Design of Hydraulic Fixtures

Fixtures are the production tools which make the standard machine tool, more adaptive to work as specialized machine tools. Manufacturing industries have brought lot of revolutions in manufacturing technology, as a consequence of which several developments like CNC lathes, CNC machine center, robotics etc. have been developed. The fixture should be designed to adopt cost effective manufacturing process so that it can be machined easily with a lesser time. The various methodology and applications by various authors are reviewed in this paper.

Index Terms—Hydraulic fixture; accuracy; Universal Fixture; Productivity

Introduction

A fixture is a work holding or supporting device used in manufacturing industry during a machining operation. Fixtures are essential elements of mass production as they are required in most of the automated manufacturing operations. The origin of fixtures can be traced back to the Swiss watch and Clock industry. The design of a fixture is a highly complex and challenging process. Hydraulic fixture provides the manufacturer for flexibility in holding forces and increased accuracy, precision, reliability, and interchangeability in parts. A clamping system that uses high-pressure liquids to power clamps and hold a work piece in place is called a HYDRAULIC FIXTURE.

Steps in Fixture Design

A proper definition of the functional requirements forms the base of the fixture design. The main task for fixture is to make the loading and unloading process simple and reduce the rejection rate. Preliminary analysis may take a few hours to several days for complicated fixture designs. Fixture design is a five-step problem-solving process. The steps are discussed in detail-

Step 1: Define Requirements

To begin with the fixture design, a clear statement of the need of the fixture should be stated. The requirements should be stated as broadly as possible to understand the problems. The designer should know the basics like ‘is the tooling be started from existing one or a completely new tooling be designed?’

Step 2: Gather Information

Collect all relevant data. The main sources of information are the part description, process sheets, and machine specifications. A thorough study of CNC machine and component drawing should be done. Work-piece specifications, operation variables, availability of equipment, and personnel must be taken into account at this stage only.

Step 3: Develop Multiple Options

This step of the process requires maximum creativity and also some experience. A designer should brainstorm for several good tooling alternatives, not just stick to one path only. The designer’s aim at this point should be in adding options and not deleting or discarding. At first the designer should draw the component and the locating points before considering sketching of the clamping devices.

Step 4: Choose the Best Option

The total cost to manufacture the product will be sum of raw material set-up and tooling cost. This can be expressed in terms of a formula:

Designer must be able to take the decision on which is the best suited model among his alternatives. To take these decisions one must take into account both the economics and ergonomics of the design.

Step 5: Implement the Design

The final step of the fixture-design process consists of reproducing the chosen design approach into reality. Final details are decided, final drawings are made, and the tooling is built and tested. Tolerances should also be mentioned. The following guidelines should be considered in improving its efficiency.

Important factors to be considered while designing a fixture:

• a. Study of workpiece and geometry
• b. Type and capacity of the machine
• c. e. Availability of indexing devices and their accuracy.
• d. f. Evaluation in the performance results of the machine.
• e. g. Rigidity of the machine tool under consideration. f.
• h. Study of ejecting devices, safety devices, etc.
• g. Required level of the accuracy in the work and quality to be produced.

Design of fixture whether it is manual or hydraulic is the same. The structure of the fixture must be sufficiently rigid and heavy to avoid deformations and vibrations produced during machining.

A. Principles of Locations The location refers to the establishment of a desired relationship between the workpiece and the fixture. The purpose of locations is to clamp the workpiece so as to restrict the movement of body in as much directions as possible. For a rigid work piece in space, there are six degrees of freedom describing the position and orientation of the work piece, as shown. They have three linear motions and three rotational motions. Fig. Degrees of Freedom of a Workpiece Guohua Qin et al., focuses on the fixture clamping sequence. J Cecil proposed an innovative clamping design approach.

B. Common locating principle is: 3-2-1 principle: Three locating pins are inserted in the base of the fixed body which arrests five degrees of freedom. Two more pins are inserted in a vertical plane of fixed body which restricts three more degrees of freedom. Another pin in the second vertical face of fixed body, arrests the other one. And remaining three degrees of freedom may be arrested by means of a clamping device. This method of locating a workpiece in a fixture is called the “3-2-1” principle or “six point location” principle.

C. Clamping Principle The main purpose of clamping is to hold the wokpiece rigidly against the locators during machining operations. The basic needs of a clamping system should be·- clamping should be easy, quick and consistent, clamping should not be affected by vibration or heavy pressure The workpiece being clamped should not be damaged due to application of clamping pressure While handling fragile components, the clamps should be provided with fiber pads.

D. Types of clamps Clamps must be selected very carefully according to the type of material and loading and unloading procedure. It also depends on the area of application of clamp on the workpiece. Some common different types of clamps available are listed below:

• - Strap Clamp
• - Heel Clamp
• - Bridge Clamp
• - Screw Clamp
• - Power Driven Clamping
• - Equalizing Clamps
• - Latch Clamp

Fixture design

To reduce design costs associated with fixturing, various clamping methods have been developed through the years to assist the fixture designer. Design of fixture whether it is manual or hydraulic is the same.

Any work holding fixture must fulfill three basic functions:

1) Position the component accurately.

2) Support the component accurately.

3) Clamp the component accurately.

For designing fixture it is necessary to study component details which include part geometry, machining process, design and interpretation rules for the machining fixture. The fixture must meet the customer needs and budget. In terms of constraints, there are many factors to be considered, mainly dealing with: shape and dimensions of the part to be machined, tolerances, sequence of operations, machining strategies, cutting forces, number of set-ups, set-up times, volume of material to be removed, batch size, production rate, machine morphology, machine capacity, cost, etc. At the end, the solution can be characterized by its: simplicity, rigidity, accuracy, reliability, and economy.

For making a fixture following steps involved are

• • Analytical design for fixture.
• • 3-D modeling in any of the modeling software (preferably PRO-E CREO, Solidworks)
• • Assembly
• • Analysis by using analysis software (preferably ANSYS).

If a fixture is pre-existing then the locations of the clamping points would be known, and it reduces the work of finding centroid of component and finding suitable locating and clamping points.

Criteria for Material Selection of Fixture

- Mechanical Properties

The material must possess a certain strength and stiffness. Selected materials are examined for strength and stiffness values

- Wear of Materials

It must be ensured that the selected materials have sufficient wear resistance.

- Corrosion

It is an important engineering design criterion for designs open to the environment for a longer period of time

- Ability to Manufacture

The material is well capable of using for the design, it may be difficult to manufacture.If this selection criteria is neglected the manufacture process might be very costly making it unprofitable as a commercial product.

- Cost

Cost is a critical fact to consider when selecting materials for a certain design for most products because they are facing a severe competition in the market.

The parts of fixture generally are:

A.Fixture Base Plate

B.Supporting Cylinder

C. Clamp and Hydraulic circuit

Fixture Base Plate

While designing base plate total length should be considered. By using the Roark’s Formula shown in for plate design the base of fixture is designed. Using this formula we can calculate the thickness of the plate and stress. Also deflection at centre is calculated. Usually the material for base plate, supporting cylinder and clamp are taken the same. Fig. Roark’s Formula After the calculation of the stresses in the plate it should be compared with the allowable stress of the material. And the plate should be safe in design. Once we get the dimensions and material, we create the model in the modeling software.

Supporting Cylinder

Hydraulic cylinders are manufactured by custom requirements or ready available cylinders are available according to the force specification. These cylinders are known as ZPS (Zero Point System) modules. These ZPS modules are to be supported by supporting cylinders which are designed using modeling software and then imported into ANSYS for analysis and verification for a safe design. The results in ANSYS are compared with the materials strength. There is no specific methods to design the supporting cylinders and hence designed for the suitable dimensions according to the module dimensions.

The module is fixed to the component by means of nipples which are inserted into this module and clamped with the hydraulic pressure. From a layout point of view, cylinders have six basic functional requirements:

Clamp

Clamping mostly depends on cutting forces Clamping force = Where, FHC- holding force Cylinder bore size = d mm Piston area = 0.785 d² Therefore, the required cylinder output force is obtained and selected.

Hydraulic Circuit

A hydraulic circuit is a system comprising an interconnected set of discrete components that transport liquid. Circuit is the routing and control of a confined liquid to apply power. This power is used to achieve a specific function resulting in work being performed. Before the hydraulic circuit can be designed, the following things must be defined

Depending on the cutting forces and requirements the number of hydraulic cylinders required for the fixture assembly is determined. After the selection of cylinders the final assembly is done on the base plate and the hydraulic lines and circuits are arranged and made ready for use.

Analysis

Analysis of fixture components is done in ANSYS for displacements and stresses individually. Then the complete assembled model is analyzed and checked for the stresses. The stresses developed should be below the allowable fracture strength.

Base structure Analysis

Displacement Result

Stress Result

Fixture Bod Fixture assembly

According to Deepak D. Pawar and Dr. M. M.Tayde The cost savings calculated for manual and hydraulic fixture is huge and in support of this statement a table formed is shown Time taken for machining one component with manual fixture =35hrs Cost of machining with manual fixture =35*600= Rs.21000 Labor cost for machining one component=35*50= Rs.1750 Set-up cost=8*50*2=Rs.800 Variable cost of one component with manual fixture=Rs.23550 Time taken for machining with ZPS fixture =21hrs Cost of machining with ZPS fixture =21*600= Rs.12600 Labor cost for machining one component =21*50= Rs.1050 Variable cost of one component using ZPS fixture=12600+1050= Rs.13650 Profit/component = (23550-13650) =Rs.9900 Another cost justification of using hydraulic fixtures is given by Komal Barge, Smita Bhise as COST JUSTIFICATION TABLE

By using this fixture Rs.80000 will be saved in one month. Hence the objectives set at the first will be satisfied if one follows the procedure.

Conclusion

Implementation of these fixtures eliminates the need of human operator for clamping of manifolds. It reduces the cycle time. The hydraulic fixture will not only provides the repeatability and high productivity, but also offers a solution, which reduces work-piece distortion due to clamping and machining forces. Fixtures have direct impact on product manufacturing quality, productivity and cost. It also gives the accuracy within 10 to 15 micron and now this is the requirement of any component or work-piece after machining. To fulfill the multifunctional and high performance fixturing requirements optimum design approach can be used to provide comprehensive analyses and determine an overall optimal design.