龙龙 2007-10-10 22:28
Inventor 3D Grips 和约束
You can use 3D Grips to edit features on your models, but how does this affect existing constraints on your sketches?
[b]You can loose constraints when you use 3D Grips.
[/b]In this short exercise you抣l learn that the first sketch of a part model can loose positional constraints when you use 3D Grips. You抣l also learn how to prevent this from happening.
First use the Standard Part template to create a 1-inch cube. Draw a rectangle on the origin of the sketch plane, and constrain the node on the origin with a Fixed constraint. Add 1-inch dimensions for the height and width, and then extrude the square 1-inch.
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[b]How to use 3D Grips
[/b]The next step is to use 3D Grips to edit the cube. Before you get started, highlight the sketch in the browser so that you can see which corner of the sketch is constrained to the origin with the Fixed constraint.
Now follow these steps.
[list=1][*]Click any surface to highlight the surface.[*]Right click and select 3D Grips.[*] Place your pointer over a grip to display a grip arrow.[/list][attach]1102[/attach]
[list=1][*]Drag the grip arrow to a new location.[*]Right click and select Done.[/list]You can practice dragging grips to change the size and shape of the cube, and while you do, make the following observations.
[list][*]The grip arrow does not appear on the faces that are adjacent to the fixed sides of the sketch profile.[*]The grips on the edge of the cube can be used to drag the geometry along two axes at the same time.[*]Once you drag the edge that抯 adjacent to the Fixed node the grip arrows do appear on the faces that are adjacent to the fixed side of the sketch profile.[/list]Before you continue, be sure to adjust the cube by dragging the edge that is adjacent to the fixed node on the sketch profile. This edge is the edge that is formed by the extrusion, it is perpendicular to the sketch profile, and one end of it is on the Fixed node. Use 3D Grips to move the edge off of the node.
[b]Find the missing constraints.
[/b]Now edit Sketch1.
If you recall, the corner of the profile was constrained to the origin of the sketch plane with a Fixed constraint. When the sketch was adjusted with the 3D grip, this constraint was automatically removed. So if you open the Auto Dimension tool you抣l see that 2 constraints are needed to fully constrain the sketch. In order to regain control over the sketch by fully constraining it, you need to re-apply the Fixed constraint.
[b]How to keep from loosing constraints
[/b]Fully constrained sketches that are located by projected geometry do not loose constraints when you use 3D Grips. So the first sketch has to be located by projected geometry.
Follow these steps to take control of the constraints. [list=1][*]Open a Standard Part file.[*]Expand Origin in the browser.[*]Right click on Center Point and select Visible.[*]Use the Project Geometry command on the 2D Sketch Panel to project a node from the Center Point onto the sketch plane.[*]Right click on the Center Point in the browser and deselect Visibility.[*]Use the green snap dot to constrain a corner of the rectangle.[*]Fully constrain the sketch.[*]Extrude the profile.[*]Use 3D Grips to adjust the cube.[/list]When you follow this procedure you抣l find that you will not be able to drag the edge of the cube that抯 adjacent to the projected node.
This sounds like a lot of steps, but if you read my Tips and Tricks for 揈ssential Templates?you抣l learn how to streamline this process.
[b]Why is this example important?[/b]
This is a very good example of why you need to fully constrain all of your sketches. Dragging the edge of the cube affected the constraints of the sketch, and the only way we knew that a constraint had been deleted was to examine the sketch. If this was a more complicated part, the effect of changing one feature may not have been as obvious. So an under-constrained sketch may change without you knowing it.
As a rule, I always suggest that you fully constrain all your sketches, and locate them on projected geometry.