Drilling cutting parameters
Los parámetros de corte fundamentales que hay que considerar en el proceso de taladrado son los siguientes:.
cutting speed
Cutting speed is defined as the tangential speed of the periphery of the drill bit or other tool used in the drill (Reamer, thread tap, etc.). The cutting speed, which is expressed in meters per minute (m/min), has to be chosen before starting the machining and its appropriate value depends on many factors, especially the quality and type of drill bit used, the hardness and machinability of the material being machined and the feed speed used. The main limitations of the machine are its speed range, the power of the motors and the rigidity of the workpiece and tool fixation.
From the determination of the cutting speed, the revolutions per minute that the milling spindle will have can be determined according to the following formula:
Where V is the cutting speed, n is the rotation speed of the tool and D is the diameter of the tool.
Cutting speed is the main factor that determines tool life. A high cutting speed allows machining to be carried out in less time but accelerates tool wear. Tool manufacturers and machining manuals offer indicative data on the appropriate cutting speed of tools for a given tool duration, for example, 15 minutes. Sometimes it is desirable to adjust the cutting speed for a different tool life, for which the cutting speed values are multiplied by a correction factor. The relationship between this correction factor and the life of the tool in cutting operation is not linear.[7].
Excessive cutting speed may result in:.
Cutting speed too low may result in:.
Bit rotation speed
The rotation speed of the drill spindle is usually expressed in revolutions per minute (rpm). In conventional drills there is a limited range of speeds, which depend on the rotation speed of the main motor and the number of speeds of the machine's gearbox. In numerical control drills, this speed is controlled with a feedback system that usually uses a frequency converter and any speed can be selected within a range of speeds, up to a maximum speed.
The rotation speed of the tool is directly proportional to the cutting speed and inversely proportional to the diameter of the tool.
Feed speed
The feed or feed rate in drilling is the relative speed between the piece and the tool, that is, the speed with which the cut progresses. The feed of the cutting tool is a very important factor in the drilling process.
Each drill bit can cut adequately at a range of feed rates per revolution of the tool, called feed per revolution (f). This range depends fundamentally on the diameter of the drill bit, the depth of the hole, as well as the type of material of the piece and the quality of the drill bit. This speed range is determined experimentally and is found in drill bit manufacturers' catalogs. Furthermore, this speed is limited by the rigidity of the workpiece and tool clamping and by the power of the machine's feed motor. The maximum chip thickness in mm is the most important limiting indicator for a drill bit. The cutting edge of the tools is tested to have a certain value between a minimum and maximum chip thickness.
The feed rate is the product of the feed per revolution and the rotation speed of the tool.
As with the tool rotation speed, in conventional drills the feed speed is selected from a range of available speeds, while numerically controlled drills can work at any feed speed up to the maximum feed speed of the machine.
Effects of ground speed.
The high forward speed gives rise to:.
Low forward speed results in:.
Machining time
In order to calculate the machining time of a drill, it is necessary to take into account the approach and exit length of the drill bit of the piece being machined. The approach length depends on the diameter of the drill bit.
Specific cutting force
The cutting force is a necessary parameter to be able to calculate the power necessary to carry out a certain machining. This parameter is a function of the feed of the drill, the cutting speed, the machinability of the material, the hardness of the material, the characteristics of the tool and the average thickness of the chip. All these factors are included in a coefficient called K. The specific cutting force is expressed in N/mm².[8].
Cutting power
The cutting power P necessary to carry out certain machining is calculated from the value of the chip removal volume, the specific cutting force and the performance of the drill. It is expressed in kilowatts (kW).
This specific cutting force F is a constant that is determined by the type of material being machined, tool geometry, chip thickness, etc.
In order to obtain the correct power value, the value obtained has to be divided by a certain value (ρ) that takes into account the efficiency of the machine. This value is the percentage of the motor power that is available in the tool placed in the spindle.
where.