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How do you drill holes to an accurate depth?

How to drill holes to an accurate depth

Shop for Dowels

Dowel holes being redrilled in a piece of wood with more accurate guidelines When drilling a hole for a dowel there are a number of things to consider.

Depth of holes

Dowel peg inserted half way into each piece of wood in a dowelling joint 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.
Image of an edge to surface joint constructed with wooden dowels 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.
Wonkee Donkee recommends that the DIYer leave a few mm of wood to support the dowel joint when drilling dowel holes
DIYer drilling a dowel hole with a power drill In this case, just deepen the hole in the edge butt to accommodate the rest of the dowel.

Allowing space for glue

Image of a pot of wood glue reminding the DIYer to leave 1mm of space for glue when drilling dowel holes 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

Making a corner joint with wooden dowels

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.

Image of an 8mm x 40mm dowel peg 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.
Diagram to show the method of working out how deep your dowel holes must be drilled 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.
DIYer with thumbs up in approval after having created a dowelling joint where neither the dowel nor the glue is visible 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.
Image of a dowel hole that has been drilled so deep that the drill bit has come out of the other side of the piece of wood

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.

Image of a joint where the dowels are visible because the DIYer drilled too far through the wood This is still workable, but undesirable as hidden joints look neater. They also allow for more glue, which makes them stronger.
Image of a board 20mm thick to be used in making an edge to surface joint with dowels 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.
Example of an edge joint created with wooden dowels including measurements 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?

DIYer wearing a blindfold to recreate the feeling of attempting measure a drill hole for a dowelling joint without being able to see the depth to which he is drilling 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?
Image showing a drill stop preventing a drill from entering a wooden board deeper than the desired length 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.
Metal drill stops which are used to prevent a drill bit from drilling a deeper dowel hole than is needed when making a dowelling joint

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.

Image to show grub screws which are the part of drill stops that allow the stop to be fastened tightly to the drill bit when drilling dowel holes Grub screws are small, headless screws with hexagonal sockets that can be turned by allen keys.
Using an allen key to loosen a clamp on a guitar 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.
2.5mm allen key which is used to tighten or loosen grub screws in a drill stop Drill stops require a 2.5mm (3/32“) allen key.
Drill press fitted with drill stop stopping the drill bit from boring too deep a dowel hole 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?

Drill bit with drill stop attached near the top for use in a dowelling project

Step 1 – Slide stop onto drill bit

Slide the drill stop onto the drill bit and push it up as far as you can.

Measuring a brad point drill bit to drill dowel holes to an accurate depth

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.

Two drill stops preset to an accurate depth

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.

Wonkee Donkee advises againt moving the drill stop when tightening it to avoid having to remeasure to ensure an accurately drilled dowel hole
Image of an arrow made of dowels to indicate that the DIYer is now ready to start their dowelling project You are now ready to start drilling!

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