How to drill holes to an accurate depth
When drilling a hole for a dowel there are a number of things to consider.
Depth of holes
Wherever possible, the dowel holes in each piece of wood should be of equal depth. This is so that half of the dowel can be inserted into each part of the joint for maximum support.
However, if you are making an edge to surface joint, such as an L-shaped, T-shaped or carcassing joint, you may find that the wood is not deep enough for this to be possible.
In this case, just deepen the hole in the edge butt to accommodate the rest of the dowel.
Allowing space for glue
Whatever joint you're making, you'll need to leave a gap of a millimetre at either end of the dowel to allow space for glue.
Example: L-shaped joint
Example 1: 25mm (1") boards
Our DIYer is making an L-shaped joint out of two wooden boards. Each is 25mm (1") deep and 100mm (4") long.
The dowels that will be used are 8mm (5/16") in diameter and 40mm in length. Length is the important measurement for calculating drilling depth.
A 40mm (19/16 dowel will ideally fit into two 20mm (13/16") holes. 1mm (1/32") must be left at either end of the dowel to allow for glue, which means that each hole will be 21mm (just over 13/16") deep.
When drilling the wood surface, this will leave 4mm (3/16") of material, which means we won't see the dowel or the glue when the joint is formed.
Example 2: 20mm (13/16") board
If the DIYer was using a board that was only 20mm (13/16") thick, they would not be able to use the measurements above as they would end up drilling right through the wood.
This is still workable, but undesirable as hidden joints look neater. They also allow for more glue, which makes them stronger.
If using a 20mm (13/16") board, they would need to lessen the depth of the hole. In this case, the DIYer could decide to drill 16mm (5/8") into the board. This would allow 15mm (9/16") for the dowel and 1mm (1/32") for glue.
As the dowel is 40mm (19/16) long, there is still 25mm (1") of dowel and 1mm (1/32") of glue to account for. Therefore the DIYer would need to drill a hole in the edge butt that is 26mm (11/32") deep.
How can you measure what you can't see?
Measuring out how deep you want your holes to be is all well and good, but if you can't see the drill bit while you're drilling holes then how can you hope to be accurate?
Obviously, it's not possible to see through solid objects, but it is possible to physically prevent the drill bit from going past a certain depth into the wood by using a drill stop.
What is a drill stop?
A drill stop is a ring made from steel or rubber, with a grub screw that allows it to be fastened onto a drill bit.
Grub screws are small, headless screws with hexagonal sockets that can be turned by allen keys.
An allen key, also known as a hex key or hex wrench, is a tool that inserts into a screw and acts as a handle, allowing it to be turned easily.
Drill stops require a 2.5mm (3/32") allen key.
Once a drill stop is attached, it will act as a physical barrier that will prevent a drill bit from drilling past a certain depth. Once the stop hits the wood surface, it can't move any further.
How do you set up a drill with a drill stop?
Step 1 - Slide stop onto drill bit
Slide the drill stop onto the drill bit and push it up as far as you can.
Step 2 - Measure drill depth
Using a rule, carefully measure the depth that you need to drill into your piece of wood, starting at the tip of your drill bit.
Step 3 - Position and tighten drill stop
Slide the drill stop down the drill bit. The stop should be attached so that the part of the drill that protrudes from the stop is the right length for the hole you are drilling. Tighten the grub screw so that it is fixed securely onto the drill.
You are now ready to start drilling!