The working principle of rivet nuts involves using a specialized tool to pull the mandrel, causing the tail of the nut sleeve to expand and form a mechanical interlock on both sides of the sheet metal. As the fundamental carrier for this process, the pilot hole’s core functions are reflected in the following aspects:
The pilot hole provides the initial installation position and axial guidance for the rivet nut, ensuring the nut remains perpendicular to the sheet metal before force-induced expansion. This prevents unilateral expansion or incomplete installation caused by tilting.
The diameter of the pilot hole is closely correlated with the expansion grooves on the rivet nut’s tail. The material around the hole wall is designed as the "deformation zone." When the nut tail expands, it exerts uniform, radial pressure on the hole wall, forcing the surrounding material to undergo plastic flow that fully fills the nut’s expansion patterns and grooves, forming a "bonding" effect.
The flange (or "blind cap") formed by the expanded nut tail on the back of the sheet metal, together with the nut’s flange on the workpiece surface, firmly "clamps" the sheet metal. A correctly sized pilot hole ensures the back flange is full and symmetrical, providing maximum pull-out force. Conversely, an oversized pilot hole causes premature material tearing, failure to form an effective flange, and a drastic reduction in pull-out force.
Hole diameter is the most critical parameter of the pilot hole—bar none.
Undersized Hole (< Lower Limit):
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Difficult Installation: The rivet nut cannot be easily inserted; forced hammering may damage threads or pre-coatings.
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Excessive Expansion Force: The tool requires higher pulling force, risking overload, tool damage, or mandrel breakage.
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Sheet Metal Tear Risk: Excessive expansion pressure may crack thin sheet metal at the expansion grooves.
Oversized Hole (> Upper Limit):
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Loose Connection: Initial clearance between the nut and hole causes wobble during installation, failing to achieve sufficient clamping force.
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Severe Lack of Pull-out Force: This is the most critical consequence. During expansion, the nut tail cannot exert adequate radial pressure on the hole wall, resulting in a thin, irregular back flange—even complete nut extraction from the hole under pull testing.
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Loss of Sealing Performance: For sealing applications, excessive clearance allows moisture and dust penetration.
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Rotation Issue: The nut may spin in the hole when tightening the matching bolt, preventing successful installation.
Thus, strictly adhering to the recommended hole diameter provided by the rivet nut manufacturer is the golden rule for ensuring connection quality. Typically, the tolerance range is controlled at +H7 (e.g., for a 6mm rivet nut, the recommended pilot hole diameter may be 6.2mm ±0.05mm).
Burrs and Flash:
Drilling, especially on the exit side, generates burrs. Failure to remove them leads to:
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Poor fit between the rivet nut and workpiece surface, creating gaps.
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Burr fragmentation under vibration, causing preload loss.
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Compromised product appearance and potential operator scratches.
Rough Hole Walls:
Rough surfaces reduce the actual contact area between the nut and sheet metal, potentially forming stress concentration points that induce cracks under long-term vibration loads.
Perpendicularity:
The hole axis must be 90° perpendicular to the sheet metal surface. An inclined hole causes the rivet nut to install askew, preventing the flange from evenly compressing the workpiece, resulting in uneven stress distribution and potential issues with subsequent bolt assembly.
Roundness:
A non-circular hole (e.g., elliptical) leads to uneven contact between the nut and hole wall. During expansion, this also causes stress concentration, fails to form a uniform flange, and significantly reduces torsion and pull-out resistance.
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Prefer High-Quality Twist Drills: Sharp, titanium-plated, or cobalt alloy drills deliver smooth hole walls.
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Drill Bit Size: Use a drill bit that exactly matches the recommended hole diameter; regularly calibrate drill wear with calipers.
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Stacked Sheet Drilling: For thin sheets, place scrap material underneath to effectively suppress exit-side burrs.
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Use Drill Jigs: In mass production, drill jigs or fixtures ensure hole position accuracy and perpendicularity.
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Deburring is Indispensable: After drilling, use a slightly larger drill bit or specialized deburring tool to gently remove burrs from both sides of the hole, creating a smooth chamfer.
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Loose Nut After Installation: First suspect an oversized pilot hole or poor hole wall quality.
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Mandrel Breaks Without Nut Expansion: Check if the pilot hole is oversized, causing premature "pull-out" rather than "expansion" of the nut tail.
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Nut Rotates with Mandrel During Installation: May result from an oversized pilot hole or improper tool bit selection; also verify if the pilot hole provides sufficient initial clamping force.
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Incomplete Back Flange on Sheet Metal: Oversized pilot hole or mismatch between sheet metal thickness and nut specification.
