Hardened steel is a term for high or medium carbon steel that received quenching treatments and then had ample tempering. This type of steel requires a different type of drill bit because of the difficulty in cutting it and the heat produced when cut. You have a variety of drill bits from which to choose if you cut hardened steel, and this selection features different styles, bit materials and sizes.
Use a special bit made for steel. If you use a wood bit, you'll break the tip off the bit. Before you begin any project, make sure the bit is the appropriate material and design for the job. Whenever you drill hardened steel, make sure that you use a lubricant while you drill to prevent the buildup of excess heat that causes the bit to dull. Also, make sure you drill using a slower drill speed, which also prevents buildup of damaging heat.
The type of bit material makes a lot of difference on its durability. High-speed steel (HSS) is the most common material for bits designed to cut hardened steel. There are other materials, however, including cobalt steel bits, which hold the hardness at higher temperatures but are more brittle than other HSS bits. If you use a Tungsten carbide bit, you can drill most all work pieces but these are also brittle and cost a lot.
Gun Drill Bits
Use gun drill bits for boring out long shafts, like the barrel of the gun. These bits, made for gun drills, have an opening at the base that allows fluid to flow through and lubricate the drill area. The lubrication cools the bit as you drill and pushes out the metal swarf that forms when you drill.
A twist bit is the most common bit, and the tip shape varies according to the material. Harder material needs a larger point angle, and the most common point angle for drilling hardened steel and metal is 150 degrees; most of these are made of high-speed steel. Some of the newer twist bits have a negative cutting edge that allows you to drill without the bit walking or a punch to start the hole. Tungsten carbide and titanium tips with a chrome vanadium shanks allow you to drill not only hardened steel but also many other surfaces.
Centre bits and spot drill are often confused. While you can use the center drill to start a hole in hardened steel, the spot drill is the best. The center drill's job is really to create holes between centers for cylindrical-grinder and lathe work. The spotting drill bit, best for starter holes, should always have the included angle at least the same as or greater than the bit to complete the hole.
Core drill bits look a great deal like a reamer and make a hole wider or finish the edges. Unlike the reamer that only widens a hole up to 1 mm, the core drill bit might widen it to as much as two times its original size. There's no cutting point on the core drill bit and no means of starting a hole. Besides removing residue left in holes, the core drill bit cleans up the edges and rounds out holes made by other bits.
Spade drill bits, which are two-part bits, have an opening in the shaft or body of the bit to run coolant through it. The tip is removable, which allows the user to exchange heads for different sized holes. Trepan drill bits leaves a central core while it cuts an annulus. These often require water to cool the tips. The ejector drill bit derived its name because it's made of two tubes, one inside the other; water goes between the tubes and flushes chips out of the center.
- Photo Credit Stock.xchng: Peter Huys (DarkSide)
- How to Drill Hard Steel
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