Are grade 8 bolts drillable? I just realized that bolts that I bought from McMaster this afternoon for making fittings are grade 8, high strength, and am a little worried that perhaps they are going to be difficult to drill (lengthwise).
They are a stone bitch to drill. IIRC, I did it just like stainless steel. Low RPM, hi feed, and oil. I used a good HSS drill bit only 'cause I didn't have carbide drills. I had several drills perfectly sharpened and changed them out every little bit. I think I had to toss a couple failed bolts.
I have the unfortunate job of occationally drilling class 12.9 metric socket head cap screws at both places I work. 12.9 screws have a higher min tensile than grade 8 bolts, as I understand (175ksi vs 150ksi).
I use a speed of about 20 feet per minute.
You will usually hear a heck of a racket as you enter and exit the bolt (as the drill goes through the case hardening). _Resist_ the temptation to increase your feed to get rid of the chatter. Feeding heavily is basically how you will destroy your cutting edge.
Drilling on a lathe is nice because of the feed control allowed by the crank on the tailstock. On a standard drill press without power feed, you'll tend to exit the bolt too quickly and wreck the cutting edge.
Once the edge is gone, don't bother wasting your energy (and patience) trying to ram the dull drill through the bolt. On soft cast iron, mild steel and non ferrous materials you can get away with it, but if you cheat with high-strength bolts, the bolt will win.
If you have many to do, this will give you lots of practice in sharpening little wee drill bits.
You could use a solid carbide center drill to get through the case hardening. Martin Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member
Well, I looked in mcmaster for cap screws, standard hex, 9/16-18, they were only in Grade 5 or 8, or stainless. There was a pack of 10 for $7.22, it was the cheapest -- but I realized that it was grade 8 when it was too late.
Thanks. I think that I will be fine, eventually, I have a drill doctor to keep the drills sharp. Just more hassle. I will do as you say, cutting slowly with a lot of cutting oil.
A side note: I have a Raytheon HT-900B "heat gun in a suitcase" with a broken tip. I will try to make a plasma torch adaptor (to hold the torch, electrical and gas connections etc) out of it. It has a few useful things like pressure gauge, air inlet etc that could be useful to make a neat package.