Xjyuk

I had to drill some 1/2" holes in 3/8" steel plates. In addition to taking something like 15 min per hole, a brand new drill bit lasts maybe two holes and it is trash.

What is the best way to drill holes in steel that thick at home, assuming no special tools like a plasma cutter?

3/8" mild (hot-rolled) steel isn't difficult to drill, but most bits will fail if you melt them. Each hole should take no more than a minute.

• Use a sequence of sizes (1/8", 1/4", 3/8"). This makes for quicker drilling and allows each bit to cool between uses.


• Use a lubricant. Common household oil is probably adequate.


• Keep the bit in alignment. Binding generates heat much more quickly than the cutting process itself.


• Cool the bits slowly while they're not being used. A fan across them would do well. Cooling quickly (quenching) can cause loss of temper (hardness) and result in premature failure.


• Keep the bits sharp. Dull bits create more heat (in the bit and in the operator). A bit sharpener is a great investment for any DIYer, but even a bit of emery cloth can help touch up cutting edges.

Feeds and speeds

This is true for generally all tool-bit-upon-steel work. You should

• cut long, continuous "chips" (strings, really) that come off like spaghetti, to the point where you actually have to intervene to break them, to keep them from rats-nesting around the drill.


• run quite cool, to where you can disengage the bit from the work and grab it with your fingers without fear, and it feels barely warm.


• move efficiently through the work

If anything else is happening, stop doing that RIGHT NOW.

Mainly, we are concerned with feeds and speeds. The rotating speed of the tool (e.g. drill bit) and the speed of movement into the material. The machine industry has exhaustive tables and formulas which serve as starting points, but it's also helpful to listen to what the tool is telling you. You are seeking the "sweet spot".

To be clear, one type of "wrong feed" is too little feed. Unfortunately with hand drills, there is no such thing as feed rate, and "feed force becomes a weak substitute. Being gentle/wimpy is the most common error.

That is one way to work-harden the surface of the steel inside the hole. This will make it more difficult to start up again, and requires decisive action to punch through it and get back into normal metal. This happens from poor cut rate, galling, or rarely heat, which is to say, ignoring the "stop doing that RIGHT NOW" advice above.

Pilot holes help

On a large bit diameter, the pressure required to hit the "sweet spot" may be impractical with a hand tool. In that case, drill a smaller hole of a size where it is practical. Then step up in increments. The hole also makes a good "pilot hole". It's hard to gauge feeds and speeds when overdrilling in small increments, but again you have it right if you have cool fast running with long chips.

Honestly, I'm a big fan of pilot holes simply because they are easier to position precisely (start with a center punch, and a 1/8" drill will stay inside the dimple much better). A hole, even a small one, also makes drilling much easier. Probably because the center of the drill has 0 speed, which makes it difficult for it to cut.

Lubricant

On lubricant, I am pretty slack about that if the cutting is going well. After all, things aren't getting hot, so it's only lubricant, not heat removal, and efficient cutting is easy on tool heads. In the shop, cutting oil or GST are right at hand, but in the field on small holes, I'll use spit, or even nothing if I'm "dialed in" and being highly productive. Really, anything will do - motor oil, 3-in-1, a spray can of Liquid Wrench, remember, we're not letting things get hot!

It goes without saying not to use Harbor Freight or other cheap Cheese drill bits. Again, whole industries are built on the premise that you can drill hundreds of holes with a single bit, so bit quality is no excuse.

Maker spaces are your friend

One more thing: you might consider using a drill press, because that allows much higher pressure and much finer control, including direct control of feed. Don't buy one (and especially don't buy cheap Cheese, as people are wont to do for one-job tools). However check out a local maker space and see what it'll take to get some time on their competent drill press.

The quickest and easiest way is to take them to a steel fabrication shop. They have hydraulic punches that would make short work of that.

The church we used to attend had to build scissor trusses to support a remodel and tried to drill the steel plates. it took forever and was costly in terms of drill bits (they bought a drill press for the job.) Took the job to the local steel fab shop, and they had it done in a matter of a few hours, at a higher quality, at a much cheaper price point (considering the labor they were paying.)

Using a good cutting fluid helps a lot keeping your drill bit from overheating. Also keep the drilling speed low prevents unnecessary heat buildup.