CNC Machining Tolerance Chart: IT Grades and What It Takes to Hold 0.005 mm

Every RFQ we receive carries at least one tight number, and most tolerance questions come down to the same three: what does this IT grade mean at my part size, what can each process actually hold, and what does the shop have to do to keep it. This page collects the answers we give quoting desks every week, in chart form, so you can check a drawing before you send it out for quote.

Five-axis machining center milling an aluminum aerospace bracket under flood coolant, the process window that decides whether a tolerance holds. — Five-axis machining of an aluminum bracket. The process window, not the print, decides whether a tolerance holds.

IT grades against real part sizes

ISO 286 defines tolerance grades IT01 through IT16. Each grade is a band whose width grows with nominal size, which is why the same IT7 reads differently on a 10 mm pin and a 300 mm housing. The table below lists band widths (total tolerance, not plus or minus) for the grades that matter in CNC work.

Grade	10 mm nominal	50 mm nominal	100 mm nominal	300 mm nominal	Typical use
IT5	6 um	11 um	15 um	27 um	Gauge-class fits, spindle bearings
IT6	9 um	16 um	22 um	40 um	Precision shafts, bearing seats
IT7	15 um	25 um	35 um	60 um	General precision fits
IT8	22 um	39 um	54 um	89 um	Standard machined features
IT9	36 um	62 um	87 um	140 um	Non-critical dimensions

Two reading rules keep buyers out of trouble. First, a band width is the full spread: an IT7 bore at 100 mm is 0.035 mm wide, so a general-purpose fit only needs the process to land inside roughly plus or minus 0.018 mm around nominal. Second, grade and cost do not scale linearly. Moving from IT8 to IT7 rarely changes the route. Moving to IT6 or tighter usually adds finish machining, temperature control, and 100 percent inspection, and the quote reflects those steps.

What each process holds in practice

Chart values describe what a well-maintained process holds on production quantities, not a best-ever first article. Our shop data across the 100+ machines on the floor, including 11 five-axis centers, supports the bands below.

Process	Aluminum alloys	Steels	Stainless	Titanium	Engineering plastics
3-axis milling, finished pass	0.010-0.020 mm	0.015-0.025 mm	0.015-0.030 mm	0.015-0.025 mm	0.030-0.050 mm
5-axis milling, single setup	0.005-0.015 mm	0.010-0.020 mm	0.010-0.025 mm	0.010-0.020 mm	0.025-0.040 mm
CNC turning, finished diameter	0.008-0.015 mm	0.010-0.020 mm	0.012-0.025 mm	0.012-0.020 mm	0.025-0.050 mm
Turn-mill, complete part	0.008-0.018 mm	0.012-0.022 mm	0.015-0.025 mm	0.012-0.022 mm	0.030-0.050 mm
Grinding or lapping, post-process	0.002-0.005 mm	0.002-0.005 mm	0.003-0.005 mm	0.003-0.006 mm	not typical

Plastics sit a full step looser than metals for a physical reason: thermal expansion. A 100 mm PEEK feature grows about 0.05 mm over a 10 C temperature rise, more than five times what steel does, so the measuring plan matters as much as the cutting plan. Where a drawing asks for tighter than the material's natural band, we say so at quotation instead of promising a number the polymer cannot keep.

Precision aluminum and steel parts checked with a digital outside micrometer on a granite surface plate. — Micrometer check on a granite plate. Hand measurement covers the fast 90 percent; CMM covers the rest.

What a 0.005 mm quote actually commits

Our standard quoted tolerance is plus or minus 0.005 mm on machined features, and that number is a package of controls rather than a slogan. Each item below exists because removing it has, at some point, cost us a batch.

Control	What it does	Typical spec on our floor
Machine warm-up cycle	Stops spindle and axis growth from moving the tool	30-60 min warm-up before first cut on tight jobs
Thermal soak before inspection	Part and master reach the same temperature as the room	2-4 h for aluminum, 0.5-1 h for steel and titanium
CMM verification	Ties every critical dimension to a calibrated reference	Bridge CMMs, 0.0015 mm rated uncertainty
First-article layout	Confirms the process, not just the part	Full layout before production release
Tool wear offsets	Compensates the drift that every cutter produces	Scheduled offset updates per process sheet

The inspection side is where promises become evidence. Our 40+ precision inspection devices include bridge CMMs for dimensional layout, height gauges and form testers for geometry, and surface testers for finish. A 200 mm aluminum part measured straight off the machine can read 0.02 mm from its true value at a 10 C difference, which is larger than the tolerance itself, so the soak schedule is not paperwork, it is the difference between a real reading and a flattering one.

Bridge CMM with granite table verifying critical features of a machined housing in a temperature-controlled inspection room. — Bridge CMM verification in a temperature-controlled room. The rated uncertainty here is a third of the quoted tolerance.

Reading a drawing before you quote it

Three checks catch most quoting surprises. Look at whether every tight dimension sits on features machined in one setup, because tolerance between setups stacks. Look at datums, because a 0.005 mm band measured to a cast or sawn datum is a different job than one measured to a machined datum. And look at geometry callouts, because flatness and position on a thin wall cost more than a linear dimension of the same size. Our reference page on tolerance standards covers the standards side, and the process pages for milling, turning, and five-axis machining show which geometry classes run on which platform.

If a drawing already carries its tolerances, send it as is through the quote form and the engineering review will flag anything the process would struggle to hold, with the reason, before you commit a purchase order. Material choice changes the answer too; the materials library lists the alloys and polymers we run with their machining notes.

Frequently asked questions

Is plus or minus 0.005 mm the same as IT5?

Not exactly. IT5 is a band width that depends on nominal size, roughly 0.011 mm at 50 mm, so plus or minus 0.005 mm (a 0.010 mm band) sits near IT5 at that size. At 300 mm the same quote is tighter than IT5. We work from the drawing's numbers rather than the grade label.

Why do plastics get looser numbers than metals?

Thermal expansion and elastic recovery. Polymers expand five to ten times more per degree than steel and recover behind the cutter, so the dimension moves between the cut and the measurement unless the shop controls temperature on both ends.

Can every feature on a part hold 0.005 mm?

Technically yes, practically it costs. Tightening every dimension slows the process and multiplies inspection time. The economical drawing puts tight bands only on functionally critical features and leaves the rest to a general tolerance.

What measurement report comes with the parts?

First-article CMM layout on request, and final inspection reports listing actual values for critical dimensions. Material certificates travel with the batch from the incoming dock.