Contact vs Non-Contact Measurement

E
#1
In my place of business there is currently a debate about the value of contact measurement (i.e. CMMs, pin gages) vs non-contact measurement(i.e. optical measurement)to verify the dimensions of machined parts. Is there any general consesus in the metrology industry on which type of measurement is more accurate or acceptable over the other? If so, why?
 
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J

Jim Biz

#2
My opinion only - Opticals (if we are talking shadow graph optical here) are "subject to light distortion" in varing degree .. contact should in theroy at least prove to be more accurate.

Regards
Jim
 

Jerry Eldred

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Super Moderator
#3
I am not a guru in this area. I took a few moments to ask one of our mechanical people an opinion, and came up with none.

I don't think there is a single answer to the question. I understand laser micrometers and certain optical measurement instruments to be very good. But bottom line (I underscore I am not a guru in this one), it comes back to comparison of the measurement and its variables and the appropriate tool. Even in the semiconductor industry use of laser (6328 Angstrom) to measure film thickness has long been one of the industry standards. But as films get thinner, X-ray becomes more desirable, and ultimately, perhaps measurement of electrical parameters on the thin films may replace the laser (i.e.: first contact, then non-contact, then back to non-contact).

In larger dimensional measurements (machined parts), some of that may also apply. Contact vs non-contact temperature measurement - some applications one is better, some, the other.

I think you have to intelligently evaluate your application, and be familiar with the state of the art tools available to measure it.

------------------
 
#4
Hi Erik,

Interesting question, but nigh on impossible to answer...

It all depends on what and how many objects you are measuring, what accuracy and speed of operation you need, etc...

Optical can be very good when you measure flexible materials for instance, but if you're measuring something with very generous tolerances it may still be "overkill". It's all down to the application..

/Claes
 
E
#5
Thank you for your replies thus far. The particular application that is receiving the most argument is measuring a .104" thru hole in stainless steel with a tolerance of +/- .001". Our Quality department is verifying the dimension on a machine vision system using profile lighting and automatic edge detection and measurement software(stated linear accuracy +/- .00025"). Our machine shop is using pin gages (class ZZ) and has a severe distrust of the vision system. I guess I am looking for a strong argument that can support one department's method of measurement over the other's to help mediate a solution. Speed is not critical in this issue, but accuracy is. Again, thank you for any help you can give me.
 
S

Steven Truchon

#6
Jim hit the major flaw in optical measurement, but it is manageable. Unfortunately most people will adjust light intensity to suit the image on the monitor which more often than not, is too bright causing the distortion that Jim mentioned. I have learned not to trust my eyes for optical parameters on a CMM. Once the optical measurement parameters are "proven" via other measurement methods, the optical method is reliable and very fast. My last experience with optical measurement was an OGP SmartScope and the only aspect that I didnt like was even though I could take one hundred plus points on a diameter, if the hole was not perfectly round I would not get the virtual diameter or the maximum inscribed circle which is what you'll get with .0001 increment gage pins for instance. For a thru hole of .104 +/-.001 diameter, I would always hard gage with pins whether I measured the part optically, or with a contact CMM or otherwise. Beyond that, air gaging is always an option but not cost effective for a .002 total tolerance.
I hope this helps

Steve
 
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