Circular saw blades have different sizes from a few inches to several feet across. Diameter is the span from one edge of the circle to the other measured along a center line. The radius is half that; it is the span from the center to the edge. A saw doesn't have the capacity to cut that deep, though. Maximum depth of cut is less than the radius. It cuts no deeper than 1/3 of its diameter. A 10" saw cuts about 3" deep at maximum capacity, and usually less. A big blade has greater cutting depth. Not only can a larger blade cut deeper, it also has a larger circumference, or the measurement around the rim. A longer cutting length around the circumference makes space for more tips overall. For this reason, bigger blades have longer endurance. Small blades have less circumference circling the rim, which explains how fast they get used up. A big one might last the equivalent of two smaller ones. However, large sizes won't necessarily allow faster feed; coarseness determines that.
RPM means Revolutions Per Minute. One revolution is one turn around in a circle. To find out how fast it is revolving, look on the motor for the RPM specification. You can also count Revolutions Per Minute with a tachometer. Revolutions are different from peripheral rim speed; also called SFM or Surface Feet Per Minute. How fast does a saw go at the rim? If a big saw blade were turning at the same RPM as a small one, it would have greater SFM and go faster at the surface. In theory, machines for similar purposes ought to have equivalent SFM, so each is built with a motor that will revolve at the appropriate rotation speed for the blade fitting it. For most saws, variable speed or RPM would be unusual. Maximum SFPM for saws is around 18,000, but recommended speed is typically lower, especially for sawing hard materials. This is why a cold saw for metal cutting is built to run at a very different, much lower SFPM than a typical saw for woodworking. For an average woodcutting example, a 10 in. miter saw turning at 5,000 RPM goes a little over 13,000 SFM. Calculate your own saw rotation speed by following the steps of this formula:
Surface Feet Per Minute
- Measure the diameter in inches and divide it by twelve to find the diameter in feet.
- Then multiply it by pi, Π or 3.14 to get the circumference in feet.
- Finally, multiply it by the R.P.M. to get S.F.M.
Surface Meters Per Minute
- Diameter in millimeters times .00314 equals circumference in meters.
- Multiplying circumference times R.P.M. equals S.M.M.
Whatever machinery you have, mount a compatible blade measuring the full size. Substituting a close enough undersized diameter may be possible, but not ideal. It won't be able to cut as deep, and it won't cut efficiently. Where a ten in. saw fits, a six in. substitute isn't practical. Since the table saw is still revolving at the same pace, a smaller substitute cuts poorly because its rim speed is too slow. Under 60% of the standard size is an incompatible risk. Hypothetically, if you could fit on an excessively large one, the peripheral speed would go too fast, but that's not usually possible because the housing would get in the way. SFPM isn't likely to go too fast unless the motor is switched to one that will revolve at a faster rotation speed, which is not recommended.
Bore and arbor bushing
One size does not fit all. The arbor of a small portable circular saw may be only 5/8 inch, because its priority is light weight. 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 inside diameter of the center bore should fit snugly on the shaft. You can't transfer it to different machines if the fit is wrong. To adapt to a larger shaft, you can get a professional to enlarge or re-bore a hole. It is easier to adapt to a smaller machine. You can take a blade that has a larger hole and put in a spacer ring made just to fit, which is called a bushing.
Reducing bore size is done by inserting a bushing that exactly fills the excess space. Bushings must be precisely accurate in both outside and inside diameter. 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 spacer rings allows you the option of reversibility.
The closer a mismatched bore size is, the more frustrating a minor incompatibility seems, because close is just not good enough. Ironically, a bigger misfit may convert downward easily with a bushing. Reduction with a 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.
Metric vs. Inch
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 brands are committed to the metric system and others specify US units. European and inch sizes are not compatible. The difference is significant. They are not the same size with a translated label. A few arbor sizes fall into a close range of arbitrary marginal differences. Some inch and metric sizes are just too close for comfort. They look nearly alike, but aren't interchangeable. At first glance, 25 millimeters and 1 in. appear the same, 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. To fill the slight space between 1 in. and 25 mm, in theory, you'd need to put in a bushing with a fragile .2 mm wall. To reduce even a 30 mm hole to fit a 1 in. shaft would still require the wall of the ring to be only 2.3 mm. The force of installation could distort it. To reduce a hole of 1.25 in. (1 1/4 in.) down to 30 mm would take a flimsy bushing insert with a wall measuring less than a millimeter. Foil and duct tape are not sustainable solutions.
Your quest for a compatible fit need not be limited to the original brand. You can try another one if it has equal measurement. If you acquire some vintage equipment without a manual, note the model, keyways and pinholes, and measure it accurately with Vernier or dial calipers. For reference, the chart below lists a few equivalent diameter examples. Mixed inch and metric sizes are sorted from small to large. In each row of the chart, the US inch size is matched to an equal metric size. In each pair, one is a common size. The other (in parentheses) is its conversion to the other measurement system. This doesn't mean you'll see many blades marked with both for accessibility. Even if both are listed together, one is the default. If its equivalent is rounded up or down, it's just for contextual reference. Measuring it yourself is the only way to know with literal accuracy. When you have what seems to be an unfamiliar fractional size, you might be surprised to find it is actually a standard in the default units of the country where the machinery originated.