What are the best bits to use for drilling bolt heads (safety wire)?

[CCX33911]

I am drilling some 1/2" UNC bolts for safety wire and having a bit of trouble. Have finished two of four, but have gone through 4 "cobalt" bits (0.063").

What are the best bits to use for drilling bolt heads?

[bkozlow]

Hi,

Why are you drilling these bolts? Drilled bolts are generally available from sources such as Aircraft Spruce etc. If they are head bolts , why aren't you using studs? Also, the drilling operation on AN type bolts is usually done wbefore they are heat treated. I question the amount of heat you are generating in your drilling actually weakening the bolt. Just my thoughts.

bkozlow

[CCX33911]

I would love to be able to use predrilled bolts, and I own a ton of them. Unfortunately I need course thread bolts 1/2" x 1.25" to mount the top of my Jag IRS diff. There is one type of course thread aircraft bolt, but I have never been able to locate them.

Do you have a source for course thread drilled Grade 8/aircraft strength bolts?

It is common practice to drill bolt heads and the temperatures are not a problem (see various books by Carroll Smith, etc.).

Thanks

[jim C]

I've had good luck drilling both grade 8 and aircraft AN bolts using a 3/32 regular high speed bit. Use a drill press and once started keep constant pressure untill all the way thru. spray wd40 or cutting oil on while drilling. Use a slow speed. Drilling the head doesn't weaken the bolt.

[scott miller]

to-g gibbs

if price is no concern try solid carbide spade
drills from industrial supply house.
next best drill for hardened steel is a carbide
tipped masonary drill.
neither will build up enough heat to be concerened with however you must use a drill
press and be light on the feed pressure.

good luck
scott miller

[ERA535]

My suggestion would first be to contact Concours West Industries - CWI - they sell the correct fastener to do what you need. They are Jag rear end specialists. Good guys.

The second syggestion is to contact Lawson Products - they have some really killer bits.

[mickmate]

Some good ideas allready posted. The originals were drilled and lock wired but you probably don't want the taper lock bolts for your application. I have found a cheapo carbon steel drill bit is hard (also brittle) but works quite well. Wear your safety glasses and keep just enough pressure on the drill to make a chip.
Good luck
Cheers Nick

[Trans-Am Jim]

Another tip is to drill on an angle so that the hole ends up on two adjoining faces of the bolt head. Less material to remove.

[CCX33911]

Finished up fine using my drill jig which requires 1/16" drills. Cobalt worked out ok. Wanted to use 3/32 but would require drilling of jig pilot which is at least as hard as the bolt heads . Main thing is to have patience. Light pressure and clean chips often. The crunching sound that happens in the middle appears to be because previous chips can not escape and get ground up. Not a great thing. Clearing chips once crunching sound starts then re-oil and continue seem to work well. Just installed, torqued and safety wired.

Thanks

[Michael C Henry]

I work for the army and removing broken bolts ( grade 8)is an everyday thing Colbalt is usualy my preferance. A sharp bit is the answer and not a loot of speed.I talked the shop into investing in a Drill Doctor.When you loose your edge just dress it and it starts cutting. If you spin a dull bit it builds up heat and becomes softer than your work and that very spot to be drilled becomes work hardened and sometimes even a carbide bitt wont cut it. Sharp ,not too fast,the right pressure, and watch out when you go through. I also have a guide for drilling safety wire holes $30 at Jeggs or Summit. Tools make all the difference when used right.