Diameter and Bore Size

Diameter size

Circular saw blades have different sizes from a few inches to several feet across. The reason is that capacity or maximum depth of cut is 1/3 of diameter. A big blade has an advantage in cutting depth. Large sizes won't necessarily allow faster feed; coarseness determines that. Smaller blades have less circumference circling the rim, so they get used up quickly. Bigger ones endure longer use.

Rotation speed

RPM means Revolutions Per Minute. Look on the motor for RPM, or use a tachometer to measure. Peripheral rim speed is SFM or Surface Feet Per Minute. At the same RPM, a big saw blade has faster SFM than a small one. Machines for similar uses ought to have equivalent SFM, so each is built with a motor going the appropriate rotation speed for the blade fitting it. 18,000 SFPM is the maximum. A typical speed is much lower, especially for sawing the hardest of materials. Calculate your own saw speed by following the steps of this formula:

For an average example, a 10 in. table saw turning at 3,450 RPM goes a little over 9,000 SFM. Whatever machinery you use, mount the full diameter that fits it. Substituting an undersized one is possible, but not ideal. If a saw is capable of holding a ten in. size, a six in. substitute isn't practical. It cuts poorly because its rim speed is too slow. Under 60% of the standard size is a risky proposition. If you could fit on an excessively large one, the peripheral speed would be too fast, but it's largely a hypothetical concern. SFPM isn't likely to go too fast unless the motor is switched for one having faster rotation speed.

Bore and arbor bushing

A small portable circular saw gives priority to light weight, so its arbor may be only 5/8 inch. A bigger machine needs power, momentum and torque, so it has a heavy duty arbor size and a large collar to improve rigidity and limit deflection.

The center bore should fit snugly on the shaft. You can't use it on different machines if it fits wrong. To adapt to a larger shaft, you can get a professional to re-bore your blades. Another way is to take one that has an even larger hole and put a bushing in it.

To adapt to a smaller machine, changing bore size can be done by inserting a bushing. Rest the blade on a small block of hardwood elevating it so the rim does not touch anything. Center the adaptor over the bore hole; it won't fit inside yet. There are often tiny serrations on the outside of arbor bushings. When you install a bushing, the metal will compress slightly. Stack another small block on top. Gently tap it until the whole circle is flush. Installing a spacer gives you an advantage of reversibility. The disadvantage of bushings is they might not be precisely accurate.

Metric vs. inch

One size does not fit all. Your quest for a proper fit need not be limited to the original brand; you can use one with equivalent measurements. If you acquire some vintage used equipment without a manual, use Vernier or dial calipers to measure it accurately. Note the model, keyways and pinholes. Standard sizes are more conveniently obtained, so it is in your interest to know which ones are currently available and familiar to your region. For example, in America it is not hard to find table saw blades with 5/8 in., 1 in., or 30 mm holes. On big machinery, it is not unusual to see a 3 1/8 in. opening that has keyways or slots, and pinholes. Odd, out of the average sizes are: 1/2 in., 3/4 in., and 25 mm.

European and inch sizes are incompatible. A few arbor sizes fall into a close range of arbitrary marginal differences. European brands are committed to the metric system and others use US units. 25 millimeters and 1 in. look alike, but a 25 mm hole won't fit on a 1 in. saw arbor. You'd need to re-bore it to 25.4 mm. Conversely, putting a 1 in. hole onto a 25 mm shaft makes a loose fit and a messy cut.

The bushing insert will work well if the difference between the arbor and opening is at least 1/4 in. for strength. The wall of the adaptor would be 1/8 in. or a little over 3 mm. Some inch and metric sizes are just too close for comfort. Converting a 30 mm hole to fit a 1 in. shaft would require the wall of the ring to be 2.3 mm. The force of installation could distort it. To reduce a hole of 1.25 in. down to 30 mm would take a flimsy bushing insert with a wall under a millimeter. To make up the difference between 1 in. and 25 mm, you'd need to put in a bushing with a fragile .2 mm wall. Foil or duct tape is not a sustainable solution.