How to drill and tap a hole

Written by c.l. rease
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How to drill and tap a hole
The size of the drill bit will determine the strength of the finished hole. (arhendrix/iStock/Getty Images)

Drilling and tapping a hole in metal is a simple process that, if performed correctly, will result in a structurally sound threaded connection. The size of the drill bit used to create the hole is the most important factor in determining the strength of the finished threaded hole. An oversized drill bit will create a loose connection between the threads of the bolt and the threads within the tapped hole. Undersizing the hole will result in broken taps and a binding connection between the bolt and the threaded hole.

Skill level:
Easy

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Things you need

  • Tape measure
  • Scribe
  • Centre punch
  • Hammer
  • Drill and tap chart
  • Drill
  • Cutting and tapping fluid
  • Drill bit set
  • Tap set
  • Tap tee handle
  • Clean rags
  • Grinder
  • Flapper wheel

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Instructions

  1. 1

    Mark the location for the threaded hole with the tape measure and scribe. With the centre punch and hammer, centre mark the hole location.

  2. 2

    Refer to the drill and tap chart to determine the correct size of the drill bit required for the tap. If the hole is larger than 1.2 cm (1/2 inch), you will need to drill a pilot hole before you can drill the final hole size. Do not use a pilot hole that is larger than 40 per cent of the diameter of the final hole size. A larger pilot hole will cause the final drill bit to bind and break, resulting in a poor final hole quality or injury to the person drilling the hole.

  3. 3

    Insert the drill bit into the drill. Apply cutting fluid to the centre mark that you placed on the metal and onto the drill bit. Drill the hole through the metal, using steady pressure. Apply additional cutting fluid as necessary to keep the drill bit cool. If you are drilling into stainless steel, pump the trigger to keep the drill bit moving slow. Increased drilling speeds will result in your drill bit heating up and losing its cutting edge. With stainless steel, slower is always better.

  4. 4

    Clean the shavings from around the drilled area with a clean rag. Insert the proper tap into the tee handle. A drill can be used to run the tap through the hole, but it is not recommended as even slight pressure other than straight down will break the fragile carbide tap.

  5. 5

    Liberally coat the tap with cutting and tapping fluid. With the tap aligned straight with the hole, turn the tee handle clockwise to start tapping the hole. If you are tapping for left-handed threads, you need to turn the tee handle counter-clockwise for the tap to start in the hole.

  6. 6

    Eliminate tap binding by turning the tee handle backwards 1/4 of a turn after each revolution of the tee handle. The 1/4 turn back will remove filing build-up from the front edge of the tap. Apply tapping fluid to the tap before continuing the tap into the hole.

  7. 7

    Reverse the tap to remove it from the threaded hole. Attach the flapper wheel to the grinder and remove the burr from the hole. Test the threads with the correct size bolt to ensure that the bolt threads correctly.

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