CNC Knowledge

How to Classify the Machining Accuracy of CNC Machining

This guide is for engineers and buyers who need to put a grade on a drawing before the quote goes out. We walk through the 4 checks that classify the machining accuracy of CNC machining: the tolerance grade you ask for, what your process can actually hold, the surface finish that comes with it, and the inspection data that proves it.

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±0.005 mm toleranceRa 0.2–0.8 μm finishIT6–IT10 reference100% inspection

Classifying the machining accuracy of CNC machining on 5-axis engine parts

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Key takeaways

Grade the tolerance firstIT grades (IT6 to IT10) tell you what fits and what does not. Pick the grade before you pick the process.

Match process to gradeMilling holds IT6 to IT8, drilling sits near IT10, boring lands at IT7 to IT9. Do not ask a drill for a bore tolerance.

Finish follows toleranceTight tolerance usually means fine finish. Ra 0.8 to 1.6 μm pairs with IT7 work; Ra 1.6 to 3.2 μm fits general milling.

Prove it with dataA grade is a claim until a CMM report backs it. Ask for the numbers before you release the drawing.

Step 1

Start with the tolerance grade, not the machine

Most arguments about accuracy start in the wrong place. People ask which machine will run the part before they agree on what the part has to do. The tolerance grade settles that question. It is a number from the ISO 286 system, written as IT6, IT7, IT8 and so on, and it links a nominal size to an allowed deviation.

A 50 mm bore at IT7 carries a 25 μm band. The same bore at IT6 carries 16 μm. That difference decides whether the assembly works or binds. So the first job is to read the drawing and find the tightest fit on the part. That fit sets the floor for everything else.

A common mistake is grading every dimension the same. If a shaft only needs to clear a hole, IT10 or IT11 is fine and costs less. Save the tight grade for the bearing seat, the sealing face, or the mating bore. Grading the whole print at IT6 adds cost with no gain.

1
IT6 to IT7Bearing seats, spigots, hydraulic bores. Needs fine finishing and stable temperature.
2
IT8 to IT9General machined fits, gearbox covers, brackets. Standard CNC milling and turning reach this.
3
IT10 to IT11Clearance holes, slots, non-critical faces. Drilling or rough milling is enough.

Step 2

Check what each process can hold

Once you know the grade, match it to a process. This is where the machining accuracy of CNC machining gets practical. Not every operation holds the same band. The rule of thumb: the more rigid the setup and the finer the tool path, the tighter the result.

Milling covers the widest range. A 3-axis or 5-axis machining center with a sharp carbide cutter and a light finishing pass holds IT6 to IT8 on most metals. That is roughly a 0.01 mm to 0.05 mm band depending on size. Deep pockets and thin walls push you toward the loose end because tool deflection grows.

Drilling is a starting operation, not a finishing one. A standard twist drill lands near IT10 with surface roughness around Ra 6.3 to 12.5 μm. If the hole has to locate a pin, drill it undersize and then bore or ream it. Reaming takes a drilled hole to IT7 or IT8 in one pass and is cheaper than boring.

Boring enlarges and trues a hole after drilling. On steel it reaches IT7 to IT9 with finish between Ra 0.16 and 2.5 μm. Boring also corrects position error left by the drill, which is why it is the normal second step on a critical bore.

1
Thin walls below 1 mmClamp pressure and cutting force bend the part. Expect one grade looser unless you use light passes.
2
Deep holes over 5xDChip evacuation and tool wander cost you accuracy. Peck drilling or gun drilling holds the line.
3
Hard materials over 45 HRCTool wear moves the size during the run. Plan a mid-run offset check.

Step 3

Read surface finish as part of the grade

Tolerance and finish travel together. You cannot hold IT6 on a face left at Ra 6.3 μm, because the tool marks alone eat the tolerance band. When you classify accuracy, read the Ra callout on the same drawing view as the tight dimension.

A fine finishing pass with a small nose radius and a high spindle speed gets you to Ra 0.8 to 1.6 μm. That pairs well with IT7 work on aluminum and mild steel. For Ra 0.2 to 0.8 μm you need a dedicated finishing cut, low feed per tooth, and often a ground or lapped step afterward.

General milling at Ra 1.6 to 3.2 μm is the normal as-machined look. It is fine for covers, brackets and non-sealing faces. Do not pay for a mirror finish on a surface that only needs to look clean and clear a screw head.

Step 4

Confirm the grade with inspection data

A grade on a print is a request. A grade on a report is a fact. Before you accept a supplier's claim, ask how they measured it. Calipers and micrometers read size, but they do not capture form, position or runout.

For true position, flatness and concentricity, a CMM or a dial indicator on a surface plate is the right tool. The measurement has to happen at a controlled temperature, usually 20 °C, because steel grows about 11 μm per meter for every 10 °C rise. A part measured hot will read undersize.

Ask for raw material certificates, in-process checks and a final inspection record. At GreatLight we inspect 100% of parts before shipment and issue reports on request. The numbers let you close the loop: if the report shows IT7 and the drawing asked for IT7, the accuracy is classified and confirmed.

1
Size onlyMicrometer or bore gauge. Good for shafts and simple holes.
2
Form and positionCMM or height gauge on a granite plate. Needed for GD&T callouts.
3
FinishSurface roughness tester. Required when Ra is specified on the print.

Do this in order

Four steps to classify a part before you quote

Work top to bottom. Each step narrows the next one.

1
Mark the critical fits on the drawingGo through every dimension and circle the ones that touch another part. Those carry the tight grade. Leave clearance holes and cosmetic faces at IT10 or looser. If the print grades everything at IT6, push back before quoting.
2
Assign an IT grade to each critical fitUse ISO 286 tables for the nominal size. A 20 mm bearing seat is typically IT6 or IT7; a 200 mm spigot may only need IT8. Write the grade next to the dimension so the machinist sees the intent, not just a plus/minus number.
3
Match the grade to a process and a datumPick the operation that holds that band with margin. Then set the datum strategy: locate from the same face for rough and finish. Refixturing between ops is the top cause of position error on tight parts.
4
Set the finish and the inspection planAdd the Ra callout that fits the grade. For IT7 work, Ra 0.8 to 1.6 μm is normal. Then name the inspection method and the sample rate. Full inspection on a 10,000-part run is expensive; agree on what gets measured and how often.

Reference

Process accuracy reference by operation

Bands are typical for steel and aluminum at sizes under 200 mm. Your part and setup may shift one grade.

Operation	IT grade	Surface finish	When to use
CNC milling (finish pass)	IT6–IT8	Ra 0.8–1.6 μm	Mating faces, pockets, profiles
CNC milling (rough)	IT10–IT12	Ra 3.2–12.5 μm	Stock removal before finishing
Drilling	IT10–IT11	Ra 6.3–12.5 μm	Clearance holes, tap pilots
Reaming	IT7–IT8	Ra 1.6–3.2 μm	Pin holes, dowel locations
Boring	IT7–IT9	Ra 0.16–2.5 μm	Large bores, bearing seats
Turning (finish)	IT6–IT7	Ra 0.8–1.6 μm	Shafts, bushings, spigots
Grinding	IT5–IT6	Ra 0.2–0.8 μm	Hardened parts, seal faces

Grade the fit, not the whole part

Classify accuracy by grading only the fits that matter. Then pick the process that holds that band with margin, set the finish to match, and confirm it with measured data. Everything else can stay loose and cheap.

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FAQs

Questions engineers ask about accuracy grades

Is IT6 the same as ±0.005 mm?

No. IT6 is a band that changes with nominal size. For a 50 mm feature it is 16 μm total, so about ±0.008 mm. For a 10 mm feature it is 9 μm, so about ±0.0045 mm.

We quote ±0.005 mm as a general capability on suitable parts. Always compare the IT band for your actual size, not a fixed number.

Can drilling hold a tight tolerance if I use a good drill?

Rarely. A twist drill follows its own path in the material and leaves a rough wall. IT10 is the realistic band even with a new carbide drill and a rigid setup.

If the hole locates a pin, budget for reaming or boring as a second operation.

Does a tighter tolerance always cost more?

Not always, but usually. The cost comes from extra passes, slower feed, more inspection and higher scrap risk. A single IT6 bore on an otherwise simple part adds little.

Grading every dimension at IT6 on a complex housing can double the cycle time. Tighten only what the assembly needs.

How do I check accuracy on a part too large for a CMM?

Use a portable arm, a laser tracker, or a height gauge on a granite plate with a dial indicator. For long parts, measure straightness and parallelism along the length, not just at the ends.

We machine up to 4,000 mm and can plan the measurement method with you before the run starts.

What causes a part to pass inspection at the shop but fail at assembly?

The usual cause is form error that size gauges miss. A bore can read on-size at two points and still be oval or tapered. Position error from refixturing is the other common one.

If assembly is critical, specify GD&T and ask for a CMM report, not just a size sheet.

When should I skip the tight grade entirely?

When the joint has a gasket, an adhesive, or a slotted hole that absorbs variation. In those cases IT10 or IT11 works and saves money.

Also skip it on non-functional cosmetic surfaces. Nobody measures the back of a bracket that never touches anything.