Introduction to robotics .............. Part-3 (Automation)

Automated Portable Hammering Machine

Hammering is the most widely used industrial as well as construction activity. Hammering or screws, metal sheets, parts etc requires a lot of time and effort. So here we propose an automated hammering system that allows for fully automatic hammering process. This allows for accurate, fast and automated hammering wherever and whenever needed using a 12V battery. The person just needs to insert workpeice and start the hammering machine. This machine can be used for automatic hammering work as and when needed. We here use a dc motor in order to move the hammer. The DC motor consists of a pulley attached to it which is connected to a larger pulley for efficient power transfer and to increase torque. This large pulley is connected to a shaft that has a connecting rod attached to it. This rod is used to achieve lateral motion from the spinning shaft. We now connect the other end of hammer to this connecting rod through a mid swinging arrangement in order to achieve desired hammer motion with enough torque. We now use a suitable bed where workpeice can be placed.

The vibration characteristics of the machine structure during the machining process. In this article, an optimization of experimental modal analysis will be presented. The classical measurement chain to perform a modal analysis is always based upon the principle of excitation, signal transmission, signal detection, and signal analysis of results. The conventional method, wherein the excitation is effected by a modal hammer and the signal detection is done by an acceleration sensor, is now replaced by a process in which excitation is achieved via an automated modal pendulum and the signal detection by means of laser or acceleration. Within the framework of this research, there are two key elements that will be discussed in detail. The first element includes the motivation for the development of the pendulum and the aspired improvements of the new model. A prototype is tested and its performance is valuated. The second key element represents an experimental analysis of the performance, including a comparison between the conventional modal hammer and the developed modal pendulum. Here it should be shown that the repeatability of the hammer strikes of the pendulum is significantly higher than that of the conventional hammer. In addition, the adjustability of the force excitation is to be ensured.

The main goal of the project is to develop an automated modal hammer in order to characterize dynamic behavior of a milling machine and workpiece. In milling, machine tool vibration and workpiece wall vibration plays an important role concerning both workpiece surface quality and tool durability. The undesirable motion, which is often referred to as chatter, can result in wavy surfaces on the workpiece, inaccurate dimensions, and excessive toolwear. In order to decrease chatter and machine the workpiece in the stable zone, a modal analysis of the machine can beperformed. For the impact testing, a modal impact hammer can be used. With the aid of impact testing, the dynamic behavior of the milling machine can be characterized. Moreover, when the results of the impact test are simulated, stability charts for the machine and workpiece can be plotted. However, in order to achieve precise results from impact testing one should minimize every possible source of error. Although modal impact hammer measurements are quick, easy and inexpensive,there are several significant challenges to overcome when striving for an “adequate linear estimate” of the structural dynamic model.

Firstly, it is difficult to control either the force level or location of the excitation point. The input force and the excitation point can differ from measurement to measurement. Therefore, the impact usually cannot be exactly replicated. Changes in the input force and excitation point location mainly depend on the skills of the operator.

Another important problem which should be minimized is a phenomenon known as “double hit”. Ideally, when a structure is struck, the impact should consist of a single contact in order to ensure clean data. However, because the impact can occur so quickly, the structure may vibrate fast enough to hit the hammer again before the user pulls it away. This results in a double hit (or more) [7]. Double hits decrease the quality of the Frequency Response Functions. Minimizing the number of double hits sometimes takes a bit of practice with operator’s technique, and less dexterous operators may never be able to achieve single hits.

Due to the problems mentioned above, it is not easy to obtain reliable results from manual impact testing. The reasons behind the idea of developing an automated modal hammer can be simply summarized as:

I. to increase the repeatability of the process,

II. to obtain a single hit in every trial,

III. to have adjustable force

IV. to reduce the manual effort associated with the repeatability of the process,

V. to reduce time and cost (high repeatability and no double hits),

VI. to achieve an operator-independent process,

VII. to improve data quality for the simulation software,

The automated hammer developed for this project should meet the objectives mentioned above.

Components

• Pulleys
• Rubber Belt
• Shaft
• DC Motor
• Hammer
• Mounts & Fixtures
• Supporting Frame
• Joints & Screws

Development

Development of an automated modal hammer in the Institute of Machining Technology was started before this project commenced. However, the existing design couldn’t meet the requirements fully. Therefore, some changes to the existing design have been made and the automated modal impact hammer has been finished in order to meet the requirements listed above.

Fig. 1. Design of the general purpose automated impact hammer

3D modeling software was used in the development of the automated modal impact hammer, allowing changes to the design to be made quickly and easily. The model was then animated for the purpose of kinematic testing and refinement. An automated modal hammer based on the refined design was produced and used in experiments for the purpose of validation.

Firstly, one general purpose hammer was designed and then some modifications were made to it in order facilitate its use

with the 3 axis milling machine. Figure 1 shows the design of the general purpose automated impact hammer, which can be used for wide range of different workpieces or different machines. As can be seen in Fig. 1, there are four main groups of

elements in the design of the automated impact hammer, namely:  main body, driver cam, hammer and stopper. The details relating to each group of the design are outlined in the following sections.

The Driver Group

The driver cam group consists of an electric motor, a cam, a connection rod, a motor coupling, a motor flange, a ball bearing and two joints. It is connected to the main body via the bottom joint. Figure 2 shows elements of the driver cam group.

This group of elements gives motion to the hammer needed to produce the required excitation force pulse. In this project, a 24V DC electric motor is used. The electric motor produces the required rotary motion, which is transferred to the cam mechanism via the connection rod and coupling. The cam mechanism consists of two moving elements, the cam and the follower. The cam has a curved outline, which, by its rotation, causes the follower to move in a specified fashion.

Fig. 2. Elements of the driver cam group

The Hammer Group

The hammer group of the automated impact hammer works like a pendulum. It is connected the body with its joints. It consists of a cam follower, connection rods, a ball bearing, a bearing housing, a spring, a hammer housing and a hammer. Figure 3 illustrates the elements of the hammer group. The hammer group of the automated impact hammer works like a pendulum. It is connected the body with its joints. It consists of a cam follower, connection rods, a ball bearing, a bearing housing, a spring, a hammer housing and a hammer. Figure 3 illustrates the elements of the hammer group.

The hammer group is connected to the main body with its joints. It works like a pendulum. The cam follower follows the predetermined path on the cam untill the tip of the cam. During this motion, the hammer group stores potential energy against gravity. When the follower finishes its predetermined path on the cam, the potential energy converts to kinetic energy and the hammer hits the target structure. Table 1 shows the motion cycle of the hammer. The magnitude of the impact is basically determined by the mass of the hammer head and the velocity with which it is moving when it hits the structure. This is due to the concept of linear momentum, which is defined as mass times velocity. The linear impulse is equal to the incremental change in the linear momentum.

Fig. 3. Elements of the hammer group

Table 1. Motion cycle of the automated impact hammer.

Automated Modal Impact Hammer

The aim of developing an automated modal impact hammer was to use it to characterize the dynamic behavior of machines, workpieces and tools. The designed hammer can be

Fig. 4. a) Dimensions of the automated modal impact hammer relative to the

milling machine

Fig. 4. b) Experiments with an unusual tool attached milling machine.

used with wide range of these. However, it has some disadvantages related to the compactness of its design. It uses gravity in order to create excitation force on the structure. Therefore, it can only be used vertically. It is not possible to use it in other orientations. Also, there must be enough space in front of the target structure in order to clamp the automated impact hammer.

The designed automated modal impact hammer has been tested with different workpieces to meet the objectives of the project. However, it couldn’t be used with the Three Axis Milling Machine. The reason for the problem was the dimension of the automated modal impact hammer and the milling machine. It could only be used with an unusually long tool attached to the milling machine. Figure 4 shows the dimensions of the automated modal impact hammer relative to the milling machine and the measurement with the unusual tool attached to the milling machine. According to 3D drawings of the milling machine, some modifications were made to the automated modal impact hammer. Figure 5 shows the 3D model of the modified automated modal impact hammer and the milling machine’s spindle.

The same principles have been used with the modified automated modal impact hammer in order to achieve the objectives stated above. Only geometrical changes have been made, mainly to the driver cam group. Firstly, the main body was tilted 45° with the aid of clamping devices. The hammer group works like a pendulum. Due to the required motion of the hammer group in an inclined configuration, the upper profile of the main body group was obstructing the motion of the hammer group. Therefore, a 45° connection part was assembled to the driver cam group in order to increase the hammer's clearance.

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Refferences: Brüggemann, T., D. Biermann, and A. Zabel. "Development of an automatic modal pendulum for the measurement of frequency responses for the calculation of stability charts." Procedia CIRP 33 (2015): 587-592.