In mechanical processing, hole processing accounts for about 1/5 of its total processing, of which drilling accounts for about 30% of the entire hole processing.
I’m sure those of you working on the front line of drilling will not be unfamiliar with the drill bits.
When purchasing a drill bit, there will be bits of different colors of different materials.
So what is the difference between different colored bits, does the color have anything to do with the quality of the bit, and which one is better?
Does drill bit color have anything to do with quality?
First of all, it is not possible to tell the quality of the drill simply from the color, the color is not directly related to the quality.
Different colored bits are mainly processed differently.
Of course, you can make a general judgment from the color, but the current poor quality drill will also have its own color treatment to achieve the appearance of a quality drill.
What’s the difference between different colored bits?
High-quality, fully ground HSS bits often appear in white, and of course, rolled bits can be made white by finishing the outer round.
The reason for the high quality is that in addition to the material itself, the quality control of the grinding process is also very strict so that the surface of the tool will not be burned.
The black is a nitrided drill bit. It is a chemical method to improve the durability of the tool by placing the finished tool in a mixture of ammonia and water vapor, after 540~560C° insulation treatment.
Most of the black drills currently on the market are only black in color (to cover up burns or black skin on the surface of the tool), but the actual use has not been effectively improved.
There are 3 processes for producing drill bits, black is rolled and the worst.
The white is clear-edged and milled.
Because, unlike rolling, there is no high-temperature oxidation, the grain structure of the steel is not damaged and is used to drill slightly harder workpieces.
The yellowish-brown drill, known in the industry as the cobalt-containing drill, is a kind of subterfuge in this drill industry.
Because the cobalt-containing drill was originally white and produced by grinding, and when it was atomized at a later stage, it became yellowish-brown (generally called amber), which is the best in circulation at present. Another type of M35 (Co 5%) is gold.
This kind of drill is called a titanium-plated drill, which is divided into decorative and industrial plating.
The decorative plating does nothing, it’s just nice and golden. Industrial plating is excellent, 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 the criterion to judge the quality of a drill, so how exactly does one choose a drill?
From experience, white is generally a fully ground HSS drill and should be of the best quality.
The gold ones are titanium nitride plated and are generally either the best or poorer at fooling around.
The quality of black also varies, some are made of very poor carbon tool steel, easy to anneal, and easy to rust, so blackening treatment is needed.
When you buy a drill, you can look for the logo on the drill shank and the marking of the diameter tolerance, the marking is clear and the quality of laser or electric corrosion will not be too bad.
If the word is molded and the edge of the word is bulging, the drill is of poor quality.
Because the bulging outline of the word causes the bit to be held with less than the required accuracy, and the word is of good quality if the edge is clear and intersects well with the cylindrical side of the bit’s stem.
The other thing to look at is the cutting edge of the drill tip, the fully ground drill has a good open edge, the spiral face meets the requirements, while the poor quality drill is not very good at the back corner face.
With the drill selected, let’s look at drilling accuracy. The accuracy of drilling is mainly made up of factors such as hole size, position accuracy, coaxiality, roundness, surface roughness and burr in the orifice.
Factors affecting the accuracy of the hole being machined during drilling:
① The accuracy and cutting conditions of the drill bit, such as tool holder, cutting speed, feeding, cutting fluid, etc.
② Drill bit size and shape, for instance, drill bit length, edge shape, core shape, etc.
③ Workpiece shape, such as hole side shape, hole shape, thickness, mounting status, etc.
The reaming is caused by the oscillation of the drill bit in the process.
The oscillation of the clamps has a great influence on the bore diameter and the positioning accuracy of the bore, so when the clamps are badly worn, new clamps should be replaced in time.
When drilling small holes, it is difficult to measure and adjust the oscillation, so it is best to use a coarse shank small-edge drill bit with a good coaxiality between the edge and the shank.
When machining with a regrind bit, the reason for the decrease in hole accuracy is mostly due to the asymmetry of the shape behind.
The control of the blade height difference effectively suppresses the amount of hole expansion.
2. The roundness of the hole
Due to the vibration of the drill bit, the hole is easily polygonal in shape and the walls of the hole appear as rifle.
Common polygonal holes tend to be triangular or pentagonal in shape.
The reason for the triangular hole is that the drill has two centers of rotation when drilling, and they vibrate at frequencies that are exchanged at 600 intervals.
The cause of vibration is mainly cutting force imbalance, when the drill bit turns first turn, due to the processing of the hole roundness is not good, resulting in the second turn of cutting force imbalance, again repeat the last vibration. However, the vibrational phase is somewhat shifted, resulting in a rifle pattern on the hole wall.
When the drilling depth reaches a certain degree, the friction between the ribbed surface of the drill edge and the hole wall increases, the vibration decays, the rifle line disappears and the roundness becomes better.
The orifice is funnel-shaped when viewed in a longitudinal section. For the same reason, pentagonal, heptagonal holes, etc. may also appear in the cut.
To eliminate this phenomenon, in addition to the control of chuck vibration, cutting edge height difference, back and flap shape asymmetry, and other factors, measures should also be taken to improve the rigidity of the drill bit, increase the amount of feed per revolution, reduce the back angle, and repair the cross edge.
3. Drilling holes on inclined and curved surfaces
When the drill bit has a beveled, curved or stepped surface, the positioning accuracy is poor, because the drill bit is radially single-sided, the tool life is reduced.
To improve positioning accuracy, the following measures can be taken:
(1) Drill the central hole first.
(2) Mill the hole seat with an end mill.
(3) Use a bit with good penetration and good rigidity;
(4) Reduce the feeding rate.
4. Treatment of burrs
During drilling, burrs will appear at the entrance and exit of the hole, especially when processing tough materials and thin plates.
The reason for this is the plastic deformation of the material being worked when the drill bit is about to drill through.
At this time, the triangular part that should be cut by the bit near the outer edge of the drill bit is deformed to bend outward after being subjected to axial cutting force, and further curled under the action of the chamfering of the outer edge of the bit and the ribbing of the cutting edge to form a curly edge or burr.
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