Z
zancky
I would like to start a thread about CMM.
I'm using a small Mitutoyo's one with Renishaw's touch probe and camera system
the handbook with the machine it is not enough for good performance, training from dealers not to be mentioned, help from Mitutoyo most of the time lacking. therefore I learned by myself how to use it, writing scripts, testing (some failures), asking help to some friends and so on. Now I have found this forum and a lot of very good technicians so I suppose it is time to to share my experience and find some suggestions and help
some the topics I would like to discuss with You:
-1) writing programs or use "learn mode"?
Personally I have promised to cut one finger to anybody in my lab. each time they try to use "learn mode" (so they have max 10 trials
)
Once a techician from Mitutoyo asked me the reason why I was so against "learn mode"
My answer was: "First are You able to elaborate a program in "learn mode" to measure a part with an accuracy about 2 microm wherever is the part, whichever is the operator?.
Second how You can optimize the program (approcing distances, speed, movements) in "learn mode"?
Third how you can compensate the algorithm bugs of Your system in "learn mode"? (I will explain after that topic)
Forth I know trigonometry,some basic other languages and programming tecnique, I can read a drawing, why I can not apply my knowledge?
Which are Your point of view and Your experiences?
-2)How You start your program, i.e. alignement of the axis and origin to the part.
Normally I spend from a third to half of the working time for alignments looking for part simmetry (luckly we are measuring deep drawing parts so there should be a revolution axis)
-3)CMM touch probe, European point of view on measuring (mean value) and Taylor principle (every point inside tolerances).
let's consider just a bore with diameter Ø12±0.1mm for a length of 20 mm. Standard CMM results will give You the mean value and the roundness of the cylinder (mean value is the way used to calibrate the machine). Asking for inner material or outer material diameter will sometime give You strange results (see how roundness changes respect to mean value one). therefore CMM touch probe seems to fit european point of view about measurements and have some problems with Taylor principle. Which are Your point of view and Your experiences?
-4)Touch proble single point bug. Try to measure a inner diameter of a known calibrated ring. if You use the circle or cylinder feature the discrepancy may be 2 microns. Now try to measure the same ring using point (edgepoint). Well if you plot the data on XY diagram You will find that at 45°, 135°, 225° and -45° the points are closer to the center of the circle while they perfecly on the circle every 90°. It seems it comes from a bug into the system. When the touch probe touches the surface it allocates the point coordinates like touching moving along X or Y whatever it is the actual direction vs surface normal. If You change the refence axis according the the direction of approcing to the surface the mistake vanishes. How many other bugs do You know?
-5)I have used CMM to measure wall thinckess on a cylinder like part. Well it took one month to establish the best fixture and the related program but at the end the correlation with the customer laboratory was better then 2 microns. Have you any strange application of CMM?
-6)I have taken the choice to ask to Mitutoyo to perform calibration so my customers are happy (traceability etc..) but I think that 95% of accuracy, while measuring a real part, depends on how I have written the program. the influence on temperature, humidity, etc is a secondary problem. Just to give an idea. Some years ago a customer asked me to measure the minimum and maximum inner diameter point to point on a cylinder like part with nominal diameter of 6.04 mm and wall thickness of 0.15mm. First I said "Sorry but at the moment I'm not able to do it". He said "ok we know a certified/accreditated external laboratory that can perform the task, they are actually calibrating our gauges". After one month we have the results: the certified laboratory declared that the minimum inner diameter was on 5 samples between 5,98 and 6.00 mm. I called them and I asked "how can you explain me that You have found 5.98-6.00 but a Ø6.01 mm plug gauge passes frely through the parts?". "May be there is a mistake somewhere" was the answer.
Do you know something similar?
-7)I would like to know how the machine calculates the features (interpolation algorithms) as asking for microns even the philosophy behing the machine software may have an influnce. What do you think?
sorry for my english and typing mistakes, but let me know if you are interested on it and your opinion
I'm using a small Mitutoyo's one with Renishaw's touch probe and camera system
the handbook with the machine it is not enough for good performance, training from dealers not to be mentioned, help from Mitutoyo most of the time lacking. therefore I learned by myself how to use it, writing scripts, testing (some failures), asking help to some friends and so on. Now I have found this forum and a lot of very good technicians so I suppose it is time to to share my experience and find some suggestions and help
some the topics I would like to discuss with You:
-1) writing programs or use "learn mode"?
Personally I have promised to cut one finger to anybody in my lab. each time they try to use "learn mode" (so they have max 10 trials
)Once a techician from Mitutoyo asked me the reason why I was so against "learn mode"
My answer was: "First are You able to elaborate a program in "learn mode" to measure a part with an accuracy about 2 microm wherever is the part, whichever is the operator?.
Second how You can optimize the program (approcing distances, speed, movements) in "learn mode"?
Third how you can compensate the algorithm bugs of Your system in "learn mode"? (I will explain after that topic)
Forth I know trigonometry,some basic other languages and programming tecnique, I can read a drawing, why I can not apply my knowledge?
Which are Your point of view and Your experiences?
-2)How You start your program, i.e. alignement of the axis and origin to the part.
Normally I spend from a third to half of the working time for alignments looking for part simmetry (luckly we are measuring deep drawing parts so there should be a revolution axis)
-3)CMM touch probe, European point of view on measuring (mean value) and Taylor principle (every point inside tolerances).
let's consider just a bore with diameter Ø12±0.1mm for a length of 20 mm. Standard CMM results will give You the mean value and the roundness of the cylinder (mean value is the way used to calibrate the machine). Asking for inner material or outer material diameter will sometime give You strange results (see how roundness changes respect to mean value one). therefore CMM touch probe seems to fit european point of view about measurements and have some problems with Taylor principle. Which are Your point of view and Your experiences?
-4)Touch proble single point bug. Try to measure a inner diameter of a known calibrated ring. if You use the circle or cylinder feature the discrepancy may be 2 microns. Now try to measure the same ring using point (edgepoint). Well if you plot the data on XY diagram You will find that at 45°, 135°, 225° and -45° the points are closer to the center of the circle while they perfecly on the circle every 90°. It seems it comes from a bug into the system. When the touch probe touches the surface it allocates the point coordinates like touching moving along X or Y whatever it is the actual direction vs surface normal. If You change the refence axis according the the direction of approcing to the surface the mistake vanishes. How many other bugs do You know?
-5)I have used CMM to measure wall thinckess on a cylinder like part. Well it took one month to establish the best fixture and the related program but at the end the correlation with the customer laboratory was better then 2 microns. Have you any strange application of CMM?
-6)I have taken the choice to ask to Mitutoyo to perform calibration so my customers are happy (traceability etc..) but I think that 95% of accuracy, while measuring a real part, depends on how I have written the program. the influence on temperature, humidity, etc is a secondary problem. Just to give an idea. Some years ago a customer asked me to measure the minimum and maximum inner diameter point to point on a cylinder like part with nominal diameter of 6.04 mm and wall thickness of 0.15mm. First I said "Sorry but at the moment I'm not able to do it". He said "ok we know a certified/accreditated external laboratory that can perform the task, they are actually calibrating our gauges". After one month we have the results: the certified laboratory declared that the minimum inner diameter was on 5 samples between 5,98 and 6.00 mm. I called them and I asked "how can you explain me that You have found 5.98-6.00 but a Ø6.01 mm plug gauge passes frely through the parts?". "May be there is a mistake somewhere" was the answer.
Do you know something similar?
-7)I would like to know how the machine calculates the features (interpolation algorithms) as asking for microns even the philosophy behing the machine software may have an influnce. What do you think?
sorry for my english and typing mistakes, but let me know if you are interested on it and your opinion
