Carriage bolts have a square shoulder resting below a smooth, domed head. The domed head eliminates snag points on the installed fastener, and the square shoulder holds the carriage bolt in place while you tighten the connection. To hold the carriage bolt in place, you need a square hole sized to match the square shoulder of the carriage bolt. Creating a square hole in material seems like a difficult task, but one process creates a square hole in common materials (metal, wood and plastic) without the need for expensive specialty tools.
Measure the width of a carriage bolt's square shoulder with a tape measure.
Select a drill bit from the drill bit set that is 1/16 inch larger than the dimension of the carriage bolt's square shoulder. For example, a carriage bolt with a 1/2-inch shoulder would require a 9/16-inch drill bit.
Drill a hole into the material with the selected drill bit and 3/8-inch drill motor.
Secure the 4.5-inch, 60-grit flapper wheel to the 4.5-inch angle grinder. Run the spinning flapper wheel along the exit side of the hole to remove the burr left by the drill bit.
Set the base of a combination square on one side of the material. Extend the combination square blade until it meets one side of the drilled hole. Drag an awl along the end of the combination square blade. Move the combination square and repeat the process to mark a straight line on each side of the drilled hole, forming a square.
Slide a square file into the drilled hole, align one corner of the file with one corner of the marked square and move the file up and down while applying pressure toward the marked corner. Stop filing when you reach the marked line. Work the file as described until all the material is removed from within the marked square hole.
Place a carriage bolt into the square hole. If the square shoulder does not enter the hole, enlarge the hole with the square file.