# Round Holes in a Square Wall: Creating Curved Perforators in Bentley Architecture

##### Did you ever feel like punching a hole in a wall? Bentley Architecture makes it easy...

When you create compound cells in Bentley Architecture, you can define a perforator that will automatically create an opening in a linear form. Because forms are planar, curves or circles used to perforate forms are forced to be segmented. The real problem is how many segments are used. The number of segments is a factor of the tolerance setting. You can find the tolerance setting in several generic forms tools in the Bentley Building products: Place Segmented Arc Form, Place Curved Form, Extrude Element to Shape(s) and Construct Openings (but not Create Slab by Flood; that is a different Tolerance setting )
The tolerance setting controls how many segments a curve is divided into by setting a maximum distance between the center of the arc and the center of the chord that makes up the segment.

The smaller the tolerance, the more segments that will be required to create the curve and the more accurate the curve approximation will be.
Here's the tricky part: It doesn't matter what the tolerance setting is when you place the compound cell that has the curved perforator. (This is strange because that is when the tolerance is used.) The important time to set the tolerance is when the compound cell is created.
Here's an example with the same round window, created with the tolerance set to 0:1, 0:0 1/8 and 0:0 1/32:

You can see that the smaller the tolerance, the more segments are created to approximate the curve.

For most BIM models, 1/8" is a small enough setting, but if you are doing a more detailed model, you may want to decrease the setting.  (You can not make the tolerance setting smaller that the accuracy setting on the working unit tab of the active design file settings.)  It is a good idea to set an acceptable tolerance in the seed file that you use for Bentley Architecture.

Relavant Key-ins:  TFCONSTRUCT OPENING - You will need this in v8i because this tool has been removed from the interface.