Machinability testing for gear hobbing applications

ArcelorMittal focuses on both mechanical performances and machinability while designing new steel grades. ArcelorMittal has developed specific programs for machinability testing in turning, low and high speed drilling and gear machining. Machinability is evaluated through cutting forces, chip shape, surface quality and tool life. Gear machining is one of the main machining operations involved in powertrain manufacturing operations. The literature proposes many papers dealing with this process however there are too few studies interested in steel machinability evaluation while gear machining. This paper focuses on a particular gear manufacturing process, i.e. gear hobbing, and more precisely on steel machinability for gear hobbing applications. Tools as well as kinematics of gear hobbing are quite complex. This paper proposes a comprehensive experimental protocol for machinability testing. This protocol is based on a European standard. Tests are performed on a machine tool using a commercially available cutting tool. Tests provide the range of cutting conditions for five different steel grades. Both steels have a ferrite-pearlite structure with yield stress from 530 to 800 MPa and ultimate tensile stress from 680 to 900 MPa. Four grades are devoted to bar machining. The last one is devoted to forming and then machining operations. Many metallurgical solutions are investigated to enhance machinability such as lead addition or increase in sulfur content or calcium treatment. This paper analyses the influence of steel composition and structure on machinability. It shows the relevance of metallurgical solutions for machinability enhancement even for powertrain applications. Cutting conditions clearly depend on the metallurgical solution even if specific cutting force is finally close. The main difference is found on tool wear with tool life ratio from 1 to 1.5

Stress and heat flux distribution in rake face. Analytical and experimental approaches

International audienceMetallurgical solutions for an improvement in machinability currently use nonmetallic inclusions in order to create solid lubricants at the interfaces between tool and workmaterial. It is possible to identify and quantify these layers afterwards. However it is important to know the conditions for which these layers appear: tribological conditions, kinematical conditions (i.e. cutting speed, feed and depth of cut) and thermomechanical conditions (stress and temperature distribution in the interfaces). We focus on the tool / chip interface and more precisely on the thermomechanical conditions. This contribution is concerned with both the analytical modeling and measurement of the temperature distribution in the chip forming zone. We propose a correlation between theoretical and experimental values

Application to crankshaft manufacturing methodology, results and analysis

The gain of productivity in machining is generally sought through tools and/or cutting conditions optimization however an increase in productivity is achievable too through the workmaterial optimization. The metallurgical structure as well as the chemical composition of steels widely influence their ability to be machined. Mittal Steel Europe R & D develops new steel grades such as the Super High Strength Steels whose tensile stresses may reach 1000 or 1200 MPa. A cooperative research program between Mittal Steel Europe R&D and ENSAM tends to propose a methodology able to sort the steel grades in terms of ability to be manufactured (in forging and machining). This study focuses on such an industrial application : the heavy vehicles crankshaft manufacturing. The operation investigated consists in deep hole drilling and is concerned with the lubrication holes. This paper proposes some relevant criteria to compare the different steel grades and/or structures. Some experimental results are proposed

Machinability in dry carbide drilling

Intensive weight savings and out-sizing programs are developed in automotive industry and lead to increase the mechanical properties of the material of the automotive parts. ArcelorMittal has developed specific steel grades known as Super High Strength Steels which are designed for both high ductility and toughness and fatigue resistance. This paper investigates machinability for a drilling operation using an experimental methodology. One of the materials is a new low bainitic steel grade. Experiments are performed with a coated carbide solid drill. Thrust force and torque measurements, chip morphology analysis, surface quality monitoring and tool wear tests are carried out. Experiments are performed with and without lubricant

Experimental study of Built-Up Layer formation during machining of High Strength free cutting steel

Machinability of high-strength steels can be improved without degrading the mechanical properties using metallurgical solutions to create or retain non-metallic inclusions. Such a metallurgical treatment usually leads, during machining, to the appearance of so-called Built-Up Layers (BULs) or transfer layers on the cutting tool. These BULs protect the tool against wear, and longer tool life or better productivity is achieved. Formation of such BULs on the cutting tool depends on many parameters i.e. tool geometry, tool material, cutting conditions. This paper proposes an experimental methodology to identify and describe BUL occurring on the tool rake face. Machining tests were carried out with a high strength free-cutting steel using an untested AlSiTiN coated carbide tool. BULs morphology and composition were determined for various cutting conditions. Temperature distributions at the tool-chip interface were measured during the cutting tests using an infrared camera. BUL appearance was then linked to the thermo-mechanical conditions at the tool-chip interface.Association Nationale de la Recherche et de la Technologi

Metallurgical Analysis of Chip Forming Process when Machining High Strength Bainitic Steels

In the following work, we propose a metallurgical approach to the chip formation process. We focus on a turning application of high strength steel in which chips are produced by adiabatic shear bands that generate cutting force signals with high frequency components. A spectral analysis of these signals is applied and highlights peaks above 4 kHz depending on the cutting conditions. A microscopic analysis on the chip sections provided data on chip breaking and serration mechanisms. Shear band spacing and excitation frequency of the whole cutting system were calculated and gave a good correlation with cutting forces spectra

Application to crankshaft manufacturing methodology, results and analysis

The gain of productivity in machining is generally sought through tools and/or cutting conditions optimization however an increase in productivity is achievable too through the workmaterial optimization. The metallurgical structure as well as the chemical composition of steels widely influence their ability to be machined. Mittal Steel Europe R & D develops new steel grades such as the Super High Strength Steels whose tensile stresses may reach 1000 or 1200 MPa. A cooperative research program between Mittal Steel Europe R&D and ENSAM tends to propose a methodology able to sort the steel grades in terms of ability to be manufactured (in forging and machining). This study focuses on such an industrial application : the heavy vehicles crankshaft manufacturing. The operation investigated consists in deep hole drilling and is concerned with the lubrication holes. This paper proposes some relevant criteria to compare the different steel grades and/or structures. Some experimental results are proposed

Machinability in drilling : mechanistic approach and new observer development

This paper focuses on machinability especially in drilling. It is usually estimated experimentally based on thrust force and torque measurements, chip morphology analysis, surface quality and tool wear tests. A specific experimental procedure has been developed and tested for many steel grades. However wear tests are probably the most critical point. Wear tests are essential for an industrial application since they lead to know the cost per hole nevertheless they are highly time and work material consuming. Wear pattern and wear rate depend on the thermo mechanical stresses exerted on the cutting tool in the contact zones. A mechanistic approach enables to estimate the mechanical stress distribution along the drill lip. A new observer based on the mechanical thrust and force measurements is proposed to estimate stress distribution dissymmetry and rapidly rank the machinability in drilling. His relevancy is checked for several steel grades and drilling conditions: metallurgical structure and hardness are different, cutting material and coating and cutting conditions vary. Experiments are performed without lubricant.International audienceThis paper focuses on machinability especially in drilling. It is usually estimated experimentally based on thrust force and torque measurements, chip morphology analysis, surface quality and tool wear tests. A specific experimental procedure has been developed and tested for many steel grades. However wear tests are probably the most critical point. Wear tests are essential for an industrial application since they lead to know the cost per hole nevertheless they are highly time and work material consuming. Wear pattern and wear rate depend on the thermo mechanical stresses exerted on the cutting tool in the contact zones. A mechanistic approach enables to estimate the mechanical stress distribution along the drill lip. A new observer based on the mechanical thrust and force measurements is proposed to estimate stress distribution dissymmetry and rapidly rank the machinability in drilling. His relevancy is checked for several steel grades and drilling conditions: metallurgical structure and hardness are different, cutting material and coating and cutting conditions vary. Experiments are performed without lubricant

Experimental investigation of the temperature and stress distribution in tool-chip contact zone

The temperature at the tool-chip interface is a good estimator of the chip formation and the mechanics taking place in the cutting area. The temperature could be used as an image of the shear stress and the knowledge of the temperature can give a lot of information on cutting phenomena. Global measured value can give a clue on the local understanding of cutting but a field measurement is a way to obtain a lot more information. Conventional experimental methods as thermocouple reached their limits when trying to obtain a temperature distribution at the very contact zone. An experimental protocol has been developed using advanced infrared imaging technology in order to measure temperature distribution in both the tool and the chip during an orthogonal or oblique cutting operation. This setup also allows the estimation of several information on the chip formation so as geometrical characteristics (tool-chip contact length, chip thickness, primary shear angle) and thermo-mechanical information (heat flux dissipated in deformation zone, local interface heat balance). A study has been carried out on the effect of cutting conditions as cutting speed, feed and depth of cut on the temperature distribution along the contact zone for an elementary operation as orthogonal or oblique cutting. Once the temperature distribution is known it is possible through an analytical model to deduce other information and especially on the contact tribological conditions as local stress, or heat flux repartition. This paper presents the capability of such an experimental setup and the influence of the cutting conditions and the possible further development

Experimental and analytical combined thermal approach for local tribological understanding in metal cutting

Metal cutting is a highly complex thermo-mechanical process. The knowledge of temperature in the chip forming zone is essential to understand it. Conventional experimental methods such as thermocouples only provide global information which is incompatible with the high stress and temperature gradients met in the chip forming zone. Field measurements are essential to understand the localized thermo-mechanical problem. An experimental protocol has been developed using advanced infrared imaging in order to measure temperature distribution in both the tool and the chip during an orthogonal or oblique cutting operation. It also provides several information on the chip formation process such as some geometrical characteristics (tool-chip contact length, chip thickness, primary shear angle) and thermo-mechanical information (heat flux dissipated in deformation zone, local interface heat partition ratio). A study is carried out on the effects of cutting conditions i.e. cutting speed, feed and depth of cut on the temperature distribution along the contact zone for an elementary operation. An analytical thermal model has been developed to process experimental data and access more information i.e. local stress or heat flux distribution