I ran into a mysterious drilling problem and am trying to find an explanation.
My wife got a greenhouse kit from Harbor Freight. Good kit but not too sturdy; the doors and some panels blew off in the first windstorm. One of the things I tried to do to strengthen it was to attach a 6-foot angle iron to the light aluminum door header, which had been torn by the storm.
I bought a 1/8 inch thick angle iron at Home Depot, 3/4 inch deep on each side. The first thing I had to do was to drill four 1/2 inch holes about evenly spaced over the bar so that the iron would fit over the four nuts protruding from the existing frame. This was done easily, not binding or overheating of the metal in the drilling process. I used my hand drill for this.
Next I drilled six 5/16 inch holes in the thin aluminum header; again no problem. I then marked the corresponding spots on the angle iron, for where the bolts would go through. These holes would be on the same face of the angle iron where the 1/2 inch holes were drilled earlier. The first hole, at the right end, was easy. The second one, at the left end, just wouldn't go through. No problem, I got another 5/16 drill bit, same type of titanium coated bit with pilot hole maker; that one wouldn't go either.
That evening, I went to Home Depot and got two 5/16 inch cobalt drill bits. This morning, I tried one in the hand drill and it wouldn't go through the iron. Just made a dimple in the iron and stopped cutting. I took the angle iron into the shop and put it on the drill press with the second cobalt drill bit. It wouldn't drill through either, just made a slight depression and started overheating, turning the metal red it the drilling area. I tried drilling all along the bar but got the same results everywhere I tried. So I gave up on that angle iron and will get another or try an aluminum bar or a 2x4.
The thing I don't understand is why this iron suddenly became too tough to drill. When I drilled the original five holes, there was no overheating which might have tempered or crystallized the bar, especially not the entire
6 foot length of it. The original five were spaced out over the entire length, so it wouldn't be a case of localized hardening of the steel. And since I tried four drill bits of two different types, I can't blame a dull bit. Any ideas on what just happened?Paul