The magnetic sine bar is the sine bar that is hinged with a magnetic base to ensure the firm placement of the measuring object on the ferrous surfaces. The magnetic base is called a sine plate professionally. The sine bar is used to measure, mark and inspect the angles on dies and other precision parts. The sine plate of a magnetic sine bar works similar to the magnetic chuck. It creates magnetic fields to attract the metal work piece so that it can stay firmly on the sine plate for measurement. The use of the magnetic sine bar is very common in lathe machining when the workings to the work pieces require accurate measurement as in the angles to take and to advance in the milling or drilling operations.
To measure the angle as accurate as possible, the work piece needs to be secured on the sine plate tightly without the interference of other objects. As a result, the clamping of the work piece cannot be done with vices or chucks, and hence the magnetic sine bar with a magnetic sine plate has become the best option. Another important note to be addressed here is that the magnetic sine bar is used only to attain the measurements of the angles; the machining process is not conducted directly on the sine plate.
To answer this question, the structure of a sine bar should be detailed first. A sine bar is a device used in conjunction with two slip gauge blocks to give the precise angular measurements, or to face locate an object to a given angle. A sine bar is usually made from chromium steel alloys which is corrosion resistant. The other three major features of the material are that, it is hardened, it is precision ground, and it is highly stabilized.
The making of the sine bar is the hardened and precision ground body with two cylinders fixed on the both ends of the bar. The cylinders are precision ground as well. The configuration of these two cylinders is that, the axes of them are mutually parallel to each other as well as parallel to the upper surface of the sine bar in equal distance. In other words, the top surface of the sine bar is also parallel to the line through the centers of the two roller cylinders. The distance between the centers of the cylinders is also precisely managed. It is chosen to be a whole number. The accuracy of the length is up to 0.01 mm/m.
Typically, when a sine bar is placed on a surface, the top edge of it will be parallel to the surface. When one of the cylinders is raised by a slip gauge block with a known distance, the top edge of the bar will tilt and form an angle of the hypotenuse of a triangle and the slip gauge block forms the opposite side. Therefore, an angle can be calculated with the sine rule. The sine rule says the height of the gauge block equals to the length of the sine bar times sine (angle), or the sine (angle) equals to the perpendicular divided by the hypotenuse. Therefore, the three most important elements to the application of a sine bar are the surface plate, the slip gauge blocks and the sine bar.
As mentioned, the magnetic sine bar cannot be applied with other clamping systems such as vices and chucks because any projections on the sine plate will cause errors in measuring the angles of the work piece. This very feature directly results in another limit to the application of the magnetic sine bar. Because it is an instrument used only for angle measuring and marking, the machining and working cannot be carried out on the sine surface of it after the measurement.
The work piece needs to be detached from the magnetic sine bar and placed to other platform for further working operations. This process takes time and limit the production lead time of the parts. Also, the size of the sine bar is fixed, so it only works with small pieces; any longer components are not applicable to the magnetic sine bars.
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