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Tapping threads in hard steel

It's impossible to answer with any certainty the OP's question about why that cheap carbide tap chipped. There's too many variables and unknowns. You'd have to run a side by side comparison with a brand name and known quality industrial level carbide tap in the next hole of that face mill and then see if the same happens. Since that's not ever going to happen..
That is generally the fault mode of most of the cheap carbides I've tried. They chip and fracture away too easily. No toughness, so they lose their edge rapidly. I appreciate the desire to find less expensive carbide inserts, but it just isn't worth all the failures, to me. Especially if one of them takes out the milling cutter.

The carbide tap chipped because it was the wrong tool for the job, nothing more, nothing less. 100% guaranteed, if OP were to do the same with an Emuge/Franken/Noris/OSG/Sandvik/whatever carbide tap the results would have been exactly the same.

Carbide taps are exclusively the domain of machine tapping with tightly controlled parameters. He did well not to just break it off in the hole.
 
The carbide tap chipped because it was the wrong tool for the job, nothing more, nothing less. 100% guaranteed, if OP were to do the same with an Emuge/Franken/Noris/OSG/Sandvik/whatever carbide tap the results would have been exactly the same.

Carbide taps are exclusively the domain of machine tapping with tightly controlled parameters. He did well not to just break it off in the hole.

You quoted my post, but I was speaking about carbide inserts, not carbide taps. There is no way to say for certain what caused the OP's tap to chip, not having been there when it happened, but cheap carbide could very well have been the reason. I've used carbide taps before, (by hand, even!) didn't chip them. Your statement is not entirely correct. They work best for new threads while held in a machine spindle, but they work fine by hand for re-threading damaged holes where alignment is good and maintained by existing threads, too. The problem is with using them by hand for new threads where rigid alignment can't be well maintained. And the proper grade of carbide can handle low speeds and even impacts. I'm betting on improper carbide grade for this cheap tap - or potentially improper hook geometry or edge prep.

Also, I agree that a brand new, sharp HSS tap should have been able to cut these threads if the material was legitimately 47 HRc. Which, again, we don't know for certain, because it was a cheap import milling cutter. The steel may well have been harder than that due to poor process control. Or OP might have been trying to use a tap that has been banging against other HSS in a drawer for years or more.
 
The carbide tap chipped because it was the wrong tool for the job, nothing more, nothing less. 100% guaranteed, if OP were to do the same with an Emuge/Franken/Noris/OSG/Sandvik/whatever carbide tap the results would have been exactly the same.

Carbide taps are exclusively the domain of machine tapping with tightly controlled parameters. He did well not to just break it off in the hole.
Agree, just to add a +1, for the Op and future newer viewers, you cant use a carbide tap by hand, or shouldn't anyway. Not really what it is for.
And goo yob not breaking it in the tool. :D
 
You quoted my post, but I was speaking about carbide inserts, not carbide taps. There is no way to say for certain what caused the OP's tap to chip, not having been there when it happened, but cheap carbide could very well have been the reason. I've used carbide taps before, (by hand, even!) didn't chip them. Your statement is not entirely correct. They work best for new threads while held in a machine spindle, but they work fine by hand for re-threading damaged holes where alignment is good and maintained by existing threads, too. The problem is with using them by hand for new threads where rigid alignment can't be well maintained. And the proper grade of carbide can handle low speeds and even impacts. I'm betting on improper carbide grade for this cheap tap - or potentially improper hook geometry or edge prep.

Also, I agree that a brand new, sharp HSS tap should have been able to cut these threads if the material was legitimately 47 HRc. Which, again, we don't know for certain, because it was a cheap import milling cutter. The steel may well have been harder than that due to poor process control. Or OP might have been trying to use a tap that has been banging against other HSS in a drawer for years or more.
Sorry for misquoting you.

There are of course applications for carbide taps in hardened steel, and carbide grades specifically for that. But most commonly carbide taps are used for wear resistance in abrasive materials rather than high hardness materials. I personally have never used a carbide tap in hardened steel, but I have used them a good number of times in other materials.
 
Sorry for misquoting you.

There are of course applications for carbide taps in hardened steel, and carbide grades specifically for that. But most commonly carbide taps are used for wear resistance in abrasive materials rather than high hardness materials. I personally have never used a carbide tap in hardened steel, but I have used them a good number of times in other materials.

That is a good point, too. Maybe this carbide tap was just not intended to be used on steel in the first place. Note to OP: carbide is not *just* carbide. There are many different grades and toughnesses available. Hook on the tap also can make a huge difference. This knowledge and how it should be applied to your application is another thing you pay for when buying the "expensive" tooling.
 
You quoted my post, but I was speaking about carbide inserts, not carbide taps. There is no way to say for certain what caused the OP's tap to chip, not having been there when it happened, but cheap carbide could very well have been the reason. I've used carbide taps before, (by hand, even!) didn't chip them. Your statement is not entirely correct. They work best for new threads while held in a machine spindle, but they work fine by hand for re-threading damaged holes where alignment is good and maintained by existing threads, too. The problem is with using them by hand for new threads where rigid alignment can't be well maintained. And the proper grade of carbide can handle low speeds and even impacts. I'm betting on improper carbide grade for this cheap tap - or potentially improper hook geometry or edge prep.

Also, I agree that a brand new, sharp HSS tap should have been able to cut these threads if the material was legitimately 47 HRc. Which, again, we don't know for certain, because it was a cheap import milling cutter. The steel may well have been harder than that due to poor process control. Or OP might have been trying to use a tap that has been banging against other HSS in a drawer for years or more.
Thank you for posting this. This is a good attempt to answer the original question. I agree the tap could have been made from the the material with insufficient toughness. How about my theory that the damage possibly happened during tap reversal under load (I did not tap the hole through)? This image from the OSG Technical Guide shows exactly what happened to my tap, although I cannot agree with their reasons.
1717157409116.png
By the way, OSG offers carbide taps specifically for hardened steel up to around 60HRC. Speaking of the hardness of my face mill, 47HRC was not a guess, but a measurement obtained on a Rockwell tester in a close vicinity of the treaded hole. I was debating which tap to buy, HSS or carbide since I did not have a good M5 tap. The material hardness was pushing the limits of the special HSS taps and their cost was comparable to carbide. I chose carbide, which may have been a mistake. Somehow I still think the tap would not chip if I could go all the way through.
 
Thank you for posting this. This is a good attempt to answer the original question. I agree the tap could have been made from the the material with insufficient toughness. How about my theory that the damage possibly happened during tap reversal under load (I did not tap the hole through)? This image from the OSG Technical Guide shows exactly what happened to my tap, although I cannot agree with their reasons.
View attachment 441591
By the way, OSG offers carbide taps specifically for hardened steel up to around 60HRC. Speaking of the hardness of my face mill, 47HRC was not a guess, but a measurement obtained on a Rockwell tester in a close vicinity of the treaded hole. I was debating which tap to buy, HSS or carbide since I did not have a good M5 tap. The material hardness was pushing the limits of the special HSS taps and their cost was comparable to carbide. I chose carbide, which may have been a mistake. Somehow I still think the tap would not chip if I could go all the way through.

That kind of chipping pretty much exclusively happens during reversal, for the two exact reasons shown, so I'm not sure what you don't agree with. In your case almost certainly chip jamming on reversal. Taps have trailing edge clearance, so during reversal there is potential for very small chips to become jammed between the wall of the workpiece and the cutting edge of the tap. Because carbide is brittle regardless of grade, this will cause fracturing and a flaking away from the cutting edge like in the photo.

Your intuition that it would probably not have happened if tapped all the way through is correct, for these reasons.

If you ever have to do anything like this again, a spiral point HSS-E powder metal TiN coated tap will be your friend.
 
What is HSS-E? A quick google didn't give specifics. I understand the HSS part, but what does the "-E" signify?
HSS-E is high in cobalt. New buzzword/fancy acronym, I guess.

HSS-E is what we in the rest of the world call 5% cobalt HSS.

The critical part of what I wrote was the powder metal part. HSS-E-PM.

Extremely tough and significantly harder than basic HSS or HSS-E. The first time you try to machine out a broken HSS-E-PM tap, you will understand the difference...
 








 
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