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How to choose the right drill bit for the job

Adam_W
Valued Contributor

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Choosing the right drill bit plays a huge role in making your drilling project is a success. It can help you use your power drill efficiently and safely.

 

However, there are many types of drill bits available, and it can be tricky to work out which one you need for your project.

 

Here is our guide to selecting and using the right drill bit so you end up with a fantastic result every time. 

Choosing bits based on material

 

The type of material you are drilling into plays a huge role in selecting the right type of drill bit for your project.

 

Here are some common materials and the type of bits best suited for them:


Timber (hardwood, softwood, MDF or chipboard)

For these natural and manufactured timbers, we advise using high speed steel bits, or HSS.  HSS bit.jpg

 

Soft metals (sheet metal, aluminium, copper or brass)

These softer metals can be drilled using a sharp HSS bit.


Hard metals (stainless steel, bronze or cast iron)

These higher hardness level metals are best drilled using a cobalt bit.


Plastics

These can easily be drilled with HSS bits. However, it is advisable to run your drill at a lower speed. A fast drill can heat up and melt the material, resulting in swarf melting and sticking onto the material you are trying to drill into.


Masonry (brick, roof, wall or floor tiles, blocks, concrete and stone)

A masonry bit should be used for these materials. In most cases, we recommend using a hammer drill on hammer setting to aid the drilling process. Note that if drilling tiles, hammer setting might crack them. For very dense igneous rocks like granite or basalt, you may need to use specialised drill bits.


Hard tiles, porcelain, glass

We recommend using a specialised diamond core bit or glass and tile spade bit for these. These bits must be kept wet while drilling. You can also use a fresh and sharp standard masonry bit. Keep area wet while drilling with diamond bitsKeep area wet while drilling with diamond bits

 

Blended materials

There may be times where you have to drill through multiple layers of materials. For example when wall framing, you may have to drill through tile and compressed cement sheets. We recommend using a multi-material or construction bit. These are designed for drilling through a range of tile, masonry, timber and metal materials.

 

Choosing bits based on hole diameter

It is also important to know the hole diameter you require for your project. This determines the type and size of bit you will need.

 

Here are two common types of holes and the types of bits most suited for them:

 
A clearance hole

This hole is the correct size for your fastener to snuggly pass through. For example, if you are using M12 bolts, you ideally want a hole slightly larger than the M12 (12mm) bolt to avoid having to hammer it through. A 12.5mm or 13mm bit is ideal.


A pre-drill or pilot hole

This is when you need to drill holes smaller than your fastener. To use our M12 example again, if you were using a large M12 coach-screw, then you would drill a pilot hole of 10mm to 10.5mm.

 

Choosing bits based on hole depth

 

In situations where you need to drill through your material, the bit should be at least 10mm to 20mm longer than the material thickness. This allows for the bit to cut a clear hole out the other side.

 

Ideally, the length of the drill bit should be measured against the length of its fluted section, not including the straight shank section at its base. This helps to avoid the risk of your chuck spinning against and damaging the face side of the material.


If you only need to drill about 30mm into a material, then use a bit that has extra fluted length. It is good practice to drill holes a little deeper than needed, especially in masonry. In these situations, drill swarf can fill the hole and reduce its depth.


Specialised bits


There are a range of bits designed for specific purposes. These include:


Spade and short auger 

These are designed for clearing short holes through timber quickly. Available mostly in larger diameters, they are used for running holes through framing for plumbing and electrical cables. Both bits have a sharp positioning and starter tips with the larger cutting section behind. Spade bitSpade bit

 

Long augers

These are used when a long hole is needed to be drilled through timber. For example, when you may need to run cables, pipes or cyclone rods through framing, floorboards and bearers or joists.


Forstner 

These bits look more like a router bit than a drill bit. They are designed for creating holes with flat bottoms. For example, when fitting dowels in furniture or concealed hinges in cabinetry.


Brad point 

These bits have a sharp position tip and raised cutting spurs on the outside edge of the flutes. They are designed for clean cutting when making furniture, or other similar projects.


Impact 

Designed specifically for use with impact drivers, these are designed to withstand percussive force emitting from the drivers.


Hole saws

These are used mainly to drill perfectly round holes. A cylindrical saw blade fits into a mount with a drill bit in the centre. It positions the saw and guides it through.Hole sawHole saw


Step drill 

These bits have tiered cutting stages. They are designed for cutting accurately sized holes through materials such as thin sheet metal, plastic, aluminium and fibreglass.


Counter sinking 

These come in a variety of forms. The basic ones are mainly used to drill wide holes for screws that sit flush with the surface. Others have an integrated pilot drill so that you can both pre-drill and counter sink.


Panel or rivet 

Very short bits designed specifically for use when pre-drilling metal for riveting. Their diameters are matched to rivet sizes. Some are also double ended.


More helpful tips for using tools 

 

Adam has shared a series of tips for drilling in our Best Advice sectionThe Bunnings Workshop community team has also shared these useful guides as part of our tools series: 

 

Need more help?

 

The Bunnings Workshop community is here to assist if you need a hand in choosing the right drill bit for your project. Don't hesitate to hit the Start a discussion button and let us know your needs.

 

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