What Is Drilling?

What Is Drilling?

Drilling is a metal processing method in which a milling tool is used to create a cylindrical hole in a workpiece. It can be done with many machine tools, such as drill presses, lathes, and machining centers. Milling tools, such as drills, taps, and reamers, are used to create screw holes and bearing holes.

Drilling is one of the most common machining operations, accounting for more than half of all processing tasks. Most of the parts we manufacture feature at least one hole, whether for screws, positioning, or bearings.

Holes are one of the most precision-critical processes in parts manufacturing. This is because inner diameter and positional tolerances (including fit and geometric tolerances) must be strictly maintained, and the inner surface of the hole often requires a high-quality finish.

Although a hole may seem simple, achieving these specifications can be challenging and costly. By aligning expectations during the design and planning stages, we can minimize unexpected costs and ensure high-quality results.

Machining Conditions

The machining conditions for drilling are the same as for milling and turning.

Milling speed (Vc)

Vc = D × π × n / 1000 (m/min)
D is the drill diameter (mm), n is the spindle speed (min^-1)

Spindle feed rate (Vf)

Vf = fr × n (mm/min)
fr is the feed per revolution (mm/rev), n is the spindle speed (min^-1)
The life of the milling tool and the amount of chips produced will vary depending on the milling speed and the feed per revolution.

Key Point

Drilling is a process that uses milling tools to add functions such as screw holes, positioning holes, and bearing holes to workpieces. It may cost more than it appears, so when consulting about processing, you can avoid trouble by checking whether the cost of drilling is higher than expected.

Types of Drilling

There are various types of drilling depending on the purpose of the holes.

Drilling

This is a basic process for drilling straight holes. There are two types of holes: through holes, which go through the workpiece, and blind holes, which do not. In basic drilling, the center hole is first drilled, and then a pilot hole is drilled. This is also common to processes such as countersinking and reaming. When drilling a large hole, the pilot hole is significantly enlarged by boring or machining with an end mill.

Countersinking (countersinking)

This is a hole processing method to make a stepped hole in a screw hole to prevent the screw head from protruding. It is also used to flatten holes in rough-surfaced products such as castings or holes in inclined surfaces so that the tightening force of the screw is uniform.

Reaming

This is a process to improve the accuracy of the inner diameter of the drilled hole. The drill marks on the inner diameter are smoothed out to improve dimensional accuracy.

Tapping

This is a process to create a female thread in a pilot hole. It is also called thread cutting.

Key Point

Drilling starts with the common process of drilling a center hole and then drilling pilot holes. Then, the shape of the hole is machined according to the purpose.

Types of Milling Tools Used For Drilling

Drill

High speed steel drill

This drill is made of high speed tool steel. It has excellent toughness and is cost-effective, but is not suitable for high precision machining.

Carbide Drill

This drill uses carbide alloy. It is harder than high-speed steel drills, has excellent wear resistance and heat resistance, and can perform high-precision machining. It is prone to breaking and chipping, so care must be taken when handling it.

Solid Drill

A general drill made of a single material. The body and shank are one piece.

Flat Drill

The tip is flat. It is used when drilling holes on inclined or curved surfaces. The lack of a point angle means that the drill does not run out even when drilling holes on curved surfaces. Burrs that appear when drilling holes through thin plates are suppressed. Because the tip is flat, the load in the axial direction is large, and the tool life is not long.

Luma type drill

For this drill type, the shank and drill diameters are different. The shank diameter is large compared to the cutting-edge diameter, and the drill itself has rigidity, so it is also used for hole drilling that requires high efficiency and precision.

Step drill

This drill has two or more stages with different diameters. It can be used for machining holes with different diameters, such as countersinking. It improves productivity by consolidating what would normally require two drills into one.

Center Drill

This drill is used to determine the center hole of a workpiece. It has a short overall length and high rigidity. It is also used to drill positioning holes to improve the machining accuracy of holes.

Throw-away drill

This is a highly convenient drill with replaceable drill tips. There are mainly two types: head-replaceable drill tips and tip-replaceable drill tips. When the cutting edge becomes dull, you can use it repeatedly by simply replacing the head and tip.

Reamer

This tool improves the dimensional accuracy of the machined surface. It also adjusts the roundness, cylindricity, and surface roughness of the hole machined surface.

Hand Reamer

It is used for hand finishing.

Taper Pin Reamer

It is used for finishing the holes into which tapered pins are inserted.

Machine Reamer

This is a reamer for machining with a cutting angle of approximately 45 degrees. Those with a short cutting edge are called chucking reamers, and those with a long cutting edge are called machine reamers.

Broach Reamer

This is a through-hole reamer characterized by its twisted blade. Thanks to the twist, cutting chips are less likely to accumulate and scratches are less likely to occur. Since the cutting resistance is small, the tool has less load and less vibration.

Tap

This is a tool used to carve the threads of a female screw.

Hand Tap

A tap for manual use. It can be used for both through holes and blind holes because the blade holds the cutting chips.

Point Tap

A tap designed to allow cutting chips to fall downwards. Used for through holes.

Spiral Tap

A tap designed to raise cutting chips upwards. Used for blind holes.

Rolled taps

A tap that is formed by applying strong force to deform the material without cutting it. No cutting chips are produced, but high precision is required for the pilot hole.

Potential Problems With Drilling

Holes are not drilled straight

When the tip of the drill enters the workpiece, it may wobble and cause the hole to be misaligned. To prevent this, guide processing with a center drill or pilot drill is often used to ensure the hole is drilled straight and accurately.

Drill breakage

A drill cannot create a hole deeper than the length of its flutes. The grooves on the drill allow chips to escape, so if the drill is used beyond the flute length, chips can clog the grooves and break the tool. Therefore, it’s important to select a drill with a flute length appropriate for the depth of the hole.

For deeper holes, cutting oil may not reach the drill tip due to the increased distance from the supply point. In such cases, a drill with a built-in oil hole at the tip ensures proper lubrication, helping to prevent overheating and tool wear.

Tap breakage

In some difficult-to-cut materials, work hardening occurs during machining, which increases the hardness of the surface, leading to tap breakage. It is necessary to select a drill that takes into account the effect of work hardening during drilling of the pilot hole.

Key Considerations for Holes in Drawings

• Pilot Hole Depth for Tapping

Pilot holes usually need to be deeper than the effective depth of the tap. Even if the drawing appears fine, drilling the pilot hole may result in a through hole, so it’s important to verify the depth carefully.

• Hole depth

Deeper holes are more challenging and time-consuming to machine. Holes exceeding 10 times the diameter (10D) may require a specialized cutter, which could increase costs.

• Effective tap depth

Similarly, deeper taps are harder to machine, especially in difficult-to-cut materials. Taps exceeding four times the diameter (4D) can be challenging and may require special considerations during machining.

Step Up Your Project with Yumoto Electric

Yumoto Electric delivers high-quality components fast, worldwide. We provide precision machining for a wide range of metals and engineering plastics. We have experience with over 200 materials including metals such as aluminum alloys, stainless steel, molybdenum, and titanium, and engineering plastics including PEEK, PPS, MC Nylon, and POM.

If you’re unsure about material selection or machining methods, we offer complimentary consultations to help optimize your design and production. For any questions, please feel free to contact us.