Tap Drill for 4-40 Roll Form Tap
What tap drill for 4-40 roll tap in brass? What cutting or lube oil? Anything thing else?
Use a #39
This is a good guide for tap drill sizes for both cut and roll (form) taps
Side Note: I really like form taps. I used to be kind of hesitant to use them, but now I use them almost exclusively for tapping 1/4-20 and smaller. And as a side question, does anyone have a reason NOT to use them? I've tried them out in all types of steel (<38Rc), SS, and of course all the softer stuff and the only time I've broken one is after it's been completely worn.
What type cutting oil or lube?