What is the shank of a router bit?

The shank is the name given to the smooth cylindrical part of the router bit that fits into the collet of the router.

An important factor to consider when selecting a router bit is the shank diameter, as this will have an effect on a number of things, including whether or not that particular cutter can be used in your router as specific collets must be used.

The most common shank diameters in the UK are imperial ¼" and ½” shanks, and 8mm metric, although other sizes are also available. Light and medium duty routers are generally supplied with ¼" collets, while heavy duty routers usually have ½" collets. Although, other collet sizes are also compatible with various routers and many routers are supplied with more than one collet.

Imperial shank diameters are ¼", ⅜", and ½”.

Metric shank diameters are 3mm, 6mm, 8mm, 10mm, and 12mm. Cutters with the smaller shank sizes are primarily used in specialist CNC routers, rather than standard routers. 

Does a larger shank mean a larger bit?

Generally, yes. A thicker shank is better able to support a larger body with longer cutting edges, and so most bits with ½" shanks will be larger overall than ones with ¼" shanks.

However, there are exceptions and you may come across ¼" router bits with larger bodies than their ½" equivalents.

You should be wary of excessively long or large bits with thinner shanks as they tend to be unstable at higher speeds and therefore, more prone to breakages. 

Which shank diameter should you choose for a router cutter?

The shank diameter you choose will depend on a number of factors, the first being the diameter of your router’s collet. The collect must be the correct size for the bit.

Collets come in different diameter sizes. All routers include one size of collet, many also come other collets of other sizes and there is usually the option for spare collets of different sizes to be bought, although these must also be compatible with your specific router. 

Router collets and router cutter shank diameters

The size of the collet will refer to the size shank they can be used with, they must always match. So for example, a ¼" collet must be used with a cutter with a shank diameter of ¼". If you want to use a router bit with different diameter shank, you will need to purchase an adapter, usually known as a collet reduction sleeve. However, these adapters are only for occasional and light work, ideally, using a collet of the correct size is usually recommended.

Trend routers have options for different sizes of collet. The heavy-duty T10  andT11  routers come with a ½” collet included and have spare collets available that are ¼", 8mm, and ⅜". Themedium-duty T5  router includes a ¼" collet, with the safety switch version of this router and the model with a carry case also coming supplied with a ¼" router. The light-duty T4  router comes complete with ¼", 6mm, and 8mm collets.

Collect reduction sleeves

Although they cannot be used for any heavy work, or for prolonged periods of time, collet reduction sleeves are an ideal stop gap if you want to use a router cutter with a shank that is smaller than the collet you have. These adapters are available in the following sizes:

• For ¼" collets -3mm , and ⅛"

• For 8mm collets -6mm , and ¼"

• For ⅜" collets - ¼", and 8mm

• For 10mm collets -6mm , and 8mm

• For 12mm collets -6mm , ¼",8mm , and 10mm

• For ½” collets - 6mm , ¼", 8mm , ⅜", and 10mm

If you do have the freedom to choose which shank diameter to use because you have a selection of collets, then you may wish to consider some of the following before making your final decision:

• Router bits with larger shanks are usually larger overall, and are therefore more stable, so are less likely to break when cutting. Router cutters with very thin shanks must be used with care as they will be prone to breakage

• A larger shank diameter has a larger circumference. This means a larger surface area, so heat can dissipate more efficiently. As a result, there is less chance of burning your material due to an overheating

• A larger circumference also means there’s more gripping area for the router’s collet, so a larger shank is less prone to slipping and vibration when working

• Bits with smaller shanks are usually less expensive initially, more widely available, and because they tend to be smaller overall, they are often the preferred choice for finer or more delicate cutting