I discovered something today that some of you may already know. I needed to drill a 7/16" hole into the chassis in order to attach a lateral support for my exhaust pipe. I wore out three titanium drill bits and spent about 20 minutes getting only about half way into the metal when I realized I wasn't going any further. Out of desperation, I tried using a self tapping hex head screw. It immediately started throwing flakes of metal and went straight in. I did a second hole with the same success. I'm stumped as to why drill manufacturers (my bits are Milwaukee and Dewalt) haven't figured out what the fastener people know. Next time you need to drill a hole because a bolt broke off, try it.
Drilling thick steel takes a very sharp bit, lots of oil, slow speed and lots of pressure.
Fast speed, dull bit, dry or inadequate pressure leads to over heating quickly-- leading to just "polish" the hole. If you don't see flakes coming off, it is not working.
Sven...They like selling lots of drill bits......$$$$
"Pulsate" the drill--in other words vary your speed. Start with a much smaller diameter drill, then go larger & larger. Main reason is that the drill point or chisel point of a larger drill has almost zero surface feet per minute (just like the center hub on a large coach or truck wheel) so the outside edge of a large drill can't cut because it's thick point isn't cutting, just rubbing. If you have the resources & ability you can thin the point web so there is less resistance. I prefer to use a smaller drill first.
Also, if you can put a 10 degree x maybe a 1/16" long chamfer (with clearance) on each flute--parallel to the drill axis-- your hole will end up cleaner, smoother, and the drill cutting edges will last longer. Almost like a two flute reamer.
(old tool engineer here)
I've tried all these strategies. I've used different size drill bits, including "pilot point" and switched between drill motors with different bits, allowing them to cool. I have a "hammer" drill and varied the speed, trying to go as slow as I could. No need to do that any more; next time, I will start out with the self tapping screws.
The self-tapping screws most likely have what is called a "split-point". This means there is no web at the point. Looking into the end of one of these you will see almost zero web at the point, thus almost zero surface feet per minute to contend with. The drills called centering drill bits have the same geometry at the point as the self-tapping screws..
The bolts that hold our RV's together are self rolling-thread bolts.
New cobalt bits, or new high speed.
New
About half of the bolts I've removed have broken off - don't try to remove any unless you are prepared to drill them out. No fun drilling through an old bolt. They may have lost their shear strength, but they're still damn hard.
Ya, what bolts are you trying to remove???
And are you making fresh breaks or are you just identifying old breaks (broken end rusted)???
You may want to try a tungsten bit or at least a carbide bit. Try going to a tooling store where machine shops buy their bits, taps, etc. They can help you with the right choice. Most of the bits you get from Lowe's and Home Depot are not up to machinist grade on any way.
John
When I re-drilled all of my Bulk head bolt holes (about 25 new holes) I used a 90 degree drill attachment.
I used a bar clamp mounted on the back of the 90 and in front of the drilling surface.
At a slow speed I was able to continuously tighten the bar clamp while shooting white lithium grease(cools,lubes and doesn't drip like oil and stays on a vertical surface) at the drill bit point.
This worked great especially when going through the thick angle iron.
I drilled all 3/8" holes without a need to down size the drill bit.
No drama, reasonably quick drill times, I used a new black tool steel bit every 5 holes ( drill bits were still good for less strenuous jobs), little fatigue on the driller (ME).
One was a bulkhead bolt (I will be tackling this job when we return from our trip), the others were on the condenser frame for the A/C and one bolt for the ground on the air dryer in the back. I was able to back out some of the bolts after heating, but others had to be drilled through. Earlier, I discussed how well the self tapping screws worked, but may not be able to count on them when I start on the bulk head. These bolts were severely rusted and had pretty much fused to the metal
Pretty ingenious. I could probably do this with my Milwaukee low angle drill, but might burn out the motor.
Just to be clear, on the bulkhead bolt:
How much torque did you apply?
Did you merely remove an already broken bolt (i.e. broken end already rusted) or did you break (clean break) the Rolock?
Quick note: Rolocks are grade 8 5/16" bolts. NOT easy to break unless WAY over-torqued or, of course due to rust jacking. If rust jacking, the bolt would have already been broken and end rusted. Very likely 4-5 threads in, where the bolt starts into the first wall of the 1.5" box beam.
And, I would not even attempt to drill out a broken Rolock. Better to access the "back" of it and with double nuts, just back it out-- see Bulkhead document. The other option would be to "sister" the broken bolt.
Sven,if you have a Fastenal store near you they would have better bits.