Transition Fit
How do I distinguish transition fit from clearance fit on the drawing?
By the tolerance zone overlap: clearance fit guarantees positive clearance, while transition fit permits either slight clearance or slight interference depending on actual size.
What assembly force should I expect for a transition fit?
Usually hand force to light press assistance, though some parts may need a soft mallet or controlled arbor pressing if the actual dimensions land on the interference side.
Which ISO fit classes are commonly used for transition fits?
Commonly cited transition-fit examples include H7/k6, H7/m6, and H7/n6.
Transition fit is a fit class where the hole and shaft tolerance zones overlap, so the assembled pair can end up with either a small clearance or a small interference depending on the actual measured sizes. In shop terms, it is the middle ground between clearance and interference fits: accurate location is the goal, not free movement, and not a heavy press fit.
On a CNC cell, transition fits are typically achieved through precision boring, reaming, or finish grinding to maintain a tight tolerance band that allows either slight clearance or slight interference. The machinist must account for tool wear, thermal growth, and machine repeatability, as even a few microns can shift the assembly from slip to press. In millwork applications, the same principle applies to dowel-referenced panels and precision jigging components where clean location without slop is essential. The practical goal is repeatable positioning with minimal shake while avoiding seizure or damage from over-tight assembly. Designers specify transition fits when accurate location is needed with only light assembly force, making them ideal for locating pins, precision sleeves, and bearing seats. Common ISO examples include H7/k6, H7/m6, and H7/n6, which ensure the overlap zone stays narrow.
Tolerance stack-up turns designed transition fit into unintended interference: Assemblies that should seat with light force require excessive force or won't go together because narrow overlap amplifies normal variation.
Assuming every transition-fit assembly will behave the same: Some parts slide in while others need a mallet, causing inconsistent takt time and rework because outcome depends on where each part lands within tolerance.
Using a transition fit where clamping or torque transfer is expected: The joint locates well but slips under load because the fit cannot transmit significant force by friction alone; transition fits are for positioning, not retention.