Drilling large holes in metal using a drill bit. How to drill with a crown.
Drilling machine from an old screwdriver: • Сверлильный станок: Чу...
1. A drop of theory.
A. Pros.
1) The main advantage of the crown in comparison with the twist and step drill is that there is no need to cut ALL material inside the hole. Here we only select the ring by diameter, the middle falls out by itself. This means that there is no need to waste time and effort on unnecessary metal removal from this part of the workpiece.
2) The large number of cutting edges allows you to work comfortably and quickly, with a minimum of vibration.
B. Cons.
1) The need for systematic removal of chips from the cutting zone. We will consider this very important point in the practical part.
2) The impossibility of using high-speed systems for drilling with bimetallic crowns. The table of permissible speeds, who needs it, in the section "practice".
B. Types and device.
There are two main types:
1) Bimetallic, that is, consisting of two parts - a carrier made of inexpensive steel with a low carbon content and a working one made of high-speed steel. In my case, this is HSS-Co 8 steel, which is close in characteristics to the familiar P 18.
2) Bits with carbide inserts.
3) Bimetallic bits have a larger number of teeth and therefore work smoother. In addition, they are decently cheaper than carbide ones. Carbide - more expensive, their teeth are more fragile, they are afraid of shock. But at the same time, they are more wear-resistant, can work at higher speeds and do not require cooling of the cutting edges.
2. Practical experience. From the ten-millimeter slab of the future grinder, which I will soon be able to show you, I need to remove some part. Hole selection is the first step in this operation and is both technologically and visually pleasing. I chose a drill with a diameter of 47 mm, because I would like to take this opportunity to check how tightly the 204th bearing fits into the resulting hole. I will need this experience in the next homemade products.
After marking and punching the centers, you need to drill the plate with a drill with the same diameter as that of the guide drill of the crown, in no case more. This is done in order to save the pilot drill from wear and tear, otherwise, if something goes wrong, you will have to select a suitable drill of exactly this diameter and grind it under the fixing bolt. In addition, if you are drilling with a drill, this operation will help you initially set a more or less correct direction perpendicular to the workpiece. To prevent long chips from escaping, I interrupt the feed periodically.
The modern shank allows you to assemble the drill with the possibility of easy subsequent disassembly: the retractable stops will prevent self-tightening. We screw the crown onto the shank until it stops, and then release it until the nearest holes coincide with these retractable clamps.
High temperatures occur in the cutting zone, they adversely affect the structure of the hardened steel of the tool and cause thermal expansion, leading to seizure. To reduce the negative of these temperatures, cooling systems are used. For this I use the simplest oil can: the oil evaporates, takes away heat from the cutting edges, keeping them sharp and prevents them from wedging in the friction zone.
And here is one of the main points of drilling with a crown, which almost no one pays attention to, but it is critical! Look: when drilling with a twist drill, the chips are perfectly removed from the zone along this very spiral. If the drill is stepped, the chips simply fall out into special grooves. In our case, the chips fly out of the zone only until the teeth are plunged into the workpiece material. Some naively believe that these holes are used to remove waste. For ventilation of the cutting area - yes. To recover metal residues - yes. For chip removal - no !!!
Chips accumulate between the teeth of the bit. Of course, the teeth are set, this allows a small part of the chips to go to the sides, but even here it is not needed: it heats up, expands, wedges.
Therefore, it is extremely important to delete it systematically. And not only from the groove to be cut, but also from the space between the teeth. In order not to violate safety standards, the machine must be stopped for this and a metal brush must be used. The deeper the crown goes into the material, the more serious the problem becomes. Wedging begins. By the way, another plus of a screwdriver machine is the presence of a ratchet that protects both the tool and the machine. To prevent the formation of such a deep groove, I recommend drilling the metal to about half of its thickness, and then turning the workpiece over. This will prevent the formation of a groove deeper than half the thickness of the metal, making our work easier and reducing wear on the bit.