Principles of drilling with diamond drills
Diamond core drill bits, including those from ASTAR, are tools that enable holes to be drilled in both natural rock materials, granite, marble, sandstone, etc., as well as artificially manufactured materials: concrete, reinforced concrete, building ceramics, asphalt, etc.
These tools are used on construction sites for drilling holes in ceilings, walls, foundations, stairs, balconies, etc., as well as in asphalt or concrete road surfaces to obtain cylindrical samples for testing, and in the stone industry for drilling in rocky materials (in quarries and processing plants). The cutting element in crowns are diamond segments correctly selected for the material to be drilled. They are fixed to the edge of the tubular drill body via a special silver-copper solder.
Holes are drilled with diamond crowns using drill rigs and special drilling machines. Depending on the operating conditions, drill rigs with electric, hydraulic, pneumatic or combustion drive are used. These machines are equipped with systems for feeding water into the centre of the tubular drill bit, as well as a multi-stage gearbox. Depending on, among other things, the power of the machine, the diameter of the hole to be drilled and the workpiece material, the following speeds are usually used: 200-500 rpm (1st gear), 400-1000 rpm (2nd) and 900-2000 rpm (3rd).
In order to work properly with the drilling machine, the exact location of the hole must be determined before drilling. Then mount the machine base at the location where the hole is to be drilled, using the following overnight methods:
- on a steel dowel and expansion bolt;
- on the strut rod (locking the drill base between the opposing surfaces of the floor and the subsoil);
- for mounting clamps or tension chains;
- by weight;
- vacuum (installation with a special pump).
| Recommended peripheral speeds of diamond crowns for selected materials (m/s)* | |
| Granite IV-V hardness class | 2-4 |
| Granite I-III hardness class | 3-5 |
| Marble | 3-5 |
| Sandstone | 4-6 |
| Concrete up to 25 MPa | 3-5 |
| Reinforced concrete 25-50 MPa | 2-4 |
| * Use speed to speed conversion and vice versa according to the formulas. V=1/60 x 3.14 x D x O, O = 60 x V/3.14 x D, where V - peripheral speed (m/s), D - outside diameter diamond drill (m), O - diamond drill speed (rpm). |
|
| Diamond drill bit feeds used | ||
| Drill diameter (mm) | Water dosing (l/min) | Drilling progress (cm/min) |
| 20-50 | 5-6 | up to 10 |
| 50-100 | 6-10 | 5-8 |
| 100-200 | 10-15 | 3-5 |
| 200-300 | 15-20 | 2-3 |
| 300-600 | 20-35 | up to 1 |
The correct and stable mounting of the drill stand ensures trouble-free, vibration-free and precise operation. Before we start drilling, we should carry out a technical inspection of the machine, paying particular attention to the efficiency of the drive and the power supply system. We can now connect the power supply (depending on the type of machine) and carry out a trial run of the drill rig on the base rail to the extreme positions. Then we start the motor to check its operation in all gears. For machines that do not have gears with synchronisers, they must be switched off before changing gears.
The drill and drill spindle threads are then lubricated and a bronze, brass or copper washer is fitted and the crown is assembled. This consists of screwing it onto the spindle threads by hand and tightening it with a spanner of the appropriate size. Using a spirit level or other instrument, we check the parallelism of the guide rail of the drill base and the outer surface of the tubular body. If there is any misalignment, make adjustments on the drill base.
The next step is to start the water metering and the machine motor. When idling, we assess the stability of the mounting of the base to the ground and check the efficiency of the drill feed and whether there is any so-called "drill run-out". If any abnormalities are found, the correct installation of the drill base, drill rig and drill bit is assessed.
When the drill rig is idling properly, we can start drilling. This is done by moving the machine with the rotating drill bit in the direction of the material by means of the mechanism used, gradually plunging the tool into the workpiece. During the drilling operation, the rotating tubular drill bit cuts the workpiece in a circular path by exerting pressure, while the water dosed into the work area causes the excavated material to be flushed out and cools the crown.
Uniform pressure should be applied during drilling, keeping an eye on the excavated material coming out of the hole. Depending on the material to be drilled, the diameter of the hole to be drilled and the power of the drill rig, different speeds, drilling progress and water dosage are used. The following basic rules must therefore be observed:
- Very hard materials - lower speed (2-3 m/s) and lower feed rate;
- Abrasive materials - higher speed (4-6m/s) and higher feed rate.
The contact pressure exerted depends on the contact surface of the diamond segments with the material to be drilled and should be 200-250 N/cm 2(approx. 20-25 kG/cm 2 ), which in terms of contact pressure results in: max. 40 kg for hand-held drill rigs and 250-400 kg for base-mounted drill rigs with mechanical feed.
Hard materials require the use of more powerful machines. If this is not possible, the speed of the diamond drill should be reduced by downshifting the drill to a lower gear and increasing the pressure of the diamond drill on the material. Reinforced concrete is one of the materials that are difficult to drill and, depending on the percentage of reinforcement, the drilling speed may slow down considerably. When drilling through the reinforcement, reduce the speed of the drill to a lower gear, reduce the amount of water dosed and the drill pressure on the material to be drilled, adjusting the drilling progress to the resistance of the reinforcement to be drilled. During the drilling process, it is important to keep an eye on the steel filings that flow out with the water. Only after drilling through the reinforcement can the drilling progress be increased.
It is worth mentioning here that the excavated material is spontaneously flushed out with water from the fissure created when the material is drilled. Lack of or insufficient water dosage can lead to accelerated segment wear. The reasons for this are:
- occurrence of excessive segment friction with the workpiece material,
- segment overheating,
- damage to the diamond drill bit, e.g. by falling off or broken segments,
- abrasion of the side surface of the metal body,
- warping of the metal body and the appearance of so-called drill run-out.
The diamond segments sharpen themselves during drilling. If a diamond drill bit becomes blunt and shows little or no drilling progress, it is necessary to sharpen the segments by drilling several holes on a block of sandstone, silicate brick, fresh concrete or other abrasive material. This will result in the so-called exposure of the diamond crystals in the segment and the symptoms of a malfunctioning diamond drill bit will disappear.
Once the hole has been drilled, the drill bit must be withdrawn from the drilled material, and the drill drive must be switched off and the material bore removed from inside the tubular drill bit. If the ceiling is drilled, the lower floor must be protected from the falling borehole.
When work is complete, the tubular drill is removed from the machine spindle using the spanners provided with the machine, then the drill and its base are dismantled. Afterwards, the diamond drill bit should be carefully washed with a strong jet of water, taking special care to carefully remove any remaining spoil from the inside. This will allow us to review the condition of the drill body and the diamond segments.
Zdzisław Figacz (Astar)