Drilling: Countersink and Counterbore

Fixing screws and bolts as well as drilling holes for them is one of the most basic tasks in woodworking. The screw is a much better and stronger fastener for joining two pieces of wood as compared to a nail or glue. The helical threads of the wood screw grip wood better and compress the two pieces to be joined tightly.

Most wood screws need to be housed in a countersink to be effective. The term countersink is defined as “A hole with the top part enlarged so that the head of a screw or bolt will lie flush with or below the surface”. If the screw is simply inserted on the surface of the wood, then the head will protrude and the gripping pressure will not be maximized.

Figure 1 This is a typical wood screw: the conical base of the head must sit in a countersink and as the screw is tightened the thread will pull the two pieces of the joint together

Figure 2 A countersink is a conical hole typically produced by the action of a countersink drill bit. The head of the screw rests in this recess

Figure 3 Countersink bits come in different sizes to match various screw head sizes

Countersink bits come in different sizes but produce bevels normally of 82 degrees, which seems to be standard. Other angles - upto 90 degrees - are also available but it is the diameter of the countersink bit that usually needs to match the head diameter of the screw. A larger bit could produce a smaller countersink if the depth of the bit is controlled; various sized countersinks are not really necessary except in a production setting where large numner of holes have to be countersunk quickly and precisely. Unfortunately, these simple but extremely useful bits are not always readily available in India. Bosch Power Tools sells a drill bit set (Bosch X30Ti 30pcs X-line Bit Set) which includes a countersink bit.

The Counterbore

The counterbore is another form of recess for housing screws and bolts. The latter usually have flat heads and undersides and often have to be set flush with the top of the wood. Some screws, particularly pan head, have a flat underside to the head and have to be set in a counterbore. A counterbore is defined as “A flat-bottomed cylindrical enlargement of the mouth of a hole, usually of slight depth, as for receiving a cylindrical screw head”.

Figure 4 This is a counterbore. Note the flat bottom and the types of screws and bolts it can take.

Figure 5 Forstner bits are great for drilling counterbores as they come in different sizes and leave a clean recess with a hole at the dead centre.

A wide variety of drill bits with various combinations are available in the West. For instance, the UK based company, Trend Machinery, sells a range called Snappy. These are combination bits which drill the pilot hole, the shank hole as well as the countersink at one go. There are other bits that go another step further and can drill all that plus a counterbore. DeWalt and a host of other manufacturers make these combination bits which can be very useful when much drilling is to be done.

Figure 6 This is what a combination drill bit is capable of. The screw can sit inside the hole while the counterbore can be covered with a wooden plug.

Figure 7 This is an example of a drill bit and counterbore combination made by Trend. This saves time and ensures accuracy without any effort.

Clearly, there is no end to the various types of drill bits and their combinations that are available. While it makes sense to buy these in a production setting, for the hobbyist they are not essential and a good set of standard brad point bits is more than enough.

Indranil Banerjie
8 September 2012