Steady Rest Research Articles

A Method of Increasing the Accuracy of Low-Stiffness Shafts: Single-Pass Traverse Grinding Without Steady Rests

Abstract The article presents a method of increasing the shape and dimensional accuracy of low-stiffness shafts manufactured in a single pass of a grinding wheel in traverse grinding. One-pass manufacturing is one of the ways for reducing machining time and increasing efficiency, thus lowering production costs. However, maintaining the necessary accuracy proves to be a challenge because the whole machining allowance has to be removed at once, leaving no room for errors that could be fixed in additional passes of the tool. It is especially true in finishing operations, such as traverse grinding. In addition, grinding the workpiece in a single pass of a grinding wheel leads to high forces, which cause elastic deformation of the part. The lower the stiffness of the part, the more difficult it is to achieve the required accuracy. As a result, there are many methods of improving the accuracy of grinding such parts, but they tend to be either expensive or reduce the machining efficiency. Thus, it is important to seek new methods that would allow improving the accuracy of the machining without reducing its efficiency. The proposed method does not require using steady rests and is based on the measurement of the normal grinding force component. Knowing the value of the grinding force when grinding with a set grinding depth, the elastic deformation of the machine tool–tool–workpiece system is calculated in each position of the grinding wheel. Based on the calculated deformation, the additional infeed of the grinding wheel is implemented in order to stabilise real grinding depth and to increase the accuracy of the produced part. The experimental tests were conducted to prove the effectiveness of the proposed method.

Improvement of Shape Error for Slender Parts in Cylindrical Traverse Grinding by Part-Deformation Modelling and Compensation

Achieving geometrical accuracy in cylindrical traverse grinding for high-aspect slender parts is still a challenge due to the flexibility of the workpiece and, therefore, the resulting shape error. This causes a bottleneck in production due to the number of spark-out strokes that must be programmed to achieve the expected dimensional and geometrical tolerances. This study presents an experimental validation of a shape-error prediction model in which a distributed load, corresponding to the grinding wheel width, is included, and allows inclusion of the effect of steady rests. Headstock and tailstock stiffness must be considered and a procedure to obtain their values is presented. Validation of the model was performed both theoretically (by comparing with FEM results) and experimentally (by comparing with the deformation profile of the real workpiece shape), obtaining differences below 5%. Having determined the shape error by monitoring the normal grinding force, a solution was presented to correct it, based on a cross-motion of the grinding wheel during traverse strokes, thus decreasing non-productive spark-out strokes. Due to its simplicity (based on the shape-error prediction model and normal grinding force monitoring), this was easily automatable. The corrective compensation cycle gave promising results with a decrease of 77% in the shape error of the ground part, and improvement in geometrically measured parameters, such as cylindricity and straightness.

The study of the curvature of the axis of the processed shaft when based in the centers using a steady rest

This paper presents the analysis and results of the study of a two-stage shaft fixed in dead centers and a rest device. Cutting forces act on the shaft, causing a bending moment. The analysis of the curvature of the axis of the workpiece, processed on a lathe when based in the centers, using a rest device, is carried out. The formula of elastic displacement of the workpiece axis at the place of the resulting cutting force is obtained. Diagrams of the stresses of the shaft axis displacements are constructed and a conclusion is made. The analysis showed that the greatest deflection is at the point of the cutting forces acting on the shaft. The quality of the part obtained after processing is characterized by accuracy. The parts mating in the product and, as a result, the overall reliability depends on how accurately the size and shape of the part will be maintained during processing. Parts with length of 10 to 12 times larger than the diameter are bent under the action of their own weight and cutting forces, as a result of which they get a barrel-like shape. It is possible to eliminate this by applying special devices for the machine. When processing long nonrigid workpieces, the tools, jigs and fixtures must evenly distribute the clamping force over the surface of the part. These conditions are well provided by technological equipment with pneumatic, hydraulic clamping devices, as well as with various collet clamps, split bushings, diaphragm or cartridges. When processing long non-rigid shafts, rest devices are used. The rest device plays the role of the main or secondary support when working with workpieces; it creates support for large, long parts during processing. It helps to avoid the risk of damage and deformation of the workpiece or the cutting elements of the machine, by giving the workpiece additional stability

Prediction of angle error due to torsional deformation in non-circular grinding

Abstract. Non-circular grinding is used in the grinding of crankshafts. In contrast to general grinding, the precision in non-circular grinding is affected by torsional deformation, which results in errors in the grinding depth. In this study, an equation to detect the angle error caused by torsional deformation is established considering the grinding force, the structure of the crankshaft, and the distribution of torque. The angle error due to torsional deformation was found to be up to 0.44 arcsec, which is 5 % of the angle error obtained from previous studies. This difference occurred as the previous studies did not exclusively detect the errors caused by bending deformation and torsional deformation. However, the established equation detects these errors separately. The fundamental cause of the two errors is the change in the structure of the crankshaft caused by bending. Further, the errors were eliminated via steady rest to reduce the bending of the crankshaft. Although the proposed equation is not entirely error-free, the results obtained by the equation have higher accuracy than those of previous studies.

Geometric stability of stepped shafts by ribbing its surface

The article discusses the technology of manufacturing low-stiff long multistage shafts made of stainless steels. A feature of this technology is the cutting of annular cooling fins on the entire surface of the shaft blank during machining and their use in thermal operations and further to finishing turning. To implement the technology, self-centering steady rests with double rollers were used during machining.

Improvement of form accuracy in cylindrical traverse grinding with steady rest by controlling traverse speed

Improvement of form accuracy in cylindrical traverse grinding with steady rest by controlling traverse speed

Determination of cutting forces based on DMG MORI CTX300 ecoline CNC lathe drive power data

Modern machine building needs demand for machines and control systems which provide higher accuracy of machining. For this purpose, setup methods are being researched and accuracy of machines and devices is being improved. The main topic of this paper is a tool path predistortion system which is necessary for improvement of shaft turning accuracy when a steady rest cannot be used on a CNC machines. Calculation of such tool path requires one to know the cutting force during turning on a CNC lathe. The research in question allowed to determine the dependence of cutting force during turning on the power consumed by the feed drive, cutting depth, cutting velocity, and feed value. An equation for further calculation applicable to a specific lathe in the turning process was derived. This equation serves as basis for adaptive turning of shafts and other parts with the aid of a mathematical model, which must account for the cutting force. Application of this formula in a CNC controlled lathe’s parametric program will provide for higher accuracy of turning without special devices.

Stabilization of the axisymmetric parts dimensions and shapes

This article reviews the method of increasing the stability of geometrics of non-rigid parts of the “shaft” type. The authors give the grounds for the necessity of treatment by the complex vibro-impact method. The authors also present the unit functional diagram, complete with the automatic control system for the implementation of the proposed method. The unit consists of a self-centering steady rest with a non-flow electrohydraulic drive; a vibro-impact drive with a waveguide converter, a workpiece temperature control device. The control system provides the workpiece axis stabilization and temperature control in the process of axial complex vibro-impacts. The article proves the expediency of using the complex vibro-impact method.

Studi Eksperimental Hubungan Feeding di Mesin Bubut CMZ T-360 dengan Kekasaran Permukaan Material St 60 untuk Shaft Steady Rest

The research tested the effects between feeding and surface roughness on shaft of steady rest which will be worked in CMZ T-360’s lathe machine. Material of St.60 was turned into shaft of steady rest in 2 CMZT-360’s machines with several of feeding like 0.045, 0.056, 0.068, 0.079, 0.90 and 0,102 mm/rev. Then the work piece was tested the surface roughness of the work piece with using roughness tester and being seen the shape of the surface with using photo macro. The result of the roughness’s test showed that CMZ-T360’s lathe machine could reach Ra 1.55 µm until 4.79 µm with several of feeding. The work piece would be assembled in steady rest and being used to support 58 kg weigth in 2 hours to know the effect of surface roughness. After testing on steady rest, would be done photo macro on the specimen to make easy on visual analysis. From the data which is obtained can conclude that feeding variation which used on this research not so affecting the quality of surface roughness of the workpiece, the value of surface roughness that resulted from this research is mostly N8 (3,25 µm- 5,29 µm) and only a few that valued N7 (1,55 µm-2,53 µm) . The second conclusion is the surface roughness which quickly wornt-out after assembled at steady rest is the rough surface (4,79 µm become 3,84 µm) compared with the smooth surface (2,23 µm become 2,03 µm).

Application of root cause analysis to increase material removal rate for productivity improvement: A case study of the press manufacturing industry

The bore machining of a large size cylindrical job on a lathe machine is a challenging job. Due to the large diameter of job or financial constraints, it is not always possible to use the steady rest. This results in the overhang of a job, which directly affects the material removal rate. This paper discusses the use of lean manufacturing in a press manufacturing industry to improve productivity through increased material removal rate of boring operation. A flange fixture is developed and it has reduced the machining time by 9.5 h.

Small Diameter-Deep Hole Drilling for Hard-to-Machine Metals ― Drilling of ϕ1.0×400 mm for Ti6Al4V ―

Small diameter-deep hole drilling is indispensable when several main parts of which airplane, automobile, machine tool and metal mould are manufactured. Especially, medical appliances, medical implants and surgical operation tools for brain and bone invariably need small diameter-deep hole drilling for hard-to-machine metals such as stainless steel and Ti6Al4V. The authors newly designed and manufactured a small diameter-deep hole drilling machine by applying technological developments which consist of a direct drive servo spindle motor controlled more precisely, removable steady rests to prevent for a slender fluted hollow shaft of gun drill to be deflected and a highly pressure circulation system of cutting fluid to prevent being clogged with chips. By using the small diameter-deep hole drilling machine and a special gun drill (specially made to order in BOTEK) a drilling of ϕ1.0×400 mm (L/D=400) for Ti6Al4V was carried out. As a result, it was ascertained that the drilled hole has high accuracy in the items of hole diameter, concentricity, roundness and inner surface roughness.

Improvement of the Form Accuracy of a Slender Workpiece in Cylindrical Traverse Grinding

During the cylindrical traverse grinding of a slender workpiece, the ground workpiece is easily bent by the normal grinding force owing to its low stiffness. Therefore, it is difficult to finish the slender workpiece with high accuracy. To prevent the elastic deformation of a workpiece during the grinding process, a steady rest is generally used. However, considerable skill of the worker is required to use a steady rest. Therefore, we developed a new traverse grinding method without any steady rest. In this method, the elastic deformation of a workpiece was kept constant by controlling the traverse speed of the workpiece. At the middle of the ground workpiece, where the elastic deformation increased easily, the traverse speed was slowed down. However, this method had a longer grinding cycle time because the average traverse speed decreased compared to that of the conventional method. To shorten the cycle time, the peripheral speed of the grinding wheel was increased to decrease the normal grinding force. Basic grinding experiments were carried out under several grinding conditions by changing the peripheral speed of the wheel. From these grinding experiments, it was confirmed that the normal grinding force and the form error of the ground workpiece decreased as the peripheral wheel speed increased. By using results obtained from basic experiments, grinding experiments involving changes in the traverse speed were carried out at two peripheral wheel speeds. The grinding cycle time was reduced successfully by increasing the peripheral wheel speed without an increment in the form error of the ground workpiece. Furthermore, a form error was observed at the end of the workpiece where the grinding wheel traveled away from the workpiece. The form error occurred because the normal grinding force decreased rapidly when the contact length between the workpiece and the wheel was decreased at the end of the workpiece. To prevent rapid changes in the normal grinding force, the traverse speed of the workpiece was increased at the end of the workpiece. By using this method, a ground workpiece with high form accuracy was obtained.

Multi Response Optimization of Turning Parameter Power Screw Hospital Bed Using Taguchi-Hybrid Method

Power screw is the driving part of a hospital bed which functions to raise and lower the bed. Power screw is an important part of hospital bed products that requires precision and accuracy, then in the manufacturing process by using CNC Turning machine with steady rest. Based on the observations, the defect of power screw caused by worn out tools. It causes chatter on CNC Turning machine. The aim of this study is to reduce the variation of power screw product by conducting experiments in the manufacturing process of power screw. The experiment was done by determining some factors such as spindle speed, depth of cut, coolant and usage time of tool. The specific purpose of this experiment is to reduce variation and simultance optimize are major diameter, minor diameter, pitch diameter, and cycle time. Taguchi multi response method was used for experiment to get simultance optimize process. Multi response case was done by using Grey Relational Analysis (GRA) and Principal Component Analysis (PCA). Experiment based on the orthogonal array of L9 with 3 replications. The results show that by using Taguchi-hybrid method, the optimum condition occurred at spindle speed is 1200 mm/min, depth of cut is 0.1 mm, coolant is 1:30, and usage time of tool is 70%. The result of the experiments carried out showed a decrease in the response variance in diameter and the cycle time faster.

Investigation of the Deep Hole Drill Stability When Using a Steady Rest

Rotary cutting tools, which are relatively long and thin spindles for deep hole machining turn out to be unstable when the cutting force FP reaches a certain critical value Fkp The behaviour of the deep hole drill stability when using a steady rest is investigated; and a diagram of the critical parameters Ic-D showing the boundary of the drill stability zone when using a steady rest located in the centre of the shank bit is represented.

Error Compensation Using Variable Stiffness in Orbital Grinding

Orbital machining is normally used for a crankshaft pin because it is difficult to process with the usual machining method. Deformation of the crankshaft in orbital grinding changes continually because of the shape of the crankshaft and the continuously changing relationship between the grinding force and the steady rest. This causes machining error. In this study, the relationship between grinding force and stiffness in orbital grinding was established and the deformation due to the grinding force was predicted. By compensating for the predicted deformation, machining error is reduced to 24% of the average deformation before compensation.

Robot Assisted Machining of Thin-Walled Structures

This paper proposes and studies the use of industrial robot manipulators to act as an active steady rest in machining thin-walled workpieces to prevent machining chatter instability. Feedback control is applied to the robotic steady rest in contrast to the other more popular active control method where the cutting tool is controlled. The entire dynamic system model is characterized by delay differential equations with time varying interactions between the tool side and work side dynamics. Semi-discretization, finite element modeling, and model reduction are applied to obtain a lower order plant model for the feedback control design. Control design and simulation results on turning of thin-walled tubes are presented to demonstrate the effectiveness of the proposed modeling and control approach for the robotic active steady rest.

Analysis of the Process of Turning of Low-Rigidity Shafts

A machining station, working together with a lathe, was designed and constructed for the stabilisation of the axis of low-rigidity parts in the process of machining. The basic element is a self-centring lunette with hydraulic drive, allowing part centring without any preliminary alignment. The methodology of calculation of the parameters of the “semi-finished product/supports” subsystem is given. The effect of various schematics of support arrangements on the dynamics of the semi-finished product was analysed. The analysis of schemes of low-rigidity part fixing in the device for the stabilisation of its axis was conducted with the method of initial parameters. The choice of supports and the positioning of backstays and/or buffering steady rests was also analysed for increasing the stationary character of the process of machining and the quality of shaping the machined surface.

Precise alignment method of the large-scale crankshaft during non-circular grinding

A large-scale crankshaft of internal-combustion engine is easy to bend and twist when clamped onto the grinding machine. The deviation of workpiece axis from its optimal machining axis has a significant influence on machining accuracy of angle, eccentric throw, and diameter and contour of the heavy crankshaft in non-circular grinding. To reduce the consumption of manual labor and setting-up time, an automatic alignment approach and apparatus is proposed and integrated into the non-circular grinder. The on-machine gauge senses X and Y components of deviation of crank journal axis and feed them back to the computerized numerical control system. The motor-driving steady rests based on slider-crank mechanism are controlled to compensate for the deviation. The algorithm with self-correcting ability for compensation value is employed to make up for the finite contact stiffness of workpiece and steady rest. The results of three alignment tests are compared, which demonstrates the better effect on alignment precision and efficiency of the self-correcting compensation.

The Design of the Fixture for the Thin Wall Cone Parts

When turning and grinding the long axis with cone thin-walled, they are clamped to be deformation easily and positioned difficultly. Based on the existing theory of clamping with the conical sleeve, we design a new type of fixture in this paper. The outer cone of the fixture cling to the inner cone of the workpiece, it is sealed hermetically at the both ends. The head of the sleeve stretch out of a cylindrical boss, we can use the three-jaw chuck to position it on the machine tool. Also we use the steady rest to fix the end of the sleeve. The steady rest connect with the shell of a rotary joint. The end of the rotary joint screw with the sleeve, the other end of it is connected with a getter pump. It formatted a vacuum between the sleeve and the workpiece by the use of holes around the circumference of the sleeve. The utility model to the fixture used for thin wall conical parts with long shaft.

Analysis and Detection of Elastic Deformation of the Large-Scale Crankshaft in Non-Circular Grinding

Due to the discontinuous axis and the big ratio of length to diameter, the large-scale crankshaft is easy to generate the complex elastic deformation under the effect of the huge gravity and the clamping force on non-circular grinding machine. The distinctive deformation deteriorates the coaxiality of all the main journals and thus the grinding precision of crankpins in non-circular grinding. In this paper, the statics analysis of large-scale crankshaft has been conducted by using the finite element simulation. The overall deformation of crankshaft and the position offset of each main journal center without or with the auxiliary supports have been simulated and contrasted. The results manifest that the auxiliary support forces produced by the steady rests have the positive effect on the compensation for crankshaft deformation. On this basis, to satisfy the demand of position adjustment of the upper and lower support pads of servo-controlled open steady rest, the on-machine measurement method and device of the position of main journal center is proposed. And then the offset calculation model of the main journal center is deduced using the measured data. The measurement method and device are beneficial for the straightening of the crankshaft and the improvement of machining accuracy of crankpins in non-circular grinding.