When modifying a model using the Modify 3D functions, you will probably encounter faces which are connected to their neighbors by smooth tangential transitions.

A Keep Tangency options enables you to choose whether to maintain tangential transitions between faces on completing a modification. The default setting is not to maintain tangencies in the part resulting from a modification. If you decide not to keep the tangential transition between faces, it will usually be replaced by a sharp edge in the resulting part. This option is available for the following Modify 3D commands:

The above figure shows a part in which a face 1 is connected to its two neighboring faces by smooth tangencies. The results of an offset operation on this face are shown on the right with Keep Tangency switched off 2 and on 3. Without the Keep Tangency function, the smooth tangency is lost in the resulting part.

To maintain tangential transitions, the faces neighboring the target face must be rotated and any gaps filled. Creo Elements/Direct Modeling uses the following color scheme to indicate the selected face and its neighbors:

A quick preview provides immediate feedback to show the new position of the target face and its neighbors if you invoke the modification. If the proposed modification does not produce the desired result you can redefine your settings as appropriate.

After selecting the Keep Tan option, the Control button becomes active so that you can use the tangency control functions. You can use these options to fix or unfix the positions of neighboring faces as required to make the desired modification. The options in the tangency control menu are as follows:

The primary neighboring faces of the selected face(s) are free to move so that the tangential transition can be preserved. The secondary neighboring faces, however, are fixed in position by default. For some modifications, you may find that it is necessary to unfix or fix a face for the modification to produce the desired result. The fix edge option is useful when modifying a model in which the primary neighboring faces of the selected face meet together at a point. A modification in this situation may produce unexpected results. Use the unfix edge option to free a previously fixed face.

For all the supported tangential transitions, Keep Tan maintains the tangency by repositioning the primary neighboring faces. In case the repositioning is not possible, Keep Tan modifies the geometrical parameters (like the cylinder radius or the cone angle) of the primary neighboring faces to maintain the tangency.

The above image shows a part in which the cylindrical face 1 is connected to two neighboring cylindrical faces 2 and 3 by smooth tangencies. When you change the radius of face 1, the radius of faces 2 and 3 accordingly adjust to maintain tangency.

When the Redo Blend option is selected, blended and chamfered edges are automatically detected by Creo Elements/Direct Modeling and will be reconstructed after making the modification. By clicking Control then Sel Blend, you can override the automatic detection of blends and manually select the blended edges to be reconstructed.

You can set the default activity of the Keep Tan and Redo Blend options in the Option Defaults menu. To open this menu, click Settings in the Modify 3D menu.

Fine control of the tangential transitions between neighboring faces can be achieved by using the Tangency Control functions. When maintaining a tangential transition, Creo Elements/Direct Modeling tries to keep the primary neighboring faces tangent to both the selected face and the secondary neighboring faces, without changing the physical position of the secondary neighbors. Occasionally, these default constraints may prevent the modification from being resolved. In these cases, you might need to fix or unfix the position of an edge or secondary face before the selected face can be modified as desired.

In the figure below, the secondary neighboring faces 3 and 4, being fixed in position, prevent the primary neighbor 2 from moving to maintain the tangential transition to the selected face 1. To solve this problem, unfix the central face 3 to allow the secondary neighboring face 2 to move so that the selected face can be offset as shown below.

When modifying parts in which it is not possible to identify secondary neighboring faces, the behavior of the Keep Tangency function may be unpredictable. For example, when two primary neighbors meet at a point, as shown in the figure below, it is recommended to fix edge 1 before modifying the selected face 2 using the Keep Tangency option.