Best tap for stainless steel?

  • Thread starter kolleamm
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I need to know what is the strongest most reliable tap for threading 6-32 into stainless steel. I used like 3 cheap steel taps and they all broke. Any advice is greatly appreciated.

Thank you
 

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  • #2
Bandit127
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I used to use spiral flute machine taps on various stainless steels, all about 30 HRc hardness. They worked OK even as hand taps but I would always use a drill press or lathe if possible. Just tighten the chuck with enough force so the tap will work, but then spin in the chuck when it hits the bottom of a blind hole and not break.
 
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  • #3
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Thanks for the advice, I went ahead and ordered some 2 flute spiral taps. Fingers crossed they work.
 
  • #4
Baluncore
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Many stainless steels work harden and so must be drilled and tapped with HSS tools.
Some require cobalt high speed steel = HSS-Co, or carbide tooling.
It is essential that you use a cutting fluid. http://www.bssa.org.uk/topics.php?article=192
 
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  • #7
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Also if you know any taps for stainless on eBay (preferably in US) please do put the link I really need to get this right.

Thanks for all your help
 
  • #8
Baluncore
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I'm on the other side of the planet and I buy unusual taps from Victor Machinery Exchange in Woodside, New York, USA. They have the contacts to get special taps made to order if needed. http://www.victornet.com/departments/TAPS/100.html

If the stainless steel you have does not finish well then you might consider drilling an oversize hole, then using an STI tap and an insert.
That will give you a very good finish on the "recoil" thread insert. It also uses a larger tap that is less likely to break.
 
  • #9
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I will have a look at them for sure.

Also here is what it says on the taps that broke --- 6-32 NC HS GH3 USA 0416M4R

By HS do they mean hardened steel? I'm guessing it's not as good as high speed steel?

Thanks
 
  • #10
Baluncore
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Cheap taps are commonly made from Carbon Steel. More reliable, harder taps are made from High Speed Steel.
HS is almost certainly “High Speed”, the steel goes without saying.
G H3 is probably the finish and oversize. H3 may mean 1.0 to 1.5 thou oversize.

Find some way to support the tap in line with the hole. Experiment with cutting fluid, oil or soap. That will almost certainly reduce breakage. Some taps in some materials produce the best finish by cutting continuously. Others require a slight advance, then a reverse to break off the cuttings. You will need to experiment to find the best technique.

Corrosion is usually the most important parameter, but if possible select a grade of stainless that is free machining.
 
  • #11
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Cheap taps are commonly made from Carbon Steel. More reliable, harder taps are made from High Speed Steel.
HS is almost certainly “High Speed”, the steel goes without saying.
G H3 is probably the finish and oversize. H3 may mean 1.0 to 1.5 thou oversize.

Find some way to support the tap in line with the hole. Experiment with cutting fluid, oil or soap. That will almost certainly reduce breakage. Some taps in some materials produce the best finish by cutting continuously. Others require a slight advance, then a reverse to break off the cuttings. You will need to experiment to find the best technique.

Corrosion is usually the most important parameter, but if possible select a grade of stainless that is free machining.
I will try to mount the tap in the drill press rather than doing it by hand in that case.
 
  • #12
Nidum
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One thing that is important when tapping difficult materials is getting the pilot hole the right diameter .

Using pilot holes slightly larger than the theoretical correct size eases the load on the tap considerably and reduces chances of breakage .
 
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  • #13
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Use a tap lubricant specifically for stainless steel. Tap Magic is commonly available. And never turn the tap more than 1/2 turn without backing off at least 1/4 turn. Other than that any decent quality tap will work. It's also a good idea to have a starter tap and a plug tap. The starter tap makes it easier to start the threads "square". If you start at an angle you will not be able to complete threading, and the tap will break.
 
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  • #14
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I have successfully threaded the holes. I used one of the broken HS taps and rounded it off a bit with my dremel. Afterwards I placed it in a drill press, with my piece placed in the vice. I used lots of lubricant and it worked like a charm without breakage. Drill press is the way to go for sure.

Thanks for the help everyone.
 
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  • #15
CWatters
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Sorry if you know this but... Are you just trying to screw the tap in or are you doing the normal half turn in and a quarter turn out?
 
  • #16
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Sorry if you know this but... Are you just trying to screw the tap in or are you doing the normal half turn in and a quarter turn out?
I tried it both ways without the drill press and it broke regardless. Funny thing though I just screwed it in in the drill press and it worked even without turning it back.
 
  • #17
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I used ridgid for most purposes, but "working a tap" not (over working it) usually produces the desired results . Use a LOT of cutting oil and if you get in a bind stop, HSS an CHS taps will snap just as quick if not quicker than any of the cheapo taps you can buy if you over work them in stainless.

These are pretty good:

http://www.northerntool.com/shop/tools/product_200624059_200624059
 
  • #18
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True, also I found a very easy way to get small broken taps out is by hole punching it about 20 times. It just flies right out.
 
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  • #19
Baluncore
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Different stainless steels work harden differently. If you “work” the tap you may harden the material, then break the tap when it hits the previously hardened wall. If you continuously turn the tap the chips will be hardened but the material being cut will be fresh and normal hardness. The speed of cutting is important because the hardening process is a chemical phase change. In some materials, if you back off torque momentarily you may hit a hardening wall.

Avoid machining old, salvaged or unidentified stainless steel. You will probably need carbide tooling.

There are different flute profile taps, spiral flute taps are designed to cut continuously while straight flute are designed to be reversed to break the chips. Don' be mislead by “spiral point”, in hard materials they are often as bad or worse than straight flute because they clog with chips, especially in blind holes. The spiral path of the flute decides the curl of the chip and the quality of the thread finish.

There are many different stainless steels. What grade material are you threading ?
How long or deep is the thread you are cutting? Blind or through hole?
Is your material new or aged? Has the material been subjected to cold temperatures? for what periods?
 
  • #20
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Different stainless steels work harden differently. If you “work” the tap you may harden the material, then break the tap when it hits the previously hardened wall. If you continuously turn the tap the chips will be hardened but the material being cut will be fresh and normal hardness. The speed of cutting is important because the hardening process is a chemical phase change. In some materials, if you back off torque momentarily you may hit a hardening wall.

Avoid machining old, salvaged or unidentified stainless steel. You will probably need carbide tooling.

There are different flute profile taps, spiral flute taps are designed to cut continuously while straight flute are designed to be reversed to break the chips. Don' be mislead by “spiral point”, in hard materials they are often as bad or worse than straight flute because they clog with chips, especially in blind holes. The spiral path of the flute decides the curl of the chip and the quality of the thread finish.

There are many different stainless steels. What grade material are you threading ?
How long or deep is the thread you are cutting? Blind or through hole?
Is your material new or aged? Has the material been subjected to cold temperatures? for what periods?
Could you explain the material hardening? I still don't fully understand it. The stainless type is 420
 
  • #21
Baluncore
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I believe your 420 grade is a “precipitation hardening” steel.
Start here;
https://en.wikipedia.org/wiki/Precipitation_hardening

Then look at this data article and data, where Dura grade 420 is listed as 4021, 4028, 4031 or 4034.
http://www.outokumpu.com/en/product...nd-precipitation-hardening/Pages/default.aspx

http://www.outokumpu.com/en/product...ining-cutting-and-grinding/Pages/default.aspx

You have chosen a difficult material to work with. Is it sheet, or bar ? Do you need it to be magnetic ?
I expect it to harden on the shelf if stored colder, or for longer. Find the data on thermal storage requirements prior to machining.
 
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  • #22
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Could you explain the material hardening? I still don't fully understand it. The stainless type is 420
Any time you are tapping a hole you are creating friction and friction creates heat, that tends to change the crystalline structure of the metal. That being said I highly doubt that will happen when hand tapping a hole in stainless steel. I used to thread pipe with a ridged pipe machine and that creates a lot of heat and I've never seen it (harden the steel). to harden any kind of steel you basically have to get it cherry red then cool it rapidly. Stainless steel tends to be a bit harder then mild steel anyway. Most of the trouble with taping steel comes from the hole itself, if you don't (size) the hole correctly for instance if the hole an tap are different sizes or if the hole is slightly (wallowed) it can grab the tap and put it in a bind. I have seen aluminum break taps, its so soft it grips the tap and the metal chips get hung up and the tap just breaks. It happens, you just have to work with it.

https://en.wikipedia.org/wiki/Hardened_steel "In order to make steel harder, it must be heated to very high temperatures"


http://www.azom.com/article.aspx?ArticleID=1141

"Hardening

Like low alloy steels, martensitic stainless steels are hardened using tempering, quenching and austenitising. Austenitising temperatures range from 980 to 1010°C."


420 stainless is "martensitic stainless"

http://www.spiusa.com/stainlesssteel_overview.php
 
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  • #23
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True, also I found a very easy way to get small broken taps out is by hole punching it about 20 times. It just flies right out.
Yeah the harder tungsten carbide taps will shatter like glass if you shock them. I have actually seen taps break simply from dropping them on concrete floors (probably cracked from use already). Most of the hardened tooling has a fracture point. I got a carbon steel piece hung up on a piranha "punch" and thought I could just hit it with a hammer and knock it off, the piranha punch broke off in the hole.
 
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  • #24
Baluncore
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I have seen aluminum break taps, its so soft it grips the tap and the metal chips get hung up and the tap just breaks. It happens, you just have to work with it.
You are being pessimistic. It is not a black art, it is a science.
There is an optimum way to perform any operation. If you are breaking tools then you do not know what you are doing. If you use a spiral flute tap made of the right material, with the correct cutting lubricant and the correct technique, then it will not break the tap.
 
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  • #25
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You are being defeatist. It is not a black art, it is a science.
There is an optimum way to perform any operation. If you are breaking tools then you do not know what you are doing. If you use a spiral flute tap made of the right material, with the correct cutting lubricant and the correct technique, then it will not break the tap.
Thanks guys I think this sort of knowledge is very good to know, and you will mess up at some point when first learning it. As long as you learn something from it that's what's important.

On another note, is there any materials out there that you can't tap with anything? I'm thinking Inconel for example.
 

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