# trim

This command allows the user to trim a surface using a curve or set of curves.

## Inline mode usage

trim -h
Displays the help file that summarizes the parameters for this command.
trim –s <surface> -c <curve>
Trims the selected surface <surface> with the selected curves <curve>.

## Interactive mode usage

Invocation
trim
Parameters
• Surface to trim.
• Cutting closed curves to be used for the trim operation.

## Example

We want to create a circular hole (with radius 1) in the center of a plane. This plane is defined as follows:

>plane
Select first point [x y z]: 0 0 0
Select second point [x y]: 4 4
> explode
Select objects on screen (Press enter when done) // Select the plane and press Enter.
> 

We need to explode the plane so it will be treated as a surface and not as an object. Next, we will define our cutting curve, in this case, a circle. This is done with the following command:

>circle
Center [x y z]: 2 2 0
Radius [double]: 1
> 

We have the surface to trim and the cutting curve, so we can proceed with the trim operation. However, the direction of the curve is going counter-clockwise (we can check this by selecting the curve and showing the normals with the button on the toolbar), so the trim operation will delete the part of the plane that is outside the circle1. We want exactly the opposite, so will need to invert the normals of the circle. This can be done simply clicking on the geometry view while the normals of the curve are being shown.

Now we can finally perform the trim operation. This is done in the following way:

> trim
Select one NURBS surface to trim (Press enter when done): // Select the plane
Select cutting closed curve(s) (Press enter when done): // Select the circle
>
1 This happens because the part of the surface that is cut off is pointed by the vector resulting from the cross product of the normal vector of the curve and the normal vector of the surface. The normal vector of a plane is pointing in the positive direction of the Z-axis by default, so the cross product vector will point outwards if the normals of the curve are pointing counter-clockwise.