I’m building a tractor tool bar and need to drill a bunch of 1/2” holes in 2 1/2” square tubing x 3/16” wall. Been using my Harbor Freight (bag over head) hoping they’ll wear out so I can buy a cobalt bit. So far the HF are holding up nicely. My father always used dark pipe tapping oil with a drop or two of Mystery Metal Mover. Slow speed, stop if it smokes, fill hole with mystery mix, repeat. Not the best machining coolant/lube but it seems to work
What did I do wrong drilling these holes?
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Sonny580
Veteran Member
I use 150 rpm on all my drills no matter what size and keep the oil flowing! Messy? ya but beats fighting bits!
Followed my own advice on spot annealing & it actually worked…. Doesn’t happen all that often
I had to drill new 1/2” bolt circle holes in a coulter disc (~1/4” thick) to fit a 4 x 4” trailer hub. No idea what steel the disc was made out of, but Ag Supply lists theirs as 1070-ish which (understatement) can be tough to drill.
No doubt it was surface hardened. A HF center drill (HSS) would barely mark it. New Dewalt cobalt bit would sink the pilot and stop. New Chinese cobalt drills would stop about 1/16” in. This was with a decent Jet drill press, 200 RPM, Mystic Metal Mover tapping fluid, keeping bit cool, trying to watch what chips there were, etc
Too cold to get much out of the oxy rig, so I tried spot annealing with a MAPP torch. Garage was 20F this AM, so I didn’t try to get it cherry red, just past the dark blue oxide coating stage.
Worked like a charm. Used Chinese bits went through it like butter
Did warp it quite a bit, but doesn’t matter for my purposes
Suspect this might not work on air hardening steel if you ran into that
I had to drill new 1/2” bolt circle holes in a coulter disc (~1/4” thick) to fit a 4 x 4” trailer hub. No idea what steel the disc was made out of, but Ag Supply lists theirs as 1070-ish which (understatement) can be tough to drill.
No doubt it was surface hardened. A HF center drill (HSS) would barely mark it. New Dewalt cobalt bit would sink the pilot and stop. New Chinese cobalt drills would stop about 1/16” in. This was with a decent Jet drill press, 200 RPM, Mystic Metal Mover tapping fluid, keeping bit cool, trying to watch what chips there were, etc
Too cold to get much out of the oxy rig, so I tried spot annealing with a MAPP torch. Garage was 20F this AM, so I didn’t try to get it cherry red, just past the dark blue oxide coating stage.
Worked like a charm. Used Chinese bits went through it like butter
Did warp it quite a bit, but doesn’t matter for my purposes
Suspect this might not work on air hardening steel if you ran into that