I need to drill a 12 mm hole in a steel piece 5mm thick, but the whole piece doesn't fit on my drill press. Can this be done anyhow?
In my previous experience drilling holes this big with a hand drill was a pain that end up dulling my drill bits.
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Use oil to cool the bit, if oil residue is not a problem. Use water if that will be a problem. Keeping the bit cool is crucial to long bit life. The larger the hole the slower the RPM are to be used to drill the hole. Slow enough it is easy to count the revolutions is a good reference for how slow. When using a drill press the RPM are usually slower, and oil or water should be used there too. If the surface is flat, and water is preferred, an easy way to keep water at the drill tip is put an ice cube beside the hole while it is being drilled. The heat will melt the ice and keep a supply of water while drilling.
Another thing I do that I feel helps the large hole get drilled is using a smaller bit to pilot a hole through the metal first. Watch the last part of the hole just before the drill goes all the way through, the flutes of the drill bit will grab hard into the hole and try to wrench the drill out of your hands. That may hurt....
You mention a drill press as the tool you had. Most of them have some form of column that is a stout round pipe, so the table/stage can be unclamped and moved up/down and around. So swing the table out of the way and put your object on the base of the drill.
Downside, the base is unadjustable so if the quill doesn't reach down enough then block up the workpiece with dunnage/scrap wood. This can get sketchy, so use clamps and common sense to secure the workpiece.
If there's still not enough vertical space, you can also rotate the head/top of the drill press so it hangs sideways. My drill press is bolted to a stand that is 140 cm above the floor, so its possible to work "over the cliff"
12mm hole is not that large - whatever drill you use, try doing it with either a series of twist drills to work up to the size, perhaps step through 3/6/9/12 mm, or you can use a step drill which does the same in often 2mm increments.
If you're hand-drilling, you can use the CD trick to try and stay vertical, whereas the pillar drill makes going off-axis a lot harder.