cmm statistical process control by hilton roberts

Coordinate Measuring Machine Statistical Process Control

PRESENTER

Hilton.L. Roberts Precision Inspection Group

Boeing Helicopters 5000 East Mcdowell Road

Mesa,Arizona 85205

602-891-7263 [email protected]

PAPER AUTHOR

Hilton.L. Roberts Precision Inspection Group

Boeing Helicopters 5000 East Mcdowell Road

Mesa,Arizona 85205

602-891-7263 [email protected]

ABSTRACf

The quality of the measurements from a Coordinate Measuring Machine ( CMM ) is in large part dependent on the stability of the environment surrounding the CMM and the stability of the processes that produce output from the collected data points.

This paper will discuss how an artifact was chosen and programmed to allow for CMM confidence checking and how Tool Qualification has been identified as a significant source of measurement uncertainty.

1998 NCSL Workshop & Symposium 673 Session 68

INTRODUCTION

A desirable outcome is that Coordinate Measuring Machines be capable of producing identical outputs from identical inputs. The measurement processes should produce results so that we may be confident that we are not accepting bad product nor are we rejecting acceptable product. Ifwe are to use Coordinate Measuring Machines efficiently, then we must be able to provide reliable and consistent results to Manufacturing.

A common method of validating the suitability of a CMM for use in measuring work pieces is the manufacturer's statement of accuracy coupled with either a laser mapping and/or a Ball Bar check. A completed ''Certificate of Accuracy" was all that was required to begin measuring and reporting values descn"bing the geometrical characteristics of ']>roduct". This approach allowed the CMM to operate in a "state of grace" until the calibration due date was reached and did not consider that the machine might not be stable for the duration. One of the reasons that CMM results (Especially those that require manufacturing to rework pieces) have been received with some skepticism in the shop is that the accuracy of the measurements is suspect. All too often when Coordinate Measuring Machines are tasked with verifying a discrepant feature, the readings do not repeat.

It seemed logical then to try to choose an artifact that could be used to test whether or not a CMM could consistently report the same values for the same features and whether or not the reported values for those features could be contained under a normal curve.

BACKGROUND

Some of the information used to conduct these tests was provided by the CMM manufacturer and some was provided by CMM Instructor Programmers. While this information was very helpful, it was considered incomplete by those with a background in Dimensional Metrology. What was missing was a lack of knowledge of how to prepare for and conduct tests to prove or disprove the accuracy claims.

The manufacturer's accuracy statements are not usually based on the configuration of the machine that is used to measure parts. The manufacturer does not know what kind of probe configuration will hang on the end of the ram to measure the parts produced. Therefore we cannot realistically expect the same accuracy on machines in their manufacturing/inspection settings. We need to know what kind of accuracy CAN be realized. We must develop test parts/programs so that baselines can be established on the CMM for use in determining when the machine begins to change.


PROCEDURE

One of the statements we heard was that "once tools are qualified in a particular configuration, they are used until either the tool configuration is changed or the machine suffers a probe crash".

Chart IB shows a baseline check of the qualification ball from point 65 to point 80. That tool file was saved and used to measure the qualification ball one day later. That chart, which starts at point 81, shows a downward shift of approximately .0002 inches. A slight upward shift one day later can be seen at point 101.

The data suggests that the CMM changed relative to the tool qualification and that change in tool offsets was reflected in the change on the charts. The data further suggests that tool qualification should take place before measuring critical parts and that "old" tool qualifications should not be kept.

The next question that begged to be answered was tool qualification itself Tool qualification is a twofold process on this CMM. First, we find the center of the qualification sphere in order to establish the coordinates of the sphere from the X, Y and Z axis resolver null points. We do this by defining to the computer a negative Z axis tool without X and Y offsets and a Z length measured to the nearest .1 inch from the center of the stylus tip to the center of the ram collett. Once this value is determined, other tools in other attitudes may be qualified. This is done by measuring 5 points on the qualification sphere by each tool The computer will determine the X,Y and Z lengths of the tools and use these "offsets" to compute geometry from measured points on parts.

It seemed reasonable to assume that if the manufacturer expressed the uncertainty of the CMM, determined by a ball bar as .00075 inch bandwidth and that the process of qualifying tools was "normal", it should be possible to qualifY several tools automatically and use one tool to establish X,Y and Z zero in the center of the sphere. The rest of the tools should then be able to measure the size and location of the sphere within that bandwidth and that those measurements would be normally distributed.

The CMM that was used in these tests was dedicated to wing measurement for a small, pilotless aircraft. The part program for measuring the wing collected several hundred data points along the upper and lower mold line as well as the leading and trailing edges and pivot hole. These data points were reduced to yield aero characteristics related to surface area as well as lift coefficients and rigging parameters of the wing. Erroneous data used to rig a wing could cause a failure in flight.

To minimize the possibility of that occurring, a part was chosen as a confidence check that had some of the characteristics that were of interest on the wings themselves. An angle in the XV plane was needed as well as a feature that required measurement from both sides of the part.


The part chosen was the Geowidgit part that is used as a training aid for programming on our manual machines. This part is aluminum and has several features similar to those of interest on the production wing .. The part is smal1 enough that it can be measured in more than one place on the CMM surface plate and can be used to monitor CMM performance in the four quadrants: X+Y+,X+Y-,X-Y- and X-Y+.

Tools in five different attitudes are used to measure the wing. A program was written to qualify those tools under numerical control and then to use the negative Z tool to establish X, Y and Z zero in the center of the qualification sphere. All five tools are used to measure the sphere 20 times to get a baseline on the machine and X Bar and R charts were constructed. These charts have been maintained since October 1989. An additional program was written to automatically align and set zeroes on the GEOWIDGET part.

To date, the charts show that this CMM does operate in a state of statistical control. Nevertheless, it is interesting to note that although the machine is not in control. certain things, when they happen, can show a sudden and dramatic shift in the charts.

The program for the GEOWIDGET is run every day. Running the program in the four quadrants on the machine surface plate yields four measured values for each feature measured. For example, the counterbore on the Y+ side of the GEOWIDGET is measured in each of the four quadrants. This provides four data values for that feature that will yield one data point for the charts on the X,Z location and Diameter of the counterbore.

A glance at chart 6A shows several interesting things. Most striking is the shift in the X location of the bore. The important thing to consider is whether or not the bore actually shifted or if something happened to the CMM to cause an APPARENT shift in the bore. The first thing that was done to determine what happened was to requalify the tools and run the program again to remeasure the Geowidget. The chart shows that requalifying the tools and rerunning the program did not change the reported location of the bore. The operator was questioned and reported that the CMM air supply had been interrupted by a high temperature alarm that triggered a compressor shutdown. The operator then reported that when the air supply fell below the pressure to keep the ram up, the ram fell and the probe hit the surface plate. An interlock that was supposed to keep that from happening was found to be set at 13 psi. The pressure required to keep the ram from falling is 60 psi.

A right angle iron was set up so that it was parallel to the Y axis of the machine and was adjusted until it was parallel within .0002 inch as measured by a negative Z axis tool. The probe was then pointed Y+ and was brought to the front face of the angle plate ( on the Y - side) and an X axis zero was set. The probe was then pointed Y - and moved to the other side (Y+ ). The face of the angle plate was measured and was found to be .036 inch out of square. It was adjusted until the out of squareness condition was less than .0005 inch. A glance at the charts shows that the process was restored. The error was caused when the tool entered the bore to measure the location. By being out of square in the XV plane, the probe shanked out and showed a shift in the X axis of the bore.


What is really important here is that the GEOWIDGET part was being used as a confidence check in a MEASURING process. Had a production part been charted as part of measuring or adjusting MANUFACTURING process capability, it is likely that extensive time and resources would have been expended looking in the wrong area to fix a problem that was showing up on a control chart.

Chart SA shows an angle in the XV plane on the GEOWIDGET. This angle is important in the measurement of the wings because of a critical bore that runs through the wing pivot plate and intersects the manufacturing chord plane. To bore this hole at the proper angle and to achieve the true position requirement of the bore, the program that measures the bore does a planar alignment on the face of the pivot plate on the Y+ side of the wing. The CMM was also used to establish the location of a seal hole on the edge of the wing when no tool was available to drill the hole. This was done by driving the CMM to the location of the hole as close as posSlole manually in X,Y and Z. The machine was then backed out ofposition in the Y axis and a dab of white lacquer was applied to the tool tip. The probe was then moved in the Y axis alone to touch the wing plank and transfer the lacquer to the wing. The hole could then be drilled manually. The procedure was to make sure the tool tip was cleaned after this operation to make sure that no lacquer remained in the tool tip. In chart SA, the probe was not cleaned after use and a flake ofpaint remained. This caused the angle on the GEOWIDGET to measure. S degree different than previous charts. An angle of this magnitude amounts to .014 per inch. Considering that the true position of the pivot bore is at a position over 2 inches away from the face of the planar alignment, it is easy to see that significant errors in the reported bore location can be produced.

The fix for this type of problem is to bore the hole oversize, tum a bushing and interference fit the bushing to the bore and reb ore the pivot hole. The fix is very expensive. Once again the charts pointed to a problem The charts did not tell us how to fix the problem nor did the chart specifically indicate what the problem was.

The drive system on this CMM has a split gear type of arrangement with a torsion bar applying torque to reduce gear backlash. Chart IB shows a typical pattern of the charted values of the X location of the Qualification sphere. At the right hand of the chart, a sudden shift in both the average and range charts indicate that something happened to the machine. Due to the noise that developed in the X axis it was determined that loss of the X axis anti-backlash adjustment was the probable cause. The machine was in warranty at the time and the manufacturer sent a technician in to fix the problem When the cover was removed from the X axis drive motor, it was discovered that the anti-backlash adjustment had slipped.

Chart IC shows that when the anti-backlash adjustment was reset, the charts resumed their previous pattern on the range chart but the average chart showed a shift. This was most likely caused by the technician using a slightly different value for the torque adjustment.

1998 NeSL Workshop & Symposium 677 Session 68

Now eight years later, we have two new Coordinate Measuring Machines. Utilizing lessons learned from the past, we decided to incorporate the qualification sphere measurement as part of the machine run off and to provide a baseline for the machine at the factory before delivery and at our location upon delivery. We wanted to know how well the machines performed here in the location they would be used in compared to what the machines delivered at the factory. We ran a 25 point baseline at the factory and compared those numbers with numbers generated here after installation. The four charts that you see are the X, Y, Z location and Diameter of the sphere. The twenty five point baselines are clearly delineated on the chart. The spike at point 51 was a wake up call that something was happening to our SPC check. All of a sudden the machine was not able to determine the location in of the qualification sphere closer than .0045 inch. What was the problem? We monitored the temperature overnight, examined the program for corruption and could not figure out what was happening. We took the existing program and ran it on the other machine and the numbers dialed in. The problem was then isolated to one machine. That did not help find the problem We looked at what it would take to make the machine behave the way it was. I thought that the last thing in the world that could cause such a problem would be tool qualification .. We had examined the hits taken for the qualifications and the Standard Deviation was. 0001 inch or less. I was thinking to myself that the only time I had seen a similar problem before was when a tool was loose ..... or a qualification sphere was loose .... I checked the tool and it was secure. I then touched the qualification sphere and felt a slight movement. I dismounted the sphere and epoxied it in place and re-ran the program As you can see by the charts, order has been restored. You could not see the sphere move. The machine probes at a pressure of approximately 3-5 grams. I would not have expected the repeat on the range chart to be as good as it was given the fact the sphere moved. Once again, measuring something periodically told me something was wrong. The information did not tell me what was wrong. Experience helped find the problem but the charts told me something was wrong before I would have found it by measuring a part used for production. We have part tolerances approaching +/.0025 inch on the entire profile. We cannot be using part tolerance up in the qualification process. To sum up what we have learned is that you simply must measure and chart some features on a part that you know in order to be able to analyze and adjust machining processes that produce the parts you measure.

CONCLUSION

These examples have shown that it is important to know the state of the measuring equipment. It is critical to have a test part that has some of the characteristics that are of importance to the production parts and that collecting and evaluating data on a daily basis becomes procedure. There is no reason why control charts should not be used to establish calibration intervals. The system of semi-annual or annual cah'brations can be replaced by a program of data collection and evaluation that will allow CMM users to evaluate the quality of their measurements being produced by their CMM and to correct any areas of deficient performance before there is an adverse effect on product quality.


seTZ. '

DEPT. 4~ PART NAME SPHERE PART rtO. HOrtE I'IACH UtE ..... 'OPERATIOft G RlR VARIABLE x-t!OORDiftATE Grand Plean = -8.888873 LCLxhar = -8.888248 UCLxbar = 8.888895 l'Iean Range = 8.888238 UCLrange= 8.888524

X BAR OIART ::= y------------------------e.~ I

-8.fIII1

-8.888193

8.888524

8.888393

8.88 HMHMMM~~~~~~~~~~~~~~~ M~~~~OO~5~~55~~M~~OO~~~

IS

82/811"98 82/82/98 82/83/98 82/85/98

UCLx

LCLx

0) co o

DEPT. 412ft PART HArlE UIDGET ° PART ftO. PIAOt UtE _

OPEMTIOfI ITDI 1 X VARIABLE X COORDINATE Grantl Plean = -8.888&18 LCLxbar = -8.888811 UCLxbar = -8.888489 I1ean Range = 8.~ UCLrange= 8.888£,29

8 •• 1588

8.1IJ1894

8.lIJI15Bl

8.1111J77

X BAR OIARTo -~-.......,z;;::::;;--""

~.~----------------------

~.881J93

8.88243

8.881823

8.881215

8.888687

-8.88

R alART

UCLr ~~~~~~~r-"~~~~~~~~~~~~bE~~~~ "~

86 18 89 12 12 8B 89 86 15 86 86 86 85 86 86 89 13 86 86 86 87 .~~~~M85~5~~~~mm~~~~~~ 83114198 83/16/98 831191'98 84/82/98 84/211'98 85/151'98 85/23198

DEPT. 41211 PART HAI'IE U (DGET MRt NO. I1AOtIPIE .-OPEMTIOft ITEtI 5 NlGLE VARIABLE-r-----GraNl I'Iea.n = 4.488898 LCLxbar = 4.467662 UCLxhar = ne.n Range = 8.828821 UCLrange= 8.863888

4.69919

4.652884

4.686579

4.568273

X BAR awn

4.5139(,8 ___________ - _ _ _ _ _ _ _ _ UCLx

~~~~~~~=r~~--~~er~~~~~~~~~~r-~ 4.467662 La.x

8.871&7

8.&58253

8.43aJ35

8.Z19418

8.88

R mART

lJCLI' Gm __ ~amaa~~~~ __ .. ma __ ~~ma __ m&~~~ .. ag=="~

~~~UU~~~5~~~~~~~n~~~~ .~$~~~~U5~~~%mm~~m~~~ 83/14/98 83/16/98 83/19/98 84/82/98 81/21/98 851'151"98 85123198

_ D.,. 41211 PART "","E SPHERE PM! 1'tO. ftOHE I'IAO-I UIE _

, 0PIlVt! 1011 GAGE RaR 'JAR lADLE X COORD IMTE GNM I'Iean= -8.~42 LCLxbar = -tL8881B4 UCLxbar:: 8.888188 .... n Ral18e :: 8.888195 UCLrange:: 8.888444

8.818123

-ft.8B8280

B.1BlB7

... 8.BIJII','53 -.,'

X BAR mART

R OIART

\

18

UCLx

LCLx "

8.1IJItt435 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - t - 0CLr

8.~17 --~ __ --__ ~ __ ~ ____ ~ ____ -+~ __ ~~ _____ ~~~ __ ~ ___ "~

8.88 '-' 89 8& 85 85 ao 85 86 87 89 fT1 11 8G 86 frl 87 89 88 8G 8& fJ7 86 5~~~tl~~~585~~~~~~~m~.~ 83119/98 83/31/98 84/1(11'98 85/14/98 85/22/98 8&/25198 81.11V98

en Cb ti) ti)

o· ::)

~

DEPT. 4~ PART fW1E P,AItT NO. HOrIE MOIlttE OPIMTIOft GAGE RAR VARIABLE G.aNt I'Iean = -8.888842 LCLxbar = -8.868184 UCLxbar PIPMt .. Range = 8.888195 UCLrange= 8.888444

X BAR mART 8 .... 123

SPHERE .... X COORD IrtATE

= 8.888UII

--r--------~--------------------

Ie..

UCLx

8.1II1II84 ~. 1\ ~ ~.MBHZ~~~\H.---------.------------------------------"~

-8.888124 .'. \/

-tt.l8I2!6

-8.

8.

8.

8.

8.

8.88 ...

~~~----------------------------- LeLx

R OWIT

UCLr

~~---------------------------------------------------- "~

861288898989898989 88 3S 48 88 88 88 88 88 88 8EV81.198 88187/98

DEPT. PART NO. OPERATION Grand Plean ftean Range

pig PART PIME none ftACHINE Sphere NeasureNent VARIABLE

= -11.888878 LCLxbar = -8.888445 UCLxbar = 8.888&49 UCLrange= 8.881378

X BAR atART

sphere s/n 8897-2346 X coordinate

= 8.888384

8.888384 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - UCLx

8.888142

-8.888821 __ ~~~~~~~~~~ __________________ r-__ i-____ ___ - "EAftx

-11.888183

-11.888346

-11.888588

8.88558

8.884185

8.88279

R CHART

8.881395 - - - - - - - - - - - - - - - - - - - - - - - -

LCLx

UCLr

8.88 ..;;;;;~;;;;;;;;;;;;===~;;=:;=:~~t==1::~== "EA"r ~~~~~~~~~~~~~~~~~~~~ 18 18 18 18 18 18 18 18 18 46 46 46 46 46 46 46 46 59 18 18 89/82/97 89/13/97 18/86/97

DEPT. PART NO. OPERATIO" Grand Plean "ean Range

8.888211

8.888134

8.888858

P i9 PART fIAI'IE none "ACHINE Sphere NeasureNent UARIABLE

= 8.888819 LCLxbar = -11.888172 UCLxbar = 8.888332 UCLrange= 8.888788

X BAR CHART

sphere 8897-2346 Y coord i nate

= 8.888211

UCLx

~----~~--~------~~--------------------r---~~4----- ~x -11.888819

-8.888896

-8.888172 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - LCLx

8.88287

8.882153

R CHART

H

DEPT. p iO PART ftME PART PIC). none I'IAQflrtE OPERATION Sphere NeaSUreMent VARIABLE Grand ftean = 8.888811 LCLxbar = -8.888141 UCLxbar "ean Ranoe = 8.888264 UCLrange= 8.888557

X BAR alART

Sphere 8897-2346 Z cooriinate

= 8.888164

8.888164

8.888183

-------------------------------- UCLx

8.888M2 ~~~~~~==~~~~~ __ ~~~~~ __ ~~~ ___ "t" IUANx

-8.888819

-8.88888

-8.888141- - - - - - - - - - - - - - - - - - - - - - - - - _ - _ _ _ _ _ LCLx

8.88251

8.881883

8.881255

8.888628

R CHART

------------------------ UCLr 8.88 ~~~~~~~~~~~~~~~~===i~~==mMr

88BB88I!J8I!J888BBI!J8888888BBI!J8BBBBBB88118686 81 81 81 81 81 81 81 81 81 82 82 82 82 82 82 82 82 88 19 18 89/82/97 89/13/97 18/86/97

DEPT. p iO PART rtA~ PART ftO. none MOl IftE OPERATIOft Sphere MeasureMent VARIABLE Grand Plean = 8.758826 LCLxbar = 8.749998 UCLxbar ~an Range = 8.888864 UCLrange= 8.888134

X BAR CHART

Sphere 8897-2346 Di.....,ter

= 8.758863

UCLx

8.7'58884

8.758859

8.758834

8.758888

8.749983

8.749958

__________________ ~~------------------+_--~~L---- ~

LCLx

R CHART 8.888134 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - UCLr

8._1 A r'\ J a -"'-7 " , "J\ 1\ l\ A J\ ,., 0 /V\ D.DDUDD ~ VV .. · r"J~ rv· V 'V~ 8.888833

8.88 88 88 88 8B 88 88 88 8B 88 8B 88 88 8B I!J8 88 88 88 11 86 86 11 11 11 11 11 11 11 11 11 89 89 89 89 89 89 89 89 81 19 19 89/82/97 89/13/97 18/86/97

1998 NeSL Workshop & Symposium 685

"EAftr

Session 68

zc WIDGET

0°0

000 °0°

1998 NeSL Workshop & Symposium 686 Session 68

PIVOT PLATE PERSPECTIVE

y

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cmm statistical process control by hilton roberts

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