Eccentric parts are not strange in the mechanical industry, this type of components are made with eccentric turning operations on the lathe machine. What is eccentric turning and how it is performed? In this article, let’s get into a deeper understanding of eccentric turning (offset turning).
Eccentric turning, also known as offset turning, is a type of lathe operation. It is typically used to machine parts with features that are not concentric to the spindle axis, such as the eccentric drive of a steam engine and a crankshaft. The main difference between turning and eccentric turning is the center of rotation of the workpiece. In the turning process, The axis of rotation of the chuck and the geometric axis of the workpiece and in alignment. While in eccentric turning, the axis of rotation and the geometric axis of the workpiece is not aligned. Most CNC turning jobs are concentric and use a 3 jaw chuck to hold the workpiece. The 3-jaw chuck moves all jaws in unison when the chuck is adjusted. While in eccentric turning, a 4 jaw chuck is usually used or the piece is turned between centers. The 4 jaw chuck independently moves the jaws which allow parts to be moved off-center.
Eccentric parts refer to the workpiece whose external axis and internal axis are not in the same straight line but are parallel to each other. This kind of workpiece can convert straight-line motion into rotary motion, so it is widely used in the field of mechanical transmission. Eccentric workpieces can be divided into eccentric shafts and eccentric sleeves according to the types of the parallel axis.
The component with axes of the outer circles are parallel and not coincident, they are regarded as eccentric shaft, the component with axes between inner and outer circles are parallel and not coincident, it is regarded as the eccentric sleeve. Eccentric shaft and eccentric sleeve are generally processed on lathe machines. The machining principles of the two parts are basically the same, mainly differs in clamping methods. When machining eccentric parts on the lathe, the center of the eccentric part to be machined must be corrected to coincide with the center of the lathe spindle. The eccentricity refers to the deviation distance between the datum axis and the actual axis. Eccentric parts are widely used in transmission devices, oil tanks, and all kinds of pumps.
How is eccentric turning done on the lathe machine? There are two methods for machining an eccentric workpiece with four jaw chuck, that is, turning eccentric parts according to marking line and turning eccentric workpieces with a dial indicator.
1. Turning eccentric workpiece according to marking
Align according to the eccentric circle that has been drawn. Because of the marking error and alignment error, this method is only suitable for eccentric turning parts with low machining accuracy requirements.
(1) Before clamping the workpiece, the chuck claws should be adjusted so that two claws are symmetrical and the other two claws are asymmetrical. The distance from the spindle center is approximately equal to the eccentricity of the workpiece. The opening distance between each pair of claws is slightly larger than the diameter of the clamping position of the workpiece, so that the eccentric circle line of the workpiece is in the center of the chuck, and then the workpiece is clamped.
(2) After clamping the workpiece, make the tailstock center close to the workpiece, adjust the claw position to make the center align with the center of the eccentric circle, and then remove the tailstock.
(3) Align the marking needle tip with the side plain line on the outer circle of the workpiece, move the bed saddle, and check whether the side plain line is horizontal. If it is not horizontal, it can be adjusted by tapping gently with a wooden hammer. Then turn the workpiece 90 degrees, check and correct another side line, and align the point of the scriber with the eccentric circle line of the workpiece section, and correct the eccentric circle. In this way, the correction and adjustment are repeated until the two sidelines are horizontal and the axis of the eccentric circle coincides with the axis of the lathe spindle.
(4) Tighten the four claws evenly. After checking and confirming that there is no displacement of the side plain line and eccentric circle line when tightening the claws, the turning can be started.
2. Use a dial indicator for alignment
For the eccentric components with small eccentricity and high machining accuracy requirements, it is obvious that the accuracy requirements can not be met by the marking alignment processing. At this time, it is necessary to use the dial indicator to align, generally, the eccentricity error can be controlled within 0.02mm. Due to the limitation of the measuring range of the dial indicator, it can only be used for the alignment of workpieces with eccentricity less than 5mm.
(1) First, use the marking line to preliminarily align the workpiece.
(2) Then the dial indicator is used for further alignment to make the axis of the eccentric circle coincide with the axis of the lathe spindle.
(3) Align the workpiece sideline to make the two axes of eccentric shaft parallel.
(4) Correct the eccentricity.
(5) The operation requirements and precautions of rough turning eccentricity are the same as those of needle alignment and turning eccentric workpiece.
(6) Check the eccentricity.
(7) Finish turning the outer diameter of the eccentric circle to ensure the machining accuracy requirements.
The clamping methods for turning eccentric parts also include three jaw self-centering chuck, double chuck, eccentric chuck, etc.