- Prefer ISO 273 series for general metric clearance holes.
- Select hole process by size, position, edge quality and quantity.
- A slot relieves one direction and may reduce slip resistance.
- Control patterns from A|B|C and allow for coating on precision holes.
Values in this guide are practical starting points for RFQ, DFM and prototype validation. They do not replace load calculations, regulations, material certificates, qualified procedures or approved production drawings.
Use Standard Clearance-hole Series
Examples from the ISO 273 medium series are M3→3.4 mm, M4→4.5, M5→5.5, M6→6.6, M8→9.0, M10→11.0 and M12→13.5. Fine assembly may use the close series, while rough or welded frames may use larger clearance. Dowel pins, fitted bolts and special shear joints require dedicated fits, not ordinary clearance holes.
Match Hole Quality to the Process
Laser cutting suits profiles and ordinary mounting holes, but holes near or below material thickness may lose roundness and gain taper. Punching is productive but creates rollover, shear zone and burr; hole diameter should generally not be materially smaller than thickness. Drilling produces round holes, reaming improves size and finish, and precision holes may be machined after bending or welding. Specify size tolerance and roughness only where function requires them.
Set Edge Distance by Failure Mode
Center-to-free-edge distance must prevent tear-out and bearing failure. For an ordinary noncritical sheet joint, ≥1.5d and preferably 2d is a useful DFM starting point, followed by load-, material- and code-based calculation; d is hole diameter. Check slots from the rounded end. High-load, fatigue and structural joints require the governing design code rather than a rule of thumb.
A Slot Relieves Variation in One Direction
One round hole plus one slot can prevent over-constraint. Set slot width from bolt clearance and length from the assembly tolerance stack plus required adjustment; the washer must fully bridge the slot. If the joint transfers load by preload friction, a slot affects slip behavior and requires a suitable anti-slip design or recalculation.
Design Threads by Effective Engagement
For metric coarse threads, D−P is a quick tap-drill estimate: M5×0.8≈4.2 mm and M8×1.25≈6.75 mm. Confirm thread class, material and tap chart. Initial engagement checks may start around 1D in steel and 1.5D in aluminum. Thin sheet often needs self-clinching, rivet or weld nuts. Self-clinching hardware must match sheet thickness/hardness and supplier edge/bend-distance limits.
Control Patterns with Position, Not Chained ± Dimensions
Establish functional A|B|C datums and control hole axes by position rather than chained ± dimensions. Maximum material condition may provide useful bonus tolerance. State whether inspection is free-state, restrained or after finish. Powder reduces hole diameter by roughly twice the film thickness; precision pin holes, grounds and threads may need masking or post-processing.
REFERENCES
ISO 273 (clearance holes for bolts and screws, reconfirmed in 2024), ISO 1101 (geometrical tolerancing), and the applicable fastener and structural-connection standards.