That's very odd. Did it drill fairly easily at the beginning? Were you trying with an annular cutter? You could try heating it red hot and letting it cool and see if it will drill easier. Annular cutters go through high tensile truck frames very easily so should work good on a moldboard. I wonder if it work hardened from a dull bit or cutter? I've had that before with drill bits. Any way you could try from the other side? If nothing else works but you have the outline of the hole partly drilled, then you could use a torch or plasma cutter to finish it. The nice thing is the holes don't have to be real precise just for bolting the cutting edge on. If the bolt fits through you're good.
Before I got an annular mag drill, I just had a 3/4 and a 1/2 mag drill I have used most of my life to drill holes in heavy equipment and truck frames, oilfield stuff, steel buildings, etc. I went thru my bin of 20-30 used drill bits and saw nothing less than what looked like an angry beaver had gnawed the ends off what was there for tooth picks. So I went to town with my checkbook and got a half a dozen brand new cobalt bits each in 1/4” and 5/8” and jugs of tap magic cutting oil from local hardware store, (precision twist drill) brand bits. I got back to the shop and took and made a drill centering tool out of an old broken plow bolt head, and used it inside the cutting edge (with several bolts in the holes) to start pilot holes and drill the rest of the mounting holes. First bit drilled 3 holes no problem. Went up to 5/8 and while it stalled on break through it still made short work of the job. Maybe like you said - drilled like butter. I moved over to start on the opposite cutting edge working center pair then outward to the far left edge of the moldboard and right smack in middle of the frog, thought I could just throw a new bit in there but it was cutting good so I should just see how far the 1st bit will go. Would not even make a single chip. I took bit out and sure enough it’s boogered up so put in a new 1/4” bit and smacked the center punch on it one more time and go to put some pressure on the bit and I’ll be damned it smeared another bit - ruined the tip. So I moved over another hole and tried another new bit, never made a chip and boogered it right away. Had to check if maybe the drill was going backwards and nope, going forwards. I just wasted $60 bucks worth of bits so I stopped there and started looking for answers.
I since got an annular cutter set and a used Hougen mag drill to try to get this done. But the shop is full of Volvos with blown out articulation pins that are higher priority. Waiting on pins to get here…. ♂️
