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Problem #1 is the Irwin tap, those are meant for rethreading and usually in mild steel or aluminum. The spiral flutes will evacuate the chips to the top of the hole vs a standard tap that pushes the chips to the front of the tap. One time I purchase an import no name (inexpensive) HSS tap and a quality brand name HSS tap to do a side-by-side comparison. Every hardware store tap I've seen is the middle plug tap, not made for starting a new hole.
Those machine taps he shows you in post 2 are much better, but they start best when held straight by a drill press or machine.
Tapping austenitic stainless always was a challenge, 'till the mid 90's you could get tapping fluids with chlorinated compounds or some would use 1,1,1 tri-chlorethane for machine tapping. The good cutting fluids, even those containing sulpher (cheap) work OK if you can keep the the part pretty cool during the drilling & tapping operation. As for the tap itself, most production folks now use one of the modified grinds that look like a short gun tap like the one Steve posted when tapping thru holes & if it's a blind hole they use the backwards spiral type that string the chip back out the whole. As for the spiral taps my experience is the spiral point gun tap (the first picture Steve showed) is the best style tap for this job. Drilling is a cutting process that uses a drill bit to cut or enlarge a hole in solid materials. Under normal usage, swarf is carried up and away from the tip of the drill bit by the fluting of the drill bit. For heavy feeds and comparatively deep holes oil-hole drills can be used, with a lubricant pumped to the drill head through a small hole in the bit and flowing out along the fluting.


In computer numerical control (CNC) machine tools a process called peck drilling, or interrupted cut drilling, is used to keep swarf from detrimentally building up when drilling deep holes (approximately when the depth of the hole is three times greater than the drill diameter). I have used this chart for along time and was unable to find it anywhere on the net so I scanned it.
NOTE ON SELECTING TAP-DRILL SIZES: The tap drill sizes shown on this card give the theoretical tap size approximately 60% and 75% of full thread depth. Do the operation as quickly as you can while still staying in your sfm & ipm requirements. Localized heat is a killer with the non-resulpherized SS as well as many of the alloyed coppers and bronzes. A long time ago people like Cat & similar had to reduce thread engagement to allow direct spindle tapping in practical use with NC machines down to the 60% range.
Combine this with the fact that they cut about 6 times easier than a normal tap and you have the winner IMHO. The cutting edges produce more chips which continue the movement of the chips outwards from the hole. A conventional drill press arrangement can be used in oil-hole drilling, but it is more commonly seen in automatic drilling machinery in which it is the workpiece that rotates rather than the drill bit. Peck drilling involves plunging the drill part way through the workpiece, no more than five times the diameter of the drill, and then retracting it to the surface.
Generally, it is recommended that drill sizes be elected in the 60% range as these sizes will provide about 90% of the potential holding power. Had 1.5" of a carbide insert parting tool disappear (melted away) while turning 316 hex stock on a cnc lathe, never will forget that sound!
The work hardening bit happens when you drill, the hole near the cutting bit end grows as it heats up and then shrinks back around the bit farther back & rubs on the margin. It cuts by applying pressure and rotation to the workpiece, which forms chips at the cutting edge.


This is successful until the chips pack too tightly, either because of deeper than normal holes or insufficient backing off (removing the drill slightly or totally from the hole while drilling). If you hear much squealing when you're drilling slow the rpms down & increase the feedrate 'till it drills quiet. If you increase the tap drill size a bit the tapping goes much easier but the engagement must now go deeper.
They're weaker structurally than the other taps and always worry me that they're going to break.
Cutting fluid is sometimes used to ease this problem and to prolong the tools life by cooling and lubricating the tip and chip flow.
A modified form of this process, called high speed peck drilling or chip breaking, only retracts the drill slightly.
This is compounded by the fact that you're trying to tap the second worst size threads you can tap.
Coolant may be introduced via holes through the drill shank, which is common when using a gun drill. This process is faster, but is only used in moderately long holes otherwise it will overheat the drill bit. When cutting aluminum in particular, cutting fluid helps ensure a smooth and accurate hole while preventing the metal from grabbing the drill bit in the process of drilling the hole. I know for a fact the tap didn?t bottom out cause I put a piece of tape on the tap showing me the exact depth of the hole, and I wasn?t anywhere near it.
I didn't use a bottoming tap, but I'm not concerned about threading all the way to the bottom.



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