Diamond Hole Saw

What type of drill bit is strongest for drilling holes on hard metals such as harderned steel?

I went to DIY shop and saw many types of drill bits such as diamond, carbide, tungsten, high speed steel, cobalt, etc. But i am a layman where those materials are concerned. Any experienced handyman here who know what type of drill bits is best (and 2nd best if the price differ a lot) to drill holes in tough metal?
Thanks.

This depends on the hardness of the steel in question.

Ordinary twist drill bits are only about 65 rockwell C. (Nearly all steel bits are high-speed steels these days.) Hence they simply will not work on the hardest tool steels, and will wear very quickly on medium-hardness, high strength steels. The most practical option is to anneal the part before drilling or machining.

Cobalt bits are only slightly harder, maximum 67-68 RC. The advantage of cobalt bits is they have better heat resistance than high-speed steel bits, hence they can be used at somewhat more aggressive cutting rates and are less likely to burn up. For example, cobalt bits work well for stainless steels which have rapid work-hardening properties.

For drilling tool steels in the fully hardened condition, you need special-purpose tungsten carbide, diamond, or boron nitride tooling. You also generally need a milling machine. (Note:Masonry bits do not work well for this because the points on these bits are too broad. Masonry bits are designed to crush and grind, not cut.)

This is usually not worth the difficulty and expense anyway, it’s much easier to anneal first, then drill & machine, then re-harden.

Lenox Diamond Hole Saw

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