Stepped Plane (SPLA)

A Stepped Plane can be ‘Open’ or ‘Closed’, the type being determined by the setting of the SPLA’s GTYP attribute. The default is GTYP OPEN. GTYP CLOSED defines a closed Stepped Plane or Enclosed Plane. The only other attribute is DIR, which determines the Plane’s extrusion direction.

An SPLA owns WPOS elements, one per plane ‘step’, whose sole attribute is POS, the step’s 3D Design position. Specifying a 3D position automatically creates a WPOS element and sets the POS attribute.

The order in which the points are defined plus the direction of the plane’s extrusion determines which side of the plane is retained. A ‘handy’ rule for determining the ‘retain’ side (with PMODE STANDARD) is to hold the thumb, index finger and middle finger of the left hand mutually at right angles; if the thumb points in the extrusion direction and the index finger points towards the last step point then the middle finger will point towards the retain side - see Figure 5:6 below. A similar ‘rule’ applies for Enclosed Planes.

Figure 5:6. Defining a Stepped Plane

The SPLA shown above would be created by a sequence of commands such as:

The STEP command will invoke the cursor, which will enable 3D positions or Design p-points to be identified, automatically creating WPOS elements. Draft imposes no limit on the number of steps, but in practice only four points can be defined by a single STEP command due to command line length restrictions. If a plane with more than four steps is required, further STEP commands will enable additional points to be appended to the existing member list. The minimum number of points required to define an SPLA is 2, which will have the effect of a Flat Plane.

WPOS elements can be created explicitly by command sequences such as:

NEW WPOS

POS E120500 N236785 U0

If this syntax is used you must leave the list of WPOS elements in the correct order for the SPLA to function.Figure 5:7 below illustrates some examples of Stepped Planes.

Figure 5:7. Use of the Stepped Plane (SPLA)

(The pictures in the left half of Figure 5:7 above illustrate the use of the SKETCH PLANE facility - see Plane Querying.)

If the STEP command is used then once a series of points are entered a check is made to ensure correct SPLANE definition; if satisfactory then a message of the form:

Splane /name is satisfactory

is output. If the plane is not satisfactory then the message will indicate what the problem is. Generally, an SPLA will be incorrect if parts of the plane overlap even if extruded to infinity at the ends; Figure 5:8 illustrates this situation.

Figure 5:8. Plane Errors

If you have defined a plane that is unsatisfactory then it will have to be manipulated. In examples A and B, the plane can be corrected by either reordering or repositioning one of the points or by adding a new point. Alternatively the plane could be made an enclosing type. In example C the plane can only be corrected by either reordering the points or by repositioning one or more of them. How to move points is discussed in the next Section.