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Counterbore

 
Counterbore

 
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Counterbore
 
 
Counterbores
A counterbore can refer to a cylindrical flat-bottomed hole, which enlarges another hole, or the tool used to create that feature. It is usually used when a bolt or cap head screw
Screw

A screw is a shaft with a helix groove or screw thread formed on its surface and provision at one end to turn the screw. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force....
 is required to sit flush with or below the level of a workpiece's surface (By comparison, a countersink
Countersink

A countersink is a Cone hole cut into a manufactured object, or the cutter used to cut such a hole. A common usage is to allow the head of a countersunk bolt or screw, when placed in the hole, to sit flush with or below the surface of the surrounding material....
 makes a conical hole and is used to seat a flathead screw
Screw

A screw is a shaft with a helix groove or screw thread formed on its surface and provision at one end to turn the screw. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force....
 ). A very shallow counterbore, such as one machined on a cast part to provide a flat surface for a fastener head, may also be called a spot face.

The uppermost counterbores shown in the image are the same tool. The smaller top item is an insert, the middle shows another three-fluted counterbore insert, assembled in the holder. The shank of this holder is a morse taper although there are other machine tapers that are used in the industry. The lower counterbore is designed to fit into drill chuck
Chuck (engineering)

A chuck is a specialized type of Clamp used to hold rotating tools or materials....
, and being smaller, is economical to make as one piece.

Counterbores are usually made with standard dimensions for a certain size of screw. The tip of the tool is called the pilot. Assuming the first, smaller-diameter hole has been drilled to the standard diameter for the particular screw size, the pilot will fit the hole with little clearance and will help to provide rigidity and hole center location (i.e., centerfinding). This is extremely helpful when running the cutter with a pistol-grip drill
Drill

A drill is a tool with a rotating drill bit used for drilling holes in various materials. Drills are commonly used in woodworking, metalworking, construction and most "Do it yourself" projects....
 (to keep the tool from wandering) and on a drill press
Drill

A drill is a tool with a rotating drill bit used for drilling holes in various materials. Drills are commonly used in woodworking, metalworking, construction and most "Do it yourself" projects....
 (for fast, easy return to coaxiality with the original hole center). The pilot matters little when running the cutter in a milling setup
Milling machine

A milling machine is a machine tool used for the shaping of metal and other solid materials. Its basic form is that of a rotating cutter which rotates about the spindle axis , and a table to which the workpiece is affixed....
 where rigidity is assured and hole center location is already achieved via X-Y positioning.

See also

  • Countersink
    Countersink

    A countersink is a Cone hole cut into a manufactured object, or the cutter used to cut such a hole. A common usage is to allow the head of a countersunk bolt or screw, when placed in the hole, to sit flush with or below the surface of the surrounding material....
  • Spot face