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Ensuring Zero Defect in Metro Coach Bogie Manufacturing by Business Process Re-Engineering

BEML LiMitED

AbstrAct

Metro bogie frame was imported at a high cost till 2015. As a part of import substitution and under “MAKE IN INDIA” policy, BEML decided to indigenize Metro bogie frame manufacturing in 2016. At the time of indigenization, distortion up to 5mm observed causing less/nil stock availability for critical machining requirement of 30 microns tolerance.

DMAIC approach was adopted to solve the problem. Distortion data collected for eight parameters using four bogie frames. Principle factor component analysis carried out and three parameters identified which contributed to 76.1% of total variation. Sigma rating calculated for three identified parameters i.e. 2.15, 1.5 & 1.66.

Based on process mapping, detailed brainstorming conducted and Ishikawa diagram was drawn for probable root causes. All probable root causes validated using appropriate validation methods like data analysis, gemba study etc.

Bevel angle, weld sequence & root gap were identified as potential root causes after analysis. Design of experiment was conducted using Taguchi L8 experiment (level 2 for 2 factors & level 4 for 1 factor) to get the optimal process parameters.

With new level of process parameters, trial implementation was done for 5 nos. Bogie frames and found no distortion. Statistical process control & process capability achieved for critical parameters.

Keywords: taguchi, Process capability, DMAiC, Zero defect, indigenization & distortion

introduction

Metro Bogie manufacturing is a giant leap towards self – reliance. BEML entered into a MoU with Centre of Railway Research, IIT, Kharagpur under Industry - Institute partnership for knowledge transfer on bogie dynamics & training. Re-engineering was taken up to convert existing Railway Bogie manufacturing facility into Metro bogie manufacturing facility. Railway Bogie manufacturing jig/fixtures & machinery were re-engineered to suit metro bogie manufacturing with an initial investment of Rs.14 Lakhs. Detailed component manufacturing started in Sept’2016. Consultancy was taken from Welding Research Institute, Trichy to improve / establish world class manufacturing process to meet stringent quality requirement of bogie. Upgraded the skill of welders by providing Skill Development training at BEML Welding School. Welders were certified for Radiography welding quality as per ISO 5817, Level B. Attitudinal change was brought in among the employees at grass root level by providing training through Centre of Excellence, BEML. Vendor development was done to outsource critical machining requirement.

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Quality Assurance in Metro Bogie Frame Manufacturing

Bogie Manufacturing is done under stringent Quality Control. Detailed QAP/ITP with 22 stages of inspection (13 random witness & 9 Hold points by customer). Welding Procedure Specification (WPS) established. Welders are certified as per ISO 9606 as extensive welding involved in fabrication. All the raw materials are 100% ultrasonically tested. All the castings are screened by 100% Radiography to E 466 Level II. Mandatory usage of jigs & fixtures (14 types) are being used for better consistency in Bogie frame manufacturing. To ensure reliable welding, weld quality ensured by Radiography testing at 24 joints in a Bogie Frame. Bogie frame manufacturing involves precision machining with 30 microns tolerance, ensured through CMM.

Selection of the Problem

The underframe of metro coaches is fitted with a framework carrying wheels attached to car body, thus serving as a modular subassembly of wheel, axle and drive. Earlier this bogie was imported in “ready to fit” condition from Rotem, Korea at a high cost. As a part of import substitution and “Make in India” policy, existing railway bogie manufacturing facility was converted into metro bogie manufacturing by business process re–engineering. Bogie manufacturing process was not a part of technology transfer. During the process of indigenization, welding distortion became a major issue of concern due to extensive welding. DMAIC approach was adopted to solve the problem in systematic approach

Presentation

Problem DefinitionDistortion observed up to 5 mm in first four bogie frames resulting in less/Nil stock for machining.

Scope : Metro Bogie frame manufacturing

Goal: Zero Defect

Project Y: To prevent any defect during Bogie Frame Manufacturing

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A detailed SIPOC (Supplier, Input, Process, Output and Customer) table was made to understand the process flow and overall mapping.

CTQ tree was made to identify critical to quality parameters for respective drivers and defect definition is made subsequently.

Data Collection Plan

Gage R & R study was carried out to ensure the error within acceptable range.

Bogie Frame Top View Dimensions

Dimension a, b and c are indicators of Distortion in Top View


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Bogie Frame Bottom View Dimensions

Diagonal difference of d1, d2, d3 & d4 are indicators of Distortion in Bottom View

Bogie Frame Side View Dimensions

Dimension j, k, l and n are indicators of Distortion in Side View.

Data was collected for four bogie frames for above distortion dimension and analyzed for normality test and sigma rating. Principal Component Factor Analysis of Parameters a, b, c, j, k, l & n was carried out. The cumulative variance of Dimensions a, b & c found to contribute 76.1% of total variation.

Variable ‘a’ is dependent on ‘b’ & ‘a’ is subset of b. Hence, parameters b, c & Diagonal Difference were selected for further analysis.

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Process Mapping of Bogie Frame Manufacturing

A detailed brain storming session was carried to identify all probable root causes that lead to distortion in bogie frame.

Identification of Probable Causes Ishikawa/Fish Bone Diagram

All the probable root causes were validated using different validation methods and found that none of the probable root caused have the significant effect on distortion. Hence a DOE by Taguchi method to determine the Optimum levels of factors viz. Welding sequence, Root Gap and Bevel angle was planned to reduce variation as a common cause strategy.


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As bogie is a high value aggregate and manufacturing cycle time is more, it was decided to conduct taguchi l8 experiment instead of 16 experiments for full factorial doe.

FactorsExperimental factors: root gap between side frame & transom, welding sequence, bevel angle of transom web plate

Control factors: Welder, weld current, weld voltage, gas flow, material

Noise factors: Ambient conditions (temp., humidity), operator fatigue etc.

No. of factors: 3Levels: 2 (for 2 factors) + 4 (for 1 factor)

Factors and levelsExperiment Design: Taguchi L8 Experimentation

Bogie Frames taken for experimentation: T3 to T6 & M3 to M6

S.No. Factors Level 1 Level 2 Level 3 Level 4

1 BEVEL ANGLE OF TRANSOM WEB PLATE

30 45 – –

2 WELDING SEQUENCE

Center to either end diagonally & simultaneously (W)

One end to other end simultaneously (X)

One end to other end sequentially (Y)

Center to either end diagonally & sequentially (Z)

3 TRANSOM TO SIDE FRAME ROOT GAP 3 mm 6 mm – –

There is a significant change in dimention 'b' when root gap, bevel angle & weld sequence are changed from 3 to 6 mm. 30 to 45 & W to Z respectively

Interaction noticed between1) Root gap & bevel angle2) Root gap & weld sequence

MAIN EFFECTS PLOTS

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There is a significant change in dimention 'c' when root gap, bevel angle & weld sequence are changed from 3 to 6 mm. 30 to 45 & W to Z respectively

Interaction noticed between1) Root gap & bevel angle2) Root gap & weld sequence

IMPLEMENTATION AND THEIR RESULT

With the new level of factors, five bogie frames manufactured for trial implementation. Distortion found within the acceptable range. No machining problem observed. No field complaint observed

There is a significant change in diagonal Difference 'DD' when root gap, bevel angle & weld sequence are changed from 3 to 6 mm. 30 to 45, & W to Z respectively

Interaction noticed between1) Bevel angle & root gap2) Bevel angle & weld sequence


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implementation and their ResultWith the new level of factors, five bogie frames manufactured for trial implementation. Distortion found within the acceptable range. No machining problem observed. No field complaint observed

Discussion

l Risk analysis is carried out at new factor levelsl Validated the modification by testing of Mechanical Properties like yield

strength, tensile strength & impact test.l Other critical requirements of weld joints like Weld penetration, Weld

Fusion, porosity etc. were verified on 6 bogie frames through Radiography testing and found OK

l Design concurrence obtained for change in levels

Initial resistance foreseen due to increase / change in work content and cost implications from all the stake-holders.

Action initiated

* Approval taken from the concerned Dept Heads for implementation of solution

* Educated employees at grass root level for Process correction and control* Sensitization of Employees thro’ several interaction* Technical closure of issue with customer

No further resistance anticipated for implementation of the modification.

Conclusionl New process updated in SAPl WPS modifiedl Drawing updated

WELD SEQUENCE

Simultaneously welding at both ends Centre to one end diagonally

One end to other end welding

Before

After

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Bevel Angle of transom Web Plate

transform to Side Frame Root Gap

Dimension 'b' Before & After improvement


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Dimension 'c' Before & After improvement

Sigma Ratting Before and After

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REFERENCES

1) Books on Six Sigma 2) Books on Design of experiment

AUTHORS:

1) Sumanta Kumar Saha, GM (Quality), BEML Ltd., Bangalore 2) G R Keshavan, AGM (Quality), BEML Ltd., Bangalore 3) K Mahadeviah, Sr. Manager (Quality), BEML Ltd., Bangalore 4) Niranjan Kumar Singh, Manager (Quality), BEML Ltd., Bangalore

ReferencesBooks on Six Sigma

Books on Design of experiment


ensuring zero defect in metro coach bogie manufacturing by

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