Chris Yates looks at getting the perfect countersinks.
There are several combi drill and countersink bits on the market, some with round shanks, and others with
1/4in hex drive, often marketed as ‘snappy’. They often come in sets for wood screws of gauges from No.4 to No.12, fitted with appropriately sized drill bits (usually 5/64in, to 9/64in in 1/64in increments). The drill bits are held in place by grub screws through the combi body and as well as enabling drill bits to be exchanged when they get blunt or damaged; it also enables the drilling depth to be set accurately when drilling blind pilot holes.
These combi bits all offer the prospect of drilling a clearance hole and countersink in a single operation, and can be quite a time saver if you need to drill lots of holes. As well as saving time, they also help to ensure that the countersink part of the bore is axially concentric with the pilot hole, something that is not easily achieved using separate countersink bits.
A further refinement with some makes of these combi drills is that a collar can be fitted over the shank and retained in place with a further grub screw, to limit the depth of countersink. However, as with all such depth limiters, some scoring of the work surface may result. My tip to overcome this scoring and to achieve perfect countersinks is to replace the twist drill in the countersink body with a drill blank. These are available from specialist drill suppliers and are inexpensive. An alternative, if you can’t find drill blanks, is to simply reverse a twist drill, so that the shank, rather than the cutting end, protrudes. This requires that you sacrifice the productivity gain referred to above, and drill the pilot holes separately. Then lay the workpiece on top of a hard surface, such as an offcut of MDF, and set the depth of the drill blank to give the correct depth of countersink by letting it bottom-out on the MDF.
The result will be perfectly bored countersinks, all of exactly the same depth, with no marring of the workpiece surface. This technique works particularly well if the countersink holes are visible in the finished workpiece, as the diameter of the tops of the holes will also all be exactly the same. If the pilot holes are first drilled using any convenient jig, or on a pillar drill, then the finished product will be perfect and far superior to that achieved by other means!