Adjusting trunion parallel to mitre slot
I picked up a new Incra mitre fence and as part of calibrating it I am checking my contractor saw allignments. I put a dial indicator on the arbor and there is almost no runout. I’m now doing the measurement of the same tooth at the front and back of the blade to check distance form the mitre slot, but, I have a question. This may be in the overkill anal category. Before I started this adjustment I put the dial indocator on the blade and started the saw (curious about what it would show). I was very careful to make sure everything was locked down and very stable so that there were no accidents. When I did that, the variation in the indcator was pretty extreme, about an overall swing of about 0.011, with 0.003 on one side of zero and 0.008 on the other. I’ve never read of anyone makng this measurement and I’m wondering if it is at all valid and should I worry about it. My concern is that if it is a problem, then adjusting the trunion doesn’t really help anything because of the overall slop in the blade travel.
George
Replies
Never thought of doing this, or heard of it being done, but I suspect that the smallest surface imperfection in the blade could cause the indicator to bounce. The return spring is not very strong, and not designed for high speed work. Did you notice if the larger vairation was on extension or retraction of the plunger? If it is on retraction, in other words, as the plunger is being pushed in, that would sorta confirm my theory. I would think the readings would be more accurate if you were turning the blade slowly by hand. And you would have to turn it by pulling on the belt or turning the motor pulley, with nothing but the indicator touching the blade, esp. not your hand, as that could deflect the blade.
Also, any measurement you do using more than just one marked spot on the blade could be measuring blade runout, even if your arbor is perfectly square to the slot. That is the reason you use that one spot to check with. Even if the blade is warped like a potato chip you are using the same point, so it travels in the same plane all the way around, given 0 arbor runout.
George,
I doubt your indications are valid but your concern is. I suspect the point of your indicator was bouncing. You can check for arbor run out by setting the indicator on a marked a spot on the blade and manually moving it to a different location. Unplug the saw, raise the blade as high as you can, mark a spot on the outer portion of the blade near the table at the back and put the point of the indicator there. Move the blade to where the marked point is at the front then see how much variation you get. The quality of saw blade plates vary and one plate is likely to wobble more than another. Some are just stamped out and others are precision ground for flatness. Having a flat saw plate is pretty important when trying to align the saw.
Even the smoothest running saws vibrate. Your dial indicator test isn't telling you anything but that.
This is an odd way to check a running saw.
If you want to check how much wobble there is in the saw, rip a groove half way through the thickness of a piece of mdf and check the width of the groove against the width of the saw blade.
Higher quality saw blades are not just stamped or stamped and ground. They are tensioned by hammering the inner area of the blade to expand the metal so that the metal is in tension close to the arbor and in compression at the perimeter. The internal stress is relieved by centrifugal force when the blade turns, and it spins truer.
A contractor saw most likely has standard (ABEC 1) bearings in it, and these chatter. They chatter differently running free and running under load. It's not a machine tool, but then it doesn't need to be to do nice work for you. If it is doing nice work, write down what numbers are for diagnostic reference in the future.
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