A surface's boundary and any intersections with other surfaces create regions which may be made visible or invisible. A surface that has invisible regions is said to be trimmed. A surface that intersects the trimmed region may be referred to as the trimming surface.
Maxsurf uses the concept of a 'region' to define an area of a surface that may be visible or invisible (trimmed off). These regions are defined by the boundaries of the surface and any intersections that the surface has with any other surfaces.
Before trimming any surfaces, it is useful to check that the surfaces intersect properly. Since, if this is not the case, it will not be possible to correctly trim the surfaces. To display the surface intersections, select Contours from the Display menu and place a tick next to the Intersections item. Surface intersections are normally displayed in yellow. Check that these are as required.
Consider the sample shape below. The left picture below shows four surfaces: a planar surface which we wish to trim, a cylindrical surface and two arc-shaped surfaces. We wish to trim the planar surface to the boundaries defined by its intersections with the other three surfaces.
Initially, all of the regions on the surface are visible and selected. We need to de-select the regions that need to be trimmed off.
All regions are initially selected, regions to be trimmed off are removed by clicking in the unwanted region.
Once we trim off the unwanted regions we are left with the required shape.
Perhaps the most important thing to remember is that for a region to be used as a trimming region, it must be fully bounded. In other words, the perimeter of the region must be fully closed. If a perimeter is not closed, a region cannot be properly formed. The diagrams below explain the difference between closed and unclosed regions.
GOOD: (Closed regions)
POOR: (Unclosed regions)
Note:
Maxsurf will find all the closed areas defined by the surface edges and any intersections. Even though two of the three intersections do not define fully bounded regions, when placed together, Maxsurf will find both regions 2 and 3.
In this example, the intersections do not define fully closed regions, so Maxsurf will find a single region, which covers the entire surface.
View a video that shows you the use of trimming after
inserting a transom surface. This video also shows how the trimmed surfaces
automatically update to changes in surface shape or position.
View a video that shows you how to quickly model a step
in a deck surface, insert a bowthruster and use right click trimming.
Trimmed surfaces allow complex shapes to be modelled in Maxsurf. Trimming is most likely to be useful when the surface is fair but there are discontinuities in the edges of the surface or a hole has to be cut in the interior.
Typical applications:
§ Deck edges
§ Transom
§ Propeller tunnel
§ Bowthruster
§ Non-tangential Skeg / Keel - hull transitions
§ Superstructure penetrations through decks
Another reason to use trimming could be when two surfaces are combined to form one shape, while they do not have the same properties such as stiffness or number of control point columns.
An example of a Chined hull using trimmed surfaces. It is difficult to control the shape of the intersection line using two trimmed surfaces. In this case it is often preferable to use bonding.
The disadvantages of using trimming for this purpose are:
§ No or little control over the shape of the intersection line of the two surfaces, especially if the intersection line is quite shallow (eg a propeller tunnel with a smooth transition with the hull)
§ It is more difficult to make changes to the model whilst maintaining trimming information. The trimming is easily lost.
It is often better to use bonded surfaces when two surfaces have to be combined into one shape.
Note:
In general it is recommended to use trimming to create discontinuities in the surfaces edge or holes in the surface interior.
Instead of trimming you can often also choose to use bonded surfaces. One of the main advantages of using bonded surfaces is the ability to join two surfaces and have control over the intersection shape. Models with bonded surfaces are easier to make changes to and export to other programs. See When to Use Bonding on page 115.
Trimming is an extremely powerful tool to model a wide variety of shapes using surfaces. However, trimming is also a common cause of difficulties with the surface model that can easily be prevented by following the guidelines below.
1: Closed regions
Make sure the region you wish to trim is completely enclosed by intersection lines or edge lines.
§ Symptom: the region cannot be selected
§ Check the intersection line carefully by zooming in.
§ Check the centreline control points are on the centreline (do a multiple selection and a control properties, check offset = 0).
2: Overlap
Use sufficient overlap on the trimming surface. Plenty of overlap helps to identify remnants of untrimmed surfaces, makes trimming easier, makes it easier to change the trimmed surface without losing trimming information etc.
§ Symptom: region cannot be selected, precision settings affect trimming. See images below for explanation why precision can affect trimming.
§ Never put the trimming surface’s edge exactly in the plane of the surface that you want to trim.
3: Over-compacted control points
Do not over-compact control points to create knuckles in the surface. See Discontinuities and Feature Lines on page 137.
4: Surfaces exactly on top of each other
Do not place two surfaces exactly on top of each other. This often happens during a Duplicate surface command without adding any spacing between the surfaces.
§ Symptom: nothing happens after Start trimming is selected
§ Make sure the surfaces are sufficiently spaced so that there is no maze of intersection lines visible.
5: Make sure trimming works at medium & highest precision
It is important to make sure the model trims at medium and highest Surface Precision. This guarantees that the model can be transferred to other programs without any problems. Sometimes it may be better to use bonded surfaces as well, see When to Use Bonding on page 115.
No trimming region at lowest precision
Trimming region at medium precision
Not only Maxsurf uses the trimmed surfaces for its calculations, also the other programs in the Maxsurf suite require the model to be trimmed properly.
All trimming rules originate from the trimming concepts, so make sure you have read the Trimming Concept section on page 107.
First have a look at the following flowchart giving an overview of the trimming steps:
Note
It is important to note that after you have taken step 2, you must do steps 3 and 4 as well. Maxsurf will continue to ask you to select your regions and trim the surface until you have finished Step 4.
Example:
The following example shows a chined vessel. The boat has been modelled with a hull and a transom. The transom will be used to trim off the aft most sections of the hull.
To trim the hull surface
§ Turn on trimming by selecting Trim Gray or Trim Invisible from the Display - Trimming menu.
§ Select the surface to be trimmed from the Surface - Start Trimming menu. (e.g. Hull)
Maxsurf will show the hull surface with all the currently visible regions selected. In this case since the whole surface is visible, the whole surface will be selected. A selected region is shown with Gray shading.
To change the visibility of a region, click inside that region. The shading will be turned off or on as appropriate to show whether it is selected or not.
In this case, to trim off the area behind the transom (i.e. make it invisible), simply click on the region defined by the edges of the hull, and the hull's intersection with the transom. When this region is clicked on, the shading will disappear.
After clicking in the region to be trimmed off, the screen looks like this:
The window may be zoomed, panned or rotated while regions are being selected.
Now that all the regions to be trimmed off have been selected
§ Choose Trim from the Surface menu (or use Ctrl T).
The contours on the design will now be recalculated and trimmed to the visible regions of the hull surface.
Trimming is often easiest in the perspective window with the “Half” display option turned on. This way you will only see the actual surfaces and not their mirror images. Rotate the view direction so that you can select the desired region. You can zoom and pan the design whilst trimming; however, you cannot change window.
Maxsurf provides the option of displaying trimmed contours in Gray or ignoring trimming altogether.
The Trimming command in the Display menu controls how Maxsurf will display trimmed surfaces. If Trimming Off is selected, Maxsurf will not display or recalculate any trimmed surfaces. This provides a method for making changes to the design without the overhead of recalculating the surface trimming after each change.
If Trim Gray is selected, Maxsurf will trim off any parts of the contours that are not in the visible regions of the surface, and show the trimmed off parts in grey.
If Trim Invisible is selected, Maxsurf will trim all contours to the visible regions of the surface and not display the trimmed contours.
If the trimming mode is set to off, and then turned on (or Gray), Maxsurf will find all the intersections, and attempt to reform any trim regions that you have selected. For example, you could work in low precision with trimming turned off, and switch to high precision with trimming turned on to have a more accurate look at the design without having to re-trim the design.
In some cases, if the design has been substantially changed, it may be necessary to re-trim some surfaces. It is possible for a complete surface to be trimmed away and hence invisible when trimming is turned on, if this happens use Trim Gray to make the surface visible or untrim the surface; see Untrim Surface in the next section.
A good design will trim consistently at all precisions. If you find that the trimming behaviour changes at different precisions, your surfaces may not intersect consistently as you change the precision. The best way to overcome this type of problem is to increase the amount by which surfaces overlap.
Also be aware that you may loose or change trimming information if you add new surfaces or move existing surfaces or control points so that the trimming regions change considerably. It is generally best to do the trimming when you have virtually completed the design.
The trimming information for any surface may be removed by selecting Untrim - Surface name from the Surfaces menu. Only surfaces that have been trimmed are listed in the Untrim menu.
Note
Only visible surfaces are listed in the trim menus.
Also see the section on Bonding Surfaces.