The Fixture Design Procedure

Prof. Ms. Amruta A. Rane

Assistant Professor,

DJSCE, University of Mumbai,

Mumbai.

Steps in Fixture Design Procedure1. Locating

2. Clamping

3. Supporting

4. Applying cutter guides

5. Drawing the fixture outline as an envelop that combines all the previously drawnelements.

In practice, one operates continuously with the part and hence the part outlineshould be drawn either in thin lines or in coloured lines.

Locating and Degree of Freedom• The forces acting (weight, clamping force, cutting forces) are not taken into

consideration with respect to their magnitude.

• Only direction of the forces are considered to ensure that part is located in aposition of static stability.

The Six Degrees of Freedom• Figure shows a body that is free in space.

• A body in this condition had six degrees of freedom, three of these are freedomsof translation and three are freedoms of rotation.

Locating and Degree of Freedom• Consider a rectangular block (casting for

forging), with unmachined rough sides.

• Locate the bottom surface on three pointsnot in a straight line.

• Assume hold-down forces acting on theblock such that it can not be lifted off.

• These three points prevent the motion inthe vertical direction and rotation arounda longitudinal and cross-wise axis.

• The block is deprived of three degrees offreedom.

Locating and Degree of Freedom• Two locating points are added against one

of the vertical sides.

• Not in same vertical line.

• Hold-down force is added.

• This prevents the motion in crosswisedirection and also rotation about verticalaxis.

• Thus deprives the block of two degrees offreedom.

Locating and Degree of Freedom• One locating point is added on a vertical

wall.

• Hold-down force against the end

• This eliminates the sixth and last degreeof freedom i.e. motion along the length.

Locating and Degree of Freedom• The addition of the forth point at the bottom surface would theoretically make

the system redundant.

Locating and Degree of Freedom• In actual fixture design the very first step is to deprive the part of its six degrees of

freedom by applying six individual locating points.

Clamping Elements• Hold-down forces in above example are clamping forces.

• In practice they are applied by using bolts, straps, cams, etc.

• The number of clamping elements used are not necessarily equal to the numberof locating points.

Clamping Elements• One vertical clamp over the centre of the black would be sufficient to take care of

three locating points.

• One clamp acting on the corner and directed along the diagonal would take careof all six locating points.

• Clamping force must be applied as directly as possible without causing anyelastic deformation.

Support• Some support is provided by locating points.

• The locators only provide sufficient support to ensure the geometrical stability ofthe part.

• The locating points are not sufficient to absorb all the acting loads withoutcausing elastic deformation of the part.

• Support – Sufficient in number and strength to absorb all acting loads.

• Should not interfere with locating points.

• Should not interfere with loading and unloading.

• Shouldn’t produce geometrical redundancy.

Cutter Guidance• To provide guidance for the cutter.

• By providing the drill bushings in drill jigs

• By providing the tool guides to position the tool prior to start of the cut in fixtures.

Completing the body• To draw the jig or fixture body that carries all the individual elements.

• Has enough strength and rigidity to hold them in proper places under load.

Jig or fixture body must accommodate following conditions:

• Must accommodate the part

• Have clearance for loading and unloading

• Clearance for chips

• Should have feet or some other supporting surface to place it on the machinetable.

• Locating elements for aligning it with machine table, machine spindle

The Six Point Location Principle• For processing the body, it is required to

restrain all the degree of freedom(DOF).

• Six pads showing a system of locationand clamping that produces the effectnecessary for complete constrain.

Six-point Location

3-2-1 Principle• It represents the minimum requirement for

the locating elements.

• The locators along with the clamps whichhold the part in place, provides equilibriumfor all the forces.

• Do not necessarily guarantee stabilityduring machining.

• Stability is satisfactory if three rest buttonsare widely spaced and resultant cuttingforce hits within the triangular areabetween the buttons.

• It resultant cutting force hits outside thetriangular area then it generates a momentwhich tends to tilt or overturn the part.

Six-point Location

4-2-1 Principle• By addition of the fourth locator in the

base, the shape of the supporting area canbe changed from a triangle to a rectangle.

• For the rough castings one of the four baselocators may be adjustable.

• If the surface is machined then all thelocators may be fixed.

References1. Jig and Fixture Design Manual, Erik K. Henrikson, Industrail Press.

2. Jigs and Fixture, P.H. Joshi, THM.

3. An introduction to jig and tool Design, M.H.A. – Kempster, III Ed.Pub ELBS.

Thank You!