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Tapping threads in hard steel

mikey553

Aluminum
Joined
Dec 10, 2013
Location
PA
I bought a 50 mm face mill from Aliexpress, which had all M5x0.8 holes for insert screws tapped too shallow. Standard screws would not clamp the inserts because of that and I did not want to cut the screws shorter. So I decided to buy a tap to cut the existing threads about 2 turns deeper. The face mill has a measured hardness of HRC47 and my regular HSS taps would not cut it. It is this face mill.

https://www.aliexpress.us/item/2255...t_main.34.21ef1802ZCyHdU&gatewayAdapt=glo2usa



I could get a carbide tap or a HSS one with a special coating. Never used them before in my life. Internet search suggested to better not do that, however I found a few success cases with brand name carbide taps. Such taps run about $100 a piece and I cannot afford that. So I ended up buying 2 carbide taps with a Klot name on them.

https://www.aliexpress.us/item/2251...t_main.17.21ef1802ZCyHdU&gatewayAdapt=glo2usa



I already have a tiny carbide dovetail end mill from Klot and its quality is very good. The taps are received and look decent. They have some kind of a black coating and are nicely ground and marked. So I applied a Moly-Dee tapping compound to the tap and went in manually with a tap wrench. I rotated the tap only in one direction and went as deep as I wanted, then reversed the tap. All 4 holes were tapped easily, but still remained blind. After that I checked the tap under a good magnification and to my surprise all teeth on the cutting edges were chipped. Chipped, not broken. It is like a thin layer from the rake surface broke off.

This tap is still usable, I fixed my face mill and I have another M5 carbide tap. In my opinion it is a success. But did I do anything wrong to chip the tap? It is carbide and this material is fragile. During tapping I stopped several times just like you do with manual tapping. But remember what happens to a carbide tool on a lathe if you stop the spindle without retracting the tool. It chips easily. Maybe carbide taps are not allowed to be used manually. What are your thoughts and experience?
 
Most mill cutter srews are the highest quality, poor quality screws often wont work..
Cutter in the photo seems to have plenty of thread depth.
Perhaps bench grind .030 off the screw ends.
 
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He paid $10 for the entire facemill. You think he's gonna spend $5 each on the proper insert screws?
I paid $31 for the face mill and a box of inserts including shipping. Rest assured, after I fixed the threaded holes this mill is every bit as good as your overpriced one. That is if you have one at all. The insert screws are just fine, there is no need for better ones.

I stated this thread hoping to get some experience with carbide tap operation and I still hope some of you guys know how to deal with them. Share your experience please. I would like to learn from you.
If you have nothing to contribute, don't respond to this thread.
 
I paid $31 for the face mill and a box of inserts including shipping. Rest assured, after I fixed the threaded holes this mill is every bit as good as your overpriced one. That is if you have one at all. The insert screws are just fine, there is no need for better ones.

I stated this thread hoping to get some experience with carbide tap operation and I still hope some of you guys know how to deal with them. Share your experience please. I would like to learn from you.
If you have nothing to contribute, don't respond to this thread.

1717017465518.png
 
HSS tap should have cut 45HRC anyway.
carbide is brittle its just going to shatter.
If the tap was low quality(Aliexpress) or hardware store, harborfreight, then that's why it wouldn't work.
 
Fine, realistically, carbide taps probably want some surface footage. What's the surface footage on an M5 tap being hand rotated?

5.0*.03937" = .19685" (nominal) Ø of the tap. Hand tapping is what... 10 RPM? 3.82xSF/.19685 = 10.... 3.82SF = 1.9685.... SF = .515

You bought a cheap tap, spun it too slow, by hand, probably not perfectly aligned, and likely ripped the tips off the teeth with the forward/reverse motion of breaking chips by hand.
 
Fine, realistically, carbide taps probably want some surface footage. What's the surface footage on an M5 tap being hand rotated?

5.0*.03937" = .19685" (nominal) Ø of the tap. Hand tapping is what... 10 RPM? 3.82xSF/.19685 = 10.... 3.82SF = 1.9685.... SF = .515

You bought a cheap tap, spun it too slow, by hand, probably not perfectly aligned, and likely ripped the tips off the teeth with the forward/reverse motion of breaking chips by hand.
So carbide taps require a decent speed to operate? This is news for me. The holes are drilled through, but I did not tap them all the way. The exit is sloped and I was afraid to break the tap there.
I did not do the forward/reverse motion since I was only cutting two additional turns of the tap. But I was stopping several times to reposition my hand. It is quite possible the chipping happened at the tap reversal, when cutting edges were still loaded. If this is true, carbide taps are only for through holes. I did my tapping by hand because I wanted a perfect alignment and a feel for it. And this tap was cutting very well and easy even with the chipped teeth. I have found the chipping only after all holes were finished.
 
So carbide taps require a decent speed to operate? This is news for me. The holes are drilled through, but I did not tap them all the way. The exit is sloped and I was afraid to break the tap there.
I did not do the forward/reverse motion since I was only cutting two additional turns of the tap. But I was stopping several times to reposition my hand. It is quite possible the chipping happened at the tap reversal, when cutting edges were still loaded. If this is true, carbide taps are only for through holes. I did my tapping by hand because I wanted a perfect alignment and a feel for it. And this tap was cutting very well and easy even with the chipped teeth. I have found the chipping only after all holes were finished.

I believe carbide taps CAN be used for blind holes, but not generally by hand. See this excerpt from McMaster:
1717026309354.png


Also note, a single carbide M5 tap from McMaster is $90.

While I don't believe that price ALWAYS determines quality, or value, when it comes to carbide cutting tools, it does more often than not, in my experience. I think you just had the perfect storm of lower quality tooling, hand tapping, and presumably holding the workpiece in your hand and not in a vise?
 
Also to note, did you buy a tight tolerance thread tap?
But also doing it by hand can oval the holes leaving the screws to be more loosely than original, this could pose a problem with keeping the inserts tight in their seats,
fingers crossed.
 








 
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