Tape Measure, the right device for 2 or 3 decimals accuracy on prints

Thomas Dorner

Starting to get Involved
#1
Hello,

I am struggling finding answers to my concern with Tape Measures. I learned that if a Tape Measure is used to verify parts it needs to be calibrated. So far so good...

Now, on the other hand I learned that a Measurement system should be 10x more accurate than the print states.
For example: Print shows 1.125" ±0.030 or 1.315" ±0.060
Doesn't this mean the Measurement System should have an accuracy of x.xxxx?
Conventional Tape Measures do have an accuracy of 1/32 or less.

My uncertainty is that a Tape Measure is more likely a tool that gives you somewhat of an accuracy, it only gives you 1/32 intervals. I understand if a tolerance of 0.060 is applied it might work, but what if when a tolerance of 0.030 is applied (1/32 = 0.03125")?

I am curious to get answers that let vanish my uncertainty...

Thank you in advance for your help,
Thomas
 
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Ninja

Looking for Reality
Trusted Information Resource
#2
Many topics and threads about tape measures on here...have a browse...

Not that a tape measure has may have RESOLUTION of 1/32...that does not mean that it has ACCURACY of 1/32. Very big difference.

Put together a hypothetical but realistic human data set of results from a tape measure and pass it through a GRR and see what you get...it should remove most of the uncertainty you may have.
Don't bother spending the effort getting real data yet...save yourself the time.
Using a gage at the limit of its resolution is not a stable approach.

And pay attention as you browse through the other threads to the comments about the end hooks vs. accuracy.

HTH
 

howste

Thaumaturge
Super Moderator
#3
I believe if you do what Ninja says and evaluate the measurement method for the example tolerances you showed that the tape measure will definitely not be suitable. I've seen tape measures used for steel joists 40 feet long with tolerances ±.25, but to use them for ±.06 you would need a much finer resolution.
 

Michael_M

Trusted Information Resource
#4
Doesn't this mean the Measurement System should have an accuracy of x.xxxx?
Conventional Tape Measures do have an accuracy of 1/32 or less.
I am not sure if I am misreading your statement here, but 10 to 1 would reflect the tolerance not the number of decimals the dimension shows. In the ±.06, 10 to 1 would be a device that is accurate to ±.006.
 

Big Jim

Super Moderator
#6
Your nuts if you expect accurate tape measuring results more accurate that 1/4".

Use the right tool. Likely a caliper and maybe even a micrometer.
 
#8
The 10 to 1 rule is referring to resolution. In the case of a tape measure, you probably don't have that. Example:

Widget A is 10.00+ .01″

If you were to follow the Rule of 10 here, you’d want to select a measurement system that has 10x the resolution of the dimension tolerance (.01″), therefore it would have to be resolute to .001″.

As others have stated, use a caliper or even a rule.
 

BradM

Staff member
Admin
#9
Hello Thomas!

It seems to me there are several issues within your question (a good one, by the way).

First, it seems you need to determine a suitable instrument to make measurements to satisfy your customers needs.

So I'll take it that your customer requires the part to be made within these specifications:

1.125" ±0.030 or 1.315" ±0.060

So... you want a device that has accuracy to at lease .060 inches or better.

Let me ask you... if those were your specifications and you were paying good money to have these parts made, would you want your customer using a ruler/tape measure? :cool::D

Let's suppose that you have a tape measure (and for arguments sake) it's calibrated and the resolution is 1/32 (.03"inch). You still have to make sure that everyone reads and interprets the analog scale the same way.

So I think you may have the minimum required instrument for the reading. However, I would suggest purchasing a digital caliper (or micrometer) have it calibrated, and use that tool to demonstrate compliance to the customers specifications.
 
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