Punch (engineering)

A punch is a hard metal rod with a shaped tip at one end and a blunt butt end at the other, which is usually struck by a hammer. Typically, woodworkers use a ball-peen hammer to strike a punch. Punches are used to drive objects, such as nails, or to form an impression of the tip on a workpiece. Decorative punches may also be used to create a pattern or even form an image.

Metal pins and similar connectors are driven in or out of holes using a pin punch.

For removal, first use a starter punch to loosen the pin, then use a pin punch to finish.

A center punch is used to mark the center of a point. It is usually used to mark the center of a hole when drilling holes. A drill has the tendency to "wander" if it does not start in a recess. A center punch forms a large enough dimple to "guide" the tip of the drill. The tip of a center punch has an angle between 60 and 90 degrees. When drilling larger holes, and the web of the drill is wider than the indentation produced by a center punch, the drilling of a pilot hole is usually needed.

An automatic center punch operates without the need for a hammer.

A prick punch is similar to a center punch but used for marking out. It has a sharper angled tip to produce a narrower and deeper point. It is also known as a dot punch. The mark can then be enlarged with a center punch for drilling. The tip of a prick punch is 40 degrees (the angle depends on what type of prick punch you are using.)

A transfer punch is a punch (usually in an index set) of a specific outer diameter that is non-tapered and extends the entire length of the punch (except for the tip). It is used to tightly fit the tolerances of an existing hole and, when struck, precisely transfer the center of that hole to another surface. It can be used, for example, to duplicate the hole patterns in a part, or precisely set locations for threaded holes (created by drilling and tapping) to bolt an object to a surface.

A drift "punch" is misleadingly named; it is not used as a punch in the traditional sense of the term. A drift punch, or drift pin, or lineup punch, is used as an aid in aligning bolt or rivet holes prior to inserting a fastener. A drift punch is constructed as a tapered rod, with the hammer acting on the large end of the taper. The tapered end of a drift punch is placed into the semi-aligned bolt holes of two separate components, and then driven into the hole. As it is driven in, the taper forces the two components into alignment, allowing for easy insertion of the fastener. Unlike most punches, force is never (and should never be) applied to the tip, or end of a drift pin.

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