reduction of reworks and rejections in manufacturing · pdf file ·...

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Int. J. Mech. Eng. & Rob. Res. 2014 P Mahesh Babu and K Sreenivas, 2014

ISSN 2278 – 0149 www.ijmerr.com

Vol. 3, No. 4, October, 2014

© 2014 IJMERR. All Rights Reserved

Research Paper

REDUCTION OF REWORKS AND REJECTIONS IN

MANUFACTURING OF A THIN WALLED AEROSPACE

COMPONENT

P Mahesh Babu1* and K Sreenivas2

*Corresponding Author: P Mahesh Babu � [email protected]

Nowadays rejection is a serious problem arising in every manufacturing industry. Due to thisreason the manufacturing industry is advancing into losses. There are many factors responsiblefor rejection and reworks, such as human errors, machine errors, process planning. Machiningof thin-walled components has increasingly become a difficulty for manufacturers. Advanceddigital analyses have been developed by many researchers to model, predict and reduce errorsinduced by machining processes. Moreover, today’s machining shop floors, characterized by alarge variety of products in small batch sizes, require flexible simulation tools that can be quicklyreconfigured. CAD/CAM systems play a vital role in design optimization and process optimizationof any component and helpful in reducing the rejections and reworks. Keeping in view theabove important aspect this project has been taken up for reducing the rejections to a minimum.The aim of the project is to reduce rejection rate rework rate. The thin wall component taken forthis project is a missile shield. The missile shield protects the missile by covering the entirebody. The missile shield design and process planning are studied in detail. This project alsodeals with development of manufacturing process plan of missile component (missile shield)using CAM software (NX 7.5) which is exclusively CAM software used to generate part programby feeding the geometry of the component and defining the proper tool path and thus transferringthe generated part program to the required CNC machine with the help of DNC lines. In thisproject efforts are made to produce different process plans in CAM software by changing thework holding systems, tool paths, cutting tools etc. Finally, recommending an optimum processplan for manufacturing of the component..

Keywords: NX-CAD, NX-CAM, Mandrel design, Manufacturing process plan, WHY-WHY

analysis, MORI SEIKI 4-axis turning machine

1 M. Tech Student, Department of Mechanical Engineering, Krishna Chaitanya Institute of Technology & Sciences, Markapur – 523 316,Prakasam District, A.P., India.

2 Professor, Department of Mechanical Engineering, Krishna Chaitanya Institute of Technology & Sciences, Markapur – 523 316, PrakasamDistrict, A.P., India.

INTRODUCTION

In the context of modern and fast emerging

industrial world, technology and automationis considered to be a big boon for the fast

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and rapid industrial development. The mainobjective of today's industries is to maximizethe production within the stipulated time usingan optimum amount of man power andresources. The trends which accounts fortoday's technological development includesCNC machining, DNC, FMS, CAD/CAM etc.The development in these factors affects theproduction very much by increasing thequantity as well as the quality. Thesedevelopments have been highly utilized inenhancing our defense warheads whichincludes several missiles AGNI, THRISHUL,PRUTHVI, AKASH, NAG etc.

A missile is a self-propelled guided weaponsystem. Missiles have four systemcomponents: targeting and/or guidance, flightsystem, engine, and warhead. Missiles comein types adapted for different purposes:surface-to-surface and air-to-surface(ballistic, cruise, anti-ship, anti-tank), surface-to-air (anti-aircraft and anti-ballistic), air-to-air, and anti-satellite missiles. The missileshield protects the missile by covering theentire body. The missile shield design andprocess planning are studied in detail.

This project also deals with developmentof manufacturing process plan of missilecomponent (missile shield) using CAMsoftware (NX 7.5) which is exclusively CAMsoftware used to generate part program byfeeding the geometry of the component anddefining the proper tool path and thustransferring the generated part program tothe required CNC machine with the help ofDNC lines. The operator thus executes theprogram with suitable requirements.

The latest CAM software introducedincludes the new NX 7.5 software, which has

important features like 2D, 3D and surfacemodeling. The component and fixture can beeither designed on this software or can beretrieved from any other CAD software. Thensequence of program such as modeling thecomponent, generating the tool path,selection of tools according to the sequenceof operations and sizes, at last the generatedNC part program is verified and sent to therequired CNC machine to manufacture theparticular missile component. Optimizing feedand speed to get high surface finish can alsobe done.

COMPUTER AIDED DESIGN

Figure 1: 2D Input of Missile ShieldFinal 3D Model of Missile Shield Using

Unigraphics NX-7.5

Figure 2: Final 3D Model

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Manufacturing process plan

• Identifying suitable machine.

• Selecting suitable tools formanufacturing thin walled component.

• Designing fixture/mandrel to missileshield component for externaloperations.

• Listing down the Sequence ofoperations performed on missile shieldcomponent.

• Generating tool path at specified cuttingspeed.

Identify Suitable Machine

MORI SEIKI 4-AXIS CNC turning machineis used for machining missile shield. DMGMORI SEIKI offers the industry's best lineupof high-performance lathes with betterprecision and rigidity, greater multi-axiscompatibility and smaller footprints.

Figure 3: MORI SEIKI 4-axis Machine

Selecting Suitable Tools

OD_80_L Facing, Roughing

Facing in the context of turning workinvolves moving the cutting tool at rightangles to the axis of rotation of the rotatingwork piece.

ID_80_L rough

Rough tool used to create deepgrooves which will remove a completed orpart-complete component from its parentstock internally.

Spot Drilling

This operation subtype allows the toolto pause at the tool tip or shoulder depth ofthe tool by a specified number of seconds orrevolutions.

Spot Drilling

This operation subtype allows the toolto pause at the tool tip or shoulder depth ofthe tool by a specified number of seconds orrevolutions.

Selection of Fixture

3-jaw chucks provide the quickest and easi-est way of holding work in the milling andlathe machines.

Figure 4: 3-Jaw Chuck

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DESIGN OF MANDREL

Mandrel is used to support missile shieldcomponent internally to allow high cuttingspeed. When external operations done on thethin walled component without any supportfrom inside it may damage or scratches mayform due to its thin wall. To overcome this,mandrel is designed to reduce rejection rateand to increase production rate.

Mandrel is modeled by considering innerdimensions of missile shield. Innerdimensions of missile shield will be outerdimensions of mandrel.

Input 2D Drawing for Mandrel.

Figure 5: 2D Input of Mandrel

Below image shows the final part of themandrel

Figure 6: Final Part of Mandrel

Sequence of operation

Set up-1

• Facing operation

• OD_Rough_Turn operation

• ID_Rough_Bore operation

Set up-2

• Facing operation

• OD_Rough_Turn operation

• Finally Drilling operation

Generating tool path

Turning operations on missile shield

Material of missile shield is aluminum alloys.Aluminium alloys are light weight and highstrength material.

Below image shows ROUGH TURN_ODoperation of missile shield with 1500rpmspeed and 0.2mmpr feed.

Figure 7: ROUGH TURN_OD operation

Below image shows verification ofROUGH TURN_OD operation

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Figure 8: Verification of ROUGHTURN_OD Operation

Milling Operations on Missile Shield

Below image shows drilling operation ofmissile shield with 1400rpm speed and240mmpm feed.

Figure 9: Drilling Operation

Below image shows verification of drillingoperation

Figure 10: Verification of Drilling Operation

The Manufacturing Process of MissileShield on CNC Machine.

Raw material is placed on the machine, anddegree of freedom is arrested using fixtures.Facing operation is general operation whichwill be done for any component, after facinginternal operations are done on the missileshield.

First Step: facing operation is done on theraw material

Second Step: internal roughing operationdone on the component

Third Step: the designed mandrel is fixedinternally in the missile shield component andexternal roughing is done

Fourth Step: the component is fixedreversely in the fixture and setup_2operations are done. Outer roughingoperation is done.

Fifth Step: drilling operation is done on finalturning component on milling machine.

Surface finish is not obtained by using nonexpandable designed mandrel, due to thegap between the mandrel and missile shieldcomponent. At high cutting speeds the loadof the tool is directly applied on the missileshield component and scratches are formeddue to the gap between the missile shieldcomponent and mandrel. Hence increase inrejection rate due to bad surface finish. Inorder to overcome from this rejection rateexpandable mandrel is designed for missileshield.

DESIGN OF EXPANDABLE

MANDREL

Below shows the 2D drawings of theexpandable mandrel with all the required

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dimensions representations the suits the bestfor manufacturing the component without anyerrors.

Figure 11: 2D Input of Expandable Mandrel

Figure 12: Final Part of Expandable Mandrel

Missile shield is again machined by usingexpandable mandrel. After completinginternal operation expandable mandrel isused as jig. Expandable mandrel expandsinside the Missile shield component andmakes contact; this contact supports thecomponent at high cutting speeds and giveshigh surface finish.

WHY-WHY Analysis

It is a method of questioning that leads to theidentification of the root cause(s) of a

problem. A why-why is conducted to identifysolutions to a problem that address it's rootcause(s). Rather than taking actions that aremerely band-aids, a why-why helps to identifyhow to really prevent the issue fromhappening again. A why-why is most effectivein a team setting or with more than oneperson involved. Involve the right people - ithelps to have those that are familiar with theprocess and the problem in the room so theyare able to answer why something happened.It is also helpful to have someone with a fresheye participate - often they ask questions thathelp those involved in the problem extractthe real reasons something happened.

Main Causes of Rejection Rate inIndustries

Operator’s negligence at work place and theirpoor knowledge in manufacturing. Rejectionrate also increased due to equipment suchas component setup, assigning impropertools, fixture design problems. Another causeof increase in rejection rate is due toprocedure of machining like mistakes insequence of operations (Turning, Milling andDrilling). Another cause is following the normsor rules of the company in impossibleconditions of machining the component.

Root Causes of Rejection Rate inManufacturing of Missile Shield

Missile shield is thin walled component it isdifficult to manufacture. Rejection rate dueto equipment such as component setup,assigning improper tools, fixture designproblems. Another cause due to improperdesign of mandrel. Going for high cuttingspeed which is not preferred for machiningMissile shield component in order to reduce

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machining time. Another cause of increasein rejection rate is due to procedure ofmachining like mistakes in sequence ofoperations (Turning, and Drilling).

Solution Obtained to Reduce Rejectionand Reworks

To reduce rejection rate the thin walledcomponent is manufactured in a sequenceas first internal operations and next by usingmandrel support external operations aredone. Proper tools are specified which willsupport for machining thin walled component.Redesign of mandrel is done to reach highsurface finish without fail.

RESULTS

Results are represented graphically to specifythe quality control of missile shieldcomponent.

Graphical Representation of Rejectionand Reworks Rate

Below graphs shows the rejection andreworks rate before WHY-WHY analysis andafter WHY-WHY analysis.

Figure 13: Rejection and ReworksRate Before WHY-WHY Analysis

Results After WHY-WHY Analysis

These four causes for rejection which arementioned above is rectified by using WHY-WHY Analysis. The following graph indicatesrejection rate after WHY-WHY Analysis.

Figure 14: Rejection and ReworksRate After WHY-WHY Analysis

CONCLUSION

By considering 2D inputs 3D model isgenerated using NX-CAD software. Tool pathis generated on missile shield using NX_CAMsoftware. The thin walled (missile shield)component is manufactured in a sequenceas first internal operations and next by usingmandrel support external operations are doneto reduce rejection rate. Non-expandableMandrel has been designed to support thecomponent for external operations. By usingNon-expandable Mandrel the rejection rateis more due to the gap between the mandreland missile shield. Proper tools are specifiedwhich will support for machining thin walledcomponent. Expandable mandrel is designedto overcome the rejection rate. Use ofexpandable mandrel results in less rejectioncompared to before mandrel. By WHY-WHYanalysis is done to check rejection and reworkrate is reduce or not. Graphical represen-

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tation of rejection and reworks rate beforeand after WHY-WHY analysis is shown inresults.

REFERENCES

1. Colvin, Fred H, Haas and Lucian L(1938), “Jigs and Fixtures: A ReferenceBook”, New York and London: McGraw-Hill Book Company.

2. Henriksen and Erik K (1973), “Jig and

Fixture Design Manual”, New York, N.Y.:Industrial Press Inc.

3. Karn G Bulsuk (2009), “An Introductin to5-why”.

4. Karn G Bulsuk (2009), “5-whys Analysisusing an Excel Spreadsheet Table”.

5. Norman Bodek (1988), “ToyotaProduction System: Beyond Large-ScaleProduction”, Productivity Press.

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