Drilling a hole in metal – what could be easier? There are, however, quite a few subtleties in this locksmith operation, regarding the correct choice of tool, sharpening and cutting conditions. Today he will tell about all the nuances of metal drilling .
To make holes in metal, drills are used – mechanical rods made of an alloy that is harder than the workpiece. Drills for metal are made from high-speed steel grades P6M5, P9, P18 under the general designation HSS, or from hard alloys: VK, T5K10, intended for processing hardened and carbide blanks.
The drill consists of three elements:
- The edges cut into the bottom of the hole and remove thin chips from it.
- Spiral cutting pushes chips out of the hole.
- The shank is designed for fixing the drill in the tool holder.
Metal twist drill design
Cutting edges are worth discussing in more detail. These are two bevels at the sharp end of the drill, which converge at the top – the most prominent point of the front part, forming a bridge. The angle at which the edges converge is called the main nose angle, and its value is standardized for various materials and processing modes:
- Hard steel and stainless steel: 135-140 °
- Structural steel: 135 °
- Aluminum, bronze, brass: 115-120 °
- Copper: 100 °
- Cast iron: 120 ° clearance angle and 90 ° edge angle
Recommended sharpening angles for metal drills
Each edge also has its own sharpening angle of the order of 20–35 °, which determines its sharpness. This angle, called the back angle, ensures that the drill touches the metal only along the edge line, while leaving free space behind them. This shape is necessary for easier removal and disposal of chips. In some drills, the edge is sharpened at a more obtuse angle, up to a right angle. These cutting edges work well on brittle metals such as cast iron, brass and bronze.
Template for checking the angle of sharpening of drills
The spiral part includes several grooves for shaving removal, at the top of which additional edges are located, the plane of which is parallel to the drill axis. This is the so-called tape, which, when the drill is dipped, cleans the walls of the hole and contributes to better centering.
Types of drills for metal and their sharpening technique
Above we examined the basic type of drills. It only takes a little knowledge and practice to understand how corners are formed when sharpening. It is best to sharpen drills on a grinding machine with a hand-hand, in the worst case, you can use a universal sharpener. It is impossible to sharpen drills on angle grinders: firstly, this contradicts safety measures when working with this power tool, and secondly, due to the high speed of rotation, the metal overheats and loosens, becoming soft.
When sharpening, the drill is placed on the handrail so that its cutting part is slightly raised. Turning the drill and moving the shank to the left, you need to ensure that the cutting edge is located strictly horizontally and parallel to the end of the circle. You need to sharpen the left and right edges alternately, removing a thin layer of metal and periodically cooling the drill in water.
If you just fix the drill in the desired position and bring it to the emery, you will not be able to properly grind the back surface. Due to the fact that the whetstone is round, the occipital part of the edge is concave. This leads to rapid edge dulling and chip evacuation problems. To avoid such a phenomenon, after touching the stone, the front part of the drill must be slightly raised, feeding forward and without releasing the pressure. This creates a convex flank surface, which takes up the cutting load much better..
Correct movement when sharpening the drill
Edge turning should be performed before removing sharp edges without chips or burrs. In this case, the removal on both sides should be uniform, which can be judged by the shape and position of the remaining bridge, as well as by the length of the edges themselves. If the bridge is misaligned, the drill will rotate eccentrically, which will increase the hole diameter. This effect can be used if the required drill diameter is not available..
When the main edges are pulled out, the lintel is sewn. To do this, the drill must be placed on a handrail at an angle of about 45 ° and pressed with its back to the edge of the circle, without touching the cutting edge. Two small notches up to 1/10 of the drill diameter are formed on the web, which act as lead-in and centering edges.
Seam drill lintel
A more specific type of drill is used for drilling thin sheet metal. When making a deep hole, the drill is first centered by the top, and at the exit is held by the ribbons of the spiral part. However, in thin metal, the apex goes right through before the ribbons hit the edges, making the hole ragged, misaligned or oval..
Drill sharpening for thin sheet metal
In such situations, it is better to use a feather drill with a centering nose. This can be made from a conventional metal drill by regrinding it in a certain way. Everything is done in the same way and with the same corners, but the edges are not turned from the top to the edges, but are brought together towards each other. The drill must be sharpened against the edge of the stone, leaving the lintel intact. Seam down the edges until the bridge forms a nose protruding 1–2 mm above the tops of the cutting part.
Step drill bit for metal
The third type of drills for metal is tapered stepped. They have several cutting edges of different diameters, which allows you to make different sized holes with just one tool. However, despite the seeming versatility, finding a really good step drill is quite difficult, and its cost will be at least $ 25. Another disadvantage is that such drills can be sharpened only on a specialized machine..
For drilling in cemented carbide and hardened steels, it is better to use concrete drill bits. Their sharpening is originally designed for crushing action, however, if you bring out the edges at an angle of about 135 ° at the top and sharpen them at an angle of 20 °, even in a very hard part you can easily make a neat hole.
How to properly drill metal
Regardless of whether drilling is performed with a drill or on a machine, the main thing is to choose the right rotation speed. In most cases, the optimal speed is in the range of 1800-2500 rpm, however, in practice, completely different values can be chosen depending on the sharpening accuracy and material properties.
For efficient and fast drilling, you cannot do without the ability to correctly correlate rotation speed and feed force. It is easy to feel how the drill cuts into the metal, continuously emitting chips, and itself begins to sink into the bottom of the hole without significant effort. In this case, the revolutions are usually quite low – about 300-500 rpm.
The best indicator that the drilling process is technologically correct, and the drill is sharpened correctly, is a uniform chip exit from both spiral flutes. Chip quality is also a significant indicator:
- when drilling steel, solid chips are emitted in the form of long spirals;
- cast iron, hardened steel and other brittle materials form a scattering of needles;
- aluminum is drilled into short curls;
- when drilling stainless steel, dust and small flakes can be obtained.
Correct chips when drilling metal
If the drill does not cut into the metal, but rubs against it with a characteristic squeak, or an atypical type of chips is formed at the exit, it is better to stop and correct the sharpening, otherwise there is a risk of metal tempering from overheating or breaking of the working part.
Comply with safety precautions! Drill should be done without gloves, protecting your eyes with metal glasses.
Before starting drilling, it is necessary to mark all the holes that need to be made in the part. The center of each hole should be marked with a center punch. First, a small hole 2–3 mm deep is drilled, and a few drops of machine oil are introduced into it. You need to learn how to let the drill do its job by itself: first, press the tool down hard, and when the edges are cut into the metal, loosen the pressure and just press down slightly, keeping a uniform rotation speed.
Other coolants can be used instead of oil. So, when drilling stainless steel, the drill must be moistened with oleic acid. Its vapors are harmful, therefore it is necessary to work in a respirator. Kerosene and soapy water are also well suited for cooling – a utility bar per liter.
Particular attention is required when the drill comes out when drilling through holes. Quite often in such cases, the thin bottom breaks through with the formation of large burrs, which fall into the spiral grooves and pull the drill forward. At the exit from the part, it is required to loosen the pressure and slightly increase the speed.
Drilling metal with a crown
It is better to drill large-diameter holes in several stages, gradually increasing the diameter of the drill. This will not only reduce the stress on the tool, but will also extend the life of the sharpener and ensure cleanliness. Holes over 13 mm in diameter are best drilled with crowns. It is recommended to use grease instead of oil, so there will be less splashing. The crown must periodically be allowed to cool down, and during operation, carefully ensure that the teeth submerge evenly, in other words, keep the spindle strictly perpendicular to the surface of the part.
The final stage of drilling is chamfering on both sides of the hole. To do this, you can use a countersink, and in its absence – a drill of twice the diameter, which is fed with minimal effort at high speeds. It is wise to use a round file and emery paper for deburring large holes..