Ask the Experts

Using Feature Selection and Prune Command

Wes Krol, Applications Engineer

A recent webcast generated some questions and comments regarding Feature Selection and Prune tools introduced in KeyCreator version 5.  This month, I’ll spend some time reviewing a few of these and pass along some tips & tricks to help you get the most out of the technology. FEATURE COMMAND Let’s start with FEATURE    When you need to execute the functions XFORM FACES, OFFSET FACES, REMOVE FACES, etc., the Feature capability button provides another means of selecting faces. Instead of using individual or window methods to select faces, you can group-select them based on a defined geometric relationship; for example the group of faces form a pocket or a boss.  Understanding how the underlying technology sees these features will provide an advantage to getting the most out of this new technology. Also, it is helpful to consider where you place your cursor and how that “feature” is attached to the body.     Bump is a generic feature made up of sets of adjacent faces forming a protrusion or cavity which are cleanly bounded by a single capping face.  Think of driving your car over a speed bump or over a pothole…what happens to the car?  Sometimes  or  will select the same entities. Fig.1    - Boss is a raised area on the body made up of sets of adjacent faces that form the walls and top of a protrusion cleanly bounded by one or more faces.  Sometimes  is what you use.     Fig.2    - Pocket is a depression in the body made up of sets of adjacent faces that form the walls and bottom of a cavity cleanly bounded by one or more faces     Fig.3    - A raised area on the body made up of sets of adjacent faces that form non-cylindrical/conical walls and top of a protrusion cleanly bounded by one or more faces.  Sometimes  is what you could use.   Fig.4    - Selected face and all attached tangent faces     Fig.5    - Constant thickness side walls, as those on a sheet metal part, made up of the face under cursor, and all adjacent faces with tangent transitions like the Smooth option, or all faces on the side wall of the solid     Fig.6   Again remember, Feature is a method of selecting faces.  You may need to add or subtract individual faces as needed because the rules and/or techniques used for the specific function you are executing still apply; (i.e. REMOVE FACES does not add or create new faces).  Sometimes, simply by using a different feature option (i.e. BUMP vs. POCKET) will select all the faces you want. I mentioned earlier that cursor placement may effect the selection.  The figure below shows what happens when the feature selection is set to . Notice how I get different face selections depending on my cursor location. Try it for yourself.  Fig.7   PRUNE COMMAND Let’s look at PRUNE                                                            Click here to see a video. The MODIFY>BOOLEAN>PRUNE  function has two options: COPY or CUT.  The COPY is more forgiving because it only makes a copy of the selected faces.  The CUT performs more operations; it tries to create a solid as the result of the feature faces selected and close up any gaps left behind on the body. This is similar to REMOVE FACES function (that’s a clue!).  See example in fig.8.  When possible, technology behind the PRUNE>CUT command tries to cap any open areas to create a solid of the feature, if it is not able to, it creates a sheet body instead. Fig.8   When you see a feature that is not recognized by the software as a feature, you may need to help it.   One way, (mentioned above), is to add or subtract additional faces yourself.  Another way is to use the IMPRINT or SCRIBE functions, MODIFY>SOLID TRIM/BREAK functions and BREAK the body into simpler shapes or remove blends or “holes” that attach the feature to the body and reapply them afterwards.  A simple example is the hinge in Fig. 9 below.  As is, neither the ribs (as I see them) nor the pockets (as I see them) will prune off.  Let’s take a look why, and how this can be resolved. I “see” the hinge knuckles as 4 “ribs” or “bosses” but when I try to prune one of them off, (cut or copy); it is not recognized as an individual feature.  The reason is that there is no logical place to limit the feature because features work on faces. In this case, the side face on the hinge is one large face attached to all the knuckles.  This is when I need to select IMPRINT, SCRIBE or SOLID TRIM/BREAK in order to help the software to recognize them.  In this case, I favor IMPRINT or SCRIBE because I only need to divide the face or faces so that all remains topologically attached and the part remains a single solid.  If I chose SOLID TRIM/BREAK, I would be breaking the hinge into multiple solids.  This would work to get the ribs, but I would lose the “pockets”.  It all depends on what you’re trying to achieve! Fig. 9 Fig. 10 shows the hinge single body imprinted vs. the hinge body broken into multiple bodies.  Fig. 10 Now I can easily prune off (cut) the boss.  Fig. 11 shows the pruned boss moved away from the body.  Notice that no gap appears in the topology!  Fig. 11 A similar problem occurs when I “see” the space between the “bosses” as a “pocket”.   In this case, the problem is the two thru-holes on either side “bosses”.  Performing the imprint does not solve this but it can remain.  Here I need to remove both holes first either by REMOVE FACES or pruning them off; it all depends on whether I need the holes.  The figure below shows the 2 holes pruned off and moved aside for clarity.   If I did not move them I could use a Boolean method to replace them if needed!  Fig. 12   Now I can easily prune off the “pocket” as a feature.  Fig. 13 shows the pruned-off pocket moved off the body.  Notice that no gap appears in the topology!  Fig.13   So, what have we learned? Feature is another way to select faces that uses feature characteristics.  The commands/functions rules apply to the faces in the same way as if you selected them separately or via windowing. You can add or subtract faces as needed. Moving the cursor over different faces can select different feature types.  That may give you the desired results. You have tools to help the software recognize features as you want to see them. Removing thru-hole conditions (shown in demo) or holes that go deeper than the mating or attaching faces (not shown) improves pruning. Sometimes it’s a two-step approach to prune off features Pruned off geometry are solids or sheet bodies and behave accordingly Prune with the CUT option must do two things in order to be successful: it must create the sheet body or solid close or fill-in the gap caused by the cut. (Hint: How does REMOVE FACE work?) Click here to see a video. That was just a start…. These are great tools for breaking down models and selecting geometry for editing. Give it a try! I’ll try to cover a few more feature related tips ‘n’ tricks in future issues! Wes Krol “Long hours don’t mean anything—results count…”    -Richard A. Moran