directions:

Now, pick up (locate) the left end of the adapter, and move in .187". At this location Y = .125" and X = .187", Center drill, drill, and tap through to the .250" slot.

Stay at the Y dimension, and move the milling table .875" along the X axis. Center drill, drill and tap through to the .250" slot.   You are going to drill and tap for 8-32 Allen Socket Head Cap Screws by 3/4" long.

The recommended tap drill size for aluminum is a # 30 drill ( .1285" diameter) for 75% thread,
or a # 29 drill ( .136" diameter ) for 70% thread.
Deburr the adapter, and you are in business.*

*Note, it is important that the top of the .250" slot coincides with the bottom of the tongue milled on the opposite side of the Adapter.

Why? In order to get optimum use of the toolbit,the cutting edge has to be on center of the lathe, or not more than .003" to .005" below the lathe center.   Then, the clearance
angles (rake) ground into the toolbit will be correct.   Anything more or less will cause the toolbit to "dig in"(more), or "heel and rub" (less), instead of cutting correctly.