Design principles of Jigs and Fixtures

The art of metalworking has a primary concern, locating the part to be machined relative to the platform. A CNC machine starts machining at a specific point corresponding to the fixture and proceeds from there. Therefore, the preciseness with which a job is machined is dependent on the accuracy that holds in the fixture. The accurate location of every part loaded into the fixture is essential. Any deviation in part location adds to the dimensional tolerance that must be assigned to the finished pieces. Furthermore, improper supporting and securing the part in the fixture affects surface finishes by temporarily or permanently deforming it. Hence, techniques for supporting, clamping, and locating must be considered together to assure repeatability from part to part.

Basic principles of Jigs and Fixtures design

LOCATING POINTS: Locating the work is a prime necessity and requires suitable facilities. The correct setup ensures smooth insertion of a workpiece in the proper position and removing a workpiece from a jig without operational hassles or time consumption. The workpiece position needs to be precise with the guiding tool in the jig or setup pieces in the fixture.

FOOLPROOF: A foolproof design of jigs and fixtures does not permit a tool or workpiece to be placed in any other way other than the intended one.

REDUCTION OF IDLE TIME: Jigs and Fixtures must be designed in such a way that ensures smooth loading, clamping, machining, and unloading of a

WEIGHT OF JIGS AND FIXTURES: A jig and fixture must be compact, easy to handle, and low cost regarding the number of materials used without giving up stiffness and rigidity.

JIGS PROVIDED WITH FEET: Some jigs require feet so that they can be placed on the table firmly.

MATERIALS FOR JIGS AND FIXTURES: Jigs and Fixtures are usually created with hardened materials to resist wear & tear and avoid frequent damage—for example, Mild steel, Cast iron, Die steel, High-speed steel, Caesium.

CLAMPING DEVICE: A suitable clamp is rated for its strength. It should be able to hold a workpiece firmly in its position while bearing the strain of the cutting tool simultaneously, without springing.

Broad rules of Jigs & Fixtures Design
  • Compare the production cost of work between the existing tools and the tool to be made and see if the manufacturing price is not more than the expected gain.
  • Determine location points and outline clamping arrangement.
  • Make sure the clamping and binding pieces are as quick to act & efficient as possible.
  • Make the jig and fixture foolproof.
  • Make sure the locating points are adjustable.
  • Do avoid intricate clamping arrangements.
  • Round all corners.
  • Make sure the operator has handles to make handling tasks easier.
  • Provide ample amount of clearance.
  • Provide holes for chips to escape.
  • Systematically locate clamps to resist the pressure of the cutting tool while machining.
  • To avoid springing action, place all clamps in proximity opposite to the bearing point of the workpiece.
  • Test the jigs before putting them in a shop.
The 3-2-1 principle

Locating a part to be machined involves mainly three steps: Supporting, Positioning, and Clamping.

Two main intentions when placing a job on a jig/fixture are:

  • Precisely positioning the part at the desired coordinates.
  • Curbing all six degrees of movement so that the part cannot budge.

An extensively used method for obtaining these objectives is the 3-2-1 principle or six degrees of freedom for part location.

 

Image Source: Quora

The 3-2-1 method is a work-holding principle where three pins are located on the 1st principle plane, i.e., either XY, YZ, ZX. And two pins are located on the 2nd plane perpendicular to the 1st plane, and at last, one pin on the plane is mutually perpendicular to the 1st and 2nd planes. The aim is to constrain the movement of the workpiece along all three axes.

Design objectives of Jigs and Fixtures

Before sitting down to design jigs/fixtures, the designer must consider the following points:

  • The tool must be foolproof to prevent any mishandling or accidental usage by the operator.
  • Easy to operate for increasing efficiency.
  • Easy to manufacture using the lowest costs.
  • Its ability to weather the tool life instead of appropriate materials.
  • Must be consistent at producing high-quality parts.
  • Must be safe and secure to use.

The designer must know the basics of the process and the tools associated with it for which the jig/fixture is designed. Overall objectives to look out for a while developing such tools are:

  • Cycle time.
  • Type of Jig/Fixture.
  • Part Assembly sequence or Machining locations.
  • Joining or machining process.
  • Clamping method and clamping sequence.
  • Required output accuracy.
  • Type of equipment to be used with the jig.
  • Method of ejecting finished output and transferring it to the next. Platform, whether the manual or automatic mode.
  • The type of material, recommended weight, number of spots involving welding.

Reference: National Institute of Technology, Calicut

 

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Designing Jigs and Fixtures

The design of jigs and fixtures is dependent on numerous factors which are analysed to achieve optimum output. Jigs should be made of rigid light materials to facilitate secure handling, as it has to be rotated severally to enable holes to be drilled from different angles. It is recommended that four feet should be provided for jigs that are not bolted on the machine tool, to enable the jig to wobble if not well positioned on the table and thereby alert the operator. Drill jigs provide procedures for proper location of the work-piece concerning the cutting tool, tightly clamp and rigidly support the work-piece during machining, and also guide the tool position and fasten the jig on the machine tool.

To achieve their expected objectives, jigs and fixtures consist of many elements:

  • Frame or body and base which has features for clamping
  • The accuracy and availability of indexing systems or plates
  • The extent of automation, capacity, and type of machine tool where jigs and fixtures will be employed
  • Bushes and tool guiding frames for jigs
  • The availability of locating devices in the machine for blank orientation and suitable positioning
  • Auxiliary elements
  • The strength of the machine tool under consideration
  • The precision level of the expected product
  • Fastening parts
  • The available safety mechanisms in the machine tool
  • The study of the fluctuation level of the machine tool

 

 

The factors below are to be reflected upon during design, production, and assembly of jigs and fixtures due to the targeted increase in throughput, quality of products, interchangeability, and more accuracy.

  • Guiding of tools for slim cutting tools like drills
  • Type of operations
  • Inspection requirements
  • Provision of reliable, rigid, and robust reinforcement to the blank
  • Production of jigs and fixtures with a minimum number of parts
  • Fast and accurate location of the jig or fixture blank
  • Rapid mounting and un-mounting of the work-piece from the jig or fixture
  • Set up time reduction
  • Standard and quality parts must be used
  • Reduction of lead time
  • Easy disposal of chips
  • Enhanced flexibility
Elements of Jigs and Fixtures

The significant features of Jigs and Fixtures are:

The body: The body is the most outstanding element of jigs and fixtures. It is constructed by welding of different slabs and metals. After the fabrication, it is often heat-treated for stress reduction as its main objective is to accommodate and support the job.

Clamping devices: The clamping devices must be straightforward and easy to operate, without sacrificing efficiency and effectiveness. Apart from holding the work-piece firmly in place, the clamping devices are capable of withholding the strain of the cutting tool during operations. The need for clamping the work-piece on the jig or fixture is to apply pressure and press it against the locating components, thereby fastening it in the right position for the cutting tools.

Locating devices: Thepin is the most popular device applied for the location of work-piece in jigs and fixtures.The pin’s shank is press-fitted or driven into a jig or fixture. The locating width of the pin is made bigger than the shank to stop it from being pressed into the jig or fixture body because of the weight of the cutting tools or work-piece. It is made with hardened steel.

Jig bushing or tool guide:Guiding parts like jig bushings and templates are used to locate the cutting tool relative to the component being machined. Jig bushes are applied in drilling and boring, which must be wear resistant, interchangeable, and precise. Bushes are mainly made of a reliable grade of tool steel to ensure hardening at a low temperature and also reduce the risk of fire crackling.

 

 

Selection of Materials

There is a wide range of materials from where jigs and fixtures could be made, to resist tear and wear, the materials are often tempered and hardened. Also, phosphor bronze and other non-ferrous metals, as well as composites, and nylons for wear reduction of the mating parts, and damage prevention to the manufacturing part is used. Some of the materials are discussed below:

  • Phosphor Bronze: phosphor bronze is used in the production of jigs and fixtures for processes that involve making of interchangeable nuts in clamping systems like vices, and also incorporated feedings that require screws. As the manufacturing of screws is costly and also wastes a lot of time, the reduction of their tear and wear is often achieved by using replaceable bronze mating nuts made with phosphor bronze.
  • Die Steels: the three variants of die steel - high chromium (12 %), high carbon (1.5 to 2.3%), and cold working steels are applied in the production of jigs and fixtures for the making of thread forming rolls, as well as cutting of press tools. When alloyed with vanadium and molybdenum for it to retain toughness at very high temperature, die steels are applied in the fabrication of jigs and fixtures that are used in high-temperature work processes which include extrusion, forging, and casting processes.
  • High-Speed Steels: High-speed steels which contain more quantity of tungsten and less quantity of chromium and vanadium have high toughness, hardness retention at high temperature, and excellent wear, tear and impact resistance. When tempered, they are applied in the production of jigs and fixtures for reaming, drilling, boring, and cutting operations.
  • Carbon Steels: when tempered with oil, carbon steels are applied in the making of some jig and fixture parts which are exposed to tear and wear like the locators and jig bushes.
  • Mild steels: Mild steel, which contains about 0.29% of Carbon, is very cheap and because of their easy availability is often the choicest material for the making of jigs of fixtures.

Other materials for the making of jigs and fixtures include Nylon and Fibre, steel castings, stainless steel, cast iron, high tensile steels, case hardening steels, and spring steels.

Reference

Charles ChikwenduOkpala, EzeanyimOkechukwu C “The Design and Need for Jigs and Fixtures in Manufacturing” Science Research.Vol. 3, No. 4, 2015, pp. 213-219. DOI: 10.11648/j.sr.20150304.19

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Geometric Dimensioning and Tolerancing (GD&T)

The design model is a depiction of a part design. However, the design model can never be an accurate representation of the product itself. Due to shortcomings in manufacturing and inspection processes, physical parts never match the design model exactly. An essential aspect of a design is to specify the lengths the part features may deviate from their theoretically accurate geometry. It is vital that the design intent and functionality of the part be communicated between the design engineers and the manufacturing unit. It is where the approach of GD&T comes into play.

Geometric dimensioning and tolerancing or GD&T is a language of symbols and standards used on engineering drawings and models to determine the allowable deviation of feature geometry. 

GD&T consists of dimensions, tolerances, definitions, symbols, and rules that enable the design engineers to convey the design models appropriately. The manufacturing unit uses the language to understand the design intent.

To master GD&T, one needs to understand the crucial concepts, which includes:

  • Machining tolerances: Tolerances mean the allowable amount of deviation from the proposed drawing. Machined parts look flat and straight through the naked eye, but if viewed with calipers, one can find imperfections all over. These imperfections or variations are allowed within the tolerance constraints placed on the parts. Tolerances should be kept as large while preserving the functions of the part.
  • The Datum Reference Frame: DRF is the most important aspect of GD&T. It is a three-dimensional cartesian coordinate system. It’s a skeletal reference to which all referenced geometric specifications are related.
  • GD&T Symbols: It is essential to be familiar with numerous symbols and types of applied tolerance in GD&T. The language of symbols makes it easier to interpret designs and improve communications from the designer to the shop. By using GD&T standard, the design intent is fully understood by suppliers all over the world.

  • Feature Control Frame: The feature control frame describes the requirements or instructions for the feature to which it is attached. A feature control frame contains only one message. If a feature needs two messages, the feature would need the corresponding amount of feature control frames for every message required.
  • Basic Dimensions: Basic dimensions are exact numerical values in theory, which defines the size, orientation, form, or location of a part or feature. 
  • Material Condition Modifiers: It is often necessary to state that a tolerance applies to a feature at a particular feature size. The Maximum Material Condition (MMC) allows an engineer to communicate that intent.

GD&T is an efficient way to describe the dimensions and tolerances compared to traditional approximation tolerancing. The engineer might design a part with perfect geometry in CAD, but the produced part, more often than not, turns out to be not accurate. Proper use of GD&T improves quality and reduce time and cost of delivery by providing a common language for expressing design intent.

 

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Types of Jigs and Fixtures

Till now, we know that jigs and fixtures are the devices which help in the machining of jobs and reducing the human efforts required for producing these parts. It has been explained before why a centre lathe is an ideal machine tool for creating individual pieces of different shapes and sizes, but for manufacturing similar objects in high number, its use is not that economical.

Different types of objects may require the use of drilling,milling, planning, and grinding machines, etc. Specific tools are necessary for producing these objects in identical shapes and sizes on a mass scale, by holding and locating tasks to minimize the repetition work. That is when various types jigs and fixtures come into play.

Considering the variety in the nature of jobs to be machined, the quality, and the associated functions, the type of jig and fixture varies as well. Following are the various kinds of jigs and fixtures.

Types of Jigs

Template Jig: The template jig is the simplest of all the models. The plate, having two holes, acts as a template which is fixed on the component to be machined.The drill is guided through these holes of the template and the required holes are drilled on the work-piece at the same relative positions with each other as on the template.

 

 

Plate Jig: A plate jig is an improvement of the template jig by incorporating drill bushes on the template.The plate jig is employed to drill holes on large parts maintaining accurate spacing with each other.

 

 

Channel Jig: Channel jig is a simple type of jig having a channel-like cross section. The component is fitted within the channel and is located and clamped by rotating the knurled knob. The tool is guided through the drill bush.

 

 

Diameter Jig: Diameter jig is used to drill radial holes on a cylindrical or spherical workpiece.

 

 

Leaf Jig: Leaf jig has a leaf which may be swung open or closed on the work for loading or loading purposes.

 

 

Ring Jig: Ring jig is employed to drill holes on circular flanged parts. The work is securely clamped on the drill body, and the holes are drilled by guiding the tool through drill bushes.

 

 

Box Jig: Box jig is of box-like construction within which the work is rigidly held so that it can be drilled or machined from different angles at a single setting depending on which face of the jig is turned toward the tool.

 

 

 

Types of Fixtures

Turning Fixtures: These fixtures are generally mounted on the nose of the machine spindle or a faceplate, and the workpieces held them. Whenever necessary, the fixture may have to be provided with a counterweight or balance the unbalance fixture.

 

 

Milling Fixtures: Milling fixtures are typically mounted on the nose of the machine spindle or a faceplate, and the work-pieces held them.The table is shifted and set in proper position, in relation to the cutter. The work-pieces are located in the base of the fixture and clamped before starting the operation.

 

 

Broaching Fixtures: Broaching fixtures are used on different types of broaching machines to locate, hold and support the workpieces during the operations, such as keyway broaching operations, such as keyway broaching, hole broaching, etc.

 

 

Indexing Fixtures: Several components need machining on the different surface such that their machined surface surfaces or forms are evenly spaced. Such elements are required to be indexed equally as many as the number of surfaces to be machined. The holding devices (jigs or fixtures) used are made to carry a suitable indexing mechanism. A fixture carrying such a device is known as an indexing fixture.

 

Grinding Fixtures: These fixtures may be the standard work-holding devices, such as chucks, mandrels, chuck with shaped jaws, magnetic chucks, etc.

Boring Fixtures: This fixture incorporates almost all the prevailing principles of jig and fixture design, their construction need not be as sturdy as that of the milling fixtures, because they never have to bear as heavy cutting loads as involved in milling fixtures, because they never have to endure as heavy cutting loads as involved in milling operations.

Tapping Fixtures: Tapping fixture is specially designed to position and firmly secure identical work-pieces for cutting internal threads in drilled holes in them. Odd shaped and unbalanced components will always need the use of such fixtures, especially when the tapping operation is to be carried out repeatedly on a mass scale on such parts.

Duplex Fixtures: It is the name given to the fixture which holds two similar components simultaneously and facilitates simultaneously machining of these components at two separate stations.

 

Welding Fixtures: Welding fixtures are carefully designed to hold and support the various components to be welded in proper locations and prevent distortions in welded structures. For this, the locating element need to be carefully; clamping has to be light but firm, placement of clamping elements has to be clear of the welding area. The fixture has to be quite stable and rigid to withstand the welding stresses.

Assembly Fixtures: The function of these fixtures is to hold different components together in their proper relative position at the time of assembling them. 

 

Source

The Engineers post, https://www.theengineerspost.com/jigs-and-fixtures

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What are Jigs and Fixtures, their Advantages, and Differences

With the rapid advancement in manufacturing technology, consumerism has increased over the years. Therefore, to meet the higher demands, manufacturers have come up with innovative methods of producing high-quality products at a much faster rate.

The production process has observed the introduction of inventive manufacturing concepts such as Lean Production System, Cellular Manufacturing, Single Minute Exchange of Dies, and Tact Time Analysis. These creative approaches require the need for a horde of efficient, cheaper tools, and work-holding devices.

The manufacturing company requires a simple work positioning strategy and devices for correct operations. This is to ensure:

  • Non-complexities in assembly and unit cost reduction,
  • Reduction in the massive manufacturing cost, and
  • Increase their profitability.

The industry has resorted to easing upthe supply chain in a bid to maintaining a low amount of inventory. This resulted in the emergence of better and cost-effective work-holding devices which ensure better quality products, increase throughput, and reduce lead time. The requirement for production standard work-holding devices has paved the way for two specific terms named: Jigs and Fixtures.

The jig is the device which guides the tool, while the fixture is a tool that securely and firmly holds the job in position during machining operations.

Jigs

In simple terms, a jig is a tool that guides the machining tool.

A common type of jig is the drill jig, which guides the drill for making holes at desired locations. Using drill jigs increases production rate drastically.

 

Fixtures

A fixture is a tool which firmly grips a workpiece on the machine bed accurately at the desired location. The fixture also reduces the loading, unloading, and fixing the time of the workpiece, which significantly reduces the non-productive hours.

 

 

Difference between Jig and Fixture

“Jig” and “Fixture” are many times referred to as the synonyms of each other while sometimes both of them are used together as jig fixture. Although both jig and fixture are used in the mass production process, functionally the two are quite different tools.

Let us go through the main points which differs a Jig from a fixture

 

Jig
Fixture

A jig controls and guides the machining tool

A fixture holds and supports the component precisely for machining operations

Jig ensures accuracy, repeatability, and interchangeability

The fixture provides a reduction in error by holding a component firmly on a table

Jigs are usually on the lighter side

The fixture is bulky, rigid and heavy

Jigs can be put in place and held by hand pressure

Fixtures are always placed firmly on a machine table

Drilling, reaming, tapping, boring are some of the standard jig functions

Fixtures are used explicitly in milling machine, slotting machine and shapers

Jigs cost more

Fixtures are not that cost-savvy compared to Jigs

Jigs require intricate design operations

Fixture design operations are relatively less complicated

 

Advantages of Jigs and Fixtures

Jigs and Fixtures have made manufacturing processes less time consuming, more precise, and hassle-free from a human factor perspective. The benefits of jigs and fixtures including but not limited to, the following:

  • Increase in production
  • The consistent quality of manufactured products due to low variability in dimension
  • Cost reduction
  • Inter-changeability and high accuracy of parts
  • Inspection and quality control expenses are significantly reduced
  • The decrease in an accident with improved safety standards
  • Due to relatively simple manoeuvrability, semi-skilled workers can operate these tools which reduce the cost of the workforce.
  • The machine tool can be automated to a reasonable extent
  • Complex, rigid and, heavy components can be easily machined
  • Simple assembly operations reduce non-productive hours
  • Eliminates the need for measuring, punching, positioning, alignments, and setting up for each work-piece thereby reducing the cycle and set up a time
  • Increases technological capacities of machine tools
  • More than one device can be used simultaneously on a work-piece
  • Setting of higher values of some operating conditions like depth of cut, speed, and rate of feed can be attained because of the increased clamping capability of jigs and fixtures.

Both jigs and the fixtures are used to ease up machining operations and reduce the non-productive time of any mass production process. The principle of location or the 3-2-1 principle, CAD tools, and FEA tools are used for the design of both jigs and fixtures. In the next article, we will go through more detailed information about 3-2-1 principle and design standards of jigs and fixtures. 

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