TABLE OF CONTENTS Press operations can produce a dimpled hole that is an effective countersink or they can dimple the material and then come back with a small-machined countersink in the dimple. The latter is done for thin metal, particularly. Countersink cutters typically provide a smoother surface. Pilot holes are typically drilled or punched before the sheet is dimpled (Anonymous 2004). The final hole size can be drilled or reamed after dimpling or countersinking if necessary. Under favorable conditions and properly designed dies, rivet clearance holes can be punched simultaneously when the dimple is formed (Walker 1986).
Shallow swaged countersinking can be performed with press tools if the depth of the countersink does not exceed 10-20% of stock thickness in light gage stock and 20-30% in heavy gage stock. Deeper swaged chamfers require more difficult and expensive operations. Swaging or coining takes the material and presses a countersink into the metal while dimpling bends the metal into a small conical pocket or flange.
As noted in Chapter 10 and a fact that is surprising to most engineers outside the aircraft industry pressing a countersink (called dimple countersinking) can produce a countersink with better fatigue life than machining the countersink does (Rans 2005). Illustrating the differences between the two approaches, Table 12-1 provides strength data on riveted joints produced by both machine countersinking and press dimpling. MIL-HDBK-5F provides more insight (Anonymous 1990).
| Sheet thickness (in.) |
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