Drill Bit Colors Explained: What Sets Them Apart?

In mechanical processing, hole processing constitutes approximately one-fifth of the overall machining activity, with drilling representing about 30% of the total hole processing. Those working on the front lines of drilling are well-acquainted with drill bits. When purchasing drill bits, you may notice they are made from different materials and come in various colors. So, what exactly is the difference between drill bits of different colors? Is there a connection between the color and the quality of the drill bits? Which color of drill bit is the best choice for purchase?

 

Is there any relationship between drill bit color and quality?

 

It is important to note that the quality of drill bits cannot be solely determined by their color. While there is no direct and consistent correlation between color and quality, different colored drill bits typically reflect variations in processing technology. You can make a rough assessment of quality based on color, but keep in mind that low-quality drill bits can also be coated or colored to give the appearance of higher-quality options.

 

What is the difference between drill bits of different colors?

High-quality, fully ground, high-speed steel drill bits are often white in color. Rolled drill bits can also be made white by fine-grinding the outer surface. The high quality of these drill bits is due not only to the material but also to the strict quality control during the grinding process, which prevents burns on the tool surface.

Black drill bits have undergone a nitriding process. This chemical method involves placing the finished tool in a mixture of ammonia and water vapor, then heating it to 540-560°C to enhance its durability. However, many black drill bits available on the market only have a black color to mask burns or imperfections on the surface, without actually improving their performance.

 

There are three main processes for producing drill bits:

1. Rolling: This results in black drill bits and is considered the lowest quality.
2. Edge Cleaning and Grinding: This process produces white drill bits, which do not experience high-temperature oxidation, preserving the steel grain structure. These bits are suitable for drilling workpieces with slightly higher hardness.
3. Cobalt-Containing Drills: Referred to as yellow-brown drill bits in the industry, these are initially white and acquire a yellow-brown (often called amber) color during the grinding and atomizing processes. They are currently the highest quality available in the market. The M35 drill bits, which contain 5% cobalt, may have a golden color.

Additionally, there are titanium-plated drills, which can be categorized into two types: decorative plating and industrial plating. Decorative plating serves no practical purpose other than aesthetics, while industrial plating offers significant benefits, boasting a hardness of HRC 78, which is greater than that of cobalt-containing drills, typically rated at HRC 54.

 

How to choose a drill bit

Since color is not a criterion for judging the quality of a drill bit, how do we choose a drill bit?

Based on my experience, drill bits come in different colors that often indicate their quality. Generally, white drill bits are made of fully ground high-speed steel and are typically the best quality. Gold drill bits are usually titanium nitride-plated and can vary in quality—they may be either excellent or fairly low-grade. The quality of black drill bits is often inconsistent; some are made from inferior carbon tool steel, which can easily become annealed and rusted, necessitating a blackening finish.

When purchasing a drill bit, you should inspect the trademark and diameter tolerance mark on the drill handle. If the mark is clear and well-defined, it suggests that the quality is reliable, whether it was made using laser or electrical corrosion techniques. Conversely, if the mark is molded and the edges are raised or bulging, the drill bit is likely of poor quality. A good-quality bit will have a clear marking that connects smoothly to the cylindrical surface of the handle.

Additionally, check the cutting edge of the drill tip. A high-quality, fully ground drill bit will have a sharp blade and a properly formed spiral surface, while a low-quality bit will exhibit poor craftsmanship, particularly at the back angle surface.

Drilling accuracy

After selecting the drill bit, let’s take a look at the drilling accuracy.

The accuracy of a drilled hole is influenced by several factors, including hole diameter, positional accuracy, coaxiality, roundness, surface roughness, and the presence of burrs.

The following factors can affect the accuracy of a processed hole during drilling:
1. The clamping precision and cutting conditions of the drill bit, which include the tool holder, cutting speed, feed rate, and the type of cutting fluid used.
2. The size and shape of the drill bit, including its length, blade design, and the shape of the drill core.
3. The characteristics of the workpiece, such as the shape of the hole sides, the overall hole geometry, thickness, and how the machining prototype is clamped during the drilling process.

 

1. Hole expansion

Hole expansion occurs due to the movement of the drill bit during operation. The swing of the tool holder significantly affects both the diameter of the hole and the accuracy of its positioning. Therefore, if the tool holder shows signs of severe wear, it should be replaced promptly with a new one.

When drilling small holes, measuring and adjusting the swing can be challenging. For this reason, it is advisable to use a coarse shank drill with a small blade diameter that maintains good coaxiality between the blade and the shank.

When using a re-ground drill bit, a decrease in hole accuracy is often due to the asymmetric shape of the bit’s backside. To effectively reduce hole cutting and expansion, it is important to control the height difference of the blade.

 

2. Hole roundness

The vibration of the drill bit can cause the drilled hole to take on a polygonal shape, with rifling lines appearing on the walls. Common types of polygonal holes are typically triangular or pentagonal. A triangular hole forms when the drill bit has two rotation centers during drilling, which vibrate at a frequency of 600 rotations per minute. This vibration is mainly caused by unbalanced cutting resistance. As the drill bit completes each rotation, the roundness of the hole is compromised, leading to unbalanced resistance during subsequent cuts. This CNC turning process repeats, but the vibration phase shifts slightly with each turn, resulting in rifling lines on the hole wall.

Once the drilling depth reaches a certain level, the friction between the drill bit’s edge and the hole wall increases. This heightened friction dampens the vibration, causing the rifling to disappear and improving the roundness of the hole. The resulting hole often takes on a funnel shape when viewed in cross-section. Similarly, pentagonal and heptagonal holes may form during the cutting process.

To mitigate this issue, it is essential to control various factors, such as chuck vibration, differences in cutting edge height, asymmetry of the back face, and the shape of the blades. Additionally, measures should be implemented to enhance the rigidity of the drill bit, increase the feed rate per revolution, reduce the back angle, and properly grind the chisel edge.

3. Drilling on inclined and curved surfaces

When the cutting or drilling surface of the drill bit is inclined, curved, or step-shaped, its positioning accuracy decreases. This occurs because, in such situations, the drill bit primarily cuts on one side, which shortens its tool life.

To improve the positioning accuracy, the following measures can be taken:

-Drill the center hole first;
-Use an end mill to mill the hole seat;
-Select a drill bit with good cutting performance and good rigidity;
-Reduce the feed speed.

 

4. Burr treatment

During drilling, burrs often form at both the entrance and exit of the hole, particularly when working with tough materials and thin plates. This occurs because, as the drill bit approaches the point of breaking through the material, the material experiences plastic deformation.

At this moment, the triangular section that the cutting edge of the drill bit is intended to cut becomes deformed and bends outward due to the axial cutting force. This deformation is further exacerbated by the chamfer on the outer edge of the drill bit and the edge of the workpiece, resulting in the formation of curls or burrs.

 

 

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At Anebon, we firmly believe in “Customer First, High-Quality Always”. With over 12 years of experience in the industry, we have been working closely with our clients to provide them with efficient and specialized services for CNC milling small parts, CNC machined aluminum parts, and die-casting parts. We take pride in our effective supplier support system that ensures excellent quality and cost-effectiveness. We have also eliminated suppliers with poor quality, and now several OEM factories have cooperated with us as well.