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Pro/Engineer   December 2005   Tip-of-the-Month


Tips & Techniques with Dimensioning Arcs

Model dimensions should reflect the design intent of the model.  Mostly that is pretty straightforward, but sometimes there is a desire to define things a little different.  Here are a few dimensioning tricks for arcs (in case you have not seen them).
 
Dimensioning Arc Length (Sketcher):
An arc can be controlled by dimensioning its length.  This can be very valuable when a model moves conditionally, as with a living hinge.

To control an arc by defining the length, sketch the arc.  Select the arc and one dimension that controls it (angular dim, radial dim, linear dim, etc.) so they highlight red.  Next, select Edit > Convert To > Perimeter > then select the dimension you would like Pro/E to vary in order to maintain the perimeter length you specify.  Figure 1 shows an animation of the procedure.

Perimeter Dimension
Figure 1.  Arc Length Dimension
(If the animation does not work, click on the image.)

 
The perimeter or arc length dimension then shows up on the screen with a "perim" after it, and the varying dimension shows up with a "var" after it.  The "var" dim cannot be modified, and if you delete it, the "perim" dimension also deletes.

TIP:  Driving the "perim" value via a relation is a great way to control the model for parts that move or bend with the model.

Note:  This functionality is not available for all features (for example: variable section sweeps).  See Pro/E Help for more info.

Note:  I don't know if it is a bug or defined functionality, but I have found that you can only make one perimeter dimension per sketch.  Hmmmmm.
 

Dimensioning an Arc Angle:
The included angle of an arc can be dimensioned directly (without centerlines, etc.)

Arc Angle Dimension
Figure 2.  Arc Angle Dimension
 
Sketch the arc.  Begin Dimensioning mode (Sketch > Dimension > ...) > Select one endpoint of the arc > Select the other endpoint > Select somewhere on the arc > Place the dimension.

TIP:  In the dimensioning selection, you can select the arc before the endpoints.  Either order will work.

TIP:  An angular dimension is often a good choice for the varying dim of an Arc Length dimension (above).

Note:  For whatever reason, the Arc Angle dimension cannot be placed on an arc after a perimeter dim is already there.  It can be made before, but not after.
 

Position Dimensioning:
Arc Position Dimensioning
Figure 3.  Arc Position Dimensioning
 
Sometimes the actual radius or length of an arc does not matter, only the position of the arc with respect to its references.  A simple scheme to accomplish this is with a centerline and a dimension tangent to the arc.  Figure 3 shows the dimensioning scheme.
 
Dimensioning Arcs in Drawings:
The same Arc Length and Arc Angle dimensions can be made in a drawing using the Create Dimension functionality just like in Sketcher.  Use Insert > Dimension > New References > then pick (just like dimensioning an Arc Angle) each arc endpoint > pick the arc > place the dimension > then choose Angular or Arc Length from the Arc Dim Type menu (Figure 5).  The result of each dimension type is shown in Figure 4.

Drawing Menu
Figure 4.  Arc Length and Angle Dimensions in Drawing Mode
 
Figure 5.  Menu
 
TIP:  Use query select to choose the endpoints to be sure you are getting the right thing.

Note:  There is the little arc length symbol over the arc length dimension.
 

For more information on any of the above topics, please see Pro/E Help, or search the knowledge base at www.ptc.com.  Of particular interest, TPI entries 35374 & 110207 deal specifically with dimensioning arcs.

 

Have a Very Merry Christmas!!
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