CENTER PUNCHES


Many applications require accurate starting dimples to position drill bits without wandering. Sizes may vary, dependent on bit size and work material. Although some are pointed, the most commonly used twist drill bits have a small flat chisel tip; which is not self starting in all but very soft materials. Resembling the rim of a volcano, a small dimple is made in the work surface with a center punch, slightly larger than the chisel tip. When drilling perpendicular holes, the rim must be of equal height around it, to avoid a weak area; which may break way. To accomplish this, the punch must be ground truly round and held perpendicular during punching. Controlled by the punch angle and the depth, the size is determined by the hammer striking force. A pin prick will not guide a 1" bit, and a large 90 ° punch will leave a crater larger than a hole drilled by a #80 bit.


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DRILL BIT CHISEL TIP


Often for very accurate alignment, this becomes a two step process; since a large center punch may block the view of the cross reference lines. Sometimes with the aid of magnification; a sharp taper, prick punch is used to make a small guide dimple to prepare for use of a larger center punch. The small dimple can usually be felt when larger point is applied. Although smaller hobby sizes should be used on models, larger ones may be useful in benchwork.

With a total angle range of 30 ° - 60 °, prick punches are normally used during layout, with very tiny dimples to allow for corrections. Very often with scribed lines, the intersection can be felt. Errors may be amended by slanting the tip in the dimple toward the desired direction and tapping. Then it is held vertically with tip in new hole and tapped again. Often machinists place prick marks along hard to see scribe lines to aid visibility during work.


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PRICK PUNCHES
very small
3/8" hex-handle w/ 30 ° total angle


A wider angle is needed for lager bits to compensate for the chisel tip.


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PLAIN CENTER PUNCH
7/16" hex-handle w/ 90 ° total angle.


In softer plastics, large, deep dimples may raise larger crater like rims around the perimeter. Light finger pressure with a very sharp prick punch, scriber or even a needle or tee pin will usually suffice on plastics for small bits used for grab-irons and small piping.

With simpler punches, point is set at the intersection of cross reference lines, and holding perpendicularly, the opposite end is tapped with a suitably sized hammer. On soft materials, care must be taken to eliminate very large dimples. Thinner sheet stock requires a hard wood, or metal, backup and light tapping to avoid denting area around dimple or punch through.


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SMALL NEEDLE TOOL
Useful as scriber or prickpunch.


At the start of DRILLING, the bit vee is barely started and then withdrawn to observe centering. Frequently for greater accuracy, machinists scribe a small circle around the prick mark to serve as a target guide. Correction is by punching a new dimple at an angle toward the right location. This is followed by a second application with punch held vertically with tip in new dimple. Earlier is better.


>Wood and leather workers commonly use sharper pointed awls to mark or start holes and to pierce through softer materials. With wooden or plastic handles, they are often pounded with the heel of the hand, but some have a metal plate on the handle end for tapping with a hammer. Their sharp points permit use as a scriber on softer materials, hence they are frequently called scratch awls., Almost extinct with the passing of the iceman, the once very common icepick has a sharper point. All these work well on plastic and, with care, wood.


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AWLS
Large awl.
Smaller awl.
Icepick.


Closely related are DRIFTPINS _for driving shafts through or aligning holes and wedge shaped CHISELS _for removing material.


For faster and easier work; adjustable, automatic, spring loaded punches are available in several sizes. They are pushed into the work piece, compressing the spring until a release point is reached. Then the spring shoots the tip into the work. Careful adjustment must be made; since in softer materials, the tip may gouge before release or excessive force may produce a very large dimple. One or two, longitudinal, screw type spring adjusters are located on the barrel.


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AUTOMATIC CENTER PUNCHES.
Maxon, with both adjustments screwed out to soften shot.
Tip is not hard enough for most steels and may be dulled.

Larger automatic center punch w/ single adjustment.



Many specialized punches are available for odd applications. Among these is a round bar stock center locater with a tapered cone around tip; often used by machinists to center drill for turning between centers on a lathe.

Since it is tricky to locate the center of existing holes in a work piece by eyeball or measurement, in order to drill mounting holes, several methods and aids have been developed. In deeper hole, a close fitting drill bit can be entered and twisted a few turns by hand. This will create a dimple sufficient for punching or starting a tap drill. Shallow holes are more difficult.

Useful around the layout or shop for locating hole centers in metal plates, hinges, brackets and braces; several sizes of centering pin punches are available. These fit into countersink holes in plates and a hammer drives a recessed center punch into wood. Used by machinists, similar punches are referred to as transfer punches, which come in large sets with many sizes and variations



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COUNTERSINK CENTER PUNCH.


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