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Chapter 3: Equipment description and preliminary testing 3.1 Introduction
In this chapter a detailed description of the apparatus used is given and modifications to the existing rig are explained.
Also, the preliminary testing carried out during the onset of the research is summarised and the results obtained are shown in the form of graphs.
The parts made in the workshop including the roller and formers are explained in detail with drawings.
Later, in chapter 6, the ranges of the suitable spindle speeds and carriage . feeds were selected after consideration of software requirements.
3.2 - Description of the experimental equipment
A lathe was needed to perform the flow-turning process. The one chosen in the workshop was manufactured by Dean Grace & Smith with 33 cm swing. There were 12 spindle speeds available, ranging from 16.8 to 750 rpm and 48 carriage feed rates, ranging from 0.226 to 12.7 mm per revolution. Plate 3.1 shows a general view of the equipment used. It was assumed that with this type of lathe the process can be carried out although the lathe is not specialised for this job but can be converted to handle this task.
3.3 - Modification to the existing rig
In order to make flow-turning possible, some components had to be made and added to the lathe. Alterations included the following
1- Modifications to accommodate the workpiece and the roller (these will be explained in this chapter).
2- Other modifications, which include attaching the transducer to the cross-slide so as to measure roller movement, selecting and fabricating the ADC and the DAC boards, mounting the shaft encoder on the leadscrew end and making the counter board; installing a relay and the necessary circuitry on the lathe electric starter so that the microcomputer SDK-85 could remotely control the ON/OFF starter. These will be dealt with in chapter 4 (the hardware).
The tooling for shear spinning consists of the mandrel, tool rings or rollers, and the tracing templates. As the mandrels must be harder than the material in the finished part, they must have a compression strength of 200,000 lb/in square. Tool steel that can be hardened to this or a higher strength is easily obtained, but for large mandrels, the cost is significant. A very satisfactory material at lower cost is high-strength nodular iron which may be cast to shape and hardened to the required strength.
The tool rings or rollers must be of high-grade tool steel in order to obtain miraimum hardness of 62 Rockwell C. This hardness is necessary to resist wear and scuffing(9).
To position the workpiece in place, it was considered best to fix by using the tailstock as a support. The tailstock was provided with a dummy rotating centre which was made specifically for this purpose to hold the blank against the mandrel end by exerting pressure on it. The centre was provided with a pig in the middle, which passes through the centre hole of the workpiece.
One thickness of commercially pure aluminium sheet (BS 1470/SIC) of 1.6 mm was used. The aluminium sheets were cut into discs of 100 mm
diameter, each with a hole drilled at the centre. High pressure grease was used as the lubricant throughout the study.
Chucks or formers are often made of cast iron, but when forming high-tensile materials or producing large quantities, chucks of hardened and ground steel may prove more economical. Generally the quality of surface finish obtainable is related to the surface finish on the chuck itself, imperfections in the chuck frequently being transferred to the work under the very heavy pressure applied(lO).
The former was made in the shape of a cone with 10 degrees semi-angle (see fig 3.1), and was used in the initial testing. It was made of steel EN8, which can withstand the compressive pressure imposed by the roller. Another former with 30 degrees semi-angle was made at a later stage of the research for the final testing (see fig 3.2). This was however made from Meehanite.
Meehanite castings satisfy the requirements for a good mandrel material. Such castings are noted for their fine grained structure, which ensures dependability and freedom from casting defects. A uniform structure provides good machinability with a compression strength of 80 ton/in square in the as-cast condition. Moreover, Meehanite type ’ GA’ is able to resist the extreme high level of external forces applied to the surface of the mandrel during spinning operations(20).
At a later stage of the research it was decided to implement the parabola contour (see figs 3.3 and 3.4). A parabolic former was made in a later stage of the work on a CNC lathe, retro-fitted with an AUDIT M268 controller. The program was generated using software package called ’path turn’ supplied by ’path trace’ Ltd. The parabola was generated with the loop command within the geometry section, this was then processed by the machining section to give tool movements in order to cut the parabola. This data was then post-processed to suit
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