Punch (metalworking)


Punch (metalworking)

A punch is a hard metal rod with a shaped tip at one end and a blunt butt end at the other that is usually struck by a hammer. A variety of punches are used in engineering, but often the purpose is to form an impression of the tip on a workpiece.

Center

A center punch is typically used as an aid to drilling operations; a drill, when brought into contact with a flat surface, will have a tendency to wander on the surface until it gains sufficient purchase to start cutting a hole. A center punch forms a small dimple in which the tip of the drill (if it is small enough) will fit. 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. A center punch usually has a tip, that when viewed on a profile, has sides at a 90 degree angle to one another.

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

Prick

A prick punch, while easily confused with the similar looking center punch, serves an entirely different purpose. A prick punch is primarily used for the purposes of layout. By design it produces a smaller indentation than a center punch, which acts as a useful datum point in layout operations. When layout is complete, the indentation made with a prick punch can be enlarged with a center punch to allow for drilling. A prick punch usually has a tip, that when viewed on a profile, has sides at a 30-60 degree angle to one another.

Transfer

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.

Pin

The purpose served by pin punches is somewhat different from the center or prick punches. A pin punch is used as a driving tool, typically, as the name implies, to drive pins that are used to affix a fixture to a rotating shaft. Pin punches are typically characterized by a hexagonal body, with a long, flat ended cylindrical section.

Doming

A doming punch is used in conjunction with a doming block to make spheres or hemispheres out of sheets of metal. The punch is generally made of tool steel, but can be made of wood. They come in a number of different sizes, the punch size determining what size the finished product will be.

Drift

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, 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.

Letter

Also known as letter stamps or number stamps. These are used to emboss the impression of a letter or number into a workpiece. They are most common in the reverse image, this allows the end result to be immediately readable, however they may be made as a positive image. This is essential in the case of die or mold making and ensures that the finished product will in fact be readable, a die is in fact a negative image already.

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