Drill & Tap Instructions


To attach removable components such as brackets, handles and bearing housings to metal or plastic, the correct size hole must be drilled and threaded with a tap. A tap is a threaded bolt-like tool, with three or four flutes ground along the shaft to form serrated cutting edges. A square section at the top of the tool fits into a “T” wrench, and the tap is screwed clockwise into a smooth hole to cut the required thread. Once threaded, the component can be bolted to the metal or plastic.

Things You'll Need

  • Set of taps
  • Number drills
  • Fraction sized drills
  • Center punch
  • Hammer
  • Drill press
  • Power drill (optional)
  • Tap “T” wrench
  • Aerosol canister of compressed air
  • Buy a small set of taps to cut the most common size threads if you don't already own a set. You should have three taps, one of each size: a taper tap to start and correctly align the thread; a plug tap with a relatively sharp taper designed to cut full-depth threads almost immediately; and a bottoming tap with no taper, designed to finish cutting threads to the end of a blind hole.

  • Consult the tap drill chart on the back of the tap set box. Most recommended sizes are number drills, with one or two fraction sizes included to suit each individual tap. Store these dedicated drills with the taps and do not use them for common drilling purposes.

  • Mark the bolt location with a center punch and hammer by placing the tapered point of the punch on the designated spot on the metal or plastic. Hold the center punch at a right angle to the surface and deliver a blow to the end of the punch with a hammer.

  • Drill a 1/16-to-1/8-inch pilot hole to guide the drill if the bolt size exceeds 1/4 inch. Use a drill press if possible. If not, use visual judgment and try to hold the drill at 90 degrees to the surface while drilling.

  • Bore the bolt hole with the correct size drill. Drill the finished hole with a drill press if possible. If you're using a handheld power drill, hold the drill as close to 90 degrees to the surface of the metal or plastic as your judgment allows.

  • Clamp the correct size taper tap into a two-handed “T” wrench. Insert the tap into the hole. Hold the tap at a right angle to the surface and exert moderate downward pressure. Turn the wrench clockwise a third of a turn, then rotate the wrench counter-clockwise far enough to break the ribbon of metal--or "swarf"--cut by the tap.

  • Continue cutting the starter thread without exerting downward pressure by rotating the “T” wrench one third of a turn clockwise, then rotating it counter-clockwise far enough to break the swarf. Continue cutting. When the starter thread reaches 1/4-inch depth, switch to the plug tap to finish cutting the thread.

  • Switch to the bottoming tap if the plug tap bottoms out when threading a hole that does not go all the way through the metal (a "blind hole"). Invert and shake the workpiece to remove any trapped swarf. If this is not possible, blow the swarf clear with an aerosol canister of compressed air.

Tips & Warnings

  • When cutting a thread in a shallow blind hole, there may be insufficient room to use a taper tap to start the thread. In this case, start with the plug tap, but hold the tap at a right angle to the surface; if the thread is started at an angle the tap will bind. If the tap is forced, it will break off in the hole and become impossible to remove.
  • A plug tap is not required when cutting threads in metal up to 1/4 inch thick.
  • A bottoming tap has no taper and cannot be used to start a thread.
  • Do not use fraction-sized drills to drill bolt holes unless designated. Some will work in a pinch. However, too narrow a hole will cause the tap to bind and possibly break, and if the hole is too wide, the thread will be shallow. This may cause the bolt to strip under pressure.
  • Always use two hands and exert even pressure when operating a “T” wrench.
  • Never use a common adjustable wrench instead of a “T” wrench when tapping. Taps are hard and brittle; they will break under the slightest sideways pressure.
  • Dip the tap into a lubricant before starting the thread. Ordinary motor oil works on brass or steel, while kerosene is best when threading aluminum or plastic.

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  • Photo Credit tools image by Witold Krasowski from Fotolia.com
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