ULTRA HIGH-SPEED BLANKING FORCES

Transcription

1 ULTRA HIGH-SPEED BLANKING FORCES by Jamie D. MacIsaac Jr., Undergraduate Research Associate Engineering Research Center for Net Shape Manufacturing and Jochen Breitling, Staff Engineer Engineering Research Center for Net Shape Manufacturing TECHNICAL BRIEF SERIES TB-ERC/NSM S October 1996 A National Science Foundation Engineering Research Center

2 ULTRA HIGH-SPEED BLANKING FORCES by Jamie D. MacIsaac Jr., Undergraduate Research Associate Engineering Research Center for Net Shape Manufacturing and Jochen Breitling, Staff Engineer Engineering Research Center for Net Shape Manufacturing TECHNICAL BRIEF SERIES TB-ERC/NSM S October 1996

3 FOREWORD This document has been prepared for the Engineering Research Center for Net Shape Manufacturing (ERC/NSM). This Center was established on May 1, 1986 and is funded by the National Science Foundation and the member companies. The focus of the Center is net shape manufacturing with emphasis on cost-effective manufacturing of discrete parts. The research concentrates on manufacturing from engineering materials to finish or near-finish dimensions via processes that use dies and molds. In addition to conducting industrially relevant engineering research, the Center has the objectives to a) establish close cooperation between industry and the university, b) train students, and c) transfer the research results to interested companies. The undergraduate internship report series is prepared as part of the requirements of the summer internship program designed to give undergraduates a feel for engineering research and to encourage them to consider formal education beyond the bachelor s level. The summer program is typically 3 months in length and is a full-time position. The student is assigned to a faculty / staff member of the ERC/NSM and works on projects in one of the trust areas of the Center. Information about the ERC for Net Shape Manufacturing can be obtained from the office of the Director, Taylan Altan, located at the Baker Systems Engineering Building, 1971 Neil Avenue, Columbus, Ohio , phone: i

4 ABSTRACT ULTRA HIGH-SPEED BLANKING FORCES By James D. MacIsaac Jr., The Ohio State University Advisor: Jochen Breitling (ERC) This report details research into the ultra high speed blanking area. It starts with a brief introduction of the blanking process and progresses through the equipment, procedure, results, and conclusions of physical experiments. This report is a continuation of already published investigations into the effects of process parameters on part quality. The goal of this project was to study the actual forces involved in the blanking process through a range of process parameters in order to develop accurate load versus stroke curves. This project yielded the following results: Increasing the blanking velocity results in a decrease in the maximum blanking force for all four materials. Also, the increase in velocity appeared to change the length and shape of the load-stroke curve. Increasing the punch-die clearance resulted in a decrease in the maximum load and more punch penetration before fracture. Since materials deform and fracture differently at higher velocities, another goal was to investigate whether the force equations derived from lowvelocity experiments are still accurate at higher velocities. The results of blanking four different materials at high velocities confirm that the standard blanking force equation is accurate for high speed blanking. ii

5 TABLE OF CONTENTS FOREWORD... i ABSTRACT... ii TABLE OF CONTENTS... ii LIST OF FIGURES... iv LIST OF TABLES... iv LIST OF EQUATIONS... v 1. INTRODUCTION Blanking The Load-Stroke Curve Blanking Parameters Shear Resistance Strain Hardening EQUIPMENT AND EXPERIMENTS Press Shut Height Stroke Length Sensors Load Cell Proximity Sensor Data Acquisition System Materials Punch and Die Clearances Punch Velocities RESULTS Velocity Influence Steel Steel iii

6 Al CU Clearance Influence Material Influence ACCURACY OF BLANKING FORCE EQUATION SUMMARY AND CONCLUSIONS LIST OF REFERENCES LIST OF FIGURES FIGURE 1: SCHEMATIC ILLUSTRATION OF THE SHEARING PROCESS...1 FIGURE 2: THEORETICAL LOAD-STROKE CURVE...2 FIGURE 3: THE LOURDES PRESS 100-OH...5 FIGURE 4: SENSOR IMPLEMENTATION...7 FIGURE 5: PUNCH VELOCITY VERSUS THE POWER LEVELS OF THE LOURDES PRESS DEPENDING ON THE STROKE LENGTH. NO CUTTING CONDITION /8/ FIGURE 6: EXPERIMENTAL LOAD-STROKE CURVE...12 FIGURE 7: VELOCITY INFLUENCE ON 1060 STEEL (0.054 INCHES THICK)...13 FIGURE 8: VELOCITY INFLUENCE ON 1010 STEEL (0.032 INCHES THICK)...14 FIGURE 9: VELOCITY INFLUENCE ON THE AL 2008 (0.041 INCHES THICK)...15 FIGURE 10: VELOCITY INFLUENCE ON THE COPPER 110 (0.016 INCHES THICK)...16 FIGURE 11: CLEARANCE INFLUENCE ON 1010 STEEL (0.032 INCHES THICK)...18 FIGURE 12: MATERIAL INFLUENCE ON BLANKING LOADS...19 FIGURE 13: GRAPH OF EQUATED VERSUS ACTUAL CUTTING FORCE...20 LIST OF TABLES TABLE 1: LOURDES 100-OH PRESS SPECIFICATIONS (ACCORDING TO THE MANUFACTURER)...4 TABLE 2: MATERIAL THICKNESSES /8/...10 iv

7 TABLE 3: PERCENT CLEARANCES FOR DIFFERENT PUNCH DIAMETERS AND MATERIAL THICKNESSES LIST OF EQUATIONS EQUATION 1: BLANKING FORCE BASED ON THE SHEAR RESISTANCE OF THE MATERIAL...3 EQUATION 2: ENERGY EQUATION...5 EQUATION 3: MATERIAL CLEARANCE...10 v

8 1. INTRODUCTION The goals of the project were: to study the influence of process parameters on the blanking loads, evaluate the accuracy of the blanking force equation for high velocities, and to further explain the mechanisms that are responsible for the benefits of high speed blanking Blanking The definition of blanking is the cutting of parts out of sheet material to a predetermined contour /1/. In blanking, the slug is the part and the material skeleton is considered scrap. Figure 1 is an illustration of the blanking process. punch motion stripper punch workpiece die Figure 1: Schematic illustration of the shearing process At the start of blanking the material experiences elastic and plastic deformation as it compresses. The sheet material is then sheared until a high enough force is reached and the material fracture. By physically measuring the blanking forces (load) versus the 1

9 punch depth (stroke) into the material, we have a better understanding of how different materials react to changes in velocity and clearance The Load-Stroke Curve The forces involved in the blanking process are best represented in a load-stroke curve such as the one seen in Figure 2. The load may be graphed versus time, crank angle, or punch penetration. The curve can be divided into the following steps: step 1 represents the elastic deformation of the material; step 2 represents the plastic deformation (rollover); step 3 is the shearing of the material; and step 4 is the fracture. Figure 2: Theoretical load-stroke curve 1.3. Blanking Parameters The following process parameters have the most influence on part quality /2/, /3/, /8/. punch - die clearance punch velocity 2

10 stock material (thickness, mechanical properties, chemical composition, microstructure and grain size) cutting tools (materials, cutting edge, tool wear) lubrication alignment of the tools and strain rate Shear Resistance To calculate the necessary blanking force of a material, the following standard utting force equation is used when blanking with a circular disk /4/. F c = π d t σ F c = blanking force s σ s = shear resistance d = disk diameter t = sheet thickness Equation 1: Blanking force based on the shear resistance of the material Though the blanking force equation does include the shear resistance of the material and the tool geometry, it does not take the blanking velocity into account. While experiments have found this equation to be acceptable accurate at low velocities, materials react differently at high velocities. Therefore, this project seeks to verify if the cutting force equation based on shear resistance of the material is still accurate at the ultra high velocities of the Lourdes Press Strain Hardening For low-speed blanking, most metals will strain harden as the deform. For ordinary steels, the shear surface will increase in hardness by a factor of about three as a result of blanking /4/. As the punch travels further into the material, strain hardening results in a 3

11 wider shear band which is very brittle. The overall results are lower part quality and higher loads on the tooling. Therefore, one goal of the project was to investigate the influence of high velocities on the strain hardening problem. Higher velocities result in high strain rates (in/in per sec) on the material. As the punch velocity and strain rate continue to increase, the yield strength of the material approaches the ultimate strength /5/. In theory, there should be an increase in the shear force at high blanking speeds due to an increase in strain hardening. 2. EQUIPMENT AND EXPERIMENTS 2.1. Press In preparation for the experiments, the punch and dies were sharpened and aligned, the press adjustments optimized, and the sensors were calibrated. The press used to conduct the trails was the Lourdes 100-OH Electromagnetic Press. Table 1 gives the specifications of the press: Force (0.030 above bottom) 10 tons Overall dimensions 10 x10 x19 Maximum work area 4.5 x10 Stroke length 0.5" to 1.5 Open height 5.0 Shut height 3.5 Approx. weight 100 lbs Approx. weight of untooled top plate 35 lbs Table 1: Lourdes 100-OH press specifications (according to the manufacturer) The press itself (as seen in Figure 3) is very small in comparison to most presses used in blanking. The standard press operates a system of clutches applied to kinetic energy 4

12 storing flywheels (of considerable size and mass) to generate the energy needed to punch through the material. This energy can be calculated with the following equation: E=½mv 2 with E = energy [J] m = weight of the moving mass [kg] v = velocity of the moving mass [m/s] Equation 2: Energy equation The standard press uses a large amount of mass at low velocity of around 0.5 ft/sec [0.15 m/s] to punch through the material. The Lourdes Press instead uses a small amount of mass (35 lb. [ 16kg] in the motion plate) at velocities as high as 12 ft/sec [3.7 m/s ] to create the same force over a much smaller time increment. Figure 3: The Lourdes Press 100-OH 5

13 The manufacturer describes the electromagnetic system which allows the press to operate at such high velocities as follows: The Lourdes Electromagnetic Press uses tractive Solenoids, comprised of a coil structure, a ferro-magnetic flux path, and an Armature, to accelerate the motion plate. The magnetic accelerator mounts directly to a precision die set /6/. The microprocessor control precisely energizes the accelerator causing the punch to be rapidly accelerated towards the die. The control regulates the tool speed and disconnects the driving forces just before the tool impacts the material. The kinetic energy or momentum of the moving tool holder is converted to work as the tooling impacts the material. Finally, any unused energy is absorbed by the urethane stops and the tool holder is converted to work as the urethane stops and the tool holder plate is returned, aided by spring force, to the initial position /7/. The tooling for the Lourdes Press consisted of a set of punches and die buttons of varying diameters. These punches and dies are mounted in ball-lock retainers to the base and motion plates of the press. In order not to influence the load-stroke curves, the urethane material stripper was removed from the punch Shut Height The shut height of the Lourdes Press is the distance between the top of the plate and the bottom of the tooling plate. It is not possible to adjust the shut height of the press without seriously affecting the press dynamics. The reason for this is a coil system which disconnects the power a set height above bottom bead center. One problem to overcome was the additional height needed to incorporate the load cell under the tooling. The solution was to shim the entire press to maintain the design specified shut height. These shims and the new plate that contains the load cell had to be designed and manufactured to very tight tolerances to maintain tooling alignment. 6

14 Stroke Length The stroke length for the experiments was considered to be the distance between the top dead center and the bottom dead center of the tooling. The stroke length can be adjusted with the urethane return stops. For the experiments a stroke length of -1.5 inches was used Sensors Figure 4 illustrates how the load cell and proximity sensor were incorporated into the press. Figure 4: Sensor implementation 7

15 Load Cell To directly measure the loads involved in the blanking process, a Helm Instruments low profile (strain gage based) compression load cell was implemented underneath the die button. The die button was free to move along the vertical axis and transmitted essentially all the force (except for the small amount of friction) to the load cell. This ring-shaped load cell allows the slug (part) to fall through. It has a 2 ton capacity which corresponds to the output range of Volts. The signal in terms of voltage is transmitted via a shielded cable to a Helm SCM-4 four channel interface which consists of: Signal conditioner filter cutting tools (materials, cutting edge, tool wear) amplifier PLC interface module Analog output The signal broad has to be calibrated in order to get an accurate tonnage range Proximity Sensor An Indikon inductive proximity sensor was used to measure the entry depth of the punch into the material. The signal was connected via a shielded cable to an Indikon signal oscillator/demodulator. After going through a voltage regulator, the signal was transmitted to the data acquisition system. Described below are the operating characteristics of the sensor: The sensor (resolution: ", linearity: ± 4%, range: 1") was mounted to the base plate and was detecting the motion plate (Figure 3). An inductive proximity sensor consists of a coil and a ferrite core arrangement. The oscillator creates a high frequency field, radiating from 8

16 the coil in front of the sensor, centered around the axis of the coil. The ferrite core bundles and directs the electro-magnetic field to the front /8/. When a metal object (target) enters the high-frequency field, eddy currents are induced in the surface of the target. This results in a decrease of energy in the oscillator circuit and, consequently, a smaller amplitude of oscillation /9/. Many static and dynamic trails were completed to verify the accuracy of the proximity sensor data. Within an inch of the target the sensor was extremely accurate and linear in its voltage response. The proximity sensor provided the punch position which was converted into the stroke on the load-stroke curve. Since the materials blanked were of different thicknesses, the load-stroke curves had to be adjusted to achieve the start of the blanking at the zero inch mark Data Acquisition System The two sensors were connected to a multi-channel data acquisition system. The data acquisition PC was equipped with a National Instruments AT-MIO-16F-5 data acquisition board in conjunction with the National Instruments LabVIEW software used for real time monitoring of the sensor signals up to 200 khz /8/. Though background interference was a problem, it was eliminated by carefully shielding and grounding all the wires. The data in terms of volts was collected by a specially written Lab VIEW program at rates of 100,000 data points per individual experiment. This data was later exported to Excel for interpretation Materials In Table 2 the materials that were selected for the experiments are listed. They were chosen for their wide range of properties and for the commonality of their use in industry. Each material came in two inch strip which manually fed through the press. 9

17 Table 2: Material thicknesses /8/ Punch and Die Clearances The punch-die clearance is defined as a relative clearance per side in percent of the material thickness (equation (3)) /7/, /8/. d c = d d 2t p 100% (3) Equation 3: Material Clearance c radial clearance [%] d d diameter of the die d p diameter of the punch t material thickness For 1010 steel, three punch-die clearances of 5%, 14%, and 30% were used to show the influence of clearance on blanking. For the other three materials a clearance of 5% was selected as the standard. Table 3 lists which punch and die button diameters were used to obtain these clearances. Table 3: Percent clearances for different punch diameters and material thicknesses. 10

18 2.6. Punch Velocities The punch velocities are based on the selected power level and the stroke length. The graph in Figure 5 was used to determine the velocities for the experiments. These values were confirmed at the start of the experiments with a velocity transducer and proved to be accurate. 15 velocity [ft/sec] stroke length 1.5" stroke length 0.5" power level Figure 5: Punch velocity versus the power levels of the Lourdes Press depending on the stroke length. No cutting condition /8/. 3. RESULTS For statistical purposes, each setup of parameters had a minimum of seven data trails. Once collected, the range of data in which blanking occurred was imported into Microsoft Excel Spreadsheets. Next it had to be converted (based on the static and dynamic calibration tests) from voltage data to inches and tons. Finally, this data had to be processes to produce the load-stroke curves. Figure 6 contains three actual load-stroke curves of the same parameter combination. The magnitude and shape of the curves are repeatable enough to furnish reliable test results. 11

19 Figure 6: Experimental load-stroke curve 3.1. Velocity Influence The main emphasis of the load-stroke curves developed for this project was to study the overall force difference between low and high speed blanking. Within this selection are the velocity influences on four different materials at a 5% punch-die clearance Steel Figure 7 shows the load-stroke curves for 1060 High Strength Steel at four different velocities. This material is inches thick and has the highest shear resistance of the four materials. The zero inch mark is where the punch contacts the material. Fracture occurs approximately at the point where the curve begins to decline sharply. 12

20 Figure 7: Velocity influence on 1060 Steel (0.054 inches thick) By comparing the curves for the low velocity of 0.5 ft/sec and the higher velocities, a lower maximum force is seen. The difference between 0.5 ft/sec and 12 ft/sec is approximately 0.1 tons or 200 pounds on a slug with a surface area of less than 0.2 in 2. These results suggest lower blanking forces at high velocities Steel The load-stroke curves for four separate velocities with 1010 steel are shown in Figure 8. This material is inches thick and is softer than the 1060 steel in Figure 7. 13

21 Figure 8: Velocity influence on 1010 Steel (0.032 inches thick) 1010 Steel appears to exhibits more plastic deformation at high velocities resulting in the maximum load being reached later in the stroke. The decrease in the maximum force between 0.5 ft/sec and 12 ft/sec is 0.05 tons. This is much smaller than the 0.1 tons decrease for 1060 Steel Al 2008 The Aluminum 2008 strips were inches thick and had lower shear resistance than both of the steels. 14

22 Figure 9: Velocity influence on the Al 2008 (0.041 inches thick) The maximum load decrease was approximately 0.13 tons when increasing the cutting speed from 0.5 ft/sec to 12 ft/sec. Again, the trend for a lower maximum force at high velocities is seen CU 110 The Copper 110 material was inches thick and had the lowest shear resistance of the four materials. 15

23 Figure 10: Velocity influence on the Copper 110 (0.016 inches thick) As with the other materials, Copper 110 has a lower maximum force when blanking with high velocities. The decrease when comparing 0.5 ft/sec to 12 ft/sec was 0.25 tons. Also, it could be seen that less punch penetration until material fracture is needed, which represents less blanking energy expended. The two obvious similarities in all of the load-stroke curves are the lower overall blanking forces and the change in the shape of the curves for high speeds (7-12 ft/sec). An explanation for these results lies in a change in material properties at high speeds. Dr. Olov Svahn found high speeds are more effective because: when force is applied, it propagated throughout the crystal structure as elastic waves. Deformations generally develop at the sites of dislocations, where atoms are bound with much less force than atoms in perfect crystals. However, research has shown that if the velocity exceeds the speed of 16

24 sound in the material, the dislocations cannot react plastically. Instead, the metal simply ruptures /1/. This rupturing of the metallic lattice from the initial shock wave could explain why the high-speed load-stroke curves have lower maximum forces and altered shapes. Another theory is that high tool velocities generate higher temperatures in the localized blanking area. The metal and the tooling are not capable of dissipating the heat generated in the shear zone fast enough. This results in a stress relief in the metal which lower the shear resistance /8/. A combination of these two mechanisms may be responsible for the lower blanking forces and the different shapes of the curves. What seems apparent is that the strain hardening effect seen at normal speeds is being offset (even at the higher strain rates) by high speed blanking. The benefit is less force on the tooling and on the press. 17

25 3.2. Clearance Influence Figure 11: Clearance influence on 1010 Steel (0.032 inches thick) Increases in clearance had the effect of lowering the maximum force on the material. Also, it appears that the process occurred over a longer stroke as the clearance increased. An increase allows for more material to deform farther into the die cavity (more plastic deformation) and prolongs the blanking process Material Influence The material properties obviously have an enormous influence on the blanking process. Figure 12 is a comparison of the force needed to blank different materials. As stated before, the start of blanking has been adjusted for the different material thicknesses. 18

26 Figure 12: Material influence on blanking loads 4. ACCURACY OF BLANKING FORCE EQUATION In Figure 13, the blanking force equation based on the shear resistance of the material appears to still be accurate for high velocities. 19

27 Figure 13: Graph of equated versus actual cutting force Though the load calculation based on the shear resistance is most accurate for thinner, softer materials, it can still be used when designing dies for high speed applications. 5. SUMMARY AND CONCLUSIONS The project produced the following results in regard to the influence of velocity on the blanking force: The maximum blanking force is measurably lower at the high velocities. With present tooling geometry, the difference in force between 0.5 ft/sec and 12 ft/sec for 1060 Steel and Al 2008 is about 0.1 tons or 200 pounds. Less change is seen for 1010 Steel and CU 110. Also, there is a change in the shape of the load-stroke curves for the different velocities. This represents a variance in the part-forming zones (deformation, shear, fracture) of the blanking process at high speeds. 20

28 To explain the dissimilar fracture mechanisms of low and high speed blanking, the shock wave fracture theory and the localized temperature increase theory were introduced. While both are reasonable, each has to be investigated further to draw any conclusions. The effect of larger punch-die clearance on 1010 Steel is to marginally lower the maximum blanking force. Additional investigations with other materials should confirm that the penetration of the punch and into the material is prolonged at large clearances. The blanking force equation that is based on the shear resistance of the material is acceptably accurate for predicting high speed cutting loads in 1010 Steel, Al 2008, and CU 110. For 1060, the thickest and hardest material, the equation is less accurate. Whether this is the influence of the material properties or of the thickness is not known. Previous investigations into the part quality improvements for high-speed blanking showed favorable results. In addition, the required loads are decreasing, which makes switching to high-speed blanking a logical step. 21

29 6. LIST OF REFERENCES /1/ Svahn, O. Super-fast blanking prevents defects, Pressworking Industry Quarterly, Volume 8, No. 4, /2/ Lascoe, O.D. Handbook of Fabrication Processes, ASM International, /3/ Lange, K. Blanking and Piercing, Handbook of Metal Forming The McGraw-Hill Book Company, /4/ Aida Engineering, Aida Press Handbook, Ltd. Third Edition, /5/ D. Schoch Dynamic Effects Created at High Press Speeds, Minster Machine Company Press/Die Application Description Form. /6/ N.N. Operating Instructions for Lourdes Electro Activated Press die sets overhead units Lourdes Systems, Inc. /7/ N.N. Brute Force vs. High Tool Speed Bulletin, Lourdes Systems, Inc. 22

30 /8/ Grunbaum, M. Influence of High Cutting Speeds on the Quality of Blanked Parts ERC Report S-96-19, OSU, /9/ N.N. Sensors catalog Turck Inc.,