Drilling a Countersunk Pilot Hole In
Today we are going to learn how to make a countersunk pilot hole in wood from our fastener expert, Bob. He told us that he has seen many people attempt to drill this hole and approach it the wrong way. Most people start the same way the would if they were using a standard drill bit. First the pilot hole, then the countersink. This however, does not work with spade bits due to the way the drill bit is engineered.
A countersunk hole is where a secondary larger hole is cut on top of the first hole so that the head of the nut or bolt can sit inside of the material and not be exposed. There are many applications for these types of holes and you probably see them around quite often without even realizing it. Fasteners may be countersunk in furniture, buildings, banisters, decks, etc.
Spade Bits have a triangular tip that digs into the wood before the outer edges of the bit do. This not only acts as a guide for the rest of the bit but as a stabilizing factor as well. Bob told us that if we drill the pilot hole first we would have a hard time drilling the countersunk area and would damage the wood.
He set out to give us an example by showing us the proper way to install a lag screw into some wood.
The first step when drilling a countersunk hole with a spade bit was to start with the countersink itself. This will allow both the larger spade bit (countersink) and the smaller (pilot hole) to grip the material and prevent “wobbling”.
So first we drilled the countersunk hole and then we set about drilling the pilot hole inside of the countersunk hole. It was a surprisingly easy task.
We tested this again by doing the process backwards and determined that what our fastener expert had said was true. Not only was it difficult to drill the holes but there was some significant chipping to the surface of the wood. While we were able to drill the hole the result was an unprofessional mess.
Our fastener expert also chose a very large spade bit for the countersink which he later explained was to allow breathing room for the socket that he would use to install the lag screw. Otherwise, it will be difficult to install without the socket getting caught up in the hole.
After comparing the two holes, we determined that the hole we drilled first (properly) was much better. The edges were smoother, barely any chipping on the outside of the wood, everything was in good shape. The second hole was abysmal. With chewed up and chipped edges from the bit bouncing around. Our fastener expert said we can do even better. Pulling out a roll of his trusty blue painters tape, he laid it over the wood and drilled the countersunk pilot hole again. This time there were virtually no chips in the wood at all. The tape re-enforced the edges of the hole being drilled to prevent chipping.
Below are the three drilled holes and the results:
- Hole 1 – Left – Was done properly with no tape. Light chipping occurred around the edges but was still a fairly clean.
- Hole 2 – Middle – This hole had the pilot hole driven first and as a result has heavy chipping around the external edges. The inside edges also weren’t smooth from so much bouncing around.
- Hole 3 – Right – Done by first covering with blue tape, then drilling the countersunk hole and then the pilot hole, this hole is the cleanest of all three with virtually no chipping and clean sides.