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Machine Shop and Tool Talk

Threading stainless steel

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Old 10-15-2009, 08:28:16 PM
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Ray Cardoza Ray Cardoza is online now
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Default Threading stainless steel

what kind of tap do i need to thread 303 stainless. i just broke one threading 10-24 thread
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Old 10-15-2009, 08:40:17 PM
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Default Re: Threading stainless steel

Ray 303 is the one of the most free cutting easiest to machine of the 300 series stainless. You shouldn't be breaking something as big a #10 tap unless there's an underlying issue. Was your tap sharp? Cutting oil? Correct size hole drilled? Were you tapping by hand, using a tapping head or some other means of alignment?

I would use a #25 drill, a sharp 4 flute HSS tap (not those cheap carbon steel taps) using Castrol Moly Dee cutting oil and would at least start the tap using some means of alignment and not free hand. Also wouldn't bother with any sort of fancy coating on the tap if this is for just a few holes.
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Old 10-15-2009, 08:59:35 PM
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Default Re: Threading stainless steel

I use two HSS flute bottom contour tap. I have often break high quality straight four flute.
I also used thread cutting oil from RIDGID cutting oil from pipe dealer supplier.

Last edited by greysteam24hp; 10-15-2009 at 09:05:16 PM. Reason: add
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Old 10-15-2009, 09:38:47 PM
D Davis D Davis is offline
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Default Re: Threading stainless steel

How thick of stain less ? Use a 9 / 64 or a 5 / 32 drill bit and plenty of oil , go slow . I do this every day . If you need some taps let me know I can send you a few .
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Old 10-15-2009, 10:27:15 PM
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Default Re: Threading stainless steel

I don't know if it will work for others as well as it does for me, but I generally have better luck "power tapping" with a drill than I do trying to tap by hand. If you can drill it straight, you should be able to tap it straight.

If you have a drill press and the part you're tapping isn't too big to fit in it, that would be even better as it will make sure the tap is straight.

Without a tapping head or anything else to align it, I've broken lot bigger taps than that, but so far I haven't broken a single tap of any size using a drill to power feed it. Still need to stop and back it out on bigger taps if they get tight, but I've run taps all the way through 1/4 & 3/8 plate steel in a single shot with no problems. Deeper holes I go 2-4 turns, back it all the way out to clear the chips, turn it back in a couple turns by hand to make sure I don't cross thread it, and then power it in a few more turns (more or less depending on how tight it gets)

Keep it well lubricated, and Jog the start button (or trigger) on & off, don't just lay into it.

Harder material or deeper holes I will often bump the drill size up to the next available size (within reason), whether I have to use numbers, letters, fractions, or metric. You don't want to go too big, but a couple thousandths more clearance can make a world of difference without affecting the percentage of thread depth too much.

If you've never tried this before, you might want to practice on some scraps to get a feel for it before tearing into something important. I tought this to a couple maintenance workers and several friends that used to break a lot of taps, and they had pretty good luck with it. I don't know if I worded it good enough here as opposed to showing someone in person, but I think if you try it a couple times, you will be happy with the results. Again though, PRACTICE on something you can't hurt first to make sure it works for you.
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Old 10-15-2009, 11:38:21 PM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Threading stainless steel

use a cutting coolant specifically designed for stainless. regular cutting oil sometines is not enough to keep the taps and dies from jambing on stainless. Sharp new dies are a must. The least bit of wear will cause the tool to try and climb out of the thread, often either tearing threads or snapping the tap or die teeth off.
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Old 10-16-2009, 08:18:40 PM
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Default Re: Threading stainless steel

If it's a through hole, I would use a spiral point or "gun" tap. No need to stop and break the chips as it's designed to push them forward. I also prefer two fluted taps since they have a more substantial web when you're dealing with little taps. Moly Dee is definitely good stuff for cutting stainless.
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Old 10-16-2009, 10:18:02 PM
KidDynamo KidDynamo is offline
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Default Re: Threading stainless steel

When yer finished, etch or engrave your tap or die with "SS" or some other marker so you always know which ones been working stainless. I've found that repeated use on stainless will wear out these tools to the point where they no longer will cut a proper sized virgin thread.

The threads they'll later cut might look real good, but might not measure and fit so good, size-wise. One or two threadings might be okay but a dozen might not.

I like to keep them for chasing and "customizing", but I want them to be identifiable so I don't use 'em again without knowing I'm doing it.
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