workholders work piece control

8/12/2019 workholders work piece control

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WorkholdersWorkholders andand

WorkpieceWorkpiece ControlControl


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Elements of a Machining SystemElements of a Machining System

Cutting Tool

Tool

Holder

Machine

Workpiece

Workholder


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Machining System AnalysisMachining System Analysis


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WorkpieceWorkpiece

A partially finished part or assembly whichA partially finished part or assembly which

does not as yet have all the part printdoes not as yet have all the part print

specifications. Thespecifications. The workpieceworkpiece changeschangesfrom operation to operation and is thatfrom operation to operation and is that

piece of material upon which work is beingpiece of material upon which work is being

performed.performed.


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WorkholderWorkholder

All devices that hold, grip, or chuck aAll devices that hold, grip, or chuck a workpieceworkpiece

to safely perform a manufacturing operation.to safely perform a manufacturing operation.


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Variables in MachiningVariables in Machining

Independent Variables:Independent Variables:

Cutting Tool Material,Cutting Tool Material,Condition, GeometryCondition, Geometry

WorkpieceWorkpiece Material,Material,Condition, GeometryCondition, Geometry

Machine Tool ParametersMachine Tool Parametersand Characteristicsand Characteristics

Cutting / Lubricating FluidsCutting / Lubricating Fluids

Cutting SpeedCutting Speed Cutting Feed RateCutting Feed Rate

Cutting DepthCutting Depth

Dependent Variables:Dependent Variables:

Type of ChipType of Chip

Forces and EnergyForces and Energy

TemperaturesTemperatures Tool Wear and Tool LifeTool Wear and Tool Life

WorkpieceWorkpiece FinishFinish

Dimensional AccuracyDimensional Accuracy

Material Removal RateMaterial Removal Rate

Type ofType ofWorkholderWorkholder


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Function ofFunction ofWorkholderWorkholder

Position or locate aPosition or locate a workpieceworkpiece in a definitein a definite

relation to the cutting tool.relation to the cutting tool.

Must withstand holding and cutting forcesMust withstand holding and cutting forceswhile maintaining precise location.while maintaining precise location.

Provide a safe operating environment.Provide a safe operating environment.


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Elements of aElements of a WorkholderWorkholder

Locating elementsLocating elementsposition theposition theworkpieceworkpiece

Tool structure or bodyTool structure or bodyprovide base forprovide base forwithstanding holding and machiningwithstanding holding and machiningforces.forces.

Brackets (ifBrackets (ifreqreqdd))provides means toprovides means to

attachattach workholderworkholderto machine bed.to machine bed. Clamping elementsClamping elementsprovidesprovides workholderworkholder

holding forces.holding forces.


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WorkholderWorkholder-- Mill FixtureMill Fixture


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WorkpieceWorkpiece ControlControl

WorkpieceWorkpieceControlControl -- To insure successfulTo insure successfuloperation of aoperation of a workholdingworkholding device, thus insuringdevice, thus insuringthat thethat the workpieceworkpiece is produced within theis produced within the

tolerances specified on the part print, thetolerances specified on the part print, theworkpieceworkpiece must be accurately located tomust be accurately located toestablish a definite relationship between theestablish a definite relationship between thecutting tool and some points or surfaces of thecutting tool and some points or surfaces of theworkpieceworkpiece. This relationship is established by. This relationship is established by

locators in thelocators in the workholdingworkholding device which positiondevice which positionand restrict theand restrict the workpieceworkpiece to prevent its movingto prevent its movingfrom its prefrom its pre--determined location.determined location.


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WorkpieceWorkpiece ControlControlconcontt..


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Geometric ControlGeometric Control

Deals with the placement of locators in order to insureDeals with the placement of locators in order to insure stabilitystability of theof theworkpieceworkpiece..

RULES:RULES:

Place locators as far apart as possible.Place locators as far apart as possible.

Place locators to insure balance ofPlace locators to insure balance ofworkpieceworkpiece..

Place locators such that holding forces will not shift thePlace locators such that holding forces will not shift the workpieceworkpieceaway from the locators.away from the locators.

Place locators such that tool forces will not shift thePlace locators such that tool forces will not shift the workpieceworkpiece awayawayfrom the locators.from the locators.

Place locators in such a way that thePlace locators in such a way that the workpieceworkpiece is easy to load andis easy to load andunload from theunload from the workholderworkholder..

Place locators such that they contact thePlace locators such that they contact the workpieceworkpiece on a solid,on a solid,stable point or surface.stable point or surface.


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Dimensional ControlDimensional ControlThat control relating to the maintenance of physical dimensionsThat control relating to the maintenance of physical dimensions as specified onas specified on

the part print. Thethe part print. The workpieceworkpiece must be produced within tolerances shownmust be produced within tolerances shown

for each dimension. Dimensional control is obtained by:for each dimension. Dimensional control is obtained by:

Selecting the proper surfaces for placement of locators.Selecting the proper surfaces for placement of locators.

Correctly positioning those locators on the selected surfaces.Correctly positioning those locators on the selected surfaces.

Rules:Rules:Place locators so that no process tolerance stacks are present.Place locators so that no process tolerance stacks are present.

Place locators so thatPlace locators so that workpieceworkpiece variation does not interfere with obtaining thevariation does not interfere with obtaining the

dimensions within tolerance.dimensions within tolerance.

Place locators so thatPlace locators so that workpieceworkpiece irregularities do not interfere with obtainingirregularities do not interfere with obtaining

the dimensions within tolerance.the dimensions within tolerance.

Dimensional control is best when locators are placed on one of tDimensional control is best when locators are placed on one of two surfaceswo surfacesto which the dimensions are shown on the part print.to which the dimensions are shown on the part print.

Dimensional control is best when locators are placed astride theDimensional control is best when locators are placed astride the centerline tocenterline to

which the dimension is shown on the part print.which the dimension is shown on the part print.

When close tolerances are required on parallelism,When close tolerances are required on parallelism, squarenesssquareness, and, and

concentricity, more than one locator must be placed on one of thconcentricity, more than one locator must be placed on one of the surfaces toe surfaces to

which the tolerances apply.which the tolerances apply.


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Mechanical ControlMechanical Control

That control relating to the proper application of forces on theThat control relating to the proper application of forces on the workpieceworkpiece -- tool forces andtool forces andholding forces.holding forces.

RULES: TOOL FORCESRULES: TOOL FORCES

Place locators opposite tool forces to controlPlace locators opposite tool forces to control workpieceworkpiece deflection.deflection.

When required, use fixed supports to limitWhen required, use fixed supports to limit workpieceworkpiece deflection caused by tool forces.deflection caused by tool forces.

When quality outweighs economy, adjustable supports may be usedWhen quality outweighs economy, adjustable supports may be used to oppose toolto oppose toolforces.forces.

Use tool forces to assist in holding theUse tool forces to assist in holding the workpieceworkpiece against the locators.against the locators.

RULES: HOLDING FORCESRULES: HOLDING FORCES

Place holding forces directly opposite locators.Place holding forces directly opposite locators.

Use supports when necessary to control deflection caused by holdUse supports when necessary to control deflection caused by holding forces.ing forces.

Holding forces must make up for the loss of the sixth locator byHolding forces must make up for the loss of the sixth locator by using friction.using friction. NonNon--rigidrigid workpiecesworkpieces require several holding forces rather than one large force.require several holding forces rather than one large force.

WorkpieceWorkpiece marring can be controlled by placing holding forces on nonmarring can be controlled by placing holding forces on non--criticalcriticalsurfaces.surfaces.

Holding forces applied by one resultant force are desirable to rHolding forces applied by one resultant force are desirable to reduce the effect of theeduce the effect of thehuman element.human element.


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Tool Design ProcessTool Design Process

Statement and analysis of the problemStatement and analysis of the problem

Analysis of the requirements.Analysis of the requirements.

Development of initial ideasDevelopment of initial ideas Development of design alternativesDevelopment of design alternatives

Finalization of the design ideasFinalization of the design ideas


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Design DevelopmentDesign DevelopmentFollowing is one structured method that can be utilized when desFollowing is one structured method that can be utilized when designingigning workholdersworkholders (jigs or fixtures):(jigs or fixtures):

1.1. Sketch or layout theSketch or layout the workpieceworkpiece in the required number of views (or obtain / create a 3in the required number of views (or obtain / create a 3--dimensionaldimensionalcomputer modelcomputer modelat full scale) to a proper scale. Theat full scale) to a proper scale. The workpieceworkpiece should be represented withshould be represented withphantom lines in an identifying colorphantom lines in an identifying colorusually red.usually red.

NOTE:NOTE: Scales are listed on the tool drawing in the form of a ratio.Scales are listed on the tool drawing in the form of a ratio. Appropriate scales are asAppropriate scales are asfollows: 1:1, 1:2, 1:4, 1:5, 1:8, and 1:10follows: 1:1, 1:2, 1:4, 1:5, 1:8, and 1:10 reducedreduced and 2:1 and 4:1and 2:1 and 4:1 enlargedenlarged..

2.2. Add reference information to the layout (blue phantom lines). TAdd reference information to the layout (blue phantom lines). This includes the following:his includes the following:

Cutting tools utilized in the operations.Cutting tools utilized in the operations.

Gages utilized to fabricate the tool.Gages utilized to fabricate the tool. Machine travel constraints / machine work envelope.Machine travel constraints / machine work envelope.

Facilities constraints (table heights, room area constraints, etFacilities constraints (table heights, room area constraints, etc.)c.)

3.3. Develop the locating system for theDevelop the locating system for the workpieceworkpiece and indicate on the drawing. (Followand indicate on the drawing. (Follow workpieceworkpiececontrol rules).control rules).

4.4. Develop a clamping system and indicate on the drawing. (FollowDevelop a clamping system and indicate on the drawing. (Follow workpieceworkpiece control rules).control rules).

5.5. Inspect for clearance or interference problems.Inspect for clearance or interference problems.

6.6. Add drill bushings if/as required.Add drill bushings if/as required.

7.7. Add cutterAdd cutter--setting devices if/as required.setting devices if/as required.8.8. Inspect for clearance or interference problems.Inspect for clearance or interference problems.

9.9. Add the tool body.Add the tool body.

10.10. Indicate the proposed mounting device if/as required.Indicate the proposed mounting device if/as required.

11.11. Check for the proper relationship between the jig or fixture andCheck for the proper relationship between the jig or fixture and the machine tool componentsthe machine tool components(spindle, cutting tool, machine bed, etc.).(spindle, cutting tool, machine bed, etc.).

12.12. In cases of Computer Numerical Control (CNC), check the programIn cases of Computer Numerical Control (CNC), check the program to insure that the tool will notto insure that the tool will notinterfere with the operation of the machine tool.interfere with the operation of the machine tool.

13.13. Initiate the final design drawings.Initiate the final design drawings.

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workholders work piece control