Simple Jig Aligns Tablesaw
You can’t cut cleanly or safely on the tablesaw if the blade is not aligned with the miter slot and fence. However, with a shopmade jig and an inexpensive dial-caliper gauge, you can check both alignments precisely.
Make the jig from a piece of hardwood. Center a hardwood runner on the bottom of the jig using countersunk screws, making sure it fits snugly into the miter-gauge slot.
Screw a dial-indicator gauge to the front of the jig. Locate the gauge so that when the jig is riding in the left miter slot, the side of the blade depresses the dial-indicator plunger about 1⁄8 in.
Elevate the blade to its maximum height, and place the plunger in contact with a tooth at the front of the blade. Check the indicator. Now rotate the blade until the same tooth is at the back of the blade, and take another reading. They should be the same. If they aren’t, the miter gauge is not parallel to the blade and you will need to either adjust the tabletop (most cabinet saws) or adjust the trunnions (most contractor saws).
Once the blade is parallel to the miter slot, you can align the fence to the blade. Place the jig in the lefthand slot and slide the fence over until it just depresses the plunger. Zero the dial, then slide the jig to the back of the slot and take a second reading. Adjust the fence until the two readings are the same. Now the fence is parallel to the blade.