Practical Measurement of True Position at RFS in GD&T

In the feature control frame, positional tolerance is assigned with three types of modifiers. They are 

• Regardless of feature size
• Maximum Material Condition
• Least Material Condition

Regardless of Feature size is used

• For critical features like where press fit needed.
• For threaded holes.

Maximum Material condition is used

• To guarantee assembly for non critical feature.
• To allow fixed gage usage when dealing with mass production

Least Material condition is used

• To ensure minimum wall thickness

Now let's discuss about how the practical measurement would be made to confirm the Pass/Fail of the each hole. 

Let's check for each condition

1. Regardless of Feature size

Many people confuse between the position tolerance and the true position.  Below picture will showcase you the difference. 

Position Tolerance & True Position

In the above picture, the red highlighted are the TRUE POSITION & the blue highlighted is the POSITION TOLERANCE.

So in general, the hole specified at RFS are often measured as circles at top & bottom in the CMM. 

Lets assume below are the measurements for the hole 1.1.

• The hole center was measured .506 from B & .504 from C at the top of the hole.
• The hole center was measured .508 from B & .494 from C at the bottom of the hole.

Measurements for the hole 1.1

Here comes the calculation part

Calculate deviations from TRUE POSITION in X & Y

 

X.dev = X measured - X nominal 

 

X.dev =  0.504 - 0.500 = 0.004

 

Y.dev = Y measured - Y nominal 

 

                              Y.dev =   0.506 - 0.500 = 0.006

Calculate Radial deviations 

R.dev = sq.rt((X.dev)^2+(Y.dev)^2)

 

R.dev = sq.rt((0.004)^2+(0.006)^2) = 0.0072

Measured Position Deviation ( M )

 

M = 2 * R.dev = 2 * 0.0072 = 0.0144

The same above calculation should be applied for all the holes. We are going to use the spreadsheet to automate the calculation.

The below table is the format for the calculation of Position Deviation ( M )

The above manual calculation is automated using formulas in the spreadsheet.

Measurements of all the holes

The values highlighted in color in M column fails the results.

The position tolerance is the 0.014. If the measured value is higher than the positional tolerance, then the hole is considered to be Fail