Background of Bit Life Prediction Formula

【Foreword to the article】

The way to maximize the cost-effectiveness of the tool is to choose the most suitable tool life, which can neither waste anything, waste the cutting performance of the current cutting edge, nor exhaust the water and lose the service life of subsequent grinding. As a typical mechanical cutting tool, the factors affecting the service life are relatively fixed and can be quantified. This lays the foundation for our tool life prediction.

The author will use two articles, combined with his own work experience, to introduce the method of predicting and improving the life of drill bits for readers. Readers are welcome to pay attention to reading.

1. Drilling and Bits

drilling

It is a common cold working process for roughing or semi-finishing hole features of small and medium diameters.

drill

It is a tool used in the drilling process, and its structure usually has two forms of integral and welded composite sheets.

Integral drill bits, that is, the material of the blade and the shank are the same, have the advantages of high geometric accuracy, good rigidity, and repeated grinding. The welded composite disc drill is made of materials with low cost and good toughness to make the cutter body, and the material with better wear resistance is welded at the cutting edge, which has the advantage of good economy.

Drill bit materials usually include tool steel (common in the handicraft industry), high-speed steel (low-to-medium speed or low-rigidity conditions), cemented carbide (medium and high-speed conditions), and superhard materials.

2. The Dilemma of Life Prediction

No matter what kind of drill bit is selected or what type of material is selected, it is difficult to set the tool life in the actual application process. The main reasons for this are:

1. The processing environment is closed and it is not easy to observe

When the sting is awakened, the yang qi rises, the temperature warms up, the spring thunder strikes, the rain increases, and all things are full of vitality. Agricultural production is closely related to the rhythms of nature.

Taking open machining tools such as turning tools and milling cutters as a reference, the operator can visually observe the machining process, and judge the tool wear through factors such as sparks, chips and workpiece surface quality during the machining process, and replace the tool in time.

Compared with turning tools and milling cutters, the drill is processed in the hole, which is not convenient for the operator to directly observe the wear of the tool, and it is not convenient to stop the processing before the tool fails. Broken, resulting in scrapped tools and even workpieces.

2. Insufficient cooling, difficult chip removal and chip breaking

Also referring to turning tools and milling cutters, the open machining method enables convenient and economical external coolant to reach the machining position of the tool tip, removing heat in time and cleaning chips.

For the drill bit processed in the hole, even if the drill bit with the internal center cooling method is used, the disadvantage of slow heat dissipation on the drill edge cannot be completely changed. The cutting heat accumulates on the drill edge, which accelerates the tool wear and shortens the service life.

3. Complex structure and many influencing factors

As the most common rotary multi-edge tool, the structure of the drill bit is more complex than that of the single-edge tool such as turning tools or milling cutters. The impact of cutting parameters and other process parameters on the wear of the key structure of the drill bit—drill tip and drill angle— The directions are different, even diametrically opposed.

To sum up, setting the service life of the drill bit is indeed a very troublesome thing.

But no matter how complex something is, if its determinants are found, it can be predicted and even influenced.

In summary

Setting the service life of a drill bit is indeed a very headache.

But no matter how complex something is, if its determinants are found, it can be predicted and even influenced.

Returning to the theme of "prediction method of drill service life", the author believes that, first of all, we must find out the main failure mode, then select the corresponding wear standard, collect and organize data, and establish a mathematical model. Predict the life of the drill bit under conditions.

The following author introduces a practical case: aiming at the improvement of ductile iron crankshaft production line and drill life, I hope it can bring inspiration to everyone!

3. Case Background Introduction

The common material of automobile engine crankshaft is ductile iron.

Ductile iron has good ductility and fatigue resistance, but because it is a medium and long chip material, its comprehensive processing performance is poor. Especially in drilling, its high unit cutting heat and chip removal are difficult, resulting in low drill life. on other cast iron materials.

At the same time, compared with the drilling conditions of box-type products and blind hole drills, the working conditions of crankshaft drills are relatively complicated. There are also radially machined oil channel drill bits with large aspect ratio characteristics, and ordinary drill bits characterized by frequent entrances and exits, such as processing flange surface threaded bottom holes, which also virtually increases the crank axis drill bit. Difficulty in life prediction.

In order to improve the service life of drill bits scattered in various processes on the crankshaft production line, the author summarizes the main wear patterns by observing the wear morphology of carbide drill bits when machining ductile iron crankshafts, and establishes a drill bit life prediction formula through experiments. Used to guide parameter adjustment and life setting.

main form of wear

Before improving tool life, we need to find out the failure mode of the drill.

Corner wear and tip wear are common forms of drill bit wear, as shown in Figure 1.

Figure 1 - Diagram of drill wear pattern

The failure form after drill angle wear is usually manifested as rough hole wall or drill bit chipping, but through passivation means, drill angle wear can be effectively suppressed or reduced, as shown in Figure 2(a)(b), after the experiment the drill bit drilled The angle wear is light, the drill tip plays an important role in guiding and centering during the drilling process, and 80% of the drilling axial force is generated at the drill tip (chisel edge). When the drill tip is worn, the axial force increases. large, the centering is unstable, and the swing of the drill bit increases, resulting in a large hole diameter, exit burrs and even the drill bit breaking, as shown in Fig.

Figure 2—Comparison of drill wear morphology

Taking the drill bit on the ductile iron QT600 crankshaft production line as an example, the failure mode of most drill bits is that the hole diameter is too large. Therefore, we believe that the wear of the drill tip is the main reason for the failure of the drill bit when processing ductile iron, and the tool should be judged on this basis. The rationality of life setting.

It should be pointed out here that this failure form is judged based on specific working conditions, and cannot be directly applied to other working conditions. Readers need to carefully observe the tool wear profile when promoting and applying, and combine the working conditions and processes. and product quality requirements for comprehensive judgment.

Article · Summary

Due to the limited space, as the first half of the article on drill life prediction, the author introduces the characteristics of the drilling process and drill, and the difficulties of life prediction.

Then, based on its own case, the first step of improving the life of the crankshaft and drill bit is introduced: the locking of the main cause of drill bit wear, which lays the foundation for the subsequent selection of indicators, design experiments and calculation formulas.

In the second half of the article, the author will focus on the form, parameters and experimental design of the life prediction formula, and introduce the advantages and disadvantages of the formula application process from both positive and negative aspects, so stay tuned.