[FuriousGeorge]

I have to drill and tap some 1/4-20 and #6-32 holes in a 0.75" plate of 4140 pre-hard. I have very little experience with this material and am looking for any advice. I plan on using a HSS #7 drill at 1075 RPM and 2.5 IPM with a peck of 1D. For the smaller holes I'll be using a HSS #36 drill at 2020 RPM and 2.5 IPM with a peck of 1D. My taps are just HSS hand taps.

[Steve Seebold]

I have to drill and tap some 1/4-20 and #6-32 holes in a 0.75" plate of 4140 pre-hard. I have very little experience with this material and am looking for any advice. I plan on using a HSS #7 drill at 1075 RPM and 2.5 IPM with a peck of 1D. For the smaller holes I'll be using a HSS #36 drill at 2020 RPM and 2.5 IPM with a peck of 1D. My taps are just HSS hand taps.

I would use a #5 drill for the 1/4-20's and a #34 or 32 for the 6-32's.

if you use the drills called for in a normal drill chart, you're gonna break a lot of taps.

[keen]

Use cutting oil rather than coolant for tapping. Don't worry 4140 is quite easy machining - compared with pre hardened stainless steel its like butter. Just use sharp tools and keep the speeds down. keen