What is Drilling?- Definition, Process, and Tips

What is a Drilling?

Drilling is a cutting process where a drill bit is spun to cut a hole in a circular cross-section in solid materials. The drill bit is usually a rotary cutting tool, often multi-point. The bit is pressed against the workpiece and rotated at rates from hundreds to thousands of revolutions per minute. This forces the cutting edge against the workpiece, cutting off chips (swarf) from the hole as it is drilled. 

In rock drilling, the hole is usually not made through a circular cutting motion, though the bit is usually rotated. Instead, the hole is usually made by hammering a drill bit into the hole with quickly repeated short movements. The hammering action can be performed from outside the hole or within the hole. Drills used for horizontal drilling are called drifter drills. 

In rare cases, specially shaped bits are used to cut holes of a non-circular cross-section; a square cross-section is possible. 

Related Posts: What is a Drilling Machine?

Drilling Process

Drilled holes are characterized by their sharp edge on the entrance side and the presence of burrs on the exit side. Also, the inside of the hole usually has helical feed marks.

Drilling may affect the mechanical properties of the workpiece by creating low residual stresses around the hole opening and a very thin layer of highly stressed and disturbed material on the newly formed surface. This causes the workpiece to become more susceptible to corrosion and crack propagation at the stressed surface. A finish operation may be done to avoid these detrimental conditions.

For fluted drill bits, any chips are removed via the flutes. Chips may form long spirals or small flakes, depending on the material, and process parameters. The type of chips formed can be an indicator of the machinability of the material, with long chips suggesting good material machinability.

When possible drilled holes should be located perpendicular to the workpiece surface. This minimizes the drill bit’s tendency to “walk”, that is, to be deflected from the intended center-line of the bore, causing the hole to be misplaced. The higher the length-to-diameter ratio of the drill bit, the greater the tendency to walk. The tendency to walk is also preempted in various other ways, which include:

• Establishing a centering mark or feature before drilling, such as by:
  • Casting, molding, or forging a mark into the workpiece
  • Center punching
  • Spot drilling (i.e., center drilling)
  • Spot facing, which is machining a certain area on a casting or forging to establish an accurately located face on an otherwise rough surface.
• Constraining the position of the drill bit using a drill jig with drill bushings

The surface finish produced by drilling may range from 32 to 500 microinches. Finish cuts will generate surfaces near 32 microinches, and roughing will be near 500 microinches.

Cutting fluid is commonly used to cool the drill bit, increase tool life, increase speeds and feeds, increase the surface finish, and aid in ejecting chips. Application of these fluids is usually done by flooding the workpiece with coolant and lubricant or by applying a spray mist.

In deciding which drill to use it is important to consider the task at hand and evaluate which drill would best accomplish the task. There are a variety of drill styles that each serve a different purpose. The sub-land drill is capable of drilling more than one diameter. The spade drill is used to drill larger hole sizes. The indexable drill is useful in managing chips.

12 Tips for Drilling Holes in Metal

For most do-it-yourselfers, there will likely come a time when you’ll have to drill a hole through a piece of metal. The tools and methods used to do it are almost as varied as the different types of metals out there.

Drilling through metal isn’t much more difficult than drilling through wood, but it requires some different techniques and a few more supplies. You can use any portable drill (or a drill press, if you have one), and the best drill bits are the same standard bits that also work for wood.

Related: What are Drill Bits?

The most important rules about drilling through metal are to follow a few basic safety tips and use the right techniques, such as drilling slowly to prevent overheating. The same methods work for most metal materials, from sheet metal to aluminum to thick steel plates.

Here are 12 tips to make the task fast, easy and safe.

1. Drill Bits Required for Metal Drilling

Almost any general-purpose twist bit will do a decent job of drilling holes in metal. In fact, most drill bits for metal are manufactured to drill through a variety of materials, including wood and plastic. The least expensive twist bits are made of high-speed steel (HSS), and these basic bits are just fine for most metal-drilling tasks.

If you’ll be drilling a lot of holes, or need to drill through hard, abrasive metals like stainless steel or cast iron, spend a couple of bucks more for black oxide or a cobalt steel drill bit for metal. These bits will bore more holes before becoming dull.

Some bits also have a special coating called titanium nitride (TIN), which manufacturers claim helps resist heat and friction better, making these bits last up to six times longer than standard high-speed steel bits.

2. Protect Your Eyes

It only takes one tiny metal fragment to cause a serious eye injury, so proper eye protection when drilling metal is an absolute must. For the best protection, choose safety glasses that wrap around the sides of your face.

3. Make a Center Punch

Drill bits have a tendency to wander when you first start drilling. To prevent this, measure and mark where you want the hole, and then use a center punch and hammer to create a small dimple. This gives the tip of your drill bit a place to ride in as you begin to drill.

4. Lubricated Bits Last Longer

For drilling holes in steel that’s 1/8 in. or thicker, use cutting fluid or a multipurpose oil like 3-IN-ONE. Lubricating the bit reduces friction and heat buildup, which makes drilling easier, and your bits last longer. For easier-to-drill metals like aluminum, brass, or cast iron, lubrication isn’t usually necessary.

5. Clamps Prevent Stitches

Never hold a piece of metal in one hand while trying to drill through it with the other. The drill bit for the metal could catch, instantly causing the workpiece to spin, strike, and slash (sharp metal edges slice to the bone!). Always use a minimum of two clamps to securely hold down your workpiece.

6. Start with a Small Hole

Need a big hole? Start small! Most twist bits are available in sizes up to 1 in. in diameter, but you’ll get the best results by starting with a 1/4 in. hole and drill successively larger holes with your drill bit for metal until you reach the size you want. Learn more about which twist drill bits to pick.

7. Deburr the Hole

After drilling a hole in metal, it’s a good idea to remove any sharp edges or burrs left behind. You can buy fancy deburring tools to smooth sharp edges, but before spending money on one, try this trick: Take a twist bit slightly larger in diameter than the hole you just drilled, and gently hand-twist it over the top of the hole. This will smooth out the edge of the hole and grind away any burrs.

8. Hole Saws Cut Bigger Holes

For large holes, a hole saw gets the job done cleanly and quickly. Like twist bits, hole saws chuck right into your drill and will cut through thin-gauge sheet metals like aluminum and steel. Use a scrap of plywood as a backer for the hole saw’s pilot bit and to protect your work surface.

9. Drill at a Slow Speed

The faster a bit spins, the hotter it gets. And heat dulls bits quickly. In general, it’s a good idea to drill through metal using as slow a speed as possible using a drill bit for metal. Hard metals like steel and larger drill bits require even slower speeds. With a small twist bit (1/16 in. to 3/16 in.), you can drill through most metals at 3,000 rpm. For larger twist bits (11/16 in. to 1 in.), 350 to 1,000 rpm is recommended.

10. Make a Sandwich

For clean, precise holes in thin sheet metal, make a wood sandwich. Simply sandwich the sheet metal between two pieces of wood and clamp everything down on a table or workbench. The wooden ‘bread’ layers of the sandwich keep the sheet metal flat and prevent the drill bit from wandering as it bores through the sheet metal.

11. Try a Step Bit

Step drill bits are designed to drill holes in sheet metal and other thin materials. They’re just the thing when you need a perfectly sized, clean hole in a steel junction box, electrical service panel or even a stainless-steel sink.

A step drill bit for metal allows you to drill holes in a variety of diameters, and some will drill through metal up to 3/8 in. thick. The downside? Step bits are more expensive than regular bits.

12. Step Up to a Drill Press

While it’s fairly easy to drill most holes in metal using nothing more than a handheld drill, you’ll almost always get greater accuracy and better results using a drill press. Most drill presses are actually built with metalworking in mind. Pulling down on the handle causes the bit to plunge straight down into a workpiece and make a very precise hole.

Drill presses also come with beefy cast-iron tables with tilt and height adjustments and allow a variety of clamping options. Speed changes are as easy as opening the lid and moving a rubber belt from one pulley to another. The most expensive drill presses are floor-standing models, but you can buy a decent benchtop unit for about $100.

FAQs