Inspection Equipment Accuracy - Measuring the diameters of small holes

[Liger]

I have a piece of inspection equipment we use for measuring the diameters of small holes (.017-.018") machined into a plate. The inspection equipment has a stated measurement accuracy of .00014". Our engineers want data reported to 6 figures. My position is the sixth figure they are requesting exceeds the capability of the machine and that it may get reported, but the accuracy of that 6th digit cannot be relied upon. Is my position correct?

 

[msec0990]

I believe you're correct, because you only have five significant digits in this case. To add a sixth would create false precision. But since I don't deal with this often, I'd like to see what others have to say.

 

[normzone]

I'm with [msec0990].

To my simple mind, if the device makes claims to about one and one half ten thousandths, then any info it gives you should be viewed as possibly plus or minus that amount.

So any number out in that fourth decimal place could possibly be out into the fifth decimal place, and the sixth decimal place is just there for entertainment purposes.

 

[Liger]

Thank-you for the quick feedback. I appreciate the information!

 

[iamtroll]

Since the stated accuracy is not .00001", even the fifth place decimal is questionable. In reality, you should be doing a gage R&R to determine the actual accuracy for the characteristic you are interested in. I had the same problem recently with a customer that wanted a six place accuracy, we told them just because you can put five zeros and a number after the decimal place doesn't mean anyone can measure it. We use lasers to measure coating thickness and they only go to five places. And don't forget temperature variation, even if you had a device that could measure to six places you would have to keep the temperature in your measuring lab at around +-.1 degree F of variation or your size will change.

 

[Miner]

I recommend perusing this article on Probable Error by Donald Wheeler. Wheeler has additional articles on this topic that may be found with a quick search.