I have a Delta Unisaw. I frequently check for the blade being parallel to the miter slot. I like to check it with the belts off the pulley for ease of rotating the blade, etc., and this way I can get accurate results less than .001. The problem arises when I put the belts back on and tighten them. Once I tighten the belts, the blade falls out of alignment by .01. I can get accurate results by tuning with the belts on, but what is causing the difference in alignment when the belts are on vs. off? Possibly my pulleys are out of alignment?
Also, it seems that my saw is out of alignment often enough that I have to check it every few weeks. I like to keep no more than .003 out of alignment but it does not take long for it to get beyond .003. Does anyone have suggestions for keeping saws aligned for longer periods of time?
(Solid Wood Machining Forum)
From contributor S:
I would say the first thing you should check on is bearing wear. With the belts off the arbor pulley, spin the arbor and try to feel for any hesitation, spots that turn harder or even grinding, etc. If you don't notice any of these traits, grab the arbor and try to wiggle it - put the indicator on the arbor while you do this and watch your needle. If you notice anything other than a smoothly rotating arbor or if your indicator registers movement when you try to wiggle the arbor, it is time for new arbor bearings. Now if you rule out bearing wear, I would say that your problem is being caused by the deflection of the arbor mount under the load you're putting on it when you tighten your belts. You may be putting too much tension on your belts. Many people have a tendency to put more tension than necessary on a belt. As long as you're not getting slippage during operation, you're going to be okay (believe me, you'll know it when a belt slips - it stinks up your whole shop). If de-tensioning doesn't help, then I would say that you have no choice but to align the saw with the belts on.
When I check the alignment on my saw, I use a "master plate" instead of a saw blade. This is basically a machined and calibrated plate of aluminum that you bolt on in place of the blade - this way you don't have to worry about pitch on the blade or running your indicator onto the carbide. You can buy it from Amazon.
It's woodworking machinery - .01" is overkill. Sure I know where you're coming from, wanting supreme accuracy, but if it's that close, you've probably got the best one ever made.
And although I have not tried to measure, I would guess a typical thin kerf blade will flex more than .003, thereby defeating the quest for super accuracy. Not to mention the tendency for most of the materials we work with to move after being machined. If after all this, you still want more accuracy, the Unisaw is a good machine, but there are certainly better ones out there. Just have to decide how much is the accuracy worth?
If your work piece, i.e. stiles and rails, are coming out not parallel, it may be that the work is pulling away from the fence as it goes past the blade. I may be wrong, but I don't believe it's the blade being out. Another thing which may help, though, is a 5" blade stabilizer.
As for tolerances, that is something we all have to find a balance with. I do try to keep my work pretty tight, but if a 30" tall overlay door is off by just over 128th from top to bottom, well I'll admit I can live with that. I'd split the difference so it's only .005 on each side, not easily visible. For your inset doors, if you still can't get the saw true, you could knock that off with two passes of a well tuned hand plane.
By the way, I'm still using my 1954 Unisaw, and although I occasionally think of upgrading, I have to admit it's a great machine.
If you are going to start looking to upgrade, I would check out the Powermatic 66, and I've also heard good things about the new Sawstop saw. But I personally don't think it's going to make much difference for a small shop. If you get it set accurately, you should be fine. You don't need the best machinery to make nice work. I would love to get the 66 myself, but my Unisaw still works fine and there are so many other things to spend my money on.