STUDY OF FIXTURES

WHAT IS FIXTURE

Fixture is a holding device used for holding the component on the machine bed without any shake while machining or other operation like welding and assembling etc.

TYPES OF MAJOR FIXTURES

Though fixtures are predominantly used for milling operation, it is also used for other operations like turning, boring, welding and grinding. Fixtures are also made for inspection and assembly works. With the use of locators and proper clamps, handling of those jobs will be made easy in fixtures than any other standard work holding devices.

MACHINING FIXTURES

Machining fixtures is mostly used for machining the components on the milling,grinding,jig boring, drilling and planer machines.while machining, the component is placed on the fixture by using various clamping methods like screw clamps, toggle clamps and swing clamps etc and the important factor to be considered while designing machining fixtures is that the fixture we use should withstand the force produced in the machining operation.

fixture with clamping devices

machining fixtures with v-clamps

WELDING FIXTURES

Welding fixtures are used to hold the parts in the required shape and is used from smaller parts to larger parts of a plane. Before welding the parts are placed and positioned for the required shape. After clamping the parts welding work will be carried out. Different type of clamping devices can be used with welding fixtures like toggle clamps,pneumatic clamps and strap clamps etc. 

simple welding fixtures with toggle clamps

welding fixture with toggle clamps
for medium size component

ASSEMBLY FIXTURES

These fixtures are used for assembling components of large size. Pipelines and other frames which are so lengthy will be placed in the fixture and assembled. As the use of fixture will be more with lengthy or large components the fixture material has to be stiffer to avoid deflection. Some parts are required to have simple operations like drilling or welding, after aligning with the adjacent parts. An assembly fixture should have to be constructed to accommodate such situations.

assembly fixture with component

At many places , outer body of vehicles and seats are being assembled with the use of assembly fixtures because of its large sizes.

fixture for body assembly

INSPECTION FIXTURES

The components manufactured newly have to be checked for its accuracy in shape or in dimension. That will be performed with inspection fixtures and they are extensively used in automotive industries. The fixture will be the master in shape and every part will be compared for its shape conformity. For checking the dimension, the fixture is prepared in such a way that it could accommodate the correct dimensioned parts only. At many industries, inspection fixtures are used for leak testing purposes. 

fixture for leak testing

tube bending checking fixture

besides the above discussed fixtures , many type of fixtures are used for various purposes in all industries.

HOW SHOULD A FIXTURE BE DESIGNED?

while designing a fixture for any operation , deep knowledge about fixture designing is required to ensure that designed fixture meets all its intended use without any flaw in the operation and final product should be free of any error for further processing and applications.

FOR ANY FIXTURE TO BE SUITED FOR OPERATION, FOLLOWING FACTORS SHOULD BE ENSURED IN BROADER PERSPECTIVES

• Fixture should be stable without any vibration or shake while operation is carried out and to ensure this, proper clamping mechanisms should be used to offset all the force produced during the operation.

• Component in the fixture should be accessible without any hindrance to the cutting tool in the machining fixture to make the machining process effective and trouble-free. If it is a welding fixture then accessibility of the welding gun and weld robotic arm should be ensured to make it productive.

• Fixture should be easy for handling, as actually it is the most important factor that affects the productivity of the fixture, and Installation of the material in the fixture and removal of the component should be easy.

• Clamping methods should be suitable and economic for the operation. When the operational force is more, Effective clamping system must be installed to withstand the force. In other way, if the force produced is less, then less-strong system may be used, and Also it depends upon the operation that will be carried out .                                                                                                            For instance, welding fixture needs only a little support force so using toggle clamp and lever clamps etc.will be enough. but As machining process involves lot of force, it needed strong clamping like strap clamping, traditional nut and bold methods and pneumatic and hydraulic clamps.   

• The fixture should be easily manufacturable and design should be simple. In anyway, it should not be complicated and Proper connecting devices like couplings and flanges should be used.

IF WE DELVE DEEP INTO THE SUBJECT, THERE ARE LOT OF FACTORS TO BE CONSIDERED WHILE DESIGNING FIXTURE

FIXTURE CONCEPT STUDY

Whatever the project is that whether it is designing a fixture for a component or modeling a component, 

First of all, you should collect all the required information before starting off that project.

For instance, lets suppose that you are designing a welding fixture for a component that has lot of parts to be welded on the base component. the details you need for this project before starting off is that,

          1. No of components needed welding

          2. Sequence of opertaion

          3. Which is the base part

          4. Clamping plans

          5. Details of the reference point.

          6. Cycle time 

          7. Types of weld

only after collecting all these details, the project should be started with basic desiging of fixture parts.

ANALYSING TECHNICAL CONSTRAINTS OF THE PROJECT 

This is the very important considerations to be done while designing any fixture.because Design should perform at most efficient manner under all conditions.

As a fixture designer , one should have a good knowledge of all technical process involved in that project like,

what type of opertaion customer going to perform on this fixture?

how is it going to be done?.

For a welding fixture, you must ensure that which type of welding is going to be performed? and who does the welding ? like whether welding is done by a person or robots. 

suppose if it is done by a robot,  how many doable movements the robots have? and how is the welding gun going to interact with a every piece-parts of the component in the fixture?

and then, one should know that how is a piece-part of the component going to interact with other parts?

Let assume that in the final component, one part should be perpendicular to another part to perform an operation in correct way. The designer should take note of this point to ensure proper geometrical position of the component after welding. if the parts are not perpendicular, we can not get the desired result with finally making the fixture a scrap. Eventually, we spent hell of the time making a scrap.

suppose if it is an machining fixture, then one should take cognizance of all features of the component that is going to be machined on the fixture . we should know,

which surfaces needs machining? and which one do not want ?

Types and size of holes and tap to be put on the component?

Which should be a reference surface? 

About tolerances that should be achieved of the component during machining? etc..

Guiding path for cutting tool if any required? etc

All these points may be looking difficult to understand what it is all about, but once you entered upon the project it all starts to make sense.

STRESS AND LOAD ANALYSIS OF THE PROJECT

The fixture or Tool we design should be enough strong to withstand the all the forces produced in the opertaion.

In case of machining fixture, the fixture will experience lot of cutting forecs like tension, compression ,torsion and bending forces. the design should overdo all the forces to perfrom well.

otherwise, the fixture would break down or be rendered useless.so, the designer must aware of all internal stress condition to make a good fixture.  

Few important factors which decides the capability of the fixture to withstand the force are,

Type of material we use for the fixture.

Material selection is very important and difficult process. unless proper material is selected , fixture may collapse. 
For instance, when the load applied is more and gradual, it is better to use MS material with some surface hardness rather than other materials like EN24, EN8 and EN353 .
EN24 has more strength and good wear resistance. so it can be used where the load is static and scratch resistance is required 

EN353 material can be used for making spindle and flanges.

OHNS material are used for making bushes because of its more wear resistance than other materials.

EN8,EN9,EN24,EN36,EN353 and OHNS are some materials used for tool and fixture manufacturing

To know more about EN materials and its properties, follow the link https://www.smithmetal.com/en36.htm

Structural arrangements of the fixture.

Fixture should be designed in a way that when it works, it should transfer the force smoothly from one part to another.

Corners must be avoided where stress concentration is more.

High ductile material should be used where load concentration is more. 

Proper joints must be preferred for energy transfer. For instance, oldham coupling is less efficient than y-shaped coupling.

METHODS OF CLAMPING

Of the all stages of consideration , it is very important to choose the proper clamping methods. In the market, lot of clamping devices are available so, judicious use of clamping devises is must.

Factors that determines the selection of clamps are,

Applications of the fixture.

The designer should take into consideration all the factors before choosing the type of clamps we are going to use and especially type of project.

For simple welding fixture, we should not use any pneumatic and hydraulic clamps it make the project more costlier and usually welding fixture do not need much force thanks to no major external forces. just toggle and lever clamps and simple screw support is enough for providing support.

For machining fixture, since the force produced in more high , here a strong counter support is needed.so, toggle and swivel clamps is unusable for this application. one can use strap clamps and bolts for clamping.

For some automatic and semi-automatic fixture, there would be a need for use of pneumatic and hydraulic clamps and grippers , when the required force is more and reaction time is less , hydraulic clamps and cylinders are preferred. in opposite way, pneumatic clamps are used like less force and prolonged reaction time.

Counter force required.

Required counter force may vary in all projects. so, based upon the requirement of force, clamps are needed to be chosen.If the force produced is more, then it is better to go with some strong clamps and hydraulic clamps and if the project is not worthwhile to use costlier clamps, then better go with bolts and strap and c-clamps.

Cost involved.

since cost is very important factor while dealing with fixture, it is prudent that total cost should be reduced as much as possible. this can be usually done by judicious use of materials and clamps, and very efficiently by simple designs and less manufacturing cost.

USAGES OF STANDARD COMPONENTS

As a designer, everyone should think of the ways to reduce the cost and time of making fixture ready so it is definitely advisable to use the standard components available in market. Now a days, Lot of standardized parts and clamps are available to purchase from the supplier and directly include it in the design. many companies like smc world, besto, missumi omron and steel smith etc are providing us with many clamps and numerous components.

Both smc world and besto is very trusted for their hydraulic and pneumatic equipments and clamps. comparatively, the cost of products in besto is little much higher than smc world products but besto products comes with good quality.

All standard components can be purchased from missumi and steel smith. Even, Some products like L-bracekts and T- brackets and adjustment shim etc are also available with some supplier so as to make our time more productive by procuring and assembling it . Standard clamps like toggle clamps, c-clamps,f-clamps,swing clamps and cam follower etc can be purchased economically from this supplier.
Omron is very much revered for its reach in the sensor world. there are many suppliers providing with us many components we use in our daily work to make our job very easier and very efficient.

From the design point of view, any fixture design should have following features

LOCATING FEATURES

Placing the component at correct position on the fixture before any operation like welding and machining is very crucial to ensure that the final product is properly manufactured. So the designer should provide ideal locating points in the fixture to install the parts on the fixture correctly to get rid of any position errors. Locating pins are mostly used to position the component on the fixture accurately. Many types of locating pins have been used in different industries. 

below, I have shared a link helping you understand everything about locating pins and its uses.


https://drive.google.com/file/d/1uBasKxLrlq3BnnEtkE-Io7kwvuHaVBQ1/view?usp=sharing

SHIMMING FEATURES

All fixtures or locators have this facility of adjusting any misalignment or variation in the position of the fixture parts by including shim with that part. Shim is nothing but a small sheet metal with the thickness varying from 1mm to 3mm.

For example,

let suppose that two resting pad or block should be at same height from the base plate to place the welding component flatly on the fixture. but while assembling the parts, possibly the top surface of the two pads would not be at the same height because of manufacturing allowances of all parts on which  the rest pad will be placed and eventually, top faces of both pads will not be parallel  to each other.
So, to make up for the error in height( bottom to top distance) or distance(side to side distance), few slim sheets(shim) will be added below the rest pad so that the surfaces of the pads would become parallel. 

As always it is very difficult to achieve accurate dimension from the manufacturing process , shimming is used to offset any variation during the assembly of the parts of the fixture.

COMPONENT RESTING PLACES

Before placing component on the fixture, a designer must ensure that all the surfaces of the fixture on which the component will be mounted is machined to flat shape with high degree of flatness. if it is not machined flatly, the component can not be balanced all along the rest pads to make a condition that only a small portion of the resting surfaces of the component mounts on the restpads and the reminder portion floats over the air not mounting on the restpad. sometimes, it creates flaws in the position of the parts of the component.

ACCESSIBILITY OF THE FIXTURE

The installation and removal of the component onto the fixture and from the fixture respectively should be easier to make the process productive. because it is this process that makes cycle time less and handling the component during operation smooth. So it is very essential to provide smooth way of exit to the component after the opertaion.

it does not end up here. it will be definitely followed by 2nd chapter of which i will write more about the fixture design, materials and calculations for making stable fixture. 

if you like to know more about fixture design,  just subscribe my blog and follow the articles to be posted here in the coming days.i am expecting your support my for my blogs. if you find anything wrong with this content , let me know of that by posting your comment in the comment box.