True Position Basics

NOTICE :

This presentation was given at the October 2007 “Lunch N Learn” at J&H Machine Tools in Charlotte.

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

True Position Basics

Understanding True Position

Part 1 : Grasping the “Basics”

True Position Basics

2.00±0.01

1.00±0.01

The Part

True Position Basics

2.00±0.01

1.00±0.01

The Tolerances

1.99 2.01

0.99

1.01

True Position Basics

1.99 2.01

0.99

1.01

The Tolerance Zone

Part 1 Center Part 2

Center

True Position Basics

1.99 2.01

0.99

1.01

The Question… Which Part is Better?

Part 1 Center Part 2

Center

True Position Basics

0.99

1.01

Part 1 is within the tolerance zone, but…

Part 1 Center

1.99 2.01

True Position Basics

0.99

1.01

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

Part 2 Center

1.99 2.01

True Position Basics

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

True Position Basics

1.99 2.01

0.99

1.01

Part 1 Center Part 2

Center

Use a Circle instead of a Square!

True Position Basics

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.

True Position Basics

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

True Position Basics

2.00

1.00

New Part Print w/True Position

No Tolerances

Ø 0.01 A B C

A

B

C

True Position

Diametral Zone

Primary, Secondary, Tertiary Datums

Tolerance

True Position Basics

2.00

1.00

Here’s an Example Calculation

Ø 0.01 A B C

A

B

C

True Position Basics

2.00

1.00

CAN WE SHIP IT?

Ø 0.01 A B C

A

B

C

Actual Measurements:

X=2.003

Y=1.004

True Position Basics

Actual X = 2.003 Actual Y=1.004

Basic X=2.000 Basic Y=1.000

Δ X = 0.003 Δ Y = 0.004

TP = 0.0032+0.0042 * 2 or

TP = 0.009+0.016 * 2 or

TP = 0.025 * 2 or

TP = .005 * 2 so…..

True Position Basics

True Position = 0.010 Hole Center

0.005 from Nominal True Position

Result = 0.010

0.003

0.00

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

True Position Basics

2.00

1.00

Ø 0.01 A B C

A

B

C

Our True Position is within the Specification….

Barely!

CAN WE SHIP IT? YES, WE CAN!

True Position Basics

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

True Position Basics

True Position Basics

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

True Position Basics

True Position Basics

2.00

1.00

Same Print, this time a “Real” Part

Ø 0.01 A B C

A

B

C

True Position Basics

On a “Real” Part:

X= 1.9619

Y= 1.0081

True Position Basics

Actual X = 1.9619 Actual Y = 1.0081

Basic X = 2.000 Basic Y = 1.000

Δ X = -0.0381 Δ Y = 0.0081

TP = -0.03812+0.00812 * 2 or

TP = 0.0015+0.0001 * 2 or

TP = 0.0016 * 2 or

TP = .0390 * 2 so…..

True Position Basics

True Position = 0.0779

Hole Center 0.0390 from Nominal 0.0390

True Position Result = 0.0779

0.0381 0.00

81

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

True Position Basics

True Position Basics

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!

True Position Basics

Let’s see how this changes things.

True Position Basics

True Position Basics

Watch out for Outliers!

True Position Basics

True Position Basics

Now that we have that sorted out,

ONE MORE TIME!

X= 1.9504

Y= 1.0104

True Position Basics

Actual X = 1.9504 Actual Y = 1.0104

Basic X = 2.000 Basic Y = 1.000

Δ X = -0.0496 Δ Y = 0.0104

TP = -0.04962+0.01042 * 2 or

TP = 0.0025+0.0001 * 2 or

TP = 0.0026 * 2 or

TP = .0507 * 2 so…..

True Position Basics

True Position = 0.1014

Hole Center 0.0507 from Nominal 0.0507

True Position Result = 0.1014

0.0496 0.01

04

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

True Position Basics

True Position Basics

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

ONE MORE THING…..

You might say :

True Position Basics

Turn on Additional Position Result in the Characteristic Settings Editor.

True Position Basics

Understanding True Position

You’ve got Questions, We have answers.

Part 1 : Grasping the “Basics”