I was trying to do a 1/4" dado with my Delta contractor's saw. My dado set is a Freud.
There seems to be a spot on the arbour for the first blade (or any blade for a non-dado cut) to sit and then there is a bit of a narrower spot before the threads start. The outer blade has a lot of play in that position. A chipper is wider and seems to be ok. The dado blade sits ok when it is on the threaded part of the arbour.
In the end I couldn't get a flat bottom on the dado cut because the leftmost dado blad had play on the arbour.
Don't know if it's been damaged? How close a look have you given the arbor? Spinning a loose blade could have damaged the leads, or your dado set may be like mine, also a Freud, but with a greater problem. When I tuck the blade up against the flange it seems loose by comparison, but seats well enough, and I don't get any more of a cut on that side than the opposite. My standard blades seem fine, so I thought it was just my Freud set.
Sure is fun trying to snake the cutters over that arbor, isn't it? Shims are really a nightmare.
What about using a spacer to shift everything over so that the entire dado stack is over the threads? While that wouldn't work for a full width dado, 1/4" should be fine.
Sounds like your threads are worn? My Delta is similar, but after close inspection with a micrometer, I discovered that isn't actually a shaft diameter difference between the threads and the "shoulder" up against hte inside stop. It "feels" that way but it's not. And the "space" between the smooth shoulder and threads starting is no larger than 1/32 so my dado blades won't go into the space by any means.
I susped either the arbor is out of spec or something (a blade got "spun" on the arbor) has worn the threads or something is out of round. That's my guess, anyway. It's easy to guess; harder when you're the one with the problem, I know.
Yes, I have encountered this exact thing, also with a Freud dado set. I used a spacer to bring everything out onto the threads of the arbor.
I've seen small packages of very thin brass at a hobby shop that might also be used over the threads instead of the shim. Then again, what about a piece of tape wrapped over the threads. Or teflon tape. You just need the height matched until you clamp down the arbor nut.
Often, the higher the blade quality the more difficult it is to mount the blade because the high quality blade manufacturers hold closer tolereances at the center hole for diameter, ovality, and true position, offering less clearance in the arbor to blade fit. Of course, that's a good thing.
Delta, (if this is a domestic saw) used functional go/no go ring gages rather than multiple tri-mikes measurements to check dia. and ovality because this guaranteed a functional arbor despite any natural process lobing. Tri mike readings might miss the " lobes" of the machining process and check good but not provide a functional fit.
But often the process of cutting an acme thread rolls up an almost imperceptible burr. The arbor would allow easy mounting of inexpensive blades with "large" center holes, but make the mounting of very high quality blades a little more difficult.
I always recommended "touching" the arbor shaft (blade removed, bump the saw on) with a strip of emory cloth to remove the burr and polish the shaft.
In the OP's case, it sounds like some damage to that portion of the arbor, or else the diameters of the blade center holes are at different spots in the tolerance range. There are other possibilities, but they are less likely.
This is the reply I got from Freud when I thought the problem was the dado: "In almost 100% of the time it is not the dado but the arbor of the saw that is the problem. Craftsman and some other contractors saw are a problem because the threaded portion is undersized. This causes the components to not be aligned properly so you never get a flat bottom."
Not to belabor the point and I can't speak for Craftsman, but a no go ring gage would go on an undersized arbor shaft causing it to be rejected. My point was that you could measure it with a tri-mike, hit the lobes, and think it was at the lower end of the range but acceptable.
However it is a clearance fit. It has to be to get the blade/chippers on the arbor. That means there can always be a variation in blade/chipper efective OD by as much as two times the amount of the clearance.
And, holding close tolerance on the center hole on a blade is much less difficult than holding close tolerance on a shaft.
And, having been involved with manufacturing both saws and to a lesser extent, blades, I would suspect variation in the true position of the blade/chipper center hole resulting in variable runnout as the primary cause of variable bottom dados.
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