Dusty - I almost had this post sent this A.M. when my Internet connection went bonkers and I lost the post. I'll try again!dusty[B wrote:...Is my perception correct?[/B] If it is not, that may explain why I don't understand runout numbers.
Jim in Tucson made the point that the blades and arbor must be clean. This causes me to think.....if there is a spot of paint on the blade, when I install it in the arbor it won't be perfectly perpendicular to the drive line...thus wobble, thus runout.
Am I getting this or no?
Yes, Your perception is right (or almost, as I understand it). Although I feel Ed covered it quite well, here's my take.
When I first came across the term "runout", it was referring to the possible wobble/wiggle in the shaft of a router. Too much runout and you either needed a new set of bearings, a new motor or a new router. Excessive runout here will produce a cut that is full of ridges and gouges. If such cuts are usable, they need much sanding. Half inch shafts give a stiffer router bit and less ridges in the finished work, but it cannot stop runout. As I understand it, the runout it is any motion of a spinning shaft that is perpendicular to the axis of the shaft itself. A quarter inch shaft bit used on a router may produce some vibration that will produce a rough cut, but that is not runout. If a half inch shaft bit smooths out the cut, there is no runout although "wobble" or vibration was seen from the smaller bit.
Runout on a table saw (sideways movement of the quill/arbor/that is perpendicular to the axis of the spinning arbor shaft) will give a wobble of the blade from side to side (as viewed from either the front or top of the spinning blade). I suppose if one puts a foreign object, such as a wedge, in the arbor before tightening this will produce blade wobble. I'm not sure one can call this runout in the true sense, but it will have the same effect.
I think all turning shafts have some amount of runout. Runout on a circle saw will always result in blade wobble. Most folks don't worry much about the small amount of acceptable runout on a circle saw the size of a table saw or hand held saw. The radius of the blade is small enough that the resulting small amount of wobble does not present a problem. The extra kerf produced by a little runout is not very detectable (if at all). I don't know what the acceptable runout is, (I have the feeling that Nick mentioned this in one of his posts) but feel if it becomes excessive the user will know it by sound and action of the saw. (see the next paragraph for effects of runout).
Excessive runout of a motor shaft and arbor on a large circle saw as used in a sawmill can, and has, caused problems. These blades usually cut a quarter inch kerf and as a log passes by the blade (or blade by the log) there is a large amount of pressure exerted on the front of the saw arbor. This can produce an effect similar to having a mis-aligned blade on a table saw as well as the other runout (wobble) forces. Chips fly, slabs fly, weird, screaming sounds happen and machinery can break. Similar things happen on smaller saws, but to a much smaller degree.
If we were to try to discuss the various vectors of force on a spinning saw blade we'd get lost in a hurry. There is at a minimum the circular torque force of the spinning arbor/blade - there is a possible left or right wobble force caused by off balance blades and wood grain - there are the forces of the wood being fed into the blade which pushes back the front of the blade and can induce other forces depending on wood grain and sharpness of the blade.
