Source: Tooling University

 

Sections:

Intro to Workholding
The role of a typical jig and fixture/ The components of a jig and fixture.

Supporting and Locating Principles
The beginning of the locating process in a how-to fashion.

Locating Devices
Common locators and their setups.

Clamping Basics
Tthe basics of clamping workpieces/ Basic clamping concerns in the context of commonly used clamps.

Chucks, Collets, and Vises
Simplest workholders.

Fixture Body Construction
Common tool body forms and the material and cost considerations associated with their construction.

Fixture Design Basics
The major factors to consider when beginning the design of a customized fixture.

Drill Bushing Selection
The major groups of bushings and their appropriate use.


 

Intro to Workholding

 

 

Term

Definition

adjustable locator

A locating component that is moveable and is used for workpieces of varying size. Adjustable locators are frequently used for cast parts.

angle-plate fixture

A fixture that positions the locating surface of the workpiece at an angle to the machining table.

assembled locator

A locating component that is fastened to the workholding device.

assembly

The process by which two or more objects are joined together.

base plate

The foundation or frame of a fixture that provides a mounting surface for locators and clamps. The base plate is also called a tool body or subplate.

blueprint

A document containing all the instructions necessary to manufacture a part. The key sections of a blueprint are the drawing, dimensions, and notes.

boring

The process of using a single-point tool to enlarge a preexisting hole. Boring can also be performed to customize chuck jaws on a lathe for a particular part.

boring machine

A turning machine used to enlarge preexisting holes with single-point and multi-point cutting tools.

bushing

A hardened steel tube used to guide cutting tools such as drills and reamers.

cam

A clamping device consisting of a lever and a circular base with an eccentric pivot. As the cam is closed for locking, increasing pressure from the eccentric base holds a workpiece in place.

cam clamp

A clamp that uses a gradually curved surface to lock itself in place. Cam clamps may be dislodged by excessive vibration.

changeover

The process of switching a machine from one part setup to another.

chuck

A workholding device with three or four jaws that clamp and hold a cylindrical workpiece. The chuck is commonly used to hold a workpiece as it rotates on a lathe.

clamp

A workholding device that maintains the position of a workpiece by holding it in place against locators.

clamping

The secure holding of a workpiece against locators. Clamping must be strong enough to resist the forces that occur during machining.

CNC machining center

A sophisticated machine tool controlled by a computer that can perform mulitiple machining operations in the same setup with a variety of tools.

collet

A slitted device that holds a workpiece in place as it rotates. A collet has a hole through which the workpiece passes, and it is designed to hold specific dimensions.

column

A large four-, six-, or eight-sided device that accommodates the mounting of fixtures, usually on a horizontal milling machine.

counterboring

An operation that enlarges the end of a predrilled hole to allow room for a head of a screw or nut.

critical surface

An important surface of the workpiece that determines the appropriate workholder design.

cycle time

The time it takes to make one part, or the time it takes to execute a series of operations on a single machine tool.

drill plate

The top plate of a jig that contains the bushings.

drilling

The process of using a multi-point tool to penetrate the surface of a workpiece and make a round hole.

external locating pin

A locating device that uses an outside surface to locate a workpiece.

finishing

The final operations performed for obtaining desired tolerance and/or surface finish.

fixed locator

A locating component that is designed to locate a workpiece in a fixed position.

fixture

A customized workholding device used on machine tools to position and hold a part during various machining operations. A fixture is built to hold a specific part design.

hydraulic clamping

A clamping system that uses high-pressure liquids to power clamps and hold a workpiece in place.

indexing fixture

A workholding device that pivots to expose additional machining areas.

indexing jig

A jig designed to position a workpiece at specific locations around a rotational axis. Indexing jigs can be used for drilling holes around a workpiece.

indexing workholding device

A workholding device that either enables metal cutting in regular increments or pivots to expose other available machining space.

inspection

The examination of a part to ensure that it meets its design specifications.

integral locator

A locating component that is built into the workholding device.

internal locating pin

A locating device that uses an internal surface to locate a workpiece.

jig

A customized workholding device used to position and hold a workpiece while guiding the location of the cutting tool. Jigs are not as common as fixtures and are not used with CNC machines.

lathe

A machine tool that holds a cylindrical workpiece at one or both ends and rotates it while various cutting tools remove material. Turning is a common operation performed on the lathe.

layout

A manufacturing process by which the component parts of a product are arranged prior to assembly.

locating

The process of positioning the workpiece in a designated location. Locating is also used to describe the precise positioning of the workpiece in the horizontal plane.

locating nest

A fixed locator that completely surrounds the dimensions of a workpiece. The workpiece rests within the locating nest.

locating pin

An assembled locating device that can be used to locate either an outside workpiece surface or an interior hole. Locating pins are available in numerous shapes and sizes.

locating point

A point on the workholding device that is meant to contact the workpiece while it is being positioned.

location dimension

A dimension that establishes the position of shapes relative to each other. For example, a hole center that is 3 inches from the edge specifies a location dimension.

locator

A workholding device used to position a workpiece within a jig or fixture. Locators establish a relationship between the workpiece and the workholding device.

lot

The number of parts made in one setup.

machine tool

A power-driven machine that uses a cutting tool to create chips and remove metal from a workpiece.

manual clamp

A clamp that is secured by hand by the operator. Cam, screw, and toggle clamps may all be manually operated.

milling machine

A machine that uses a multi-point tool to remove metal from the surface of a workpiece. Milling machines are commonly used to machine slots, grooves, and flat surfaces in rectangular workpieces.

modular fixture

A workholding device that uses standard reusable components to construct a customized workholding device.

plate fixture

The most basic type of fixture that contains mounted clamps and locators on a plate for holding the workpiece parallel to the machine table.

pneumatic clamping

A clamping system that uses high-pressure air to power clamps and hold a workpiece in place.

power clamping

A clamping system that converts hydraulic or pneumatic power into mechanical clamping forces.

reaming

The use of a cutting tool to smooth or enlarge a previously drilled hole.

repeatability

The ability of a workholding device to position workpieces in the same place, part after part.

rest button

A short locating pin that is used to both support and locate a workpiece.

screw clamp

A type of clamp that locks securely in place by the turning of threaded devices. A screw clamp is often slow but secure.

spindle

The part of the machine tool that spins or rotates. On the mill, the spindle holds a cutting tool. On the lathe, the spindle holds the workpiece.

spring locating pin

An adjustable locator with a metal or plastic bulb that is used to push a workpiece up against fixed locators on the opposite side.

spring stop button

An adjustable locator with a metal button or tang that pushes a workpiece up against fixed locators on the opposite side. Spring stop buttons exert more force than spring locating pins.

strap clamp

A type of clamp that reaches over the workpiece to hold it in place. Strap clamps are often used when extra toughness is required.

supporting

The secure location of a workpiece that typically contacts the bottom surface of a workpiece. Supporting is one of the three roles of a workholding device.

swing clamp

A clamp containing a swinging arm that moves to facilitate the quick loading and unloading of workpieces.

tapping

The process of cutting internal threads in a hole with a rotating multi-point tool.

testing

The examination of a part to ensure that it performs its intended function.

threaded adjustable locator

A locating device consisting of a threaded screw that adjusts to the varying dimensions of workpieces.

toe clamp

A type of clamp with a serrated surface that reaches forward and down to grip the workpiece.

toggle clamp

A type of clamp that operates on a pivot and lever system. Toggle clamps lock just past the center of the pivot points.

tolerance

The unwanted but acceptable deviation from the desired dimension.

tombstone

A large two-sided rectangular device that accommodates the mounting of fixtures.

vise

A workholding device with one fixed jaw and one moveable jaw. Vises are often used to hold simple rectangular or cubic workpieces on a mill or machining center.

V-locator

A fixed locator that uses an angled interior to position and center the cylindrical surface of a workpiece.

ways

Two precisely measured, parallel tracks that support and guide the movement of the carriage and cross slide of the lathe.

workholding

The process of securely supporting, locating, and clamping a workpiece for a manufacturing operation.

workholding device

A device used to position and hold a workpiece. The workholding device references the tool performing the operation on the part being held.

workpiece

A part that is being worked on. It may be subject to cutting, welding, forming, or other operations.

 


 

 Supporting and Locating Principles:

 

 

Term

Definition

12 degrees of freedom

The collective range of possible directions that a workpiece could move. An unrestricted workpiece has twelve degrees of freedom.

3-2-1 method

A workholding rule defining the minimum number of contact points necessary to properly locate a rectangular part in three planes. The primary datum requires three points, the secondary datum two points, and the tertiary datum one point.

adjustable locator

A locator that can be adjusted to accommodate variations in workpiece dimensions. Adjustable locators are less accurate than fixed locators.

auxiliary locating feature

A part of a workpiece included for the purposes of workholding that is not related to the use or function of the workpiece.

axes

Imaginary lines that pass through the center of an object. Axes are used to define the location of objects in the Cartesian coordinate system.

axial degrees of freedom

The potential direction of linear workpiece movement along its axes.

Cartesian coordinate system

The system that describes the location of an object by numerically expressing its distance from a fixed position along three imaginary linear axes.

clamp

A workholding device that maintains the position of a workpiece by holding it in place against locators.

concentric locator

A locator whose center aligns with the center of a circle in a workpiece.

conical locator

A locator used to position a cylindrical workpiece by self-centering the workpiece on a cone.

cutting force

A force generated by the motion of the cutting tool.

datum

A surface that is assumed to be geometrically perfect or that acts as a reasonable surface for the purpose of workpiece location.

fixture

A customized workholding device used on machine tools to position and hold a part during various machining operations. A fixture is built to hold a specific part design.

holding

Maintaining the positioning of a workpiece. Workholding devices maintain, or hold, the workpiece in its desired location usually with the aid of clamping devices.

locating

The process of positioning the workpiece in a designated location. Locating is also used to describe the precise positioning of the workpiece in the horizontal plane.

machine tool

A power-driven machine that uses a cutting tool to create chips and remove metal from a workpiece.

machined hole

A hole that has been shaped by a cutting tool. Machined holes are more accurate than cast holes.

machined surface

A surface created by the removal of metal during a machining process. Machined surfaces are generally accurate and are preferable for locating a workpiece.

metal cutting

A machining process that uses a tool to create chips and remove metal from a workpiece.

milling machine

A machine that uses a multi-toothed milling cutter to remove metal from the workpiece surface to create flat and angular surfaces and grooves.

perpendicular

An angle formed by two lines at a right angle. The corner of a piece of paper is formed by perpendicular lines.

positive stop

A locator into which the part is directed by primary cutting forces generated by the cutting tool.

primary datum

The first plane of the 3-2-1 method determined by three points. The primary datum usually coincides with the largest surface of the workpiece.

radial degrees of freedom

The potential rotational direction of workpiece movement around its axes.

redundant location

A situation in which a workpiece has more locating points than it actually needs. Redundant locators increase the chances of errors in manufacturing processes.

secondary datum

The second plane of the 3-2-1 method determined by two points. This plane must be perpendicular to the primary plane and is usually the second largest surface of the workpiece.

sight location

A rough locating method used as a first step. Machined details are usually created in this step for further use and better precision.

support

The process of locating from underneath the workpiece. Supports generally restrict motion down along the Z-axis.

tertiary datum

The third plane of the 3-2-1 method determined by a single point. This plane must be perpendicular to both the primary and secondary planes and is usually the smallest surface of the workpiece.

unmachined surface

A surface that is less accurate than a machined surface. Cast parts have rough, unmachined surfaces.

vise

A workholding device with one fixed jaw and one moveable jaw. Vises are often used to hold simple rectangular or cubic workpieces on a mill or machining center.

workpiece

A part that is being worked on. It may be subject to cutting, welding, forming or other operations.

X-axis

The linear axis representing motions and positions along a line parallel to the longest edge of the worktable.

Y-axis

The linear axis representing motions and positions along a line parallel to the shortest edge of the worktable.

Z-axis

The linear axis representing motions and positions along a line perpendicular to the worktable.

 


 

Locating Devices:

 

 

Term

Definition

adjustable locator

A locator that can be adjusted to accommodate variations in workpiece dimensions. Adjustable locators are less accurate than fixed locators.

alignment pin

A pin that is used to lock a workpiece into position on an indexing jig. An operator pulls the alignment pin into and out of a hole that lines up with another hole in the workpiece.

assembled locator

A separate locator device that is attached or fastened to the tool body of the workholding device. Assembled locators can be easily replaced if damaged.

cast

Material that has been poured into a mold as a liquid and cooled into a solid form.

clamp

A device that resists secondary cutting forces during a machining operation. Clamps are used to hold a workpiece against the locators.

cutting force

A force that is generated by the cutting tool as it machines the workpiece. Cutting forces are divided into primary and secondary cutting forces.

external locating pin

A locating pin that is used to locate an outer surface of a workpiece.

fixed locator

A locator that is specifically designed to maintain a fixed position for a workpiece dimension.

horizontal plane

An imaginary plane that is parallel to the ground floor. The top of a table is typically positioned on a horizontal plane.

indexing jig

A jig that is specifically designed to locate a part in multiple positions, one after another. Indexing jigs are often used to drill holes around the surface of a cylindrical workpiece.

integral locator

A locator that is built directly into the body of a workholding device.

internal locating pin

A locating pin that is used to locate an interior hole of a workpiece.

locating nest

A fixed locator that completely surrounds the dimensions of a workpiece. The workpiece rests within the locating nest.

locating pin

An assembled locating device that can be used to locate either an outside workpiece surface or an interior hole. Locating pins are available in numerous shapes and sizes.

locator

A device that resists primary cutting forces during a machining operation. Locators can be used to support a workpiece from below or locate it on a horizontal plane.

partial locating nest

A locating nest that surrounds only portions of the workpiece surfaces.

primary cutting force

A cutting force that is directly generated by motion of the cutting tool during machining. Primary cutting forces occur in the same direction as cutting tool movement.

production run

The collective processes that are necessary to manufacture a group of similar or related parts.

redundant location

The use of an extra, or duplicate locator that does not add to the accurate positioning and supporting of a workpiece surface. Redundant locators potentially introduce errors in workpiece tolerancing.

relieved locator

A locating pin with a diamond-shaped head that is used to position a workpiece in only two opposite directions. Relieved locators are most often used with an internal locating pin.

rest button

A short locating pin that is used to both support and locate a workpiece.

retractable plunger

A spring-loaded device with a metal ball that is used to lock a workpiece into position on an indexing jig. The spring forces the round ball into a hole on the workpiece.

secondary cutting force

A cutting force that is generated in response to primary cutting forces. Secondary cutting forces include vibration during machining and forces that attempt to lift a workpiece after a drill penetrates the workpiece.

shoulder

A ring or collar on a locating pin that enables the pin to withstand greater loads. Shoulders can also be used to support a workpiece.

spring locating pin

An adjustable locator with a metal or plastic bulb that is used to push a workpiece up against fixed locators on the opposite side.

spring stop button

An adjustable locator with a metal button or tang that pushes a workpiece up against fixed locators on the opposite side. Spring stop buttons exert more force than spring locating pins.

threaded adjustable locator

An adjustable locator with a threaded knob that is turned to push up against the workpiece surface.

tolerance

The unwanted but acceptable deviation from a desired dimension. Workholding devices must have a tighter tolerance than the desired tolerance of the workpiece.

tool body

The foundation of a workholding device. The various components, such as locators and clamps, are fastened to the tool body to create a customized workholding device.

vise

A workholding device with two jaws that grip and hold a workpiece in place. A vise is mostly used to hold rectangular or cubic workpieces with fairly simple dimensions.

V-locator

A fixed locator that uses an angled interior to position and center the cylindrical surface of a workpiece.

workholding device

A device used to locate and hold a workpiece. The workholding device references the tool performing the operation on the part being held.

workpiece

A part that is being worked on. It may be subject to cutting, welding, forming, or other operations

 


 

 Clamping Basics:

 

 

Term

Definition

cam clamp

A clamp that uses a gradual curved surface to lock itself in place. Cam clamps may be dislodged by excessive vibration.

C-clamp

A screw clamp in a C-shaped frame. The C-clamp is a versatile clamp and is widely used for woodworking.

clamping

An operation that holds the workpiece against the locators. Clamping resists secondary tool forces.

finger clamp

A small strap clamp.

fixture

A workholding device used on machine tools to position and hold a part during various machining operations.

fulcrum and lever

A system in which a rigid bar, the lever, pivots around a fixed point, the fulcrum.

hydraulic power

Power created by water or liquid pressure. A hydraulic system converts hydraulic energy of pistons and cylinders into mechanical motion.

inclined plane

A plane set at an angle. The inclined plane can be used to raise or lower a load by rolling or sliding.

involute curve

A curved line that gradually becomes more distant from a center point. An involute curve can be traced by a point on a taut string as it unwinds from a cylinder.

lever

A device that pivots on a fixed point to transfer force and motion.

locator

A workholding device used to position a workpiece within a jig or fixture. Locators establish a relationship between the workpiece and the workholding device.

pneumatic power

Power caused by air used under pressure. Air power converts pressurized air into mechanical motion.

positive locking position

A clamping position in which additional cutting forces encourage the locking position.

power clamping

A clamping system that uses a means other than manual power to secure the clamps. Hydraulic and pneumatic systems are power clamping systems.

primary tool force

A force that is resisted by locators. Primary cutting forces include the rotational forces in drilling or grinding as well as the downward force of the tool.

rise of the cam

The degree of elevation on a cam clamp.

screw clamp

A type of clamp that locks securely in place by the turning of threaded devices. A screw clamp is often slow but secure.

screw thread

A spiral ridge cut into metal. The number of threads in a given space determines the speed and clamping force of the screw.

secondary tool force

A force that is resisted by the clamps. Drilling generates secondary cutting forces when it penetrates the workpiece and attempts to lift it.

serrated

A surface notched with a series of ridges or grooves. Serrated surfaces improve gripping ability.

straight strap clamp

A basic type of strap clamp that looks like a bar with an oval opening in its center.

strap clamp

A type of clamp that reaches over the workpiece to hold it in place. Strap clamps are often used when extra toughness is required.

swing clamp

A clamp containing a swinging arm that moves to facilitate the quick loading and unloading of workpieces.

throw of the cam

The maximum distance a cam clamp travels to create the rise.

toe clamp

A type of clamp with a serrated surface that reaches forward and down to grip the workpiece.

toggle clamp

A type of clamp that operates on a pivot and lever system. Toggle clamps lock just past the center of the pivot points.

torque

The force exerted in rotation.

U strap clamp

A type of strap clamp that is open on one end and forms a "U" shape.

wedge clamp

A clamp that uses a wedge to gradually exert pressure on jaws located on each side of the wedge. Wedge clamps push out and down and typically hold a workpiece from the side.

workholding device

A device used to support, locate, and hold a workpiece. The workholding device establishes a relationship between the cutting tool and the workpiece.

 


 

Chucks, Collets, and Vises:

 

 

Term

Definition

bar stock

Stock that is sold in the form of long cylindrical, hexagonal, or square bars.

cast aluminum

Aluminum that is poured as a liquid into a mold and cooled into a solid shape. Chuck jaws are often made of cast aluminum because it is easily machined.

center

The device located in the tailstock of a lathe or turning center that supports the end of a cylindrical workpiece opposite the spindle.

chuck

A workholding device with three or four jaws that clamp and hold a cylindrical workpiece as it rotates on a lathe or turning center.

collet

A tapered workholding device with prongs that grip a workpiece passing through a hole in the center. Each collet is designed to match a specific workpiece diameter.

counterweight

A weighted device that is used to properly balance a rotating workpiece or workholding setup on the lathe or turning center.

faceplate

A special fixture that is designed to hold a workpiece as it rotates on the lathe or turning center. A faceplate is often required if the workpiece is not cylindrical.

fixture

A customized workholding device that is designed to effectively support, locate, and hold a specific type of workpiece. A workpiece with multiple, complex dimensions often requires a dedicated fixture.

four-jaw chuck

A chuck that uses four jaws to surround the part. Most four-jaw chucks are independent chucks, with jaws that open and close independently of one another.

hydraulic power

Power created by the motion and pressure of fluids.

independent chuck

A chuck with jaws that open and close independently. Independent chucks can be adjusted to accommodate irregularly shaped workpieces.

lathe

A machine tool commonly used to create cylindrical forms. A lathe holds a cylindrical workpiece on one or both ends. The cutting tool is gradually passed along the surface of the rotating part.

low-carbon steel

Carbon steels that contain less than 0.3% carbon. Low-carbon steels are easy to form, and they are used to make machinable chuck jaws.

milling machine

A machine that uses a multi-toothed milling cutter to remove metal from the workpiece. A milling machine is most often used to machine flat or rectangular workpieces.

range

The difference between the smallest and widest workpiece diameter that a chuck can hold between its jaws.

self-centering chuck

A chuck with jaws that open and close together. Self-centering chucks accurately position a workpiece along the centerline of the chuck.

serrated

Having a surface with a series of small teeth or notches. Serrated vise jaws offer improved gripping strength.

spindle

On a lathe or turning center, the part of the machine that rotates while the workpiece is held in a chuck, collet, or faceplate.

stock

Raw material that is used to make manufactured parts. Stock is often sold as bar stock or flat plate stock.

tapered

Gradually decreasing in size from one end of the object to the other. Collets are tapered.

three-jaw chuck

A chuck with three jaws. Most three-jaw chucks are self-centering, with jaws that open and close in unison.

turning

A machining operation used to make cylindrical parts. A single-point cutting tool passes along the outer surface of a cylindrical workpiece as it rotates, and gradually removes a layer of material.

universal workholder

A workholder that is designed to accommodate a variety of workpiece sizes and shapes. Chucks, collets, and vises are universal workholders.

vise

A workholding device with one fixed jaw and one moveable jaw. Vises are often used to hold simple rectangular or cubic workpieces on a mill or machining center.

workholding device

A device used to locate and hold a workpiece. The workholding device references the tool performing the operation on the part being held.

workpiece

A part that is being worked on. It may be subject to cutting, welding, forming, or other operations.

 


 

Fixture Body Construction:

 

 

Term

Definition

6061 aluminum

A wrought aluminum alloy containing small percentages of silicon, copper, magnesium, and chromium.

aluminum

A silvery white metal that is soft, light, and has a high strength-to-weight ratio.

aluminum tooling plates

A pre-formed tooling material made of aluminum that is ground to highly accurate dimensions.

brace

A device that steadies or supports other components.

built-up fixture bodies

A fixture body created by adding components to a standard tooling plate. The pieces are assembled with dowels and pins and are used for precision machining operations, inspection, and some assembly tools.

cast aluminum

Aluminum that is fabricated by allowing it to cool in a mold.

cast bracket

Brackets available in cast iron and cast aluminum in lengths of 25 in. (63.5 cm). They are designed as structural elements of a workholding device.

cast fixture bodies

A fixture body that is produced as a casting. Cast fixture bodies provide excellent dimensional stability and vibration dampening. They are used for permanent workholding devices that will not be drastically changed.

cast iron

A type of steel that is formed into its final shape from the molten state. It contains at least two percent carbon, as well as percentages of silicon and sulfur.

chuck

A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.

clamping

In workholding, the operation which holds the workpiece against the locators. Clamping resists secondary tool forces.

collet

A slitted device that holds a workpiece in place as it rotates. A collet has a hole through which the workpiece passes, and it is designed to hold specific dimensions.

doweled

Held together by small metal rods called dowels.

fixture

A workholding device used on machine tools to position and hold a part during various machining operations.

fixture body

The foundation of the workholding device. Various components, such as locators and clamps, are fastened to the fixture body to create a custom workholding device.

inertia

The tendency of a body to move in a straight line or remain at rest. On a machining center, it is very difficult to overcome the inertia of large workpieces.

locating

In workholding, the process of positioning the workpiece in a designated location. Locating is also used to describe a step in the process that corresponds to positioning the workpiece in the horizontal plane.

mechanical fastener

A fastener that is held in place by force. A screw or bolt is a mechanical fastener.

precision ground materials

Stock that is finished by very accurate grinding.

rib

A structural component that provides shape and support.

steel

A metal consisting of iron and carbon, usuallly with small amounts of manganese, phosphorus, sulfur, and silicon as well.

structural sections

A pre-formed tooling material made of steel, aluminum, or magnesium that is available in standard shapes and is used to save weight.

subassemblies

Semi-assembled pieces that will be attached to a larger piece in another assembly operation.

supporting

The process of locating from underneath the workpiece. Supports generally restrict motion down along the Z-axis.

tool steel

A type of steel designed with high wear resistance, toughness, and strength.

vise

A workholding device with one fixed jaw and one moveable jaw. Vises are often used to hold simple rectangular or cubic workpieces on a mill or machining center.

web

A structural component providing support through its web-like pattern of sections.

welded fixture bodies

A fixture body that is created by welding pieces together. They are used for roughing rather than finishing operations. Welded fixture bodies run the risk of heat distortion during welding.

 


 

Fixture Design Basics:

 

 

Term

Definition

3-2-1 method

An effective method for locating a rectangular workpiece. Three datum points support the workpiece, two datum points locate a flat workpiece surface, and a single datum point locates a second surface perpedicular to the previous surface.

base plate

A precisely ground plate that acts as the foundation of a fixture. Various components, such as locators and clamps, are fastened to the base plate.

clamping

The appropriate forces used to hold a workpiece against the locators during the machining operation.

external locating

The use of external surfaces to locate a workpiece. The 3-2-1 method is an external locating method.

fixture

A customized workholding device used on machine tools to position and hold a part during various machining operations. A fixture is built to hold a specific part design.

ground

Machined with an abrasive to achieve highly accurate measurements.

hydraulic power

Power created by water or fluid pressure.

inspection fixture

A fixture used to hold a workpiece while it is being examined. Inspections usually involve comparing workpieces with desired measurements for accuracy.

internal locating

The use of internal surfaces to locate a workpiece. The most common internal locating method is the use of a round and relieved pin to locate two machined holes.

locating

The accurate positioning of the workpiece in a horizontal plane to establish a relationship between the workpiece and cutting tool.

locating pin

An assembled locating device that can be used to locate either an outside workpiece surface or an interior hole. Locating pins are available in numerous shapes and sizes.

lot size

The number of parts created during the use of a particular tooling setup.

nonproductive time

Time that is spent without the machine in operation producing chips. Nonproductive time includes setup time, changing of parts, equipment failure, etc.

plate fixture

A fixture consisting of assembled components such as a base plate, locating pins, rest buttons, clamps, etc.

pneumatic power

Power created by air pressure.

power clamping

A clamping system that converts hydraulic or pneumatic power into mechanical clamping forces.

relieved pin

A locating pin with a diamond-shaped head that is used to position a workpiece in only two opposite directions. Relieved locators are most often used with an internal locating pin.

rest button

A short locating pin that is used to both support and locate a workpiece.

setup time

Time that is spent setting up the fixture, calculating tool offsets, and performing all the necessary tasks to produce the first accurate part.

strap clamp

A type of clamp that reaches over the workpiece to hold it in place. Strap clamps are often used when extra toughness is required.

supporting

The process of locating from underneath the workpiece. Supports generally restrict motion down along the Z-axis.

toe clamp

A type of low-profile clamp with a serrated surface that reaches forward and down to grip the workpiece on its edge.

toggle clamp

A type of clamp that operates on a pivot and lever system. Toggle clamps have a limited clamping range.

tolerance

An unwanted but acceptable deviation from the specified dimension.

tool steel

A specialized type of alloy steel that has excellent strength, toughness, and wear resistance. Tool steels are used in cutting tools, punches, and other industrial tooling.

workholding device

A device used to support, locate, and hold a workpiece during machining. The workholding device accurately references the tool performing the operation on the part being held.

 


 

Drill Bushing Selection:

 

 

Term

Definition

ANSI

The American National Standards Institute. ANSI is a non-profit organization that works to standardize many aspects of the business marketplace.

arbor press

A device used to install a bushing in a jig plate. The use of an arbor press prevents any distortion of the bushing.

bushing

A hardened steel tube used to guide cutting tools such as drills and reamers.

case-hardened steel

Steel that has a surface layer hardened by special heat treating methods.

concentricity

The degree to which a given dimension resembles a perfectly round circle or cylinder.

counterbored

A hole that has been slightly widened from one end to a particular depth. A counterbored hole typically provides space for a recessed bolt or screw head.

flat clamp

A small clamp with a flat edge that is used to lock a bushing in place.

ground

A surface that has been machined with an abrasive to improve finish and achieve accurate dimensions.

gun drill bushing

A special type of bushing that is used to guide a drill for deep-hole drilling operations.

head press fit bushing

A basic type of bushing with a rim or collar at the top for extra loads. Head press fit bushings are permanently installed.

headless press fit bushing

The most basic type of bushing. A headless press fit bushing is simply an accurate, hardened metal tube. These bushings are permanently installed.

interference

The degree that the diameter of an internal part exceeds the diameter of the external part when being fitted in a hole.

jig

A workholding device used to position and hold a workpiece while guiding the location of the cutting tool. Jigs are the only workholding device that use a bushing to guide the tool.

knurled

A surface that has been marked with small diamond-shaped impressions. Knurled surfaces are easier to grip.

liner bushing

A type of bushing that acts as a sleeve or receptable for slip and slip-fixed renewable bushings.

load

The weight or burden that is supported by a material.

oil groove bushing

A special type of bushing that contains interior grooves to encourage the flow of cutting fluids during machining operations.

reaming

The use of a cutting tool to smooth or enlarge a previously drilled hole.

removable bushing

Another term for a slip renewable bushing.

replaceable bushing

Another term for a slip-fixed renewable bushing.

slip renewable bushing

A type of bushing that can be inserted or removed between operations and held in place with a lockscrew or clamp. These bushings are replaced after excessive wear.

slip-fixed renewable bushing

A type of bushing that can be installed and held in place with a lockscrew or clamp. These bushings are replaced after excessive wear.

soft material bushing

A special type of bushing with a serrated or grooved exterior that helps hold the bushing in place when installed in softer materials.

tolerance

The unwanted but acceptable deviation from the desired dimension.

tool steel

A type of steel designed with high wear resistance, toughness, and strength.

workholding device

A device used to position and hold a workpiece. The workholding device references the tool performing the operation on the part being held.