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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?

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    Are you using anything to cool or lubricate the bits when drilling? Is this with a drill press or a cordless drill or what? – JPhi1618 Jul 8 at 14:39
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    What quality of bit are you using? – Solar Mike Jul 8 at 14:41
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    8 holes, I'm on my 3rd drill bit. No oil used but I do cool the bit plunging it into water – amphibient Jul 8 at 16:31
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    Plunging your bit into water is what's killing your bits. It's making them dull when you heat them up too much, then brittle when you quench them. – computercarguy Jul 8 at 22:47
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    Also, if you are calling bits "trash" because they are dull, most drill bits can be sharpened. If you are shattering them, it's because you are getting them too hot then quenching them. – computercarguy Jul 8 at 22:57
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3/8" mild (hot-rolled) steel isn't difficult to drill, but any bit will fail if you get it hot enough to melt the cutting edge. 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. If you only have "pilot point" bits on hand, use a starter bit that's no larger than the pilot on the next bit to keep things centered.
  • 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 while they're not being used. A fan across them would do well, or lay them on a damp rag.
  • 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.

More tips

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    The only thing I would add, is to use quality bits. The cheaper brands (e.g. Powerfist, Ryobi, etc.) simply don't use hard enough steel to cut steel effectively. – J Crosby Jul 8 at 17:03
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    I'd skip the 1/8 bit and use a punch to make a dimple where you need to drill. Don't run the drill at its fastest speed and NEVER put it in water to cool, sure it will get hot but that's okay use plenty of used motor oil. I think isherwood is right on the mark though. – user1946891 Jul 8 at 21:59
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    They won't lose temper by quenching, if they are hot enough that the temper will be affected it is already too late and no cooling except specifically the one they were made with is sufficient to avoid damage... – Stian Yttervik Jul 9 at 9:08
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    Please note it is harder to keep the hole at the right place when you step the hole so many times. It is also more likely for your hole to "triangle" with a two flute bit. – Keeta Jul 9 at 14:34
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    A big reason for the stepping up of sizes is that the drilling speed can only be right for one point on the flute, inside of that it is too low and outside too high. By using a sequence of drills you cut each region of the hole closer to the ideal settings, and you accomplish the "never going to be right" of the center of the hole at a drill size where you can keep the pressure (force in a small area) high. – Chris Stratton Jul 9 at 14:51
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Cutting should be easy

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

  • cut long, continuous "chips" (strings, really) that come off like pasta, to the point where you sometimes 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.

Continuing not only wrecks the bit, but can work-harden the surface of the hole, which will make it harder to start doing it properly.

Feeds and Speeds

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" is an imprecise substitute. Being gentle/wimpy is the most common error.

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. Punch a dimple with a center punch, and start with a small drill, which will stay in the dimple better than a big one. My go-to is 1/8”. A hole, even a small one, also makes drilling much easier. Probably because the larger drill doesn't have to cut the center, which is difficult because the center of the drill is at the wrong speed (nearly 0).

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.

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    Most of the makerspaces you'll find use the HF Chinesium drill bits for the "average user". They are cheap to replace when they inevitably break due to inadequate user experience/training/skill/patience/etc. Sure, use their drill press, but if you want good cutting bits/saw blades/etc, bring your own. – computercarguy Jul 8 at 22:54
  • @computercarguy yeah, that goes without saying... TechShop probably used cheapies in the tool drawer but were happy to sell you good ones... – Harper Jul 9 at 1:08
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    "Probably because the center of the drill has 0 speed, which makes it difficult for it to cut. " also more importantly the center of a drill bit has a "web" this is needed to hold the drill bit together but makes the center of the bit rather ineffective at cutting. – Peter Green Jul 9 at 5:24
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    +1 for FEED AND SPEED. Hand drills often have max speed of about 3000 rpm. That converts to 390 SFM for a 1/2 inch hole. For mild steel, you want to aim for HALF this speed. For hardened steel, you want a QUARTER of this and for high temp alloys you want a SIXTH of this. Working in this range will make your bit heat up a lot less, but will make your drill motor heat up a lot more. This is why a drill press is so helpful; slower speed and bigger/stronger motor. – Keeta Jul 9 at 14:29
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    One of these days I'll figure out what you actually do, but apparently at some point you were a machinist, +1. – Mazura Jul 9 at 22:37
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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 (Good steel is tough on bits.) They bought a drill press for the job, was drilling slowly and using oil, but the number and size of the holes was just taking too long. They 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.)

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  1. Use a 'speeds and feeds' calculator to determine the ideal combination for the material, diameter, etc. in question. This should help you get a feel for the best possible circumstance and see how close you can get to it with the stuff you have available. Here is an example: https://www.whitneytool.com/SpeedAndFeedCalculator.aspx

  2. When you get into this level of thickness, a center drill is your best friend as long as you can keep from tilting it during drilling, in which case it becomes your worst enemy, breaking off in the material. In other words: drill press=good; handheld=bad.

  3. Unless you don't care where your hole ends up, center punching alone instead of drilling a pilot hole is a losing proposition: too big a final diameter against too much material thickness. If a center drill is not in the mix, 3/32 to 1/8" is near ideal for doing pilot holes.

  4. A carbide drill bit is the real deal, but usually not cheap--check eBay. A titanium coated metal-cutting bit is arguably the next best thing. Regular high-speed steel bits--meh.

  5. If the bit is sharpened incorrectly, that's probably going to ruin your day. It's not easy to hand-sharpen a drill bit exactly correct; a mechanized sharpener is a good investment if you use your drill bits with any regularity. You're NOT going to be able to sharpen a carbide drill bit btw. Dull bits will 'walk' the hole location before you know what's happening.

  6. Chilled cutting coolant is the way to go but that's usually only available on big cutting machines (I used to work in machine shops and studied tool/die.) Cutting oil is cheap, works pretty well, makes quite a mess.

  7. You're not likely to EVER get a proper sustained 'curl' without a mechanized feed, to which you don't seem to have access. Your scenario is more likely to produce small, potato-chip-like chips combined with the occasional hair-thin curl. Just telling what you should probably expect.

  8. The only way you're going to control heat apart from a cooling agent is to graduate from pilot hole to final dimension in increments--probably four drill bits ending in 3/8". This has the potential to skew the final location of your hole, but nowhere near as bad as just center punching and going for gold straightaway.

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Do not under any circumstances use used motor oil, as has been suggested. As well as being filthy it is seriously carcinogenic and an environmental hazard and should only ever go to your local recycling site. You really need machine cutting oil, which is much more of a coolant than a lubricant, and needs to be supplied continuously in adequate quantity, normally the machine having a pump and recovery tray. The idea is to prevent the cutting edges of the drill from overheating, not to lubricate the cutting process.

Drilling a pilot hole is usually a good idea as has been suggested. However, it is important only when using inappropriate drill tip geometry. There is a way of sharpening the drills, called split point geometry, or four facet geometry, which is far better than the standard cheap and nasty two facet geometry, which is not centre cutting, so needing a pilot hole.

Good quality drills, properly sharpened, and used in a rigid drilling machine with the workpiece securely held, should not be giving the problems you are having.

If you have access to a vertical milling machine you could try a carbide tipped slot drill, which is centre cutting, not an end mill, which is not. One advantage is that the mill enables the exact position of each hole to be set and locked, effectively using the machine as a jig borer. Vertical mills should have a coolant pump.

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You have work-hardened the holes by letting the bit turn without removing metal. Work-hardening can increase the steel strength by 6 times+ ;like from 50 ksi tensile to 300 ksi tensile . One the hole surface in the plate is hardened above 200 ksi , it is too hard for a new sharp bit to cut. Every turn should remove metal or the bit is burnishing/hardening the work. I am guessing you need more pressure and lower RPM...... If you look at the drill point , the small straight line that crosses the center is the chisel; it does not cut, it pushes metal out of the way . So if you drill a smaller hole first ,it removes the metal from the center of the hole where the chisel would need to push, that helps because pushing is more difficult than cutting.

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I had to drill 1/2" holes in hardened steel mower blades. The blades were meant to go on a 5/8" arbor 5ft mower deck. After a lot of research on machinist sites & discussion boards I found that these guys were recommending to use carbide tipped concrete bits.

So off to Lowes .. I found a Bosch 1/2" bit that had an aggressive carbide tip. Now I ain't gonna lie, it did get dull but not like a HSS or cobalt bit would. I dressed it up w/a grinder and she kept on plugging away. 1 bit did three blades.

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

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