Threading & Fits

H7/h6. Press fits.
Thread classes.
Read the drawing.

Fit designations like H7/h6 and thread classes like 6H/6g look cryptic but encode precise tolerances. This reference decodes the ISO 286 system, thread classes, and practical fit selection.

Free DFM Review Thread standards

01 · ISO 286 basics

IT grades and position letters.

ISO 286 specifies tolerances as a combination of IT grade (tolerance magnitude) and position letter (where the tolerance zone sits relative to nominal size).

IT grade

Tolerance magnitude

IT01 tightest, IT18 loosest. Common: IT6 (precision machining), IT7 (standard precision), IT8-9 (general machining), IT11+ (rough/cast).

Position letter

Where zone sits

Capital letters for holes (H, J, K...), lowercase for shafts (h, j, k...). H/h: zone starts at nominal. Letters earlier in alphabet: zone below nominal (smaller). Later: zone above nominal (larger).

H7/h6

The classic fit

H7 hole: zone from 0 to +IT7. h6 shaft: zone from -IT6 to 0. Slight clearance fit. Workhorse of machined assemblies.

Typical for machining

IT6 to IT9

Machining economics: IT9 is standard milling. IT7-8 is standard turning. IT6 needs grinding or boring. Below IT6 usually needs grinding and lapping.

Reading fit spec

Hole/shaft

Always written as hole/shaft. H7/h6 means H7 hole and h6 shaft. H7/k6 is H7 hole with k6 shaft (transition fit). H7/p6 is H7 hole with p6 shaft (interference).

Preferred fits

Use standard

Stick to preferred combinations: H7/h6, H7/g6, H7/k6, H7/p6, H8/f7 etc. Non-standard combinations can be produced but check tooling/gaging availability.

02 · Fit types

Clearance → transition → interference.

Fits fall into three categories based on whether the shaft is always smaller, equal, or larger than the hole.

Fit type	Example	Typical use	Assembly
Loose clearance	H11/c11	Agricultural, low precision	Slip on
Free running	H9/d9	Rotating shaft with oil	Slip on
Close running	H8/f7	Precision shafts, gear shafts	Slip on
Sliding	H7/g6	Sliding without play	Slip on with slight force
Location clearance	H7/h6	Precision located, stationary	Hand press
Location transition	H7/k6	Accurate location, may bind	Slight force
Location transition (tight)	H7/n6	Fixed location, needs force	Press or shrink
Location interference	H7/p6	Press fit for permanent assembly	Arbor press
Medium drive	H7/s6	Permanent press fit	Hydraulic press
Force fit	H7/u6	Heavy interference, shrink fit	Heat/cool assembly

03 · Thread classes

Metric & inch thread tolerance.

Metric external

4g 6g 8g

4g: tightest common. 6g: standard (default if unspecified). 8g: loose for coarse conditions. Letter g means slight clearance for plating allowance.

Metric internal

4H 5H 6H 7H

4H/5H: precision. 6H: standard default. 7H: loose. Upper case H means basic nominal diameter (no plating allowance).

Metric fit spec

M10 × 1.5 - 6H/6g

M10 nominal with 1.5mm pitch, internal 6H and external 6g. Typical drawing callout — both halves specified.

Inch Class 1

1A / 1B

Loose — rarely used. Historical. For agricultural or rough assemblies.

Inch Class 2

2A / 2B

Standard — default class for most fasteners and threaded holes. 98% of inch threads.

Inch Class 3

3A / 3B

Tight — precision applications, aerospace, safety-critical. Higher cost for tapping and thread verification.

04 · Press fit practical

Press fit selection in practice.

Light press (H7/n6): Interference 0.005-0.015 mm per 10 mm diameter. Held by friction after assembly. Assembly: arbor press at room temperature. Can be disassembled with more force. Used for bearings in housings, shafts in pulleys where assembly/disassembly is occasional.

Standard press (H7/p6 or H7/r6): Interference 0.015-0.035 mm per 10 mm diameter. Permanent assembly — disassembly damages parts. Assembly: hydraulic press 2-10 tons typical. Used for permanently installed bearings, gears onto shafts, permanent bushings.

Shrink fit (H7/u6 or H7/x6): Interference 0.035-0.08 mm per 10 mm diameter. Requires heating housing (or cooling shaft) for assembly — won't press at room temperature. Assembly: heat housing to 200°C, drop shaft in, cool to room temp. Very high holding force. Used for railway wheel on axle, heavy-duty industrial gears.

Calculating holding force: Simple approximation: holding force ≈ π × d × L × µ × P, where d is diameter, L is engagement length, µ is friction coefficient (0.1-0.15 steel/steel, up to 0.3 with knurling), P is pressure from interference. For typical H7/p6 fit, holding force 10-50 kN per 10mm of engagement.

Design considerations: (1) Lead-in chamfer on both parts for smooth assembly. (2) Account for stress in outer member — too much interference can crack a thin-walled housing. (3) Surface finish Ra 0.8 µm or better on both parts. (4) Clean and lightly oil before assembly. (5) For critical applications, document assembly force as process parameter.

FAQ

How do I know what fit to specify?

Work from function: Rotating shaft in bearing → H7/g6 or H7/h6. Permanent bushing in housing → H7/p6. Bearing race pressed in → H7/p6 or H7/r6. Gear on shaft for removable assembly → H7/k6 or H7/n6. Alignment pin in reamed hole → H7/p6 or H7/r6. When in doubt, H7/h6 is the default clearance fit — small gap, easy assembly.

What's the cost difference between IT6 and IT9?

IT9 holes: drilled or rough milled — standard speed, cheapest. IT7 holes: reamed or boring bar — 1.5-2× time. IT6 holes: precision reaming or grinding — 3-5× time. Below IT6: specialty precision machining or grinding — 5-10×. For same part, specifying IT6 everywhere vs IT9 everywhere can double total machining cost. Specify IT6 only on critical fit features.

Thread class 2B vs 3B — when does it matter?

For standard industrial fasteners and hardware, 2B threads are adequate — fasteners rarely fail due to thread class. Specify 3B when: (1) Precision alignment is critical (instrument threads). (2) High-cycle fatigue (aerospace engine mounts — tighter thread is stronger). (3) Safety-critical applications. (4) Customer specification requires it (aerospace, ASME specs). Tight threads require precision taps and more careful production — 30-50% cost premium.

Running fit vs sliding fit?

Running fit (H8/f7, H9/e9): continuous rotation with lubricant. Clearance large enough to maintain oil film. Used for: bearings, rotating shafts in journals. Sliding fit (H7/g6): intermittent sliding without rotation, lighter loads. Smaller clearance, less play. Used for: location shafts, piston pins, adjustable slides. For continuous high-speed rotation: specify running fit with proper lubrication.

Dowel pin holes — what fit?

Standard dowel pin holes: H7 on hole, pin typically ±0.005 mm (dowel pin supplied precision). Result: 0.005-0.015 mm interference. Press fit with hand pressure or light tap. For tooling requiring repeatable removal: H7/p6 acceptable. For permanent alignment: H7/r6. Ream holes to final size (drilled holes not precise enough). Hold reamer perpendicular and clean bore for proper fit.

Shaft tolerance beyond H7/h6?

For diameters below 3 mm, H7/h6 zones become very small (~0.01 mm total). Reachable but expensive. For very precision (scientific instruments, measuring equipment): H5/h5 or H6/h5, requires precision grinding. Above 500 mm diameter, tolerances grow significantly with IT grades — H7 on a 500mm hole is ±0.04 mm, reasonable for large shafts. Use ISO 286 tables for exact values — don't calculate from memory.