Question
I am curious about how saw arbors are produced. They have an arbor that is surrounded by a bearing. The bearing is somehow captured by some kind of casting. Is this a press fit or is there heat involved? How is the assembly installed so as to keep the arbor from drifting side to side along the length direction of the arbor?
Forum Responses
(Cabinetmaking Forum)
From contributor G:
Basically two bearings, one shoulder and one nut. The bearings are retained in two seats by virtue of being on opposite sides of the casting. Have never torn one down but I imagine, at least in theory, a semi press fit on both shaft and casting. I doubt that they use clearances that require heat shrink fits.
The lateral (axial) play (and radial play for that matter) of the arbor (and blade) is at the mercy of the fit of the ball bearings in their races. (If you want better, one goes to a higher class bearing, or to a system using bearings with greater thrust capacity and preloads them so that the pair of bearings are forced to ride at either their axial maximum or minimum separation. Which reduces both axial and radial play. (Compare to zero lash CNC axes drives.) Or was that what you wanted to know?
Rim speed = How fast the rim is traveling... If the circumference, the distance around the disk, is 36 inches and the angular speed is 1000 rpm, then the rim speed is circumference times RPM, here 36 x 1000 or 36,000 inches per minute. That equals 3,000 feet per minute (divide by 12). It also equals 600 inches per second (divide by 60) or 50 feet per second (divide by 12 and then 60.) Think of it as how fast the wind or scenery is going past if you are riding on the edge of the disk.
To imagine the why of rim speed, just think of each tooth as a wood plane... How fast do you want that plane to be traveling? You are dealing with heat from friction, the ability of the tool to clear chips, the amount of horsepower (push) you got, whether the material behaves better by taking a deep slow cut or a shallow fast one, how fast the material is fed. If you feed maple too slow or run the tool too fast each little bit of maple sees too many passes with heat being generated by each cut, or the blade just skating over without cutting, and the heat burns the wood.
The circumference of a disk is its diameter (or twice its radius) times pi (3.14159). So rim speed is (diameter times 3.14159) times RPM.
The other way of thinking about it is to unwind the disk. Take the teeth off the cutter or saw and stick them on a long string. Bingo - you got a hand saw or a band saw.
From various book tables or the manufacturer, you can get feeds and speeds, which is how much the tool is able to cut per tooth per pass. The combination of rim speed and feed speed. Too deep and you lose temper from heat, load the gullet with cuttings or stall the saw. Too little and you lose production, may have heat problems, or the teeth may not engage but just skate over the work piece.
If you use that VW strobe and it is calibrated in RPMs, it can give you the RPMs. If you know the motor speed and work through the pulley changes, you can get arbor RPMs. Say 1850 RPM on motor, 2 inch pulley on motor and 5 inch on arbor, then every turn of the motor gives you 2/5 turns on the arbor. 2/5 times 1850 is 740 rpm on the arbor. If you have an 8 inch blade on the arbor it has a circumference (distance around) of 8 x 3.14159 or 24.9 inches so rev times distance is 740 times 24.9 or 18,426 inches per minute, or 307 inches per second or about 25.6 feet per second.
Also people get sloppy. Since they have worked out what the rim speed should be for a given feed rate they may just tell you, "For hand fed lumber just run that cutter at such and such RPM." They are giving you a figure that applies to that tool's diameter and is easier to say and set on the machine, which only knows RPMs, than referring to the rim speed, but the tool's performance is measured in its rim speed nonetheless.
And to your question, yes - if the diameter goes up, the rim speed goes up, assuming a fixed RPM. In fact if the tool is twice the diameter, the rim speed will be double. (And in theory you should either double the feed speed or half the RPM if you double the cutter diameter... all else being equal.)
To find the rim speed, which is usually shown as inches or feet per minute, multiply the diameter of the blade or cutter by PI (3.1416) and multiply that figure by the shaft RPM.
A 10 inch blade on a 5000RPM shaft would be:
10 x 3.1416 = 31.416 x 5000 = approximately 155, 680 inches per minute.
A 1" diameter router but in the same machine would have an edge speed of 15,560 inches per minute.
Cutter speed (rim speed) is a lot more critical in metal working that wood, but this is why it is difficult to use router bits in a shaper. The shaper has a slower shaft speed, mainly due to its being generally designed for larger diameter cutters. Likewise, the same reasoning applies to why you slow down a router as the bit diameter increases. It all relates to the force being applied to the cutting edge.
