ISO 2768 grades, common tolerance classes, surface finish callouts, and GD&T basics — written for buyers and engineers writing real drawings.
If you can change only one thing on a CNC drawing to lower price without compromising function, tighten the tolerances less. Tolerance class is the single biggest cost lever a buyer controls. A plus/minus .005 inch hole is a drill. A plus/minus .001 inch hole is a drill plus a reamer. A plus/minus .0002 inch hole is a drill, a reamer, a separate inspection, and probably a controlled-temperature gauge. This guide walks the tolerance classes you will actually see on a CNC drawing, explains what they mean in shop practice, and shows where the price cliffs are. If you are still in the buyer's process, start with how to get a part CNC machined; if you are writing an RFQ, also read how to request a CNC quote.
Most CNC drawings carry a default-block tolerance — the tolerance that applies to any dimension on the drawing that does not have an explicit feature-level tolerance written next to it. There are two common ways to specify it.
ISO 2768 is the international standard for general tolerances. It defines grades for linear dimensions and geometric (form, position) characteristics:
Grade f (fine): tight default; appropriate for precision instrument work. Grade m (medium): the standard CNC default; appropriate for most prismatic and turned parts. Grade c (coarse): looser default; appropriate for weldments and rough work. Grade v (very coarse): rarely used on CNC.
Grade H (fine), K (medium), L (coarse). Most CNC drawings combine a linear grade with a geometric grade in a single title-block callout: "ISO 2768-mK" means medium linear, medium geometric. "ISO 2768-fH" means fine linear, fine geometric.
U.S. drawings often skip ISO 2768 and use an inch-based default block: typically plus/minus .010 inch on a one-place decimal, plus/minus .005 inch on two-place, plus/minus .001 inch on three-place, and plus/minus .0005 inch on four-place. Angular default is usually plus/minus 0.5 degrees or plus/minus 1 degree. Either system is acceptable on a CNC drawing; pick one and stick with it.
Routine CNC work. Achievable on most features in one operation. No special fixturing. No special inspection beyond standard hand gauging. This is the right default for non-functional surfaces, clearance holes, and dimensions that do not affect assembly.
Standard precision CNC work. Achievable on most features without a second setup. Requires good machine condition, calibrated cutters, and proper finishing passes. Parts shipped under our precision machining service routinely land here without uplift through our production network.
Begins to require careful fixturing decisions. Holes at this tolerance usually require reaming or boring rather than drilling. Length dimensions at this tolerance require thermal stability during the cut. Inspection typically includes calibrated bore gauges or pin gauges for holes and high-precision micrometers for length.
Our routine tight-tolerance class delivered through our production network. Requires dedicated workholding (often soft jaws cut on the same machine), single-setup machining to avoid stack-up, controlled spindle warm-up, and a separate inspection step on a benchtop optical comparator and calibrated hand gauging. This is the floor of what CNC machining holds reliably in production; tighter than this typically belongs to grinding or honing operations.
On the right CNC lathe, repeatability between parts in a run can reach plus/minus .0001 inch. That is not the same as holding a part at plus/minus .0001 inch from drawing nominal — that takes grinding. If your drawing calls for plus/minus .0001 inch on a feature, expect the shop to either propose a secondary grind or ask whether the requirement is really repeatability between parts.
Surface finish is specified in microinches Ra (roughness average) in U.S. drawings, or micrometers Ra (sometimes Rz) in metric drawings. Lower number means smoother surface. Common callouts:
125 Ra: as-machined rough finish, no specific call-out. 63 Ra: standard CNC finish, common default if no specific value is shown. 32 Ra: good CNC finish, achievable with appropriate finishing pass and tool. 16 Ra: excellent CNC finish; may require specific tooling or a secondary operation. 8 Ra and below: typically requires polishing, lapping, or honing rather than CNC finishing alone.
Specify finish only on the surfaces that need it. Calling out 16 Ra on every surface of a complex part can multiply cycle time when only the sealing face actually needs it.
Geometric Dimensioning and Tolerancing (GD&T) is the symbolic language for specifying the geometric relationship between features — not just where a feature is, but how it relates to other features. The common controls are:
Form controls (no datum required): flatness, straightness, circularity, cylindricity. Orientation controls (datum required): parallelism, perpendicularity, angularity. Location controls: true position (the workhorse), concentricity, symmetry. Profile controls: profile of a line, profile of a surface. Runout: circular runout, total runout (turned parts).
Use GD&T when functional fit depends on how features relate to one another — mating bolt patterns (true position), sealing faces (flatness and parallelism), rotating shafts (runout). Use plus/minus dimensional tolerances when absolute size and location is what matters. Most aerospace, defense, and instrumentation drawings use a combination.
Standard CNC routinely holds plus/minus .005 inch on default-block features. Precision CNC routinely holds plus/minus .001 inch. Tight-tolerance routine work holds plus/minus .0002 inch, and CNC lathe repeatability reaches plus/minus .0001 inch on high-precision platforms. Tighter than .0002 inch typically requires grinding rather than machining.
ISO 2768 is the standard for general (default-block) tolerances. Grade mK (medium for linear, medium for geometric) is the most common default for CNC parts; grade fH is tighter, vL is looser. Specify the grade in the title block and tighten individual features as needed.
Yes, but not linearly. Plus/minus .005 inch to plus/minus .001 inch is usually a modest uplift. Plus/minus .001 inch to plus/minus .0002 inch can multiply cost on the affected features because it requires careful fixturing, often a separate inspection, and sometimes a different machine. Tighten only what the assembly requires.
63 microinches Ra is routine off the machine. 32 Ra is common on finished features. 16 Ra requires fine-finish tooling and feeds and may require a secondary operation. Below 16 Ra typically requires polishing, lapping, or honing rather than machining.
For most prismatic parts, plus/minus dimensional tolerances are adequate. GD&T is required when functional relationships between features matter more than absolute dimensions. Most aerospace and many precision parts use GD&T.
Plus/minus .0002 inch is roughly the thickness of a sheet of plastic wrap divided by ten. It's the tolerance class where temperature affects measurement, where the part must be allowed to stabilize before inspection, and where the inspection plan starts to matter as much as the machining plan.
Our plus/minus .0002 inch routine in production.
Standard precision CNC with documented inspection.
RFQ checklist and quote review.
The five-step buyer's process.
Equipment, envelope, tolerance class.
Send a drawing to be quoted.
