bmw mini screwdriving analysis

PRODUCTION LINE MINI

GLASS PANELS SCREWDRIVING

PROCESS ANALYSIS

28 February 2014

Milan Gacík-Repčík

Internal and Confidential P. 2

DMAIC DEFINE

Occurence of loose GD screws at customer

A problem statement

Occurence of loose GD screws at customer

The customer

BMW

Critical to Quality — what are the critical process outputs?

Proper clamping of joined parts (Final Torque and angle reached)

Project targets

Target of project is to analyze nonconform screwdrivings on BMW

production line

Project boundaries

Production line Sunroofs assembly BMW MINI

BMW MINI 998.73701X


DMAIC DEFINE

Screwdriving process is defined with 6 individual phases with 3 phases

monitoring screw joint performance/quality:

Phase Description

P1 – Prevailing torque monitoring phase intended to monitor screw joint

conditions while screw is going into nut thread

P2 – Prevailing torque monitoring phase intended to monitor screw joint

conditions during screwing while screw is not yet in contact with

GD insert

F – „Final tightening“ = Tightening with Torque as principal value and

Angle as monitor value


DMAIC DEFINE

Screwing process steps P and F determine torque and angle settings which

are essential for final tightening of screw – fixing glass pane to mechanics.

FACTS:

- Typically screw is turned 4,2 – 4,5 times (1500 - 1620°) until it

reaches final position.

- Witness marks on Glass pane insert are created between 3,75 – 4,5 screw rotations (1350 – 1620°)

- Screw thread is covered with Loctite securing layer between screw

thread 2 – 4 (720 – 1440°)


DMAIC MEASURE

Process critical Xs (inputs) and Ys (outputs)

X1: Parts (Screw + „Nut“)

X2: Workforce

X3: Tool

Y: Clamped assembly

Measurement plan

Measurement of torque and angle reached at the end of screwdriving

process

Test the measurement system

MS test are performed regularly by Q Department (Verification and

calibration)

Data collection

Data collection is performed on regular base downloading


DMAIC MEASURE

Screw 5

Screw 2

Screw 6

Screw 3

Screw 1

Screw 4

Screw 11

Screw 8

Screw 12

Screw 9

Screw 7

Screw 10

Data collection

Data collection has been

performed to evaluate

performance of screwdriving

process.

Data from 25 PaDa´s has been

recorded


DMAIC MEASURE TOTAL ANGLE

Values measured and calculated

Difference 0,285 mm in screwed in lenght



VARIATION

DMAIC MEASURE TOTAL SCREWED

LENGHT

VARIATION



DMAIC ANALYSE PERFORMANCE

Gaps between current performance and goal performance

Current performance: within limits

(First time through rate > 97%)

How the process inputs (Xs) affect the process outputs (Ys)

Parts quality > Torque and Angle out of specification

Parts design > Torque and Angle out of specification

Working method habits > Interruptions of process by worker

Occurrence of worker induced process interruptions

Wear of screwdriving tools > Increase of process standard deviation

List of potential opportunities to improve

Parts quality (screw and thread in mechanics according to specs,

without impurities and manufactured with correct tools)

Parts design (correct dimensioning and tolerancing of thread in

mechanics)

Adjustment of screwdriving strategies based on collected data


DMAIC ANALYSE PERFORMANCE

Data collection

Data collection has been performed

to evaluate performance of

screwdriving process.

Data from 25 PaDa´s has been

recorded

Screw 5

Screw 2

Screw 6

Screw 3

Screw 1

Screw 4

Screw 11

Screw 8

Screw 12

Screw 9

Screw 7

Screw 10

Capability (total angle): Screw 1 2,19 Screw 2 1,83 Screw 3 1,90 Screw 4 2,02 Screw 5 1,79 Screw 6 1,27 Screw 7 1,14 Screw 8 1,77 Screw 9 1,52 Screw 10 1,75 Screw 11 1,09 Screw 12 2,31


DMAIC ANALYSE VARIATION

Sources of variation

Parts > Dimensional deviations from specification – see next slide

Process adjustment > GD position in X and Z

Working method habits > Worker assumptions about process

performance (tendency to overrule measurement system)

Wear of screwdriving tools > Decrease of process capability and

performance


DMAIC ANALYSE PARTS

DIMENSIONS

3,5

–

5,7

mm

Amounts of turns within specs according to parts design: 3,50 – 5,70 = 1260° - 2052° Mean = 1656° 1,1 - 1,4 mm

4,6 – 7,1

mm

1,0 – 1,4 mm

Amounts of turns within specs as measured: 4,25 – 4,60 = 1527° - 1655° Mean = 1574°


DMAIC ANALYSE PARTS

DIMENSIONS


DMAIC ANALYSE PARTS

DIMENSIONS

GD1 GD2


DMAIC ANALYSE POSSIBLE FAILURE

MODES

Possible failure modes in regular process and their detection:

Missing screw

Prevention: Automatic feeder with link to PLC

Detection: Poka Yoke sensors

NOK screw joint (screw – thread in mechanics combination)

Prevention: Quality check at incoming inspection

Prevention: Design change of parts tolerances

Detection: Prevailing screwdriving phases Nr. 1 and 2

Crossthreading:

Prevention: Screwdriving tool

Detection: Prevailing phase Nr. 1


DMAIC CONCLUSION

Variation of total angle at screwdriving is lower than variation from limit values of parts tolerances.

There have been acchieved screwdriving angles within range 0,285 mm which should lays within tolerance limit of washer paws 0,2 – 0,3 mm.

Screw thread lenght is well within allowed limits

Thread in front and rear mechanics are well cut and cleaned

Screwdriving process is capable

Working discipline is as expected

Lower capability of final angle reached on screws 6, 7 and 11 shall be analyzed on sample with readjusted glass panels 1 rear and 2 front to slightly higher position from deepest acchievable position.

Based on measureed and evaluated results is screwdriving process safe as defined in relevand BMW GS standards.


DMAIC SCREWDRIVING PROCESS

THANK YOU FOR

YOUR ATTENTION

bmw mini screwdriving analysis

Automotive