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 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
If you don't care for the strength of the bolts why purchase grade 8
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
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.
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
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.
Safety is not involved, I am making a fitting for compressed air,
70-90 PSI. (I need to make an oddball fitting with 9/16-18 thread,
hence purchase of the bolts).
Nah, all I want is for the weld not to fall aparta and to not leak
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.
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
I have welded and turned grade 8 on the lathe both worked fine. Can't say O
have done much drilling, grade 8 turns to a nice finish on the lathe. The
welds I did have held up fine, not much or any real concern for safety with