Drilling into a seized bolt

I recently had to replace 2 license plate bolts on a matrix. They were probably m6 and unfortunately had slot heads (not hexagonal). The previous owner had stripped these slots, and so I undertook to grind off the stripped heads, drill into the bolt, and use a bolt extractor to remove. Unfortunately I was not able to drill into the bolt very well. I started with a center punch and a small bit, but wasn't taking off much metal. I bought some cobalt bits (generic brand, but still $40 for 13-piece) and that didn't really improve things.

I've drilled out a stuck bolt in the past (didn't use an extractor, just oversized the hole and re-threaded), and have other experience drilling 1/4" steel (on trailers). Sometimes this was slow going, but I was always able to get through with a little patience. Never resorted to Cobalt bits before.

I ended up taking the vehicle (a matrix) to a shop, and they said it was an hours labor, although they charged me for 1/2 that. Said they needed to use a torch. I was expecting they would have had better drills and would have been able to go right through the bolt in 10 minutes and re-thread.

So my question; is there some grade of bolt that is so impervious to drilling that I (and the shop) should have had so much trouble with this? Or did I simply have low-quality cobalt bits, and the shop didn't do things in the most efficient way?

 

It depends on the grade of bolt used. I wouldn't think a license plate bolt is grade 5 or 8 or similar. A good quality HSS should do it. Your mistake was buying cheap ass cobalt bits. Lol

Example of said awesome bits if you want to get some for christmas. These are the bits I have. Their cobalt bits are almost twice as expensive, but cobalt is overkill for all but some of the toughest applications.
http://www.harryepstein.com/index.php/norseman-hi-molybdenum-set-29-pc.html

 

Some bolts are harder than others. I've had the same problem. But I'm too broke to buy a nice set of drill bits

 

That is my suspicion too.

It's making me think twice about the shop I used. They have a good reputation in my town, but I find it hard to believe an experienced mechanic would take an hour on this job if it was simply a matter of using appropriate drill bits. They used a torch and bolt extractors...but in my opinion there was no reason not to have drilled right through and either re-threaded, or just used a bolt and lock-nut to attach the plate, as the other side of the mounting is accessible from inside the car.

Must have been some tough bolt :S

 

I don`t care how expensive of drill bits one buys .

You know the $25.00 per bit kind.

Trying to drill through a bolt by hand is hard work .

Have the same piece same drill bit set up in a drill press you go through it like butter.

 

Plastic cuts Skin -> Aluminum cuts Plastic -> Steel cuts Aluminum -> Tungsten Carbide cuts everything else

Remember if it's important to do it easily, quickly and with accuracy a good tungsten carbide bit goes a long way. They are expensive but invaluable when you get stuck. Also starting with a good center drill first helps too.