the method I would use requires the following tools/ supplies:
magnetic base
dial indicator
flashlight
10 inch test bar for your taper
1. put the test bar in the spindle
2. install the indicator on the mag base on b axis table
3. indicate the sides of the test bar at 0 and 180 as Busted Knuckle describes
on absolute encoder machines this should coincide with x home when both sides read "0" to your satisfaction whether that is .0001 or 0.001 inches is up to you
4. clear the test bar from the indicator, rotate the table 90 away from the spindle
5. come in to your same "0" on the dti.
refer to your machine range drawings to verify, but the test bar length plus 0.5 x test bar diameter is the distance your spindle face is from the center of the pallet. You can back up a known distance and set home on absolute encoder machines or merely find how far from home you are in incremental encoder machines with home dogs. You can even do this a couple times to see if home is repeating well if you have home dogs/ switches.
most of the time in smaller machines I am inside the machine with the flashlight reading the dti and I have an assistant I trust to do exactly as I say helping me reach the handwheel. If you are alone an ispection mirror can be nice for the back side dti reading
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