There is a trick that most machinists know, but not really necessary in your case. If you have to drill and tap a hole or several holes and make sure they are very square, you do it like I did the other day.
I had to drill and tap about 20 1/2 inch NF holes in some 3/4 inch steel. Since I was mounting a stand on it that would have a lot of force, I wanted the bolts to be drilled square to the surface of the plate, and the threads exactly square with the holes.
I used a drill press and a gun tap. Squared the plate up on the table, moved it to the first hole location, locked it down.
Drilled the pilot hole, removed the bit, installed the main bit, drilled that hole.
Then I chucked the tap up in the drill without moving anything in the X/Y axis. Turned on the drill, moved the tap down until it bit, and just let it feed its self into the hole.
Hit the off button a second to two before it finished, so it coasted to a stop. I did not want to let the tap reach the end of the threads and bind.
Then just backed the tap out by turning the chuck by hand.
Since nothing moved from the time I drilled the first hole to the tapping, everything was correct.
Of course, you must use a "gun tap" which shoots the chips ahead of the tap, and the hole has to be either a through hole, or deep enough to hold all the chips.
Naturally in a main shop, you would do it with a "tap-a-matic", which chucks up in a press or mill, and has clutches, reduction gearing, and an instant reverse built in.
But given the few holes I have to tap, its too rich for my blood.
... it's a crapshoot.
