Help tapping thick 304 stainless | The Hobby-Machinist

Yet another tapping question. Sorry guys. Need to tap several 1/4x20 in 0.5" thick 304 stainless (through hole). I am looking at doing it by hand and my poor little 4 flute Irwin tap is going to break, I just know it. Just got done doing 3/8x16 holes in in the same material and barely got through it without breaking that tap. Any suggestions? I see folks recommending a spiral point 2-flute good for power tapping aluminum. How about hand tapping stainless? I've got both Tapmagic and some stinky dark sulfur cutting fluid. Is one better than the other for this? I've done a fair bit of tapping in 304 stainless and it sucks. Best thing to do is throw out that Irwin tap because it is garbage and find a new one with ground flutes. Honestly the cheap import taps cut better than Irwin. The edge sharpness makes a huge difference in the tapping force. I'd recommend a spiral point tap too if you're doing through hole tapping. The chips eject out of the bottom of the hole and the body diameter of the tap is larger which means you are less likely to break it.

Another things is the cutting fluid. I prefer a sulphurized tapping oil. WD-40 is fine for aluminum but seems to work less well in stainless.

EDIT: Taper or Plug length taps are great for through holes as well. They cut straighter and form thinner chips over a longer length of the tip of the tap. That helps with the cutting forces a lot too. Through holes? then I would use a 2 flute gun tap(spiral-point flute) gh1 or 2 ,with a good quality #3 or 4 drill. A #4 drill will give you 63% of full thread. Use your drill press or mill to hold straight.Clear chips often.Make sure drill hasn't work hardened the stainless. slow speed , fast feed.
If your tap doesn't show gh1 ,gh2 etc. it most likely is a lesser quality tap.

I've done a fair bit of tapping in 304 stainless and it sucks. Best thing to do is throw out that Irwin tap because it is garbage and find a new one with ground flutes. Honestly the cheap import taps cut better than Irwin. The edge sharpness makes a huge difference in the tapping force. I'd recommend a spiral point tap too if you're doing through hole tapping. The chips eject out of the bottom of the hole and the body diameter of the tap is larger which means you are less likely to break it.

Another things is the cutting fluid. I prefer a sulphurized tapping oil. WD-40 is fine for aluminum but seems to work less well in stainless.

EDIT: Taper or Plug length taps are great for through holes as well. They cut straighter and form thinner chips over a longer length of the tip of the tap. That helps with the cutting forces a lot too.

For true; tapping stainless is a difficult operation and requires professional grade taps. Also, drill the holes on-size with a sharp new drill, lightly countersink the entry side, finally, don't try it freehand. At the very least, set up your DP to hold the tap perpendicular and provide some downforce; this is better done as a four-hand operation. If turning the tap wrench by hand, turn 1/4-turn, back up to break the chip, turn another 1/4-turn, back up. This helps in some grades of stainless.

jack vines I forgot to say I did drill the holes with a #4, 60% engagement, hoping for an easier time with it. Ran the mill at 140 rpm with cutting fluid. Hopefully that didn't work harden it too much.

I started the tap in the hole with the chuck so it's good and straight. But I'm only a few turns in and I know this tap will break. Sucks to hear the Irwin's are crap. The set wasn't all that cheap.

The stinky cutting fluid I have is high sulphur cutting oil. I was trying Tapmagic but I'll go back to it.

I will hold off on trying to go further and order a good spiral 2-flute to finish this project. I've been working for days on this part and I really don't want to break a damn tap off in it at the last step and have to start all over. Found this on Ebay, hopefully it's good to go.

1/4-20 2 flute GH2

I forgot to say I did drill the holes with a #4, 60% engagement, hoping for an easier time with it. Ran the mill at 140 rpm with cutting fluid. Hopefully that didn't work harden it too much.

I started the tap in the hole with the chuck so it's good and straight. But I'm only a few turns in and I know this tap will break. Sucks to hear the Irwin's are crap. The set wasn't all that cheap.

The stinky cutting fluid I have is high sulphur cutting oil. I was trying Tapmagic but I'll go back to it.

I will hold off on trying to go further and order a good spiral 2-flute to finish this project. I've been working for days on this part and I really don't want to break a damn tap off in it at the last step and have to start all over. Found this on Ebay, hopefully it's good to go.

1/4-20 2 flute GH2

OSG makes great taps. I think you'll be very happy and impressed with that one.

Irwin taps are fine for general work, so the set wasn't wasted, but if you want to work on harder materials or hold good thread tolerances, you need nice taps. I have a couple of Irwin taps. They work alright, but don't hold a candle to my higher end taps. Most of them are OSG or YG1 spiral flute HSS taps. Those things cut like a hot knife through butter in comparison. I don't even hesitate to chuck them in the BP and let it rip. I have run them in 304 and they work well.

If you want some that REALLY suck, I'll send you a couple from my HF set. Those are better used as nails than as taps. What is the tapped hole for? If you don't need half inch of threads to hold what you need you could drill 1/4 inch down with an oversized drill that would not get tapped. So you tap only 1/4 inch of material. Depends on what you need the tapped hole to do. 1/4 inch stainless steel tap is very strong. May not need whole 1/2 inch tapped.