How to Make a Square Hole for a Carriage Bolt in Metal

Written by c.l. rease
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How to Make a Square Hole for a Carriage Bolt in Metal
A triangle file is essential in making a square hole in metal. (file image by Aleksandr Ugorenkov from Fotolia.com)

Carriage bolts have a half-round head that overhangs a square base, followed by the treads of the bolt. The square section of the carriage bolt head holds the bolt head in place while you tighten the hex nut to secure the part being held by the bolt. To hold the bolt head in place, you need to create a square hole in a piece of metal. The square hole needs to be slightly larger than the square head of the carriage bolt. However, if you make the square hole too large, the hole will not hold the carriage bolt.

Skill level:
Moderately Easy

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Things you need

  • Carriage bolt
  • Tape measure
  • Drill bit set
  • 3/8-inch drill motor
  • Safety glasses
  • Combination square
  • Awl
  • Triangle file

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Instructions

  1. 1

    Measure the square head of the carriage bolt with the tape measure.

  2. 2

    Select a drill bit from the drill bit set that has the same diameter as the width of the carriage bolt's square head.

  3. 3

    Secure the drill bit into the chuck of the 3/8-inch drill bit and put on your safety glasses.

  4. 4

    Drill a hole through the sheet metal at the location you need to install the carriage bolt.

  5. 5

    Extend the blade of the combination square to reach past the drilled hole.

  6. 6

    Place the base of the combination square on one edge of the metal and align the edge of the combination square blade with one edge of the hole.

  7. 7

    Drag the awl along the aligned edge of the combination square blade to make a reference line on the surface of the metal. Move the combination square to a different edge of the metal and repeat the described process to mark another reference line. Continue moving and marking the metal until you have a square marked on the metal with the drilled hole at the centre of the square.

  8. 8

    Slide the triangle file into the drilled hole. Align one of the corners of the triangle file with the intersection of two of the lines you scribed on the surface of the metal with the awl.

  9. 9

    Move the file up and down while applying pressure toward the marked corner. Continue filing until two sides of the triangle file meet the awl's scribed lines. Move the corner of the file to another marked corner and repeat. Continue moving the file and filing until you have a square hole in the metal.

  10. 10

    Slide the threaded section of the carriage bolt through the square hole until the square head of the bolt slides into the hole. If the square head of the carriage bolt does not slide into the hole, enlarge the hole with the triangle file until the square head of the carriage bolt fits into the square hole you created in the metal.

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