Tight Tolerance CNC Machining Services: How to Judge a Supplier

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  What matters most
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  What to weigh before you send an RFQ
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  Match the tolerance to the feature, not the drawing note
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  Settle the geometry question: wall thickness and aspect ratio
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  Read the inspection plan before you read the price
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  Lead time, MOQ and the cost of switching shops
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  When tight tolerance CNC machining is the wrong choice
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  Six steps from drawing to first article

A title block that reads ±0.005 mm across every dimension is a warning sign on the customer side, not on the shop side. That number applies to millimetre-scale features held in a single setup: a bore, a slot, a bolt circle, a shoulder. It does not scale linearly to a 900 mm rail with a 3 mm wall. When we quote tight tolerance CNC machining work, the first step is splitting the print into features that carry the tight callout and features that ride along at ±0.05 mm.

Size matters in both directions. A Ø6 mm pin pressed into a Ø6 H7 bore needs the roundness and the fit, not a blanket tolerance. A 750 × 1,150 × 550 mm envelope part usually needs flatness and hole position more than it needs every outside face at ±0.005 mm. Loose the faces that do not touch anything and the part gets cheaper without losing function.

Look at the datum scheme next. If datums A, B and C sit on three different faces that are machined in three different setups, the stack-up will eat the tolerance before the cutter touches metal. A cleaner print puts the primary datum on a face that gets machined first, then dimensions the rest from it. That single change often turns a part that needed five-axis work into a three-axis job with a fixture.

Heat treat is the other trap. A part machined to ±0.005 mm and then hardened will move. If the print calls for 440C at 58 HRC with a tight bore, plan a rough-machining step, heat treat, then a finish pass with a ground or hard-milled bore. Shops that quote the whole thing as one operation are telling you they have not read the heat treat note.

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  Tight callouts belong on mating featuresBores, slots, shoulders, hole positions.
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  Loose the cosmetic facesOutside surfaces at ±0.05 mm cut cycle time.
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  Datums first, dimensions secondA clean datum scheme reduces setups.
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  Leave stock for heat treatRough, treat, then finish the critical bore.

Tight tolerance CNC machining gets hard when the part is thin. A wall under 0.8 mm on aluminium will deflect under cutting force and spring back after the vise opens. The micrometer reads the relaxed shape, and the relaxed shape is not the machined shape. For thin walls, ask how the shop plans to support the part: sacrificial tabs, a wax or low-melt fixture, or light finishing passes with a small radial depth of cut.

Aspect ratio is the second number to check. A 10 mm diameter bore that is 120 mm deep is a 12:1 ratio. Boring bars and end mills flex over that length, and the hole will come out tapered or bell-mouthed. A shop with a mill-turn center can sometimes drill from both ends or use a line-boring setup. If nobody mentions the ratio, the quote is a guess.

Pocket depth follows the same logic. A 4 mm wide slot that is 40 mm deep needs a long, thin cutter. Tool runout shows up as a wall that is not parallel. Reducing the depth, widening the slot, or adding a corner radius all help. These are design changes, so raise them before the order, not after the first article.

Material choice sets the floor here. Aluminium 6061 and 7075 cut cleanly and hold a finish well. Titanium TC4 and Inconel work-harden and push heat into the tool, so light passes and rigid setups are the only way to hold a tolerance. Stainless 316 moves more than 17-4PH after machining. The right answer depends on the alloy, so quote the alloy, not just the shape.

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  Walls under 0.8 mmExpect to discuss support and light finishing passes.
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  Aspect ratio over 8:1Hole straightness becomes a process question.
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  Deep, narrow pocketsLong tools deflect; add radius or reduce depth.
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  Alloy drives the resultInconel and TC4 need different parameters from 6061.

Two quotes for the same part can differ by 40% and both be honest. The difference is usually in how much of the part gets measured, and how often. Our standard flow is a raw material check, in-process monitoring at defined checkpoints, and a final inspection before shipment, with 100% inspection for the parts that carry the tight callouts. Reports go out on request.

What you want on the report is a datum-referenced CMM result, not a stack of caliper readings. Calipers tell you a size. A CMM with the drawing datums tells you position, perpendicularity, and whether the bore is round or three-lobed. For a mating part, the second answer is the one that predicts assembly.

Temperature is the part nobody asks about. Aluminium grows about 23 μm per metre per degree Celsius. A part measured at 28 °C and used at 20 °C in a metrology lab has a real offset. Shops holding ±0.005 mm on long parts either measure in a controlled room or apply a correction. Ask where the final measurement happens.

For regulated industries, add the certificate to the checklist. ISO 9001:2015 covers the general quality system. IATF 16949:2016 adds automotive traceability and change control. ISO 13485:2016 covers medical device work. ISO 27001:2022 covers how your CAD files and drawings are stored. If your program needs one of these, confirm it before the RFQ, not after the first article.

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  CMM over calipersPosition and form, not just size.
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  Ask about the metrology roomTemperature drift moves long parts.
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  Match the certificate to the industryAutomotive, medical and data security have different rules.
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  Reports on requestFAI, material certs and inspection data.

A quote that arrives in three days is not a fast quote; it is a slow start. We return a quotation and a free DFM analysis within 12 hours, and production can begin within 24 hours once the design is frozen. Parts typically ship in 3–5 days. Historical late delivery sits below 2%, which matters more than the fastest single job.

MOQ is where prototype programs die. Some shops will take one piece but put it on a slow queue, or charge a setup that makes the second unit absurd. We run no minimum order quantity, so a single prototype and a 10,000+ part run can share the same process plan. That continuity is worth more than a lower unit price on the first article, because the fixture and the inspection plan carry over.

Think about the second operation before you commit. Anodizing, electroless nickel, black oxide, bead blasting and laser marking all happen after machining. Moving a part between vendors adds shipping, handling and a second inspection. If the finish is in-house, the tight dimensions stay in one quality system. Laser marking has a floor of 1.5 mm character height, so plan the mark layout accordingly.

Capacity is the last check and the easiest to verify. Ask what machines the tight work will run on. Our floor has 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, 16 mill-turn centers, 12 four-axis mills and 27 three-axis machines. A 4,000 mm maximum processing size covers large frames; a Ø400 mm rotary table covers round parts that need multi-face work.

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  12-hour quote and DFMEngineering feedback arrives with the price.
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  No MOQOne piece to 10,000+ on the same plan.
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  In-house finishingFewer handoffs, one inspection system.
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  Machine list on requestMatch the part to the right spindle.

Not every part should be machined to microns. If the geometry is a complex organic shell with internal channels, 3D printing gets you a functional prototype in days, and you can machine only the sealing faces afterward. If the annual volume is 50,000 identical small parts, die casting or injection molding will beat CNC on unit cost once the tool is amortized, and you can hold the critical bores with a secondary machining pass.

If the part is a flat bracket with generous tolerances, sheet metal fabrication is faster and cheaper. If it is a one-off fixture with no wear surface, a ±0.05 mm result is fine and the ±0.005 mm callout is wasted money. Paying for precision you cannot use is the most common overrun we see on RFQs.

The reverse also holds. A part that must seal against a gas or fluid path, a bearing housing, a spindle mount, an optical bench component, or any surface that sets a fit class, belongs in tight tolerance CNC machining. So does any part with a tolerance stack that feeds a safety function. In those cases, cutting the precision to save 15% is a bad trade.

One more boundary: quantity and repeatability. CNC holds a tolerance across a run because the setup and the inspection plan are fixed. Processes that rely on a mold inherit the mold's wear. If your program needs the tenth part to match the first, and the thousandth to match both, stay with machining and lock the process plan early.

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  Organic shells with channelsPrint the body, machine the sealing faces.
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  High-volume small partsCast or mold, then machine critical features.
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  Flat, generous bracketsSheet metal is the cheaper route.
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  Sealing and bearing fitsKeep these in CNC, do not downgrade.