Drilling Grade 8 bolts

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.

Karl

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.

Regards,

Robin

Thanks, Robin and Karl.

I do not care one bit for their strength.

Can I, perhaps, anneal them in a fire? Would that help with hardening any?

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

Rob>

Ignoramus,

If you don't care for the strength of the bolts why purchase grade 8 bolts?????

Grade 2 or 3 would be much cheaper and easier to drill.

As stated by others, grade 8 bolts can be machined, but use a low cutting speed, say 40 fpm, use cutting oil, and keep the drill producing chips.

Wolfgang

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.

Forgot to say, I only need to drill a narrow hole, say, 1/8" would be fine. That should make things easier.

Can these cap screws be welded? Just wanted to make sure.

Yes.

Why not just go get what you originally needed, rather than fighting with what you got? Ken.

I am not sure what you mean here. Do you mean to get a proper fitting? I could find no such thing.

Ig asked:

Yes they can be welded, but, unless you REALLY know what you are doing it is not recommended if personal safety is involved.

Quality welding of grade 8 bolts would require a knowledge of the chemistry of the steel (which is not standard but left to the discretion of the steel mill).

Next pick the appropriate filler wire and welding process.

Determine the pre-heat temperature required and how to maintain it.

Determine the post-weld heat treatment and cool-down rate required and how to control it.

Determine subsequent heat treatment if the grade 8 properties are to be re-established.

As you can see this is quite a hand-full of things to take care of and falls into the area of expertise of the welding engineer.

If you just want to stick things together for fixturing or ornamental stuff......braze it using oxy-fuel torch and bronze rod. Cool slowly, but this will destroy the grade 8 heat treatment properties.

Wolfgang

Safety is not involved, I am making a fitting for compressed air,

Nah, all I want is for the weld not to fall aparta and to not leak air.

snip----

But should they be? Welding on heat treated items is not good practice without proper follow-up heat treatment. I'd be inclined to think there will be some serious issues in the heat affected zone.

Harold

One word - Cobalt.

He's making a low pressure air fitting out of it, something with 1/4" holes at most and running at

I have done it with good old HSS bits, and tapped holes onto them to with HSS taps. Grade 8 bolts are hard as hell though, and it will not be easy going!! Greg

Yes, that's right.

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.

I mean put the grade 8 on the shelf for use in another project sometime and go get some plain bolts....*someone* has them. Ken.