolerance, certification.

The first mistake is quoting a 3D model without a 2D drawing. The model shows nominal geometry. It does not show which faces are datums, which holes are press-fit, or which surface must seal. Without that, the shop guesses, and the guess shows up in the first article.

The second is putting a blanket tolerance in the title block. A ±0.01 mm general tolerance on every dimension makes the part expensive and hard to inspect. Call out tight tolerances only on the features that need them. Leave the rest at ±0.1 mm or looser.

The third is ignoring the setup plan. A part with features on four sides quoted on a 3-axis machine needs four fixtures. If the quote does not mention that, it is understated. Ask before you place the order.

The fourth is skipping the DFM review. A free DFM check catches deep pockets that need a long-reach cutter, thin floors that will chatter, and holes that break into a wall. Fixing it on the drawing costs nothing. Fixing it after the first article costs a week.

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  Model onlyNo datums, no fit intent. Send a 2D drawing or a PDF with callouts.
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  Blanket toleranceTight title-block tolerance inflates cost across the whole part.
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  Missing setup planMulti-face parts cost more than the hourly rate suggests.
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  No DFM checkCatch chatter and tool-reach problems before cutting metal.