Tricks of the Trade
Q: I have a job that requires a coined countersink in 1/8" (3mm) aluminum. I took the dimensions of the screw and then ordered a coined countersink tool. I was surprised when I discovered that the screw did not sit flush with the sheet. The coin diameter* of the tool matched the screw size. Does material spring-back cause the coin diameter to shrink?
 A: No. The reason for the screw not sitting flush or below the sheet is a result of making the coin diameter the actual size of the screw. Normally, screws are tumbled after being manufactured to break the sharp edges. The dimension required to make the screw sit flush with the sheet is the "theoretical sharp" dimension. The drawings below illustrate the difference.
It is best to get the fastener dimensions from a machinist's handbook or order the coining tool by screw type (i.e. 8-32 Phillips head, 82º screw). It is important to give the head type, because hex head screws for example, have a larger head diameter than Phillips or slotted type screw heads. Screw sizes come in 82º or 100º angles. All metric screws are 90º.
We recommend making the coin diameter approximately .010" (0.25mm) larger to ensure the screw sits below the sheet.
*Coin diameter is the major hole diameter at the top of the sheet.
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