Fixture Design Basics 201

12 degrees of freedom

The possible directions of movement for an object. The 12 degrees of freedom include linear movement along three axes and rotational movement around those axes.

3-2-1 method

A method of external locating that involves placing three locators on the bottom surface of the workpiece, two on one side of the workpiece, and one on the side right next to the two locators. The 3-2-1 method will restrict nine of the 12 degrees of freedom.

active surfaces

A section of a workpiece that will be machined to create a finished part. Active surfaces must be, as much as possible, left open to the cutting tool when designing a fixture.

adjustable supports

A support component with movable parts that can be shifted to suit the shape of a workpiece. Adjustable supports allow for more flexible fixture setups.

aluminum

A nonferrous, silvery-white metal that is lightweight, highly conductive, and non-magnetic. Aluminum workpieces are soft and must be supported, located, and clamped carefully to avoid damage.

angle plates

A workholding device that consists of two surfaces connected at a 90° angle. Angle plates can securely hold rectangular workpieces or raise a workpiece from the worktable surface.

assembled

Produced by connecting and joining separate components together. A fixture is assembled through joining key components such as body fixtures, locators, supports, and clamps.

axes

An invisible straight line that helps describe the movement of an object in three-dimensional space. The axis usually passes through the center of an object.

base plates

A flat, rectangular fixture body with holes, slots, or both to allow the placement of fixture components. Base plates are set on a machine worktable for milling, drilling, or grinding operations.

binding

The unwanted fusing of the workpiece and locating device. Binding usually occurs when a locator is placed inside a hole and the cutting forces cause them to stick together.

blocks

A rectangular piece of metal used to reinforce a workpiece during a machining operation. Blocks provide a large support area but must be precisely machined.

bolt

A cylindrical fastener with exterior threads that mate with a nut. Bolts fit in holes on fixtures to allow operators to attach supporting, locating, and clamping components.

button

A fixture support component with a flat, circular head. Buttons fit in the holes on fixtures and provide a large support area.

cam clamps

A fixture clamping device that smoothly moves into place as a lever is moved. Cam clamps grip less tightly than other clamps but also cause less damage to surface finish.

cast

Manufactured through a process of pouring a liquid into a hollow mold and allowing the liquid to solidify. Cast parts have irregular surfaces.

cast iron

An alloy of iron, carbon, and silicon that contains at least 2% carbon. Cast iron is a hard, heavy material.

centers

A conical device that is inserted in the end of a cylindrical workpiece to allow it to rotate on a lathe. Centers provide support for workpieces during lathe operations.

chucks

A workholding device that secures a workpiece as it rotates on a lathe or other machine. Chucks commonly have three or four adjustable jaws that can be moved to fit workpieces of a variety of sizes.

clamping arm

The component of a clamp that makes contact with and grips the workpiece. Clamping arms may have heads or contact pads to reduce potentially damaging force they exert on the workpiece.

clamping range

The measurement span between a clamp's most open position and its closed position. Clamping range determines the minimum and maximum workpiece size a clamp can fit.

clamping surfaces

A section of the workpiece that can be gripped during a manufacturing operation. Clamping surfaces are usually separate from support and locating surfaces, though they may overlap with active surfaces.

clamps

A fixture component that tightens to hold a workpiece in place when it is exposed to the cutting forces of manufacturing. Clamps include strap clamps, screw clamps, and toggle clamps.

columns

A special type of pillar-shaped tombstone fixture with four, six, or eight identical sides. Columns can hold workpieces or vises on all their sides.

computer-aided design

CAD. The use of computers for the design and drawing of a part or other product. Computer-aided design drawings are precise and can be used by engineers to design a fixture for manufacturing the part.

computer-aided fixture design

CAFD. The process of using computer-aided design (CAD) based software to assemble and test possible fixture designs. Computer-aided fixture design allows engineers to quickly test a variety of possible fixture setups, optimizing the end result and increasing design speed.

concentric locator

An internal locator that restricts nine of the 12 degrees of freedom but allows some rotational movement. Concentric locators prevent movement along both the positive and negative portions of the X and Y axes and the negative portion of the Z axis and rotationally around the X and Y axes.

cutting forces

The various stresses involved in a machining process. Cutting forces include the pressure that a tool places on a workpiece when they come into contact.

cutting tools

A device with one or more edges used to create chips and remove material. Cutting tools include cutting inserts and drill bits.

datums

A point of reference for machine tools, programs, and fixtures from which measurements are taken or positions are located. A datum can be a hole, line, or any three-dimensional shape.

dedicated fixture

A custom workholding device that is machined or otherwise constructed for a specific workpiece. Dedicated fixtures are more expensive than modular fixtures but lead to better productivity and part tolerance.

deforming

Distorting or changing shape permanently. Deforming may occur with softer workpieces that are not carefully supported by a fixture during manufacturing.

diameter

The distance from one edge of a circle to the opposite edge, as measured through the center. Diameter describes the size of a cylindrical or round workpiece.

diamond pins

A type of fixture locating device with a polygon-shaped head. Diamond pins are used as internal locators that restrict workpiece rotation.

digital models

An electronic, three-dimensional (3D) version of a part or other device. Digital models allow fixture designers to create a variety of fixtures on the computer before creating the physical fixture.

dimensions

The measurements of a part. Dimensions for a part are usually described through the measurements of its height, width, and length.

dowel pins

A type of fixture locating device with a cylindrical body and flat surface as a head. Dowel pins are the most commonly used external locating device.

drilling

A machining operation in which a multi-point tool is fed into a workpiece to create a round hole. Drilling is often used as the first step in a hole-making process, followed by more precise procedures such as boring and reaming.

ductile

Able to bend, stretch, or form without breaking. Ductile materials are prone to deforming under the cutting forces of machining.

duplicate locators

A locator that restricts the same degree of freedom as another locator. Duplicate locators are inefficient as they require time and resources to implement and serve no purpose.

edge clamps

A fixture clamping device that grips a workpiece from the side. Edge clamps must generate great pressure in order to prevent the workpiece from lifting off the fixture.

equalizing supports

A support component with two heads that each move based on the position of the other head. Equalizing supports are best suited for irregularly shaped parts or parts with irregular surfaces.

external locating

The process of restricting workpiece movement by placing locators on outside surfaces. External locating is most commonly used for parts with flat outside surfaces and no holes.

faceplate

A circular fixture body with holes, slots, or both to allow the placement of fixture components. Faceplates are primarily used for lathe operations.

fasteners

A device that holds two or more objects together. Common fasteners include bolts and screws.

features

A defining characteristic on a component or part. Features include corners, edges, holes, and grooves.

feeds

The rate at which a cutting tool and/or workpiece move in relation to one another during a machining operation. Feeds affect the amount of cutting forces that an operation produces and must be accounted for in fixture design.

fixed supports

A support component made of a solid piece of material. Fixed components provide the most stable support of all support components.

fixture bodies

A component that provides the base for the other elements of a fixture. Fixture bodies include faceplates for lathes and base plates, angle plates, and tombstones for milling and drilling.

fixture designers

A person who makes all decisions in selecting and placing fixture components for a specific workpiece and operation. Fixture designers sometimes create the physical fixture, or they create a print that machine operators use to create the actual fixture.

fixture tolerances

The acceptable deviation from the ideal design for the fixture components, including the fixture body, supports, locators, and clamps. Fixture tolerances can refer to a component's position, size, or both.

fixtures

A customizable, modular workholding device created by configuring locators, supports, and clamps on a body fixture. Fixtures are useful when holding irregularly shaped workpieces or when holding multiple workpieces for a single operation.

fool-proofing locator

A locator that prevents an operator from loading a workpiece incorrectly in a feature. Fool-proofing locators reduce the possibility of cutting a workpiece incorrectly due to operator error.

grinding

A machining process in which an abrasive is used to precisely cut and shape the surface of a workpiece. Grinding operations are often used to remove small amounts of material from a workpiece and create good surface finish.

heads

The component on a support or locating device upon which the workpiece rests. Heads can be flat, conical, rounded, or hemispherical, among other shapes.

high-carbon steel

A steel containing more than 0.45% carbon. High-carbon steel is extremely hard and requires rigid fixtures to prevent vibration during machining.

holemaking

The act of machining a hole into the interior surface of a workpiece. Holemaking can be accomplished by drilling, boring, reaming, and tapping.

hydraulic

A system that creates power and motion through the controlled movement of pressurized liquid. Hydraulic systems are the preferred power source for power clamping because they take up less space than pneumatic systems.

internal locating

The process of restricting workpiece movement by placing locators on inside surfaces. Internal locating is more efficient and accurate than external locating.

lathe

A machine tool that holds and supports a cylindrical workpiece at one or both ends and rotates it while a single-point cutting tool removes material. Lathes are commonly used to perform turning operations.

length

The measurement of a component from one end to the other, rather than along its side. Length is one of three key dimensions, along with width and thickness, when assessing a fixture body.

level

Completely horizontal and in alignment. Level surfaces on fixtures ensure that a part is machined accurately.

leveling

Keeping the surface of one or more components completely horizontal and in alignment. Leveling surfaces on fixtures ensure that a part is machined accurately.

linear

Motion in a straight line along a single axis. Linear movement includes up and down, left and right, and back and forth.

locate

To establish a workpiece in a set location. A fixture locates the workpiece through the use of specific components, such as locating pins.

locating surfaces

A section of the workpiece that can be placed against a locating device to restrict movement. Locating surfaces will usually exist on three sides of a part.

locators

A fixture component that positions the workpiece and restricts its movement. Locators include locating pins, locating cylinders, and locating screws.

locking screw

A threaded device that can be opened or closed to hold a component in a particular location. Locking screws are used with springs in some adjustable supports.

lot size

The number of similar or identical parts that will be made using a specific workholding or cutting tool setup. Lot size helps determine the cost effectiveness of investing in a fixture.

machining

Using a material removal process to create a part. Machining processes include turning, milling, drilling, and grinding.

marring

Creating surface defects. Marring occurs when soft workpieces are supported, located, or clamped improperly in a fixture.

maximum-material condition

MMC. A state where a workpiece or part has the largest amount of material remaining while still being in tolerance. Locators designed for the maximum-material condition will always fit a workpiece or part that meets tolerance, while locators designed for the minimum-material condition will not.

milling

A machining operation in which a multi-point horizontal or vertical cutter is used to remove material from a workpiece. Milling is often used to create flat surfaces or to create grooves or pockets in a flat surface.

modular

Consisting of interchangeable and discrete parts. Modular fixtures have individual pieces that can be replaced more easily than if the whole fixture had to be replaced.

motion economy

A way of moving that eliminates all unnecessary motion to perform an action. Motion economy should be maximized in order to ensure efficient productivity.

nonproductive time

Any period in a manufacturing operation during which a part is not actively being created. Nonproductive time includes setup, pauses during manufacturing, and finishing steps and should be reduced where possible.

nut

A fastener with a hole and internal threads that mate with a bolt or screw. Nuts can serve as the head for support and locating fixture components.

operators

A person trained to run a specific machine. Operators are often responsible for setting the workpiece in the fixture for operation.

parallels

A pair of identical support blocks. Parallels help reinforce and raise workpieces in a variety of milling, drilling, and grinding operations.

pins

A cylindrical device, sometimes with a shaped head, used on fixtures to restrict workpiece motion. Pins are the most commonly used fixture locating component.

pivot

A central point around which a component swings or rotates. Pivots allow components to move in precise relationships to one another.

pivot pin

The point on a toggle clamp around which the clamping arm rotates. The pivot pin locks the clamping arm in place when operators pull down a lever to lock the pivot pin.

pneumatic

A system that creates power and motion through the controlled movement of compressed gas, usually air. Pneumatic power sources are used with some power clamping systems.

pocket

An interior recess that is cut into the surface of a part. Pockets can be circular, rectangular, or irregular.

post

The solid component of a clamp that holds the clamping arm and is secured to the fixture body. Posts are usually secured to the fixture body using bolts or other fasteners.

power clamping

The use of a fixture securing system energized by hydraulic or pneumatic power sources. Power clamping has a range of advantages over manual clamping, including faster loading and unloading of parts, greater and more consistent pressure, and the ability to adjust pressure during an operation.

pressure

The exertion of mechanical force on an object. Pressure in fixtures occurs when a clamp is tightened against a workpiece.

print

A document containing all the instructions necessary to manufacture a part. A print, or blueprint, includes a part drawing, dimensions, and notes that will describe the order and type of manufacturing operations and important part specifications.

production rates

The speed at which a manufacturing operation produces parts. Production rates can be improved through the selection of proper workholding.

radial locator

An internal locator that restricts rotational movement around the Z axis. Radial locators restrict three of the 12 degrees of freedom.

rests

A device used to support long workpieces on a lathe. Rests are circular in shape with adjustable fasteners or wheels that are tightened to grip the workpiece.

rework

The process of running a part through an additional and unplanned manufacturing step. Rework is necessary when a part is not produced within the correct tolerance or has poor surface finish or other defects.

rigid

Stiff and inflexible. Rigid machine setups reduce issues like vibration and lead to better tolerance and surface finish on a part.

rotational movements

Motion in a circular path. Rotational movements can occur in both directions around an axis.

scrapped

Discarded because of poor tolerance or other defects. Scrapped parts increase production costs because of the lost time and material.

setups

The position of tooling and fixturing for a machining operation. The setup of a machine for manufacturing will affect the proper placement of components on a fixture.

slots

A narrow channel in the surface of a part or component. Slots allow fixture components to attach to fixture bodies.

speeds

The rate at which a workpiece or machine spindle rotates during a machining operation. Speeds affect the amount of cutting forces that an operation produces and must be accounted for in fixture design.

spherical-head pins

A type of fixture locating device with a smooth dome head. Spherical-head pins spread cutting forces more efficiently when used as external locators and their smooth surfaces reduce binding when used as internal locators.

spring

A flexible device made of coiled material that yields under force and returns to its original shape when the force is removed. Springs are used in some adjustable supports to allow for easy movement of the support component.

stability

The ability of an object to remain firmly in position. Fixture stability can be improved by using thicker fixture bodies made of heavier materials.

steel

An alloy of iron and carbon containing less than 2% carbon. Steel comes in varying levels of hardness and is the most common manufacturing metal.

step blocks

The component of a strap clamp that raises the strap to the appropriate height. Step blocks have a series of risers so that a single step block can be used to move a strap to varying heights.

strap clamps

A fixture clamping device made from a series of interchangeable components. Strap clamps can quickly be assembled and adjusted to suit a wide range of fixturing requirements.

straps

The component of a strap clamp that makes contact with and grips the workpiece. Straps are generally strips of metal with a curved end that holds the workpiece.

support surfaces

A section of the workpiece that can be reinforced during a manufacturing operation. Support surfaces are where engineers will place supporting components when creating fixtures.

supports

A fixture component that provides reinforcement to the workpiece during machining to prevent it from breaking or deforming. Supports include rest buttons, support pins, and centers.

surface area

The total area of an outer surface of an object. The surface area of a support surface refers to the amount of the component that will be in contact with and supporting the workpiece.

surface finish

The degree of roughness and variation on the surface of a part after it has been manufactured. Surface finish can be damaged by excessive pressure from clamps, vibration against fixture components, or improper securing and location.

swing clamps

A fixture clamping device that rotates around a fixed axis. Swing clamps exert a large amount of cutting pressure because they must resist their rotational movement when tightened.

thickness

The measurement of a component from top to bottom. Thickness helps determine the stability a fixture body provides.

thread

A long spiraling groove machined in the interior or exterior of a part. Threads allow compatible fasteners to connect to and move about each other.

toe clamps

A fixture clamping device that grips a small portion of the workpiece with a large amount of pressure. Toe clamps are useful for workpieces with small clamping surfaces or to provide more access for the cutting tool.

toggle clamps

A fixture clamping device that locks into place when an operator moves a lever. Toggle clamps are easy to operate and produce strong pressure but can only be adjusted slightly to accommodate various workpiece thicknesses.

tolerance

The acceptable variation from a specific dimension. Tolerances represent the amount a part can deviate from its specifications while still performing its desired function.

tombstones

A vertically oriented rectangular fixture that can hold workpieces on two parallel surfaces. Tombstones are often used to machine a series of workpieces at the same time.

T-slots

A T-shaped channel in a component. T-slots enable engineers to place fixture components anywhere along the component where the slot runs.

turning

A machining operation that guides a single-point tool along the length of a rotating cylindrical workpiece to remove material. Turning often involves the use of a faceplate as its fixture body.

variables

A value, factor, or element that is liable to change. Variables in fixture design include workpiece material and operation speed and feed.

v-blocks

A workholding component with a v-shaped groove down the middle of its rectangular body, and sometimes including an adjustable gripping component, such as a screw. V-blocks, or v-locators, are used to hold and support cylindrical workpieces for milling, drilling, and grinding.

vibrate

Move quickly back and forth. Hard workpiece materials sometimes vibrate during machining, which can damage the tool and workpiece.

vises

A workholding device with two jaws, usually one fixed and one movable, that grip and hold a workpiece in place. Vises are often used to hold workpieces for milling, drilling, or grinding operations.

v-locators

A workholding component with a v-shaped groove down the middle of its rectangular body, and sometimes including an adjustable gripping component, such as a screw. V-locators, or v-blocks, are used to hold and support cylindrical workpieces for milling, drilling, and grinding.

width

The measurement of a component along its side, rather than from end to end. Width is one of the three key dimensions, along with length and thickness when assessing a fixture body.

workholding

A device used to secure, support, and locate a workpiece during a manufacturing operation. Workholding for irregularly shaped workpieces often involves the use of fixtures, which can be custom designed.

workpiece

An unfinished part that is subjected to one or more manufacturing operations in order to create a finished part. Workpieces can be shaped and finished through the various machining operations, such as metal cutting and grinding.