How to Drill Metal
Drilling is an essential aspect of most metalworking projects. Whether you’re boring holes for bolts, drilling out broken fasteners or removing spot welds for automotive disassembly, understanding the basics of this process can make the difference between success and failure. No matter what metal you’re working with, the right drill bit can help ensure that your metal drilling project is successful.
Types of metalworking drill bits
At first glance, metalworking drill bits may seem identical to their woodworking counterparts. But a few key differences set them apart. First, a high-quality metalworking bit features a 135-degree split point that prevents skating and doesn’t require a center punch to create a starting point. Woodworking bits (and some inexpensive metalworking bits) typically feature a 118-degree point that is more prone to skating across a metal work surface
A 135-degree split point will keep the drill bit from walking on the smooth metal surface.
In addition, there are differences in overall length, shank diameter and flute design as well. The type of task you’re undertaking will have a direct bearing on the type of drill bits you need. Depending on the application, you can choose from these types of metalworking bits:
The most common type of drill bit, twist bits are designed so that the front edges (also called the lip) cut the material and the spirals along the length of the shaft (called flutes) remove the curls of debris (called swarfs) from the hole. Twist bits are either straight-shank or tapered-shank (also referred to as Morse taper bits). Tapered-shank bits have more overall rigidity and strength, and they’re typically used in larger drill machines that are outfitted with a tapered socket chuck.
A common industrial drill configuration for metalworking (and the type you’re most likely to find at a home center), this type of bit features somewhat longer flutes and is longer overall than standard twist drill bits. Jobber bits are available in 29 fractional sizes from 1/16 in. dia. to ½ in. dia.
Made for jobs where space is at a premium, this type of bit is shorter than a standard jobber bit.
Available in 6- and 12-in. lengths, this type of bit provides longer reach for drilling into hard-to-reach spots. As with jobber and mechanic’s bits, aircraft bits are available in 29 fractional sizes.
Silver and Deming bit
also referred to as an S&D bit, this type is used to create holes with a diameter larger than 1/2 in. All S&D bits are 6 in. long, have 3-in. flutes and feature a ½-in.-dia. shank to fit a standard ½-in. chuck.
Similar to an S&D bit, this type features a 3/8-in.-dia. reduced shank to fit into the smaller 3/8-in. chuck.
This is used for drilling multiple sizes of holes in thin metal without having to swap bits. Step drills features incrementally increasing diameter steps, often spaced 1/8 in. apart, and are often used in hand drills by electrical, plumbing and HVAC contractors.
A step bit is like having an entire drill bit set in one bit. It allows you to bore different sizes of holes in thin metal without changing bits.
Like its woodworking counterpart, this bit allows you to create a countersunk hole for driving screwheads flush with the adjacent surface.
Countersink bits are ideal for deburring and countersinking holes for flush or countersunk screwheads.
Metalworking drill bit materials
All of the bits described above are available in a variety of materials, each of which is best for drilling specific metals. You can choose among bits made of these materials:
Titanium nitride (TiN) — Easily identified by its gold color, this coating increases the hardness of the drill bit and extends its life by as much as six times compared with black oxide bits.
Black oxide — Identified by its shiny black appearance, this coating enables drill bits to last 50 percent longer with three times the penetration speed of standard HSS bits.
High-speed steel (HSS) — The most common material for working with metal, HSS is harder than carbon steel, which is typically used in woodworking drill bits. HSS can better withstand the higher temperatures generated by metalworking.
Cobalt -– Made from an alloy that resists temperatures of 1,100 degrees Fahrenheit, cobalt bits are typically used for drilling in extremely hard and abrasive materials such as treated stainless steel, cast iron and titanium. But just because it can drill through very hard materials does not make cobalt the best overall choice. “Many consumers make the mistake of thinking that cobalt bits are a step up,” says Tim Kruto, drill bit product manager for Bosch Tools. “Cobalt is a specialty material that is engineered to be specifically used for very hard materials and at very low speeds. Using a cobalt bit too fast may actually harden your workpiece and make it harder to drill.”
Metal drilling techniques
Before you can accurately drill any work piece, you’ll need to mark it with layout lines. The most common layout tools are a center punch and a scriber. A center punch is made from hardened steel and creates a dimple on the work piece that helps prevent the drill bit from skating — especially important when you’re using a hand drill. Some center punches require a hammer blow to mark the work piece, whereas automatic center punches contain a mechanism that initiates an impact when downward force is applied.
A scriber and a center punch are vital for marking metal workpieces. An automatic punch like the one shown here does not require a hammer blow to dimple the metal.
A scriber can be either a short length of hardened steel with a sharp point or a pencil-like marking tool with a carbide tip. In either case, it is used to scratch a fine line on metal. Because scribed marks can be hard to see on reflective metal surfaces, a coating of fast-drying blue layout dye can be applied to provide contrast. In a pinch, you can use a felt-tip marking pen instead of the dye.
Layout dye enables you to make easy-to-see marks on what is otherwise a difficult-to-read material.
Once you know where you want to drill a hole, you’ll need to properly brace the workpiece. No clamping is necessary for workpieces that are either too large for a vise or so heavy that they are inherently stable(such as an automobile), but most workpieces will need to be secured to prevent them from spinning once you begin drilling. One of the best tools for this purpose is a cross vise — a clamp that you can adjust very accurately along both the X and Y axis. By turning a crank handle you can easily align the center of the marked hole location with the drill bit.
Because it is adjustable along both the X and Y axis, a cross vise lets you make minute adjustments to a workpiece.
If you’re drilling several holes in thick metal – a process that will build up a lot of heat and could damage the drill bit – you can coat both the drill bit and the workpiece with cutting oil, a specially formulated lubricant that extends the life of the drill bit by reducing the amount of heat created during the drilling process. “But for most projects that consumers tackle,” Kruto points out, “cutting oil is usually not needed. Coatings such as titanium provide enough lubricity to easily get the job done.”
Cutting oil is typically needed only for high-heat applications, such as drilling multiple holes in thick metal.
As you drill, work slowly, with the drill press or hand drill set at a low speed. In general, the harder the material you’re drilling, the slower the speed of the drill press or hand drill needs to be. Just as the hardness of the workpiece comes into play, so does the hardness of the drill bit. Follow the drill bit manufacturer’s recommendations when selecting the right speed for the material you’re drilling.
If you’re drilling large holes, start small. Choose two or three bits of incrementally larger diameters, and starting with the smallest, drill a pilot hole. Change to the next larger bit and enlarge the hole; then repeat the process until the hole is the right size.
Patience is essential when you’re drilling metal. Keep the drill at a low rpm setting, and let the bit do the work. As with many of life’s challenges, if you rush, the results will probably be disappointing. Take your time and make your project a success story.