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A bit more elaboration on Jakes response below.
Screws that move devices on precision machinery are not perfect. Even the most precision thread ground to the closest tolerance will have a "lead" error. You can feel this lead error in the tilting wheel if you turn it slowly. It's very slight, but enough to give you a problem. The proper way to set the blade to 90 degrees is to back up at least 10 to 20 degrees "short" of 90 and then go forward. This will take care of the "lead" error. If you miss the 90 degree mark using this technique, back up short of 90 degrees and try again. Don't use that degree scale. It's off by a country mile. Use a square of known precision (many are way off) and stay off the blade teeth, as described in the manual. ALWAYS make a point of rechecking this adjustment every couple weeks or every hundred cuts or so. This goes for all adjustments.
Don't tighten that set screw too tight. In fact, never tighten locking screws very tight on any machinery. Inaccuracies in the equipment will be manifested if too much tightening is done.
As an example, in the machining trade where tenths of thousands tolerance are required, the locks that lock tables are never tightened at all. There's "slop" in all gibs, screws, etc. When you set up the machine for a dimension and then torque down a lock, your setup is going to get thrown way out of wack. On a Bridgeport milling machine the common practice is to just let the weight of the tightening handle "fall" in the tightening direction only. The lever weighs maybe eight ounces. Tightening any further will really mess things up.