I need to drill and tap 50 holes in mild steel with 3-48 thread. The blind holes are drilled to a depth of .25" and I'm trying to get about 3/16" of thread depth. I drilled four test holes with a 5/64th (.078") drill bit and subsequently broke taps in two of the test holes. I started with a taper tap which worked well but bottomed out after just a few threads. I followed with a plug tap (3-flute) which broke toward the bottom of the hole. I enlarged the next test hole to .081", used plenty of tapping fluid and used compressed air to blow any chips out but still managed to break the plug tap. I was only successful with two of the four holes (I had no issues using the bottoming tap). Any advice on how to tap these holes successfully would be very much appreciated.