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madman
02-25-2009, 07:00 PM
I been tapping 10-28 and 10-24. 8-32 tapped holes WOW (snapp em off pretty quick) I even bought special stainless taps drilled .005 to .007 thou over the tap drill size and have had one tap snap just a turn into the material. Man this sucks Any tips Guys?

Just Bob Again
02-25-2009, 07:21 PM
50% thread depth. Make real sure you're not work hardening the material when drilling. Sharp drill and low speed and high feed and lots of coolant. Tapping fluid is as important as the tap. Molly Dee.

Ries
02-25-2009, 07:25 PM
I do a lot of these- I drill oversize, especially on blind holes- then I use plenty of good tapping fluid- I like Tapmagic natural, as the old style stuff tends to take the paint off my car even when its parked outside...

Then, I use a fixed, not racheting tap handle, I go real slow, and I back it off sometimes every 1/8" turn. Pull it all the way out and blow it out often, brush the chips off the tap every time, and most important- Focus. I cant do tapping like this when anyone else is around, I gotta be totally into it.

I still break the occasional tap, but with stainless, you can tig weld it back to flat, redrill, and tap it again, most times.

motorworks
02-25-2009, 07:25 PM
First
Is the application under a lot of stress etc?

You can go to 55% for your tap drill size:

10-24 use # 22 for 55%
8-32 use #28 for 51%
(Kennametal chart)

You need very good quality taps for stainless
I find the "blue" coated types work best
Do not use the ones with a bright finish
Lots of tapping oil.
(seal oil works best ;) )

eddie

Hogridnfool
02-26-2009, 03:41 PM
Had the exact same problem. Plant engineer said to countersink the holes first. Did what he said and that was the end of the problem.:)

Mark Hockett
02-26-2009, 04:21 PM
Mike, Ries,
If you do break a tap off in stainless it is very simple to chemically remove it with a Tap Out kit. It will remove the tap and leave the threads in prefect condition so you can finish threading the hole with a new tap. Swift Tool in Kent WA (800-562-0900) sells the kits for about $35 if I remember correctly.

Fasttrack
02-26-2009, 04:53 PM
You can also torch the tap out if you have a deft hand. SS doesn't burn well so if you warm up the tap and then blast it with oxygen, the tap blows out and leaves the threads intact.

I recently tapped 32 holes in 1/4" 304 SS with a #4-40 tap. Only broke on tap and I torched it out. I use a high quality spiral point tap and I do it by hand in the mill - meaning I use a large drill chuck and tighten the chuck by hand around the round body of the tap. Then I feed it by hand, one hand providing slight downward pressure and the other spinning the tap. Once you get the hang of it, the tap will slip in the chuck before breaking. I cut these threads at ~55% thread engagement.

Work hardening is a big issue. Like others have said you need a sharp drill and plenty of feed. I also tapped 100+ 10-32 holes in 3/8" 304 SS. I did those under power with a high quality spiral point tap. Doused it with tapmagic, turned on the mill and let it eat. Keep a hand on the brake and use the drill chuck again. Used two taps for the entire operation and never broke one. Those were at ~70% engagement.

:edit: You can also buy spiral flute taps. I got a a few spiral point 8-32 and 8-36 spiral flute taps with a black oxide coating. I've used them once or twice in SS and was pleased with their performance.

Blind holes are more of a pain. You have to back it off every few turns and remove the chips.

304, at least, is really not that tough to machine. You just need to get a feel for the appropriate speeds and feeds. In some ways, I prefer working with it over some "gummier" materials, like some of the soft aluminum alloys.

madman
04-29-2009, 05:04 PM
Well my small holes i go to 50 percent tap drill size minus a couple thou. Worked AWESOME BUT that pipe threading sucks. i think i will have to go with a taper pipe reamer and see next time .