Clear CNC Machining Service: What to Check Before You Order

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  What clear machining actually means
• 2
  Five checks, and what a strong answer looks like
• 3
  Tolerance claims need a feature size attached
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  Inspection paperwork separates a clear CNC machining service from a clean-looking one
• 5
  Setup count drives both accuracy and price
• 6
  Lead time: when does production actually start?
• 7
  MOQ, materials and finishing: the small print that moves cost
• 8
  A five-step RFQ process that surfaces problems early

A tolerance number on its own means very little. ±0.005 mm on a 12 mm bore is a different job from ±0.005 mm across a 900 mm plate. Thermal drift, tool wear and fixture rigidity all scale with the feature, so ask the shop to state the tolerance band and the size range it covers. If the answer is a single number for the whole drawing, the quote is guesswork.

The second question is which features actually carry the tight call. Most drawings have two or three critical fits and a long tail of loose surfaces. Machining everything to ±0.005 mm wastes cycle time and raises cost without improving function. Mark the critical dimensions and let the rest run at general tolerance.

On 5-axis work, the tolerance also depends on whether the feature is cut in one continuous pass or reached after a rotary reposition. A Ø400 mm rotary table with a good post-processor holds position well, but the error budget is still larger than a straight 3-axis cut in the same plane. Ask how the feature will be reached before you accept the number.

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  State the feature sizeA tolerance without a size range cannot be verified.
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  Mark the critical fewTwo or three fits, not the whole print.
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  Ask about the approachContinuous pass versus rotary reposition.

A part can look perfect and still be out of tolerance in a place nobody checked. The difference between shops shows up in the report. Ask for a sample dimensional report from a similar part before you place the order. It should list the measured value, the nominal, the tolerance band and the datum it was taken from. A sheet that only says 'passed' tells you nothing you can audit later.

The inspection plan should also match the part's risk. A first article on a new geometry is different from a spot check on a repeat run. For medical and automotive work, the plan usually follows a control plan format. For a one-off prototype, a shorter report on the critical features is often enough.

We inspect 100% of parts before shipment and keep raw material check, in-process monitoring and final inspection as separate steps. Reports are issued on request. If a supplier cannot describe those three steps in order, the quality claim is probably a marketing line rather than a process.

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  Sample report firstMeasured value, nominal, band, datum.
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  Match the plan to riskFirst article versus repeat-run spot check.
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  Three steps, in orderMaterial check, in-process, final.

Every time a part leaves the fixture and comes back, you add stack-up error. You also add labor. A part that needs four setups is not just slower; it is harder to hold. When you review a quote, ask how many setups the shop plans and which datum carries the chain. A good answer names the datum and explains why the feature can be reached from it.

This is where 5-axis capability changes the economics. A part with undercuts, deep cavities or compound curvature can often be cut in one setup on a simultaneous 5-axis center, where a 3-axis machine would need three or four. The savings are real, but only if the shop programs it that way. A 5-axis machine run as a 3-axis machine with a tilting head gains nothing.

For long parts, the travel envelope decides the setup count before anything else. We run up to 4,000 mm on the large travel, with 750 × 1,150 × 550 mm and 600 × 600 × 600 mm machines for mid-size work and 500 × 500 × 450 mm or 500 × 310 × 200 mm for compact parts. If your part sits near an envelope edge, ask how it will be held.

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  Name the datum chainA setup plan without datums is not a plan.
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  5-axis only helps if programmed that wayA tilting head alone saves nothing.
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  Check the envelope earlyNear-edge parts need a holding plan.

Quoted lead time usually counts from the day the shop starts cutting, not the day you send the PO. The gap in between is where schedules slip. Ask two separate questions: how fast is the quote and DFM feedback, and how fast does the spindle start after you approve? We quote and return DFM analysis within 12 hours, and production can start within 24 hours after approval.

The delivery window then depends on process, finish and quantity. A bare machined aluminum bracket is a different schedule from a hardcoated part that also needs laser marking. Add the finish steps to your timeline before you commit a build date, and ask which steps run in-house.

It also helps to ask about the shop's own late-delivery history rather than a promised date. A number they can support is more useful than a date they cannot. Our historical late-delivery probability is below 2%, and parts normally ship in 3–5 days.

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  Split the two clocksQuote clock and cutting clock are not the same.
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  Add finish stepsAnodizing and marking extend the timeline.
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  Ask for the historyA supported number beats a promised date.

A low MOQ is only useful if the process is the same at one piece and at one thousand. Some shops quote a prototype on a manual mill and then switch to a different process for the run, which means the prototype does not validate the production part. Ask whether the same machine class and the same inspection plan carry through. We run from one prototype to 10,000+ part runs with no minimum order quantity.

Material choice also changes the tolerance you can hold. Aluminum 6061-T6 and 7075 cut cleanly and hold ±0.005 mm well on stable geometry. Stainless 316L and 17-4PH work-harden, so light passes and sharp tooling matter. Titanium Ti-6Al-4V and Inconel move more under heat. If your part is thin-walled in one of those alloys, expect a conversation about stock removal sequence.

Finishing is the last place cost hides. Clear anodizing, hardcoat, electroless nickel, black oxide and bead blasting all change dimensions slightly. If a masked surface must stay at nominal, say so on the drawing. Laser marking needs a minimum character height of 1.5 mm to stay legible after coating.

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  Same process across volumesA prototype must validate the run.
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  Alloy sets the real toleranceWork-hardening and heat shift the band.
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  Finishes move dimensionsCall out masked surfaces on the print.