A carriage bolt, also known as a coach bolt, has a large round head and is normally used with wood. Carriage bolts are often seen on wooden picnic benches. They are usually mounted so that the heads are flush with the wood. This is known as "countersinking," and involves making a shallow hole in the wood for the flat part on the head of the bolt to fit in.
Use a pencil to mark the wood where you want to insert the carriage bolt.
Measure the diameter of the head of the bolt. For example, the head is 1 inch in diameter. Use a 1 1/8-inch spade bit so the head of the bolt will fit in the hole.
Use a tape measure to measure a distance of 1/2 inch on the flat part of the spade bit. Mark this with a magic marker and draw a line across the bit. This way you can tell when to stop drilling with the spade bit and make all the holes the same depth.
Put on a pair of safety glasses. Mount the spade bit in a drill. Place the point in the center of the spade bit on the pencil mark and drill straight down until you reach the mark on the bit.
Remove the spade bit from the drill. Mount a regular drill bit in the drill that is the same size as the threaded part of the bolt. For instance, if the threaded part of the bolt is 3/8 inch, you would use a 3/8-inch drill bit.
Use the small hole left by the point of the spade bit as a guide for your hole. Drill a hole through the wood. Slide the bolt through the hole. Mount a washer and nut on the other side, and use a wrench to tighten the nut. This will fully seat the head of the bolt in the wood.