A metal detector search coil is the rounded coil of wire on the end of the metal detector. The coil is fed a signal by the electronics in the body of the detector and transmits to the area surrounding it via an oscillating electromagnetic field. When the field comes into contact with a metallic object, its shape is altered. This is detected by the coil, which sends a signal back to the electronics in the metal detector, causing a sound that notifies the user of the presence of metal.
Draw a large circle on your block of wood using the pencil and compass. The size of the circle may vary, but the depth of penetration of the metal detector is proportional to the size of the detector coil; so the larger you can make it, the better. Draw a second circle inside the first circle, leaving a gap of between 10 and 15 cm/2 and 3 inches between the circles.
Place the block of wood on the band saw. Carefully cut the outer circle of wood out using the band saw. Choose one spot on the circle and cut a line through it to access the inner circle. Cut out the inner circle of the wood using the band saw, leaving you with a circular wooden shape with a small cut in it. Remove the circle from the band saw and discard the excess wood.
Wrap the copper wire tightly around the circle, starting on the outside and turning the copper so it passes through the hole in the middle and comes back on itself. Continue until you have wrapped the copper wire all around the circumference of the circle, creating a circle of looped copper.
When wrapping copper wire around your wooden block, be sure to leave between six inches and a foot of copper wire loose at the start and end points of the loop as these will connect to the circuitry of the detector. You could use less wire for these leads, but that would mean placing the detector equipment closer to the coil, which may cause interference.