Broken Taps

Hey guys. I'm trying to tap "C-1020 / 1026 Steel Tubing ASTM A-513" and I keep breaking taps. The tap we are trying to use is a 1/4-20 NC GH3 SP/FL bottoming tap. We are already drilling oversized with a #6 drill (.204). We are drilling 1.1" deep and tapping .800" deep @ 100 rpm with a feedrate of 5"/min.

Any ideas on where to start trouble shooting? I'm new at this. We didnt have much issue with this until recently. The last tap only tapped about 4-8 holes before breaking.

I'd switch to a roll tap.

How are you holding the tap?
Your post implies you are just feeding in at the pitch and expecting things not to break.

Matthewkral said:

Hey guys. I'm trying to tap "C-1020 / 1026 Steel Tubing ASTM A-513" and I keep breaking taps. The tap we are trying to use is a 1/4-20 NC GH3 SP/FL bottoming tap. We are already drilling oversized with a #6 drill (.204). We are drilling 1.1" deep and tapping .800" deep @ 100 rpm with a feedrate of 5"/min.

Any ideas on where to start trouble shooting? I'm new at this. We didnt have much issue with this until recently. The last tap only tapped about 4-8 holes before breaking.

Click to expand...

That tubing is gummy, and your RPM is WAYYYYYY to slow.
You're at 7 sfpm.
You want at least 70 sfpm.
Go 1000 rpm and use a good tapping fluid/oil.

note we run our taps in self centering extension/compression holders

If this is tapping an id on a small thick-wall tube having a thru hole it should not be chip-packing sine you are using a spiral point tap. Depending on the holder you are using and if there is no slight countersink to help it locate, it could not be centering, and being a bottoming tap it will not be as forgiving as a plug or taper would on a misaligned hole.

If its a blind hole and breaking in the bottom id safely say you are probably chip-packing with only .300 clearance with that deep of a hole and a spiral point. It has happened to me on ductile iron with a spiral point tap on a deep blind hole (only tap we had at the time), also our spindle wasn't rigid tap and would do an extra rev or two before reversing which was just enough to break one off in every other hole or part (it ran five good parts before breaking one). we noticed some of the parts still had the chips packed in the bottom as a solid slug that had to be hit with a hand drill to remove, drilling the holes with .500 clearance fixed it. I would look at switching to a plug or semi-bottom spiral flute so the chips come back out of the hole.

If its on the od and breaking at the top defiantly look at putting a .25 countersink so the high spots are not forcing the tap to cut off center.

Also look at a guhring green or yellow ring tap we love them they seam to last quite a while and cut smooth. In low/med steel we will run a green ring @ 611rpm and .050"/Rev

Also you can go to a .208Dia and still keep 65%thread so one more thing to try before buying as list of taps.

Where in the process are the taps breaking?

On entry- tap not coaxial with hole
On plunge- chip issue, coolant issue, SFM
At reversal- spindle or tool holders issue, depends on if rigid tapping or not
On retract- feedrate issue, possibly a chip issue

Programmed via Mazatrol

what kind of tap? OSG, Emugee, or hardware store ass secretion?

Matthewkral said:

Hey guys. I'm trying to tap "C-1020 / 1026 Steel Tubing ASTM A-513" and I keep breaking taps. The tap we are trying to use is a 1/4-20 NC GH3 SP/FL bottoming tap. We are already drilling oversized with a #6 drill (.204). We are drilling 1.1" deep and tapping .800" deep @ 100 rpm with a feedrate of 5"/min.

Any ideas on where to start trouble shooting? I'm new at this. We didnt have much issue with this until recently. The last tap only tapped about 4-8 holes before breaking.

Click to expand...

Why a bottoming tap ?