GlossaryKind of term: Manufacturing termCountersink and counterbore
What are countersinks and counterbores?
A countersink is a conical and a counterbore a cylindrical recess at the top of a hole, in which a screw head sits below or flush with the surface.
A countersink takes a countersunk screw. For metric countersunk screws the included angle is 90°; inch screws often use 82°, and 100° is common in aerospace. A counterbore is a cylindrical recess with a flat floor, for example for a socket head cap screw. The drawing gives the diameter and the angle or depth at the hole; drawings to ASME use the symbols ⌵ for a countersink and ⌴ for a counterbore.
What does the standard say about countersinks and counterbores?
The sizes follow from the screw. For countersunk screws with a head to ISO 7721, such as ISO 10642, ISO 15065 gives the countersink dimensions. For cylindrical head screws such as ISO 4762, DIN 974-1 gives the counterbore diameters, in series that differ in clearance and in room for a washer. The depth follows from the head height, any washer and how far below the surface the head must sit; if the drawing does not give it, agree it with the customer.
How do countersinks and counterbores work in practice?
A countersink or counterbore is an extra pass on the same hole, unless a combination tool makes hole and recess in one pass. A countersink of the right angle covers several diameters, because the depth sets the diameter at the surface; a piloted counterbore usually belongs to one screw size. An end mill on a circular path makes counterbores of several sizes with one tool, at more time per hole; a chamfer mill of the right angle does the same for countersinks. A recess on the back face needs a second setup or a back-countersinking or back-counterboring tool. The depth decides whether the head sits clear of the surface, so it is checked; on a countersink this is often done through the diameter at the surface.
What are countersinks and counterbores often confused with?
A counterbore is not a pocket: a pocket is a milled cavity with its own contour, a counterbore a cylindrical recess around one hole. Nor is it a spotface, which only machines a shallow flat seat for a head or nut on a rough or sloping surface. And a countersink is not a chamfer: a small chamfer only breaks the edge of a hole and carries no head.
How do you see countersinks and counterbores in Blake?
When a counterbore on the back face needs a second setup, you enter that setup on the route in Blake and accept it yourself. Setup then becomes its own line in the estimate, per piece and per batch, and is shown as an estimate. More on the factory.
Frequently asked questions
What is the difference between a countersink and a counterbore?
A countersink is conical and lets a countersunk screw sit flush with the surface. A counterbore is cylindrical with a flat floor, for a socket head cap screw. A counterbore tool usually suits one size; a countersink covers several diameters through its depth.
What angle does a countersink have?
For metric countersunk screws 90°. Inch screws often use 82°, and 100° is common in aerospace. Match the angle to the screw on the drawing; with the wrong angle the head bears on an edge instead of across the cone.
How deep should a counterbore for a cap screw be?
Deep enough for the head, with any washer, to sit below the surface. The diameters per screw size are in DIN 974-1. Take the dimension from the drawing, and ask when it only says ‘counterbore for M6’.