Arbor

The arbor is the shaft, spindle or mandrel which the circular saw blade mounts to. Typically, it is the motor shaft, which has been specially designed for blade mounting. So, what we refer to as the arbor on a circular saw blade is more correctly the arbor hole. This center bore is responsible for power transmission from the circular saw motor.

Sizes

Arbor holes can be a variety of sizes, depending upon the size and style of the saw. The most common sizes are:

As you can clearly see, as a general rule, the larger the saw blade diameter, the larger the arbor hole. The most common circular saw blade sizes are 7-1/4”, 10” and 12”. The 7-1/4” size is used for handheld saws, while the larger sizes are used for stationary circular saws. The size of an arbor hole is 5/8-inch for an almost every circular blade, although small woodcutting saw blades have 1/4″ arbor for 3″ diameter, or 1/2″ for 6″diameter. It’s very important to look at this measure before buying: 5/8″ equals to 15.875 mm for European standards. A bigger shaft of 1″ is characteristic of powerful circular saw since more power and torque require larger spindle. Some blades smaller than 12” may still come with a 1” arbor hole or a diamond shaped one. Since the blade must fit fairly tightly to the arbor, to maintain concentricity, this excess space is taken up by a reduction bushing. Most saw blades come with this.

Worm Drive Saws

Whiles the saw blades have rounded shape of the bore, worm drive saws have a diamond shaped arbor hole because of the higher torque which these saws develop. These are handheld saws, designed for use by carpenters. Cutting through large quantities of dimensional lumber quickly on a construction site is important. Therefore, worm drive saws are popular amongst professional carpenters. They have an engine vertically directed to the saw blade plate offering more torque power, under the cost of lower motor speed. The worm drive mechanism greatly increases the available torque, with little sacrifice of RPM in the process. This allows the carpenter to cut through dimensional lumber rapidly, even when the wood is wet, contains knotholes or has nails in it. Some saw blades have diamond knockout arbor, a rectangular-shaped slot around the rounded arbor that should be removed in order to use blade on a worm saw. Most people simply knock out the diamond arbor with a hammer.

Arbor Reducer Bushing

When selecting circular saw blades, it is important to ensure that the arbor hole is the same size as the arbor on your saw. It should match exactly in order to spin and cut efficiency.  In case bore doesn’t match the shaft, there’s an alternative to buying a new blade. For too small spindle you can put a bore reduction bushing, a spacer ring that adjusts the sizes between the blade and the saw to make it usable. Arbor bushing is easily to install, cheap and saves money if working with different sizes of shaft is needed. Never mount a saw blade to a saw, if the arbor doesn’t fit snugly. This can cause a severe out-of-balance situation, which can be extremely dangerous.