Jig and Fixture Design

Topics: Machining, Milling machine, Screw Pages: 11 (3520 words) Published: March 21, 2013
This paper proposes a design of jigs and fixtures for the mass production manufacturing of Sportsman Race Series aluminum slipper pistons (Probe Industries) that are used in the Chevrolet LS Series V8 engines. The designs of two sets of jig and fixture designs are proposed. The first set (two vee-blocks and a drill bushing) is designed for the process of drilling the wrist pin holes in the piston. The second design (two vee-blocks and a pin locator) is used in the milling of the skirts and lower form of the piston. The designs use principles of manufacturing facilities design, research on manufacturing practices and 3D modeling software (Catia) in order to optimize quality (accuracy), costs and productivity in the manufacture of such pistons.

1. Introduction

1.1 The Piston
An important mechanical component of an internal combustion engine, the piston is basically a solid part in an airtight cylinder that moves under the pressure of a fluid and transfers the force of the expanding gas to a crankshaft through a connecting rod (The American Heritage, 2005). Pistons typically consist of a few main features. Pistons are cylindrical. The top part of the piston that comes into contact with the expanding gas is the crown. All pistons have a wrist pin hole to contain a steel wrist pin that attaches the piston to a rod that is connected to the crankshaft. The skirt of the piston is the end opposite the crown and is often milled in the sides to reduce weight.

1.2 Selected Component (Chevrolet LS Series V8 Piston)
The specific component chosen for the design project is the Sportsman Race Series aluminum slipper piston (Probe Industries) used in the Chevrolet LS Series V8 engine. The high-speed nature of a V8 engine requires pistons to have reduced weight to improve engine balancing. Unlike older pistons that were made with cast iron, this piston is made with forged aluminum alloy to reduce its weight as well as improve fatigue life. Furthermore, this specific piston is a slipper piston with a length of only half its diameter and has some parts milled off in order to reduce size and weight dramatically.

1.3 Objectives, Scope, and Assumptions
The aim of this design project is to design jigs and fixtures for the mass production of the aluminum slipper piston used in the Chevrolet LS Series V8 engine. The scope of the design project is limited to designing jigs and fixtures for the only two processes in the manufacture of such pistons that require jigs and fixtures, specifically the drilling of the wrist pin holes and the milling of the skirts and lower form of the piston. The design project does not account for the exact design, accuracy or efficiency of the drilling and milling machines. The strength and hardness of the materials are not considered. The time and motion in between the processes are not measured for optimization.

2. Manufacturing the Component

2.1 Component Design
The exact specifications and design of the piston for the Chevrolet LS Series V8 engine were taken from a manufacturer of such pistons, Probe Industries.

The bore (diameter) of the piston is 4 inches and the height is 2 inches. The wrist pin hole is 0.927 inches in diameter and centered at 1.155 inches below the crown. The wrist pin length is 2.5 inches, meaning that the skirt below the oil grooves is milled at each end of the wrist pin hole up to 1.25 inches away from the center of the cylinder (Probe Industries, 2012). These dimensions are important in the milling and drilling processes in the manufacture of the piston. The image below shows the rest of the dimensions and the tolerances (calculations shown later).

2.2 Machining Processes in Manufacture of Component
After forging a solid aluminum slug into the basic shape of the piston with the desired strength and stability, the component undergoes several machining processes. The first machining process involves using a lathe or a CNC turning machine to make the...
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