In mechanical processing, hole processing accounts for about 1/5 of the total processing, and drilling accounts for about 30% of the entire hole processing. I believe that everyone working on the front line will not be unfamiliar with drill bits. When purchasing drill bits, there will be drill bits of different materials and colors. So what is the difference between drill bits of different colors? Is there any relationship between color and drill bit quality? 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 distinguish the quality of drill bits simply by color. There is no direct and inevitable relationship between color and quality. Different color drill bits are mainly different in processing technology. Of course, a rough judgment can be made from the color. However, at present, poor quality drill bits will also process their colors to achieve the appearance style 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 achieve white by fine grinding of the outer circle. The reason why it is said to be high quality is that in addition to the material itself, the quality control during the grinding process is also quite strict, and there will be no burns on the tool surface. The black one is a drill bit that has been nitrided. It is a chemical method that places the finished tool in a mixture of ammonia and water vapor and heats up at 540℃~560C° to improve the tool’s durability. Most of the black drill bits on the market are only black (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 one is edge cleaning and grinding. Because it does not produce high-temperature oxidation like rolling, the grain structure of the steel is not destroyed, and it is used to drill workpieces with slightly higher hardness. The yellow-brown drill bit is called cobalt-containing drill in the industry. Cobalt-containing drills are originally white, and are produced by grinding. When they are atomized later, they are made into yellow-brown (generally called amber), which is the best in the current industry. M35 (Co 5%) also has a gold color. This 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 good and golden. Industrial plating is great, with a hardness of HRC78, which is higher than the hardness of cobalt-containing drills (HRC54).
How to Choose a Drill Bit
Since color is not a criterion for judging the quality of a drill bit, how do you choose a drill bit?
Judging from experience, white ones are generally fully ground high-speed steel drill bits, and the quality should be the best. Gold ones are plated with titanium nitride, which are generally either the best or relatively poor. The quality of black ones is also uneven, and some are made of very poor carbon tool steel, which is easy to anneal and rust, so they need to be blackened.
Generally, when buying a drill bit, you can look at the trademark on the drill handle and the diameter tolerance mark. If the mark is clear, the quality of laser or electrical corrosion will not be too bad. If the word is molded, if the edge of the word is bulging, the drill bit is of poor quality, because the bulging outline of the word will cause the drill bit clamping accuracy to fail to meet the requirements, while the word edge is clear and well intersected with the cylindrical surface of the drill handle, the quality is good. In addition, we need to look at the cutting edge of the drill tip. The fully ground drill has a very good edge and the spiral surface meets the requirements, while the quality of the back angle surface of the poor quality drill is very poor.
Drilling Accuracy
After choosing the drill, let’s take a look at the drilling accuracy. The accuracy of the hole is mainly composed of factors such as the aperture size, position accuracy, coaxiality, roundness, surface roughness, and hole burrs.
Factors affecting the accuracy of the processed hole during drilling:
- The clamping accuracy and cutting conditions of the drill, such as the tool holder, cutting speed, feed rate, cutting fluid, etc.;
- The size and shape of the drill, such as the length of the drill, the shape of the blade, the shape of the drill core, etc.;
- The shape of the workpiece, such as the side shape of the hole, the shape of the hole, thickness, clamping status, etc.
Hole Expansion
Hole expansion is caused by the swing of the drill during processing. The swing of the tool holder has a great influence on the aperture and the positioning accuracy of the hole. Therefore, when the tool holder is severely 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 coarse shank small-diameter drill with good coaxiality between the blade and the shank. When using a reground drill, the reason for the decrease in hole accuracy is mostly due to the asymmetry of the back shape. Controlling the height difference of the blade can effectively suppress the cutting and expansion of the hole.
Hole Roundness
Due to the vibration of the drill, the drilled hole shape is easy to be polygonal, and rifling-like lines appear on the hole wall. Common polygonal holes are mostly triangular or pentagonal. The reason for the triangular hole is that the drill has two rotation centers when drilling, and they vibrate at a frequency of 600 exchanges. The main reason for the vibration is the unbalanced cutting resistance. When the drill rotates one turn, the roundness of the processed hole is not good, causing the resistance to be unbalanced during the second turn of cutting, repeating the last vibration again, but the vibration phase has a certain offset, causing rifling lines to appear on the hole wall.
When the drilling depth reaches a certain level, the friction between the edge of the drill blade and the hole wall increases, the vibration is attenuated, the rifling disappears, and the roundness becomes better. This hole type has a funnel-shaped opening when viewed from the longitudinal section. For the same reason, pentagonal and heptagonal holes may also appear during cutting. To eliminate this phenomenon, in addition to controlling factors such as chuck vibration, cutting edge height difference, and asymmetric back and blade shape, measures should be taken to increase drill bit rigidity, increase feed per revolution, reduce back angle, and grind the chisel edge.
Drilling on Inclined and Curved Surfaces
When the cutting surface or drilling surface of the drill bit is an inclined surface, curved surface or step, the positioning accuracy is poor. Since the drill bit is radially single-sided cutting at this time, the tool life is reduced.
To improve 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.
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 is about to drill through, the material being processed undergoes plastic deformation. At this time, the triangular part that should have been cut by the cutting edge near the outer edge of the drill is deformed and bent outward under the action of the axial cutting force, and further curled under the action of the chamfer of the outer edge of the drill and the edge of the blade, forming a curled edge or burr.
