Local Ops: Adding Draft

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Contents

• 20.1 Introduction
• 20.2 Reference entities
  • 20.2.1 Examples
• 20.3 Step tapering
• 20.4 Taper methods
  • 20.4.1 Curve-based and surface-based tapering
  • 20.4.2 Offset tapering

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20.1 Introduction

Drafting, or tapering, functionality is typically used to model parts for injection molding/casting. Adding draft allows the part to be easily withdrawn from the mold. There are two stages:

• analysis - divides the body into a set of faces requiring the addition of draft, and a set of faces which don't. Typically some faces are split;
• tapering - the surfaces of the faces which require drafting are replaced by isocline surfaces.

During both stages, the functionality requires a draw direction vector pointing into the mold and a draft angle.

Analysis

During this stage of the process PK_FACE_imprint_cus_isoclin is used to create and imprint isocline curves on a set of faces and to report whether the resulting faces are steep. The set of faces must all come from the same body.

Faces are deemed to be steep when, at all points, the face normal makes an angle with the draw direction which is greater than (90 - draft angle);

Tapering

This stage of the process uses PK_FACE_taper to create isocline surfaces to replace the surfaces of the selected set of faces

Definitions of isocline curves and surfaces

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20.2 Reference entities

PK_FACE_taper requires a reference entity for each face it tapers. The reference entity is used to define the isocline surface. It can be one of the following:

If a face's reference entities are a set of edges plus a surface, the required operation is then a step tapering.

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20.2.1 Examples

The following examples are intended to illustrate the different aspects of this functionality.

Example 1:

Splitting a single face and adding draft to two of the resulting faces

In Figure 20-1 (a) the curved face has been analyzed and split by two isocline curves into three faces of which two need draft. (b) shows the drafting stage complete and illustrates the isocline surfaces meeting the original face smoothly.

Figure 20-1 Splitting a single face and adding draft to two of the resulting faces

Example 2:

Selecting edges as reference entities

In Figure 20-2 both faces f0 and f1 require draft. In this instance you may identify edges e0 and e1 as the reference entities which both retain their geometry after the draft is applied. In this example the draft surface for face f0 is a plane and for face f1 a ruled B-surface.

Figure 20-2 Selecting edges e0 and e1 as reference entities

Example 3:

Using an adjacent face as the reference entity

In Figure 20-3 two faces (f0 and f1) need drafting. Drafting face f0 modifies edge e0, which means that e0 cannot itself be used as the reference entity for f1.

In this case Parasolid allows the reference entity of f1 to be specified as f0, which in turn means that f1 is to acquire the same isocline surface as f0. Figure 20-3(b) shows the effect of using this specification and since f0 and f1 are adjacent they are merged into one face.

Figure 20-3 Using an adjacent face as the reference entity

Example 4:

Edges to be fixed during drafting

In Figure 20-4 both edges e0 and e1 are required to be fixed during the drafting of face f0. This is achieved by specifying both e0 and e1 as references for f0. The drafted body must then split at the common vertex of e1 and e0 into faces f0 and f1, with each drafted appropriately.

Figure 20-4 Edges e0 and e1 are to be fixed during the drafting of f0

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20.3 Step tapering

In step tapering, the taper is based on both a reference surface and a parting line of reference edges. The specified faces are tapered in the same way as in regular tapering, but, contrary to regular tapering, these faces are not split into new faces stemming from the reference edges. Any of the reference edges that do not lie in the surface produce a new step face which extends from the reference edge to the tapered face.

A typical example of step tapering is shown in Figure 20-5. The face to taper is marked f0, the reference surface is the plane marked s0 and the reference edges form the parting line. The taper direction is the upward vertical.

Figure 20-5 parting line of four edges

As in regular tapering, the reference edges, the intersections of the faces to taper and the reference surface, are kept fixed. The reference surface, though, does not have to intersect the faces to taper, as shown in Figure 20-6.

Figure 20-6 Closed parting line of four edges

The reference surface may also split reference edges into new edges as shown in Figure 20-7, leading to the creation of the new-edge associated step faces.

Figure 20-7 Edges of parting line are split

Step surfaces

The surfaces attached to the step faces created from reference edges during a step taper operation can be of two types:

• Tapered surfaces These surfaces are generated in the same manner as the main tapered surfaces, they therefore satisfy the taper condition.
• Normal surfaces These surfaces are ruled surfaces perpendicular (normal) to the relevant original main taper face. They do not necessarily satisfy the taper condition.

Figure 20-8 Step-tapered body with normal step face

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20.4 Taper methods

Parasolid allows you to create a number of different types of taper:

• Isocline-based tapering is the standard taper method as described in previous sections. It is particularly suited to creating molds.
• Curve-based and surface-based tapering are methods that are particularly suited to aesthetic tapering. See Section 20.4.1.
• Offset tapering is particularly suited when creating a taper against a single curved top surface.

The method field in the options structure for PK_FACE_taper lets you choose which method to use. This field can take the following values:

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20.4.1 Curve-based and surface-based tapering

Curve-based tapering lets you create tapered surfaces in cases where the taper condition is not satisfied (such as when the taper reference curve is in the same direction as the draw direction). It is particularly suited to aesthetic tapering.

Surface-based tapering also lets you create tapered surfaces in cases where the taper condition is not satisfied, using a different method of calculation. It is particularly suited to aesthetic tapering.

In many cases, surface-based and curve-based tapering yield identical solutions, as shown in Figure 20-9. Note that body b0 in this figure could not be tapered using the isocline-based method.

Figure 20-9 Creating surface and curve-based tapers

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20.4.2 Offset tapering

Offset tapering lets you create tapered surfaces in which the curve of the reference edge is projected onto a single top surface, creating a new "offset curve" that appears offset from the curve of the reference edge when viewed from the direction of taper, as shown in Section Figure 20-10.

When producing this type of taper, only one top surface is allowed, and it must be the surface of a face in the body. It should not be the surface of any taper face.

Figure 20-10 Creating an offset taper

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