How to Drill a Hole Through Metal

Rotating drill bits can be hazardous if your metalworking rig is not set up correctly. To safely drill metal, use a vise to secure the metal piece to a stable work surface before drilling.

You can drill metal in a variety of ways using a portable drill or a drill press.

Using a drill-press

Because of the force of its rotating bit, a drill press can be hazardous. Work clamped too loosely can be caught by the spinning bit and sent flying, or clothing can get tangled in the bit.

• To prevent mishaps, never use your hands to hold the work you are drilling. Use at least two C clamps to fasten the work to the machine's table. Special drill-press vises are good for holding small or odd-shaped objects.
• Roll up your sleeves and tuck in your shirt for safety's sake. And never wear work gloves when you use a drill press. The loose fabric of the glove could be caught in the bit.
• Always wear safety goggles when operating a drill press — or using a hand-held drill.

Choosing the right drill speed for the job

Choose the most effective drill speed, expressed in revolutions per minute (rpm), by finding the hole diameter in the left-hand column and the metal you are drilling in one of the three right-hand columns. These figures are approximate and may not correspond exactly to the speeds that are recommended by the manufacturer of your drill press. In general, the smaller the metal bit and the softer the metal, the faster the speed you will need.

Drilling high-precision holes in metal with power tools

You may frequently need to make holes in sheet metal to accommodate fasteners and other mechanical parts that require an opening of an exact size. Although an ordinary electric hand drill bores a hole that is precise enough for some of these needs, use a drill press for more professional work. This machine combines a powerful, variable-speed drill with a secure, adjustable work surface. You change the drill's speed by moving a motor-driven belt from one pulley-step to another. In this way, you can vary the torque, or turning force, of the drill to suit the hardness of the metal.

With any type of drill you determine the size and shape of the hole you'll make by the drill bit you choose. A common twist bit a pointed metal cylinder with a helical channel that spirals down its length — makes a straight (but sometimes inexact) hole. A twist bit bores into the metal and carries the cut chips up the channel and out of the hole. In soft sheet metals such as aluminum or copper, such a bit often cuts a ragged hole; for these metals you should choose a sheet-metal bit, which has a point flanked by two cutting flanges.

Always use high-speed drill bits for making holes in metal. Fashioned of the hardest steel, these bits range in diameter from less than 1/64 inch up to 1 inch. Sizes are designated in different ways: by letters from A to Z, by wire-gauge sizes, in millimeters, in inches, or by numbers. To make holes larger than 1 inch, you can fit the drill with a hole saw — a toothed cylinder that cuts holes up to 6 inches in diameter and up to two thirds the depth of the hole saw itself..

For perfectly precise holes up to 1-½ inches in diameter, you will need to ream the hole after you drill it, either by hand or machine. Hand reamers are straight bits with four or more lengthwise cutting edges. They gradually pare away the inside of the hole as you rotate the reamer. Adjustable hand reamers are available, but they are less exact than fixed-size reamers. Machine reamers fit in a drill press.

Anytime you drill a hole, you will need to use a special metal-cutting fluid that reduces friction and cools the bit as it turns. You will also need clamps and vises to hold the work exactly where you want it and to prevent it from being wrenched from your grip. To mark the place where you will drill, use a center punch to make a small dimple for the drill point. You can find all of these metalworking supplies at most hardware stores.

If you don't need the precision of a drill press you can use a heavy-duty electric hand drill, which has the advantage of being easily transported to the job site. But unless it is a variable-speed drill, you can only slow it for metalwork by a technique known as feathering — pressing the trigger in repeated short bursts.

Drill press and bits for working with metal

A drill press consists of an adjustable table beneath a motor-driven spindle and bit-holding chuck that lower the spinning drill into the metal. Inside the machine's head, a pair of four-step pulleys and a vee-belt connect the electric motor to the spindle.

The chuck grips the drill bit. You lock the bit in place by tightening the chuck with a cogged key. Pulling on the feed handle lowers the bit. The operator controls the hole's depth either by setting the depth stop or by raising or lowering the drill-press table. The spindle, rotating inside a sleeve called the quill, retracts into the head when not in use.

You'll use two kinds of drill bits for metal. For drilling metal thicker than ¼ inch, the high-speed twist bit (inset, left) has a shank, a helical channel called a flute that carries drilled debris out of the hole, and a land — the untooled section of the shank, rimmed with a cutting edge beside the flute. The flat cutting tip is called the web. A sheet-metal bit (inset, right) cuts a clean edge around a hole in thin metal, thus preventing any tearing of the metal.

Marking the Hole on Metal

To locate the hole on the metal workpiece, use a ruler and a scriber to draw two short lines intersecting at right angles at the center of the hole. To align the bit and help keep it from wandering, make a tiny dent at the center point, using a center punch and a ball-peen hammer.

Adjusting the drill speed on the drill press

Determine the necessary drill speed, then set the drill to operate at that speed. Open the drill press belt guard and release the tension on the pulleys by unscrewing the motor-housing knob. To change speeds, push the motor frame forward and slip the belt from one pulley level to another, according to the speed ratings printed on the nameplate of the drill press or listed in the owner's manual. You'll obtain the slowest speeds and the highest torque at the lowest tier of pulleys, with the belt looped between the smallest motor pulley and the largest spindle pulley. Conversely, you'll obtain the fastest speed when the belt is looped around the highest pulleys.

Make sure the belt runs horizontal between the pulleys. Then push the motor back to its original position and tighten the motor-housing knob.

Drilling the hole

Put the drill bit into the chuck and tighten the chuck. Adjust the drill press for the depth of the hole by lowering the bit alongside the work and turning the depth stop to the desired point on the calibrated stop rod. Then raise the bit just enough to slide the work under it, using the quill lock to hold the bit so that you can position the punched hole exactly under the point. Clamp the work well with C clamps. Do not hand-hold the work, clamp it in a vise or to the machine table.

Loosen the quill lock and turn on the power to the drill press. Place a drop of cutting fluid on the hole you punched previously, pull on the feed lever and begin drilling. Apply even pressure. Use a brush to remove chips and shavings as they collect, adding more cutting fluid as you work. If smoke begins to drift out of the bored hole, ease up on the feed handle and check the color of the metal chips. They should be silver or straw yellow. If they are bluish the metal is overheating, and you should add more cutting fluid and reduce the speed of the drill press. When you finish the hole release the feed handle slowly to retract the bit, then turn off the power.

Reaming a Drilled Hole for Maximum Precision

If you plan to use a reamer for extra precision, drill the hole in the metal 1/16 inch narrower than it will be when finished. To center the reamer, deburr the hole by countersinking it slightly. Select a hand reamer of the appropriate size. Attach a tap wrench to it by tightening the wrench's handle. Lubricate the hole with cutting fluid, insert the reamer in the hole and turn the reamer clockwise, working slowly and evenly with slight pressure, until it can go no farther. Then twist the reamer out of the hole, still turning it clockwise.

To ream a hole with a drill press, use a spiral-flute machine reamer (inset). Attach the machine reamer. Finish the hole with the reamer using the slowest speed possible.

Cutting your own threads with taps and dies

You can cut threads in metal using high-carbon-steel taps and dies that come in standardized sizes. The taps cut threads inside a hole; the dies thread the outside of pipes or rods.

When the outside diameter is less than ¼ inch, taps and dies are marked with a gauge number that corresponds to the gauges of machine screws. For example, a tap stamped 8-32NC will cut threads for a No. 8 machine screw with 32 threads per inch in the National Coarse designation. A die stamped ¼-28NF will cut 28 threads per inch on a ¼-inch-diameter rod in the National Fine series.

In the metric system, the designations are slightly different. The outside diameter is given in millimeters, and so is the thread pitch, which corresponds to the threads-per-inch designation of the American National system. In addition, every metric tap or die bears a class number from 1 to 3 indicating tightness of fit: Class 1 is loose, Class 3 is close, or tight. In this system, tightness of fit is analogous to National Coarse and National Fine, in that it makes distinctions between more-precise and less-precise work. 

When you are cutting threads to match those on an existing screw, bolt, or nut, you will need to measure the number of threads on the fastener with a screw-pitch gauge in order to select the correct tap or die. 

Before tapping a hole for a screw or a bolt, you may have to drill the actual hole. This hole must correspond to the size of the tap and must be made with a special tap drill bit. Most tap-and-drill sets contain a tap drill chart to help you select the right drill bit. The charts are available in hardware stores. The chart will indicate, for example, that you would need a No. 29 bit for a hole to accommodate an 8-32NC tap.

If you have never used taps or dies before, practice on a piece of scrap metal. The keys to cutting clean, accurate threads are to work with sharp taps and dies, and to keep the work square and well lubricated if lubrication is called for. When tapping, use a try square as a guide to keep threads straight; when cutting with a die, use the guide on the diestock. You should lubricate the tools and work area liberally with the appropriate cutting fluid when you are working with steel, copper, bronze, or aluminum. Brass and cast iron are cut dry.

Using a portable drill

Holding the Drill

You can increase the precision of holes drilled with a portable drill. Clamp the work in a vise or to a bench. Then, with the power to the drill off, place the tip of the bit in the punched starting hole. To enlarge the starting hole, rotate the chuck by hand several times, pressing down on the bit. Apply cutting fluid, turn on the power, and hold the drill body steady with your left hand while pushing with your right as the drill speeds up. If possible, set an upright try square beside the drill to help you align the drill bit vertically. If you are not using a variable-speed drill, vary the speed by feathering the drill — intermittently squeezing the trigger and releasing it. Ease up on the pressure as you reach the bottom of the hole, but let the bit continue spinning as you remove it from the hole.

Correcting a drifting drill bit

Use a cold chisel and a ball-peen hammer to make a groove from the middle of an off-center hole to the intended center. If the metal is soft or the hole is smaller than ¼ inch in diameter, scratch the groove with a half-round chisel. Then use the groove to guide the bit's point back to the original center. Caution: This works only for a twist bit, which bores with its tip. It will not work for a sheet-metal bit, which cuts with its side edges.