Complete Tolerance & Standards Guide
Specifying tolerances correctly is the difference between a part that works and a part that breaks your budget. This guide covers ISO 2768, GD&T fundamentals, ISO fits, surface roughness standards, and process-specific achievable tolerances , everything you need to write a drawing that gets the result you want.
ISO 2768 General Tolerances
ISO 2768 is applied to all drawing dimensions that do not carry an explicit tolerance callout. We use ISO 2768-m (Medium) as our default. Specify ISO 2768-f in the title block for finer general tolerance.
Part 1: Linear Dimensions (mm)
| Nominal Range | Fine (f) | Medium (m) ★ | Coarse (c) | Very Coarse (v) |
| 0.5–3mm | ±0.05 | ±0.10 | ±0.20 | — |
| 3–30mm | ±0.05 | ±0.20 | ±0.50 | ±1.0 |
| 30–120mm | ±0.10 | ±0.30 | ±0.80 | ±1.5 |
| 120–400mm | ±0.15 | ±0.50 | ±1.20 | ±2.5 |
| 400–1000mm | ±0.20 | ±0.80 | ±2.00 | ±4.0 |
| 1000–2000mm | ±0.30 | ±1.20 | ±3.00 | ±6.0 |
| 2000–4000mm | ±0.50 | ±2.00 | ±4.00 | ±8.0 |
★ ISO 2768-m is Samshion’s default for all CNC parts. State ISO 2768-f in your title block for finer general tolerance.
Part 1: Angular Dimensions
| Shorter Leg (mm) | Fine (f) | Medium (m) | Coarse (c) |
| Up to 10 | ±1° | ±1° | ±1°30′ |
| 10–50 | ±0°30′ | ±0°30′ | ±1° |
| 50–120 | ±0°20′ | ±0°20′ | ±0°30′ |
| 120–400 | ±0°10′ | ±0°10′ | ±0°20′ |
| Over 400 | ±0°5′ | ±0°5′ | ±0°10′ |
Part 2: Geometrical Tolerances (Form & Position)
| Feature Range | H | K | L |
| Up to 30 | 0.05 | 0.1 | 0.2 |
| 30–100 | 0.1 | 0.2 | 0.4 |
| 100–300 | 0.15 | 0.3 | 0.6 |
| 300–1000 | 0.2 | 0.4 | 0.8 |
| 1000–3000 | 0.25 | 0.5 | 1 |
GD&T — Geometric Dimensioning & Tolerancing
GD&T provides unambiguous definition of part geometry. We accept ASME Y14.5-2018 and ISO 1101 callouts. Our CMM verifies all geometric controls.
Form Controls
Flatness
CMM verified
Circularity
CMM verified
Cylindricity
CMM or roundness tester
Straightness
On shaft axis or surface line
Orientation Controls
Perpendicularity
CMM verified with datum
Angularity
CMM calculated
Parallelism
CMM or surface plate gauge
Circular Runout
Dial indicator on lathe / CMM
Orientation Controls
True Position
CMM with RFS or MMC
Concentricity
CMM median point verification
Symmetry
CMM median plane
Profile of Surface
CMM scanning probe
Common GD&T Callout Examples
Position of bolt hole pattern
Perpendicularity of a boss
Flatness of a sealing face
Runout on a turned shaft
Cylindricity of a bore
Profile of a curved surface
ISO Hole & Shaft Fits
ISO fits define the intended relationship between mating parts. The fit class controls whether parts rotate freely, locate precisely, or press together permanently.
| ISO Symbol | Fit Type | Condition | Typical Clearance/Interference | Torque to Assemble | Applications | Notes |
| H11/c11 | Loose Running | Large clearance | +60 to +240µm | Hand assembly | Non-precision pivots, agricultural, flexible shafts | Not for precision location |
| H9/d9 | Free Running | Significant clearance | +20 to +105µm | Hand assembly | Easily rotating shafts, lubricated bearings | Good for dirty or hot environments |
| H8/f7 | Close Running | Small clearance | +6 to +41µm | Hand assembly | Precision spindles, sliding fits | Needs good lubrication and alignment |
| H7/g6 | Sliding Fit | Minimal clearance | +2 to +29µm | Finger/hand | Precision location + free rotation | Bearings, pulleys, gears on shafts |
| H7/h6 | Easy Location | Zero to light clearance | 0 to +25µm | Hand | Standard precision shafts in housings | Most common bearing fit specification |
| H7/k6 | Transition | May be clear or light press | −10 to +18µm | Hand / light press | Precision location, repeatably removable | Gear hubs, locating pins |
| H7/n6 | Snug Fit | Light interference | −13 to +15µm | Light press | Parts located and occasionally removed | Bushings, disc hubs on shafts |
| H7/p6 | Push Fit | Light press | −21 to +6µm | Arbor press | Permanent location, light drives | Bushings pressed into housings |
| H7/s6 | Drive Fit | Medium press | −35 to −9µm | Hydraulic press | Coupling hubs, bearing inner race | Standard shrink-fit application |
| H7/u6 | Force Fit | Heavy press | −51 to −25µm | Heat shrink required | Permanent joints, safety-critical | Calculate interference stress carefully |
Surface Roughness Reference (Ra / N Grade)
Ra is the arithmetic mean deviation of the surface profile. Specifying Ra correctly saves money — don’t over-specify what your application doesn’t need.
| Ra (µm) | Ra (µin) | N Grade | How Achieved | Visual Description | Cost Factor | Typical Applications |
| 25 | 1000 | N12 | Rough milling / saw cut | Very rough — visible cut marks | 1× | Rough stock surfaces, non-functional |
| 12.5 | 500 | N10 | Rough milling | Coarse tool marks clearly visible | 1.2× | Non-mating rough machined surfaces |
| 6.3 | 250 | N9 | Standard milling | Tool marks visible on inspection | 1.4× | General machined non-contact surfaces |
| 3.2 | 125 | N8 | Standard CNC milling | Light marks, typical CNC output | 1.0× (baseline) | Default machined surface — most CNC parts |
| 1.6 | 63 | N7 | Fine milling / turning | Very smooth to touch | 1.3× | Sliding fits, non-sealing contact faces |
| 0.8 | 32 | N6 | Fine turning / reaming | Smooth, barely visible marks | 1.7× | Bearing fits, O-ring grooves, sliding seals |
| 0.4 | 16 | N5 | Grinding / fine turning | Near-mirror — reflects light | 2.5× | Precision bearing seats, hydraulic seal faces |
| 0.2 | 8 | N4 | Precision grinding | Mirror-like appearance | 4× | High-speed bearing seats, lapping joints |
| 0.1 | 4 | N3 | Lapping / honing | True mirror | 7× | Gauge surfaces, optical flats, precision hydraulics |
| 0.05 | 2 | N2 | Super-finish / lapping | Optical mirror quality | 15× | Metrology, optical mounts, CMM references |
Surface Roughness Design Rules
Only specify Ra where it matters
Leave unspecified surfaces at Ra 3.2µm (N8) default. Every Ra callout that requires additional operations adds cost and time.
Consider Ra vs. surface function
Sealing surfaces: Ra 0.4–0.8µm. Sliding fits: Ra 0.8–1.6µm. Static contacts: Ra 1.6–3.2µm. Rough machined non-contact: Ra 3.2–6.3µm.
Check finishing effect on Ra
Bead blasting raises Ra to 0.8–2.5µm. Anodizing adds minimal Ra change. Hard chrome brings Ra down. Electropolish reduces Ra by ~50%.