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NOTICE :

This presentation was given at the October 2007 “Lunch N Learn” at Ellison Technologies in Charlotte.

The presentation is meant to be viewed with the narration and detailed explanation of the presenter.

Understanding True Position

Part 1 : Grasping the “Basics”

Understanding True Position

1.00±0.01

2.00±0.01

The Part

Understanding True Position

1.99

2.01

1.01

0.99

1.00±0.01

2.00±0.01

The Tolerances

Understanding True Position

1.01

Part 1 Center

Part 2 Center

0.99

1.99

2.01

The Tolerance Zone

Understanding True Position

1.01

Part 1 Center

Part 2 Center

0.99

1.99

2.01

The Question… Which Part is Better?

Understanding True Position

1.01

Part 1 Center

0.99

1.99

2.01

Part 1 is within the tolerance zone, but…

Understanding True Position

1.01

Part 2 Center

0.99

1.99

2.01

Part 2 is closer to the Nominal Center and will assemble easier than Part 1

Understanding True Position

How can we set up a tolerance band that fairly and functionally judges the position of features?

Understanding True Position

1.99

2.01

1.01

Part 1 Center

Part 2 Center

0.99

Use a Circle instead of a Square!

Understanding True Position

Definition Time!!!

The result of a True Position Characteristic is the diameter of a circle, centered on the basic dimensions from the specified datums, that contains the actual center point.

Understanding True Position

Mathematically Speaking….

True Position = ΔX2 + ΔY2 * 2

ΔX = Difference from Basic X value from Actual X value

ΔY = Difference from Basic Y value from Actual Y value

Understanding True Position

Diametral Zone

Tolerance

True Position

Ø 0.01 A B C

No Tolerances

A

Primary, Secondary, Tertiary Datums

1.00

2.00

B

C

New Part Print w/True Position

Understanding True Position

Ø 0.01 A B C

A

1.00

2.00

B

C

Here’s an Example Calculation

Understanding True Position

Ø 0.01 A B C

A

Actual Measurements:

X=2.003

Y=1.004

1.00

2.00

B

C

CAN WE SHIP IT?

Understanding True Position

Actual X = 2.003Actual Y=1.004

Basic X=2.000Basic Y=1.000

Δ X = 0.003 Δ Y = 0.004

TP = 0.0032+0.0042 * 2or

TP = 0.009+0.016 * 2or

TP = 0.025 * 2or

TP = .005 * 2 so…..

Understanding True Position

True Position = 0.010

Hole Center 0.005 from Nominal

0.005

True Position Result = 0.010

0.004

0.003

Nominal Center at X=2, Y=1 from Basic Dimensions

Understanding True Position

YES, WE CAN!

Ø 0.01 A B C

A

Our True Position is within the Specification….

Barely!

1.00

2.00

B

C

Understanding True Position

Let’s do this very example in Calypso and see if the math works out!

Understanding True Position

Understanding True Position

This is a Theoretical feature with X Y coordinates placed exactly at 2.003 and 1.004.

Understanding True Position

It Works!

Understanding True Position

Ø 0.01 A B C

A

1.00

2.00

B

C

Same Print, this time a “Real” Part

Understanding True Position

On a “Real” Part:

X= 1.9619

Y= 1.0081

Understanding True Position

Actual X = 1.9619Actual Y = 1.0081

Basic X = 2.000Basic Y = 1.000

Δ X = -0.0381 Δ Y = 0.0081

TP = -0.03812+0.00812 * 2or

TP = 0.0015+0.0001 * 2or

TP = 0.0016 * 2or

TP = .0390 * 2 so…..

Understanding True Position

True Position = 0.0779

Hole Center 0.0390 from Nominal

0.0390

0.0081

0.0381

True Position Result = 0.0779

Nominal Center at X=2, Y=1 from Basic Dimensions

Understanding True Position

What???

Understanding True Position

Explanation Time!!!

The GD&T standard specifies that the FUNCTIONAL center be used for calculations. This means for a hole, that the Maximum Inscribed fit be used. Calypso does this automatically!

Understanding True Position

Let’s see how this changes things.

Understanding True Position

Understanding True Position

Watch out for Outliers!

Understanding True Position

Understanding True Position

Now that we have that sorted out,

ONE MORE TIME!

X= 1.9504

Y= 1.0104

Understanding True Position

Actual X = 1.9504Actual Y = 1.0104

Basic X = 2.000Basic Y = 1.000

Δ X = -0.0496 Δ Y = 0.0104

TP = -0.04962+0.01042 * 2or

TP = 0.0025+0.0001 * 2or

TP = 0.0026 * 2or

TP = .0507 * 2 so…..

Understanding True Position

True Position = 0.1014

Hole Center 0.0507 from Nominal

0.0507

0.0104

0.0496

True Position Result = 0.1014

Nominal Center at X=2, Y=1 from Basic Dimensions

Understanding True Position

YES!!!

Understanding True Position

ONE MORE THING…..

You might say :

“So if True Position only gives me distance from center, how do I know how to make my offsets?”

Understanding True Position

Turn on Additional Position Result in the Characteristic Settings Editor.

Understanding True Position

You’ve got Questions, We have answers.

Understanding True Position

Part 1 : Grasping the “Basics”

Understanding True Position