What are the differences between drill bits of different colors?

　　What are the differences between drill bits of different colors?

In mechanical processing, hole processing accounts for about 1/5 of the total processing volume, and drilling accounts for about 30% of the entire hole processing. I believe that engineers working on the front line of drilling will be familiar with drill bits. When purchasing drill bits, there will be drill bits made of different materials and colors. So what are the differences between drill bits of different colors? Is there any relationship between color and the quality of the drill bit? Which color drill bit is better to buy?

Is there any relationship between drill bit color and quality?

First of all: it is impossible to tell the quality of drill bits simply by color. There is no direct and inevitable relationship between color and quality. Drill bits of different colors are mainly due to different processing techniques. Of course, you can make a rough judgment based on color, but the current poor quality drill bits are also Will process its own color to achieve the appearance of high-quality drill bits.

　What is the difference between drill bits of different colors?

High-quality fully ground high-speed steel drill bits often appear in white. Of course, rolled drill bits can also be made white by finely grinding the outer circle. The reason why they are high quality is not only the material itself, but also the quality control during the grinding process. It is quite strict and there will be no burns on the tool surface. The black ones are nitrided drill bits, which is a chemical method to improve the durability of the tool by placing the finished tool in a mixture of ammonia and water vapor and undergoing a heat preservation treatment at 540~560C°. Most of the black drill bits currently on the market are only black in color (in order to cover up burns or black skin on the tool surface), but the actual use effect has not been effectively improved.

There are three processes for producing drill bits. The black one is rolled, which is the worst. The white ones are edge-cleared and polished. Because unlike rolling, no high-temperature oxidation occurs, the grain structure of the steel is not destroyed, and it can be used to drill workpieces with slightly higher hardness. Yellow-brown drill bits are called cobalt-containing drill bits in the industry. This is an unspoken rule of the drill bit industry. Cobalt-containing diamonds are originally white and are produced by grinding. When they are atomized in the later stage, they are made into yellowish brown (commonly called amber). They are currently the best in circulation. M35 (Co 5%) also has a gold color. This kind of drill bit is called titanium-plated drill, which is divided into decorative plating and industrial plating. Decorative plating has no effect at all, it just looks nice and golden. Industrial plating is very good, the hardness can reach HRC78, which is higher than the hardness of cobalt diamond (HRC54).

　　How to choose a drill bit

Since color is not a criterion for judging drill bit quality, how do you choose a drill bit? Judging from experience, generally speaking, white drill bits are generally fully ground high-speed steel drill bits and should have the best quality. The gold ones are titanium nitride-coated, and are generally either the best or the worse ones that fool people. The quality of black also varies, and some are made of poor carbon tool steel, which is easy to anneal and rust, so it needs to be blackened.

Generally, when buying a drill bit, you can look at the trademark and diameter tolerance mark on the drill bit shank. The marks are clear and the quality of the laser or electric corrosion will not be too bad. If it is a molded word, if the edge of the word bulges, the quality of the drill bit is poor, because the bulging outline of the word will cause the drill bit clamping accuracy to fail to meet the requirements, and the edge of the word is clear and well connected with the cylindrical surface of the drill bit shank. The borderline ones are of good quality. In addition, look at the cutting edge at the tip of the drill bit. The cutting edge of the fully ground drill bit is very good, and the spiral surface meets the requirements, while the quality of the poor quality drill bit at the rear corner surface is very poor.

　　Drilling accuracy

After selecting the drill bit, let’s take a look at the drilling accuracy. The accuracy of the hole is mainly composed of factors such as aperture size, position accuracy, coaxiality, roundness, surface roughness, and orifice burrs.

Factors that affect the accuracy of the hole being processed during drilling:

①Drill clamping accuracy and cutting conditions, such as tool holder, cutting speed, feed amount, cutting fluid, etc.;

②Drill bit size and shape, such as drill bit length, blade shape, drill core shape, etc.;

③ Workpiece shape, such as hole side shape, hole shape, thickness, clamping status, etc.

1. Expansion

Hole expansion is caused by the swing of the drill bit during processing. The swing of the tool holder has a great influence on the hole diameter and hole positioning accuracy. Therefore, when the tool holder is seriously worn, a new tool holder should be replaced in time. When drilling small holes, it is difficult to measure and adjust the swing, so it is best to use a drill bit with a thick handle and a small blade diameter that has good coaxiality between the blade and the shank. When using regrinding drill bits, the reason for the decrease in hole accuracy is mostly due to the asymmetry of the subsequent shape. Controlling the edge height difference can effectively suppress the cutting expansion of the hole.

2. Roundness of the hole

Due to the vibration of the drill bit, the hole drilled can easily become polygonal, with lines like rifling lines appearing on the hole wall. Common polygonal holes are mostly triangles or pentagons. The reason for the triangular hole is that the drill bit has two rotation centers when drilling. They vibrate at a frequency that is exchanged every 600. The main reason for the vibration is the unbalanced cutting resistance. When the drill bit rotates for one turn, the roundness of the processed hole will be uneven. Well, the resistance is unbalanced during the second cutting turn, and the last vibration is repeated again, but the vibration phase is shifted to a certain extent, causing rifling lines to appear on the hole wall. When the drilling depth reaches a certain level, the friction between the drill bit edge surface and the hole wall increases, the vibration attenuates, the rifling line disappears, and the roundness becomes better. This hole type has a funnel-shaped opening when viewed in longitudinal section. For the same reason, pentagonal and heptagonal holes may also appear during cutting. In order to eliminate this phenomenon, in addition to controlling factors such as chuck vibration, cutting edge height difference, back surface and blade shape asymmetry, etc., measures should also be taken to improve drill bit rigidity, increase feed per revolution, reduce relief angle, and grind. Chisel blade and other measures.

3. Drilling holes on inclined and curved surfaces

When the cutting surface or drilling surface of the drill bit is a bevel, curved surface or step, the positioning accuracy is poor. Because the drill bit is cutting on one side in a radial direction, the tool life is reduced.

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

1) Drill the center hole first;

2) Use an end mill to mill the hole seat;

3) Choose a drill bit with good penetration and good rigidity;

4) Reduce the feed speed.

4. Burr treatment

During drilling, burrs will appear at the entrance and exit of the hole, especially when processing tough materials and thin plates. The reason is that when the drill bit is about to drill through, the material being processed undergoes plastic deformation. At this time, the triangular part that should be cut by the edge of the drill bit near the outer edge is deformed and bent outwards by the axial cutting force, and is bent outward at the outer edge of the drill bit. It is further curled under the action of chamfers and edge surfaces to form curled edges or burrs.