We use hardened steel hex bolts, hardness C38-45, shaft diameter 3/8, head diameter 9/16, head length 3/8, to fasten large and expensive climbing holds to a plywood wall, with a nut behind the wall. Some nuts cannot be reached, the bolt sticks in the nut, and the nut may spin behind the wall. Rather than sacrifice the holds that are stuck, we would like to remove the head of the bolt, so the hold may simply slide off the bolt shaft (the bolt is a bit recessed into a roughly 10/16 hole in the climbing hold). We tried a 9/16 carbide extraction bit, but 15 minutes of drilling only removed at most half the head. Is there a better method? A cutting wheel with aluminum oxide quickly cuts the bolts (albeit with many sparks), but for large holds a cutting wheel cannot reach behind the hold to cut the shaft. Is there a burr or drill bit with aluminum oxide (or any other equipment) that we could use to buzz off the head, by inserting the device into the opening in the hold containing the bolt? I'm not sure this is the correct forum for this query, so let me know if it should be posted elsewhere.
Unfortunately, we cannot reach the bolt shaft with any tools that fit between the climbing hold and the wall. Thus we need to 'buzz off' or 'drill off' the bolt head from the front opening.
Is there a way that you could cool off the head of the bolts using liquid Nitrogen to get them brittle enough that you could break them off with a chisel and hammer?
!!! Caution using the liquid Nitrogen !!!
Only other idea is to get out the cutting tourch and protect what you do not want burned and cut the bolt heads off if you can reach them this way... of coarse have water and a fire extingwisher available...
First off I am wondering how the bolts are tightened if the nuts cannot be reached.
Seems like a bad design.
I assume the drill is not cutting because the bolt is turning.
If so take a socket and weld a handle on to the side so you can hold the bolt head while drilling down thru the socket.
Would be best if you had somone with a lathe bore out the socket and put a drill bushing in it to guide the drill.
For a 3/8 bolt I would only use a 3/8 drill with this system, no reason to make more chips then needed.
A good cobalt drill bit should work and be less likly to brake.
You're right not to use the head size. That's almost 4 times the material to remove. I'd go a touch bigger to account for off-centre drillling, say 7/16 for a 3/8 bolt. It may help to centre drill the bolt with a 1/8 bit first or even to use a solid carbide centre drill for the job.
Another thought is to use studs in the future. You should be able to buy decent studs that have a female allen key depression in them. This way both the stud and the nut can be held from the outside.
JB-weld (or other tough glue) an impact socket with a retention ball hole to the bolt head.
Get an impact driver with the retention ball in tact and line it up with the hole in the socket. You should not be able to separate the socket and the driver easily with the ball engaged in the hole.
Pull on the driver while spinning CCW. You can probably generate enough friction to grab the nut on the back side.
back on the ground, you'll be able to hammer (or burn and hammer) the bolt back out of the socket for re-use. If it works, buy a bunch of the sockets and glue them all at once.
You could use riv-nuts in the wood - if they have them with enough grip length - so that the nut part is more secure. They install from the accessible side, too.
No time to do it right, plenty of time to do it twice.
How about just grinding the heads off with a regular hand held grinder? Yeah, lots of sparks, but it would be done quickly. (watch the heat buildup though...)
I am not an expert on drilling out hardened steel bolts, but I do have a few suggestions that are a little more exotic than the usual advice.
Don't know if this will work in your situation, but it works to drill a hole in a hardened motor shaft or a hack saw blade. Try using a rod the same size as the drill bit or the back end of the drill bit or a bit you have already wrecked, etc. Put it in the drill and pretend you are trying to drill as fast as possible. This is to spot anneal the bolt. Then use the real bit. But first let it cool before applying the drill bit (lest you wreck its temper) or applying cutting fluid (lest you quench it). This beats ruining the temper on the bolt and the drill bit at the same time. And keep the real bit cool and lubed. Spot anealing might not go deep enough, unless you repeat the process (using a rod with a cone tip that matches the drill bit/hole). Or
You need a bit that is harder than the steel you are drilling. Carbide, diamond, titanium, cobalt, abrasives, etc.
There are a couple tips here about using masonry bits in hard steel:
You need some sort of cutting fluid. Unfortunately, that can be a problem at that angle. You will have cutting fluid everywhere but where you wnat it.
Drilling concrete with masonry bits, I have ruined multiple bits in one hole but adding a little water one drill does many holes. A little reminder of the importance of cooling when drilling hard stuff. So spray water in the hole. Given your vertical orientation, you might try a water based gel such as K-Y jelly - works drilling teeth. Cuts down on flying chips, too (just drilled a u-bolt plate). It has to boil the water before it can wreck the bit - provided the water is in the right place.
Whatever you use for a cutting fluid, the drill bit is pumping it out of the hole so you want to replace it frequently. Also, carbide doesn't like intermittent lubrication (thermal shock leads to chipping). It doesn't like hand drilling much either. If you have one of those drill press stands used to convert a hand held drill into a drill press, try rotating it 180 around the column so it can steady the drill. Since this is a climbing wall, supporting the weight of the drill/drill press should be no problem at all (belay on). You might even bolt the base to the wall. Not exactly a uni-mill or mag drill press but might help if you find you are chipping carbide.
You might also use progressivly larger drill bits. Reduces the amount of metal you are trying to remove at one time. Also, you don't technically need to drill out either the bolt flange or the core - the only metal you actually need to remove is a hollow cylinder about the size of the bolt diameter -i.e. a coring drill.
If you don't have enough pressure, you will generate more heat than chips. A piece of 1" webbing between two climbing attachment points and behind your shoulders may help.
Another option is to take a sawzall to the plywood, then remove your bolt the old fashioned way, and cut a new piece to cover the hole and give your climbing wall some detail. Or tack weld another bolt head on and use a slide hammer to extract the bolt, climing anchor, and nut through the plywood.
Someone else has a similar problem:
Of course, this being CNCzone, no response would be complete without suggesting you hammer a piece of PVC pipe over the bolt head, fill it with deionized water, and mcgyver an EDM hole popper and post the video on youtube. :-)
Another thought, is there any way you can use a magnetic base drill? That way you could get nice even pressure that is dead square to the head of the bolt. They also have a nice low rpm so you won't destroy drill bits. You can usually rent them. You may need to temporarily attach a steel plate to the wall but it might be worth it.
It strikes me that the more time you spend setting this up, the easier the job will be.