Pre-assembly of Components
Positioning Parts
Assembly Conditions
Introduction
When assembling parts together, certain rules must be followed — this is what we will cover here.
Surfaces must be mated, facing parts must comply with a minimum edge gap and the edge flushness of edges must be maintained. All assemblies must be produced without stress and must be free of interference.
Contact Fit
Contact fit: refers to two parts face-to-face; the bearing area of the mating surfaces must be perfect.
The allowable gap between mating surfaces depends on the zone and type of assembly (dry or with interposition).
As a general rule, a gap of 0.1 mm is permitted between fasteners and at edges.
Contact Fit
A contact fit defect can be caused by foreign matter between parts (swarf, for example).
Edge Gap
Edge gap: parts must never touch edge-to-edge under any circumstances. The only permitted contact is face-to-face. The edge gap is compliant when a clearance exists between the edges.
This clearance is standardised; its value is given on the drawing. It depends on the aircraft zone and the thickness of the parts.
Edge Gap
Note here the gap between the two skins with a regular edge gap.
Edge Gap
Here we can see the gap around the central door with a regular edge gap.
Flush Fit
Flush fit refers to two parts whose edges are aligned.
It depends on the zone, but two edges are generally aligned to avoid potential interference with other elements.
Flush Fit
Here, we can see that the angles are correctly flush with the skins. The radii also follow the edges in flush alignment.
Flush Fit
Here, we can see that all parts are correctly flush with each other. The radii also follow the edges in flush alignment.
Interference
The interferences between edges and radii, flanges, etc. are forbidden. More generally, any contact other than face-to-face is forbidden.
This issue is resolved by repositioning the parts if possible (checking clamps), by shimming, or by machining a chamfer or radius (with supervisory approval).
Interference
Here, for example, an interference at the radius of a bent part. The part is too long and interferes with the angle.
Stress
A stress condition is considered to exist when mating parts requires more force than a finger would normally apply.
Forcing parts into contact (pressing two parts together) using a pin or clamp is forbidden.
06-01-01-Assembly_Conditions.pptx
Lauak
Created on June 23, 2026
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Transcript
Pre-assembly of Components
Positioning Parts
Assembly Conditions
Introduction
When assembling parts together, certain rules must be followed — this is what we will cover here.
Surfaces must be mated, facing parts must comply with a minimum edge gap and the edge flushness of edges must be maintained. All assemblies must be produced without stress and must be free of interference.
Contact Fit
Contact fit: refers to two parts face-to-face; the bearing area of the mating surfaces must be perfect.
The allowable gap between mating surfaces depends on the zone and type of assembly (dry or with interposition).
As a general rule, a gap of 0.1 mm is permitted between fasteners and at edges.
Contact Fit
A contact fit defect can be caused by foreign matter between parts (swarf, for example).
Edge Gap
Edge gap: parts must never touch edge-to-edge under any circumstances. The only permitted contact is face-to-face. The edge gap is compliant when a clearance exists between the edges.
This clearance is standardised; its value is given on the drawing. It depends on the aircraft zone and the thickness of the parts.
Edge Gap
Note here the gap between the two skins with a regular edge gap.
Edge Gap
Here we can see the gap around the central door with a regular edge gap.
Flush Fit
Flush fit refers to two parts whose edges are aligned.
It depends on the zone, but two edges are generally aligned to avoid potential interference with other elements.
Flush Fit
Here, we can see that the angles are correctly flush with the skins. The radii also follow the edges in flush alignment.
Flush Fit
Here, we can see that all parts are correctly flush with each other. The radii also follow the edges in flush alignment.
Interference
The interferences between edges and radii, flanges, etc. are forbidden. More generally, any contact other than face-to-face is forbidden.
This issue is resolved by repositioning the parts if possible (checking clamps), by shimming, or by machining a chamfer or radius (with supervisory approval).
Interference
Here, for example, an interference at the radius of a bent part. The part is too long and interferes with the angle.
Stress
A stress condition is considered to exist when mating parts requires more force than a finger would normally apply.
Forcing parts into contact (pressing two parts together) using a pin or clamp is forbidden.