My job at work today was to drill and tap a bunch of M2x4.5mm blind holes in 4140. This is an ongoing job, so we have various jigs to hold the parts how we need them. We power tap the first 2mm in an old Brother Hi-Tap to make sure the tap goes in square and finish the rest by hand. We have 3-flute spiral taps bought specifically for the job, but even with the started thread, plenty of oil and frequent backing out to clear the chips, I still managed to snap 3 of them over 40 holes. No one was particularly surprised or upset, but I feel like I can do better. We're going to try drilling the next lot at 1.7mm instead of the normal 1.6mm in the hope that it reduces the cutting pressure just that little bit.

I had no problem feeling resistance, just found it tricky to tell if it was normal cutting force or that tiny bit extra that was more than the tap would handle. Are there any tricks to keeping small taps alive, or is breaking them pretty much inevitable with a combination of tiny thread and fairly tough material?


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