Main wear forms of carbide drilling bits
Drill angle wear and drill tip wear are common wear forms of carbide drilling bits. The failure form after drill angle wear is usually rough hole wall or bit edge collapse, but the drill angle wear can be effectively restrained by passivation. The drill angle wear is light after the test. The drill bit tip plays an important role in guiding and centering in the drilling process, and 80% of the drilling axial force is generated at the drill tip (transverse edge). When the drill tip is worn, the axial force increases, the centering is unstable, and the carbide drilling bit swing increases, resulting in large hole diameter, outlet burr and even carbide drilling bit fracture. Drill tip usually seriously worn after working, and the transverse edge on the tip is almost ground flat.
Taking carbide drill bit on the nodular cast iron QT600 crankshaft production line as an example, the failure form of most carbide drilling bits is that the hole diameter is too large. Therefore, the wear of drill tip is the main cause of carbide drilling bit failure when machining nodular cast iron, which should be used to judge the rationality of tool life setting.
Carbide Drill bits life formula
In practical application, in addition to observing the wear morphology of the bit, there are two common methods to measure whether the setting of the bit life is reasonable. One is to consider the total depth of the holes processed within the bit life (referred to as the total depth), and the other is to consider the processing distance of the bit shoulder (referred to as the total distance). The former is easy to calculate, and the total depth can be calculated by multiplying the number of pieces by the hole depth, but the influence of cutting speed and feed rate on tool life is not considered. Although the cutting speed and feed rate are considered in the calculation of the latter, it is still unable to quantify the impact of the change of cutting parameters on the tool life. By changing cutting parameters and tracking the change of tool life, the formula of bit life is established.
According to the classical cutting theory, the most influential factors on tool life are cutting speed, feed rate and cutting depth. In drilling, the cutting depth is determined by the cutting speed and feed rate. The formula of bit tool life only needs to consider the cutting speed and feed rate as variables, which is expressed as
T = aVF
In order to facilitate the calculation, the equation is transformed into
lnT = lna + blnV + clnF
In which T is the tool life defined by the crossing distance of the bit shoulder (m), V is the cutting speed defined by the linear speed of the bit shoulder (M / min), and F is the feed rate (mm / min)
Test scheme and formula solution of carbide drill bits on ductile cast iron
The dynamic balance station of crankshaft production line is to meet the requirements of crankshaft dynamic balance by drilling a series of balance holes with different angles (phases) and depths (balance amount) on crankshaft blades. In order to accurately control the depth of the balance hole, the equipment monitors the axial force received by the carbide drilling bit in real time. This function also plays a role in monitoring the tool wear. When the axial force exceeds the set alarm value, it will prompt to replace the tool and display the total depth of the working hole within the tool life. The balance hole station bit not only has constant tool wear standard, but also can accurately record the service life (total depth) of the test bit. Therefore, it is selected as the test tool. Combined with the application practice, the alarm value of drilling axial force is set to 5500no
According to the processing beat of the equipment and the use of tools, set the test scheme: the rotating speed is gradually reduced from 3300r / min to 2750r / min and 2250r / min, and the feed rate is reduced from 660mm / min to 600mm / min. the test scheme and results are shown in Table 1.
There are three test results, and the following three equations can be obtained
InT1= Ina + blnV 4- c × lnFl （ 1）
lnT2 = Ina + blnV2 + clnF2 （2）
Table 1 test scheme and results
|Test scheme||Tool speed n（ rpm）||cutting speedV( mm/min)||Feed speed F( mm/min)||Total processing depth||Cutting depth|
InT3 = Ina + blnV3 + clnF3 (3)
To facilitate the solution, define
Feed index c = (A2B3-a3बी2)/(c2बी3 -c3बी2), Speed index b = (A2-C2 × c)/B2,Formula coefficient a =lnT1 – b・V1-c・f1。
After substituting the test data, a = 3959619943, b = -0. 20209,c= -1.98728, Therefore, the tool life formula should be
T = 3959619943 V－0.20209F-1.98728
Formula extension and application limitations of carbide drill bits
(1) Derivation of the carbide drill bit working time formula
Because only three groups of representative tool data are needed (if the service life exceeds the set tool life, the hole diameter will be too large), the life prediction formula of drill tools on the same production line can be calculated. Therefore, this method has strong popularization. Taking other curved axis tools as an example, the cutting parameters are substituted into the above formula, the theoretical tool tip distance is calculated, and the theoretical life is calculated according to the machining depth of a single piece.
As shown in Table 2, there is little difference between the theoretical and actual life of most tools. It can be seen that the tool life still has high accuracy within a certain range.
Table 2 theoretical and actual life of cutting tools
|Tool number||cutting speed||Feed speed||Drillspeed||Actual tool tip distance||Theoretical tool tip distance||Error ratio||Current life|
|( mm/min)||( mm/min)||( mm/min)|
(2) Formula limitations
In the process of popularization, it is found that the formula has large deviation for the prediction results of large length diameter ratio and large diameter bits. Tools with large aspect ratio generally refer to tools with the ratio of length to diameter greater than 12, such as crankshaft oil passage hole drill. Due to the large aspect ratio and insufficient rigidity, it is more sensitive to the swing in the drilling process. In addition, the long drilling time and poor chip removal effect lead to the burning of the cutting edge after the accumulation of cutting heat. Therefore, the predicted life is higher than the actual life. Large diameter bit usually refers to the bit with a diameter greater than 12mm, such as the bit for drilling the center hole at the flange end of the crankshaft. The reason for the nonconformity of large diameter bit is not clear and needs to be studied in the future.
- When machining nodular cast iron with carbide drill bits, the main wear form is drill tip wear. The common failure form after drill tip wear is large aperture or burr of machining port. The most serious case is broken blade or broken blade
- The wear degree of drill tip can be monitored by detecting the axial force of drilling. When machining QT600 nodular cast iron crankshaft, the limit value of axial force is recommended to be 5500n.
- The life prediction formula of carbide drilling bit is applicable to the carbide drilling bit used in most mechanical crankshaft production lines. However, for bits with aspect ratio above 12 and diameter above 12mm, the prediction error of bit life is large.