If router bearings have ever left you scratching your head, you're not alone. With so many sizes and configurations available, it's easy to feel overwhelmed. But once you understand how bearings are categorised and what they’re designed to do, their purpose becomes clear. This article breaks down the key types and uses, helping you work smarter with your router bits.
Common Router Bearings Sizes
Router bearings are best understood by looking at their internal and external diameters. The internal diameter determines which bits they can be used with. There are three common sizes:
- 8mm internal diameter bearings are used with slot cutters, which have a threaded end that suits this size perfectly. Regardless of whether the cutter itself is a quarter-inch or half-inch, the threaded end remains the same.
- ¼ inch (6.4mm) internal diameter bearings fit smaller router bits typically used with compact or trim routers.
- ½ inch (12.7mm) internal diameter bearings are used on larger router bits that fit into standard half-inch shanks.
Internal sizes are matched to a variety of external diameters, which control how deep the bit cuts into the timber.
How Bearing Size Affects Cutting Depth
For slot cutters, changing the external diameter of the bearing can dramatically affect how deep your cutter goes into the material. For example, a 22mm bearing might allow a 14–15mm deep trench — ideal for a number 20 biscuit. If you're using a smaller biscuit, you can increase the bearing’s diameter to reduce the cut depth. This approach gives you flexibility for fine-tuning joinery cuts without changing the bit itself.
Shank-Mounted Bearings
Bearings fitted directly to the shank of a router bit opens up a range of new possibilities. For example, a straight bit can be turned into a pattern-following bit simply by adding a bearing. This allows you to follow templates or guides with precision.
Large profile bits like windowsill or finger pull bits are often used exclusively in router tables due to their size. However, by fitting a bearing, you can use them with a handheld router to follow a guide or template. This makes it feasible to apply detailed profiles to large workpieces that are too cumbersome to manoeuvre on a router table.
Fine-Tuning Moulding Bits for Decorative Profiles
Standard round over bits come with a bearing that matches the curve of the cutter. But if you replace that bearing with a smaller one, you can transform the bit into a beading bit, creating a stepped decorative edge. This simple change adds versatility to your standard round over bit, enabling more ornate finishes without buying new tools.