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1 Sounds Sketchy – Design Intent in Autodesk Inventor Walt Jaquith – Simplex Manufacturing MA219-5 There is nothing more important in 3D design than Design Intent. While there are many ways to produce a 3D model, when building in Design Intent the model becomes more than simple geometry -- intelligence is placed into the model. Design Intent can be built into sketches, parts and assemblies. Proper sketching and modeling techniques not only allow you to build more stable designs but will also allow you the flexibility to easily make changes to the design. Both new and experienced Autodesk Inventor users will leave this session with a better understanding on how to structure sketches, parts, and assemblies to best capture Design Intent. About the Speaker: Walt is an independent design contractor in the aerospace industry. A user of Autodesk Inventor since its beginning, he has also been using Autodesk 3D design products for 14 years. Prior to his work in the aerospace industry, Walt worked “the other side of the wall” as a fabricator, mechanic, and general troubleshooter. He has experience in most mechanical engineering disciplines, and has served as a manager of CAD departments and fabrication crews. [email protected]

Sounds Sketchy – Design Intent in Autodesk Inventor · 1 Sounds Sketchy – Design Intent in Autodesk Inventor Walt Jaquith – Simplex Manufacturing MA219-5 There is nothing more

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Sounds Sketchy – Design Intent in Autodesk

Inventor Walt Jaquith – Simplex Manufacturing

MA219-5 There is nothing more important in 3D design than Design Intent. While there are many ways to produce a 3D model, when building in Design Intent the model becomes more than simple geometry -- intelligence is placed into the model. Design Intent can be built into sketches, parts and assemblies. Proper sketching and modeling techniques not only allow you to build more stable designs but will also allow you the flexibility to easily make changes to the design. Both new and experienced Autodesk Inventor users will leave this session with a better understanding on how to structure sketches, parts, and assemblies to best capture

Design Intent.

About the Speaker: Walt is an independent design contractor in the aerospace industry. A user of Autodesk Inventor since its

beginning, he has also been using Autodesk 3D design products for 14 years. Prior to his work in the aerospace industry, Walt worked “the other side of the wall” as a fabricator, mechanic, and general

troubleshooter. He has experience in most mechanical engineering disciplines, and has served as a manager of CAD departments and fabrication crews. [email protected]

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Sounds Sketchy – Design Intent in Autodesk Inventor

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COURSE INTRODUCTION

To many new users, Autodesk Inventor starts off as somewhat of a mystery. The basic workflows can be very different than what the user is familiar with, which can result in lost time, frustration and raised eyebrows among management. In the rush to learn what to do, the question of why is often pushed aside to be answered later. In the mean time, the poor designer is thrust into production with a tool that they don’t yet fully understand. The result can be a poor first impression of the software. The purpose of this session is to give the beginning Inventor user a clearer understanding of why Inventor’s workflows are arranged like they are, and how the new tools can be put to work to make more stable, accurate and flexible models.

Inventor is not difficult to learn, and its workflows are quite logical—as long as the concepts underlying the process are understood. The fundamental principles that drive 3D parametric design are different than those which form the foundation of a 2D design package such as AutoCAD. A firm grasp of these principals is essential to shorten the learning curve and get the user productive in Inventor. One of the most important of these concepts is Design Intent.

WHAT IS DESIGN INTENT?

Design intent is the art of imbedding intelligence into a model in order to convey more information about its nature and purpose than is possible by simply defining it dimensionally. The concept of design intent is not new, of course. The idea of doing everything possible to convey the significance of a part rather than just its dimensions is at least as old as drafting itself. The recent revolution in parametric 3D design, however, has taken the potential for capturing design intent to entirely new territory.

At its core, Inventor is a relational database. While the theory behind relational database design is a worthy study in itself, for our purposes all that’s needed is to understand that a relational database attempts to do two things:

FIG 1: JUST SHOWING OFF. WHETHER THE SKETCH IS A SIMPLE SQUARE PROFILE, OR A COMPLEX MASTER

SKETCH FOR AN ENTIRE ASSEMBLY SUCH AS THIS ONE, THE DIMENSIONS AND CONSTRAINTS TELL A STORY OF

WHICH RELATIONSHIPS ARE IMPORTANT. CAREFUL ATTENTION TO PROPER SETUP WILL ALLOW EVEN A SKETCH

AS BUSY AS THIS TO BEHAVE IN A STABLE AND PREDICTABLE MANNER.

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1. Clearly define an object. This obvious first step is shared by all CAD packages and databases. Whether the description is graphical or uses other methods, the goal of accurately and virtually representing the real-world object is universal.

2. Define the relationships between the object an all other objects with which it interacts. This idea applies not merely to machine components, but to every element that makes up a design, right down to the individual lines that describe its basic profiles. This is where design intent lives. While all CAD applications have some level of ability to capture design intent, the fact that a parametric solid modeler such as Inventor is based on relational database technology means that capturing design intent is an inherent part of the program. If the software is used as it was intended, design intent will be captured.

Close attention to design intent will return big payoffs over the duration of a project. The main objective, of course, is a clear and accurate dataset (either electronic or in drawing form) which can be used to quickly and easily manufacture the design. Inventor is capable of producing drawings of unprecedented clarity, but that’s just the beginning. Other benefits come in the form of a stable, easily modified set of models that are readily reusable. Most Inventor users have at some time experienced the frustration of dealing with a fragile assembly that is constantly experiencing sick constraints, red crosses and other unhealthy behavior. Other assemblies seem nearly impossible to break, taking any modifications in stride with a minimum of fuss. What’s the difference between the

FIG 2: AVOIDING THE SHAFT. THESE TWO JACK SHAFTS ARE DIMENSIONALLY IDENTICAL, BUT WERE MODELED IN VERY DIFFERENT WAYS. THE MAIN BODY OF THE FIRST WAS REVOLVED FROM A SINGLE

SKETCH, WHICH ALSO DEFINES THE HOLE IN THE FAR END. THE SECOND WAS EXTRUDED FROM A SERIES OF CIRCLE PROFILES, EACH PLACED ON THE END OF THE PREVIOUS ONE. NOT ONLY IS THE FIRST EXAMPLE A MORE STABLE MODEL BECAUSE THE VARIOUS EXTRUSIONS ARE LESS DEPENDENT ON EACH OTHER, BUT THE INTENT OF THE DESIGN CAN BE SEEN AT A GLANCE WITHOUT LOOKING

THORUGH SEVERAL FEATURES TO FULY UNDERSTAND THE VARIOUS RELATIONSHIPS.

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two? The extra layer of intelligence, embedded by the designer into the models. The good news is that the modeling practices that promote stability and maintainability in an Inventor Project are the same ones that accurately capture design intent. If you dissect a strong, stable assembly, you will find that its creator deliberately built that stability into the models while simultaneously imbedding the why element into each component.

THE HIERARCHAL MODELING SYSTEM

Before we dive into sketches themselves, there’s another thing to note, and it has to do with Inventor being a feature-based, hierarchal modeling system. What does this mean? Sketches, features and other objects are added to a part one at a time, with each building on the ones that came before. When one feature is placed on another, a dependency is formed, since the second feature will be orphaned if the first feature is deleted. In this way a hierarchy of features is built up. With a complicated part, the interacting matrix of dependencies can become quite complex.

Modeling for Stability and Intelligence

As I mentioned earlier, the techniques for capturing design intent and the techniques for building stable, easily editable models are often the same. The hierarchal nature of Inventor is one area where they overlap strongly, and so I want to take a few moments to discuss the impact of the feature hierarchy that is found in a part model’s browser tree.

FIG 3: EDGY HOLES: TWO EXAMPLES OF A THREE-HOLE PATTERN ON THE EDGE OF A BAR. IN BOTH CASES THE CONSTRUCTION LINE THAT PLACES THE CIRCLES

IS CONSTRAINED ON ITS MID POINT TO THE CENTERLINE OF THE BAR. THE PATTERN ON THE NEAR SIDE WILL GROW AND SHRINK WITH THE LENGTH OF THE BAR, ALWAYS KEEPING THE OUTER HOLES .375 FROM THE ENDS. THE ONE ON THE FAR SIDE WILL

MAINTAIN THE HOLE PATTERN AT 3.25, EVEN THOUGH IT MEANS THE PATTERN WILL FAIL IF THE BAR GETS MUCH SHORTER. THERE ARE MANY, MANY MORE

WAYS THE PATTERN COULD BE ARRANGED. WHICH IS THE “RIGHT” WAY IS TOTALLY DEPENDENT ON THE

REQUIREMENTS OF THE DESIGN. THE EXTRA INTELLEGENCE THAT IS BUILT INTO THE MODEL WHEN

THE BEST OPTION IS USED CONTRIBUTES SIGNIFICANTLY TO THE STABILITY, USABILITY AND

EDITABLITIY OF THE PROJECT.

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Looking at the browser tree of a typical Inventor part, we can see the various features and objects laid out in a vertical pattern. The ones at the top were added first, and the one at the bottom was typically the last feature added to the part. As we go down the browser, the matrix of dependencies gets more complicated. While in reality it is more common for several trails of dependencies to be scattered through the browser tree, it is possible for the last feature in the browser to be dependent for its existence on every other feature, all the way up to the first one. That would mean that any change in any other feature could have a detrimental effect on that final feature. We can see that in any hierarchal system, the features and objects at the bottom of the browser tree will naturally be less robust than the ones at the top, simply

because they depend more on other features. If our hypothetical, dependency-ridden part is placed in an assembly, and happens to be constrained in such a way that the constraint is dependent on that least-secure bottom feature, then any change in the part might not only cause instability in the part model itself, but also in the assembly model.

Fortunately it’s not difficult to establish modeling practices that result in healthy, stable parts, which in turn result in good assemblies. There are some basic principals involved in good part modeling, and we’ll be covering them in a bit. Avoiding sloppy modeling techniques should be a priority for any Inventor user, since bad practices at the part level will almost inevitably cause headaches later down the line. This is where the relationships in a relational database come into play. Throughout the rest of this lesson we will be discussing methods for minimizing potential instabilities in our models while at the same time capturing design intent.

FIG 4: BETTER AND WORSE. A COMPARISON BETWEEN THE BROWSERS OF THE TWO JACK SHAFTS FROM FIGURE 2. ON THE LEFT, THE ENTIRE MAIN SHAFT IS REVOLVED IN THE FIRST FEATURE, WHICH ALSO SHARES A SKETCH WITH THE CROSS HOLE. THIS PART MODEL WILL BE VERY STABLE AND EASY TO

MAINTAIN. ON THE LEFT, THE MAIN SHAFT TAKES FOUR SEPARATE EXTRUSIONS, AND THE FAILURE OF ANY ONE OF THEM CAN CASCADE DOWN AND CAUSE THE CROSS HOLE TO FAIL. IF THAT HAPPENS, ANY ASSEMBLY CONSTRAINTS THAT REFERENCE THE CROSS HOLE WILL GO SICK. A LOT

MORE EFFORT WILL GO INTO UPDATING AND MAINTAINING A PART FILE THAT IS SET UP THIS WAY.

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ORIGIN GEOMETRY IN INVENTOR

Any discussion of design intent in the context of an Inventor sketch has to start with Inventor’s origin geometry. An empty part file presents us with a set of planes on which the part’s initial sketch can be placed. Since the default setting for Inventor is to automatically place and edit a sketch when a new part file is created, many new Inventor

users proceed from that point without giving the origin geometry much thought. This is a mistake, as it bypasses one of Inventor’s most powerful and elegant tools for both capturing design intent and imbedding natural stability into a part model.

In our earlier discussion of part file stability, we noted that the higher up the browser, the more inherently stable the features become. So what’s at the very top of every part browser? Three planes, three axis and a point—the part’s origin geometry. When used correctly, the origin geometry is much more than merely a place to put the part’s first sketch. It is an absolutely stable set of features that can be utilized to make your part files more robust, and also to more clearly communicate the part’s relationship to the rest of the project.

As an example of how this works, consider the ‘good’ shaft part in figure 2. This part was

modeled in such a way that the geometry is revolved around the X origin axis. When it’s time to place that part in an assembly, it will surely be constrained to some other part by its shaft centerline. There are lots of ways to do this; any of the shaft’s cylindrical surfaces could be chosen for a mate constraint. However, The X axis itself might also be selected instead of any of the part’s actual geometry. Is there an advantage to doing so? Absolutely. Any part geometry is subject to modification as the design progresses. Sometimes things change so radically that the features on which assembly constraints are dependent get deleted, and then the constraint will get “sick”. This will never happen to a constraint that is referenced to origin geometry. Even if all the features in the part are deleted altogether, that constraint will not fail. In any assembly, there are plenty of things to keep track of, and skilful management of constraints is required to make sure the assembly behaves as expected. Having a few less constraints to worry about is always welcome.

FIG 5: A POWER USER’S SECRET WEAPON. A PART’S ORIGIN GEOMETRY IS

ABSOLUTELY STABLE, AND SHOULD BE EXPLOITED FOR EVERY BENEFIT IT CAN

OFFER. NEW INVENTOR USERS SOMETIMES IGNORE A PART’S ORIGIN GEOMETRY, AND THEN WONDER WHY

THEIR PROJECTS AREN’T AS ROBUST AS THEY SOULD BE.

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In beginning any part file, the critical first decision must be made: How should the first sketch be arranged? In making these decisions, the orientation of the part’s origin geometry should always be considered in order to put it to the best possible use. If the part is symmetrical on any plane, it will probably be positioned later according to that centerline. Thus, the sketch should be positioned so that one of the three origin planes run up that centerline. It’s true that a work plane and axis could be added later to accomplish the same thing, but it is not only needless duplication, but those work features will be potentially less stable than the origin plane, because they are dependent on other part geometry, while the origin plane is not.

One thing that should never be neglected is to lock down the initial sketch in some manner. Always at the very least project the origin point onto the sketch, and constrain the sketch to it in some way. Besides wasting the valuable resource of correctly positioned origin geometry, unconstrained, floating sketches simply add needless ambiguity to a part, which is counterproductive to the idea of capturing design intent. Such careless modeling practices make a part more difficult to modify, and can contribute to instability.

FIG 6: ALIGNED PLANES. A VERY SIMPLE LIBRARY PART. HOWEVER, THE MODEL WAS CONSTRUCTED SO THAT THE PART’S ORIGIN GEMOETRY IS SITUATED WHERE IT CAN BE PUT TO USE IF NEEDED. THE EXTRA TIME

TAKEN TO DO THIS WILL BE WELL REPAID AS THIS HEALTY MODEL IS USED AGAIN AND

AGAIN.

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FIG 7: THE DESIGN INTENT IS BUILT IN. WHEN PLANNING TO MODEL THIS CONNECTING ROD, THE OBVIOUS CHOICE WAS TO SKETCH THE OVERALL SIDE PROFILE IN A SINGLE SHARED SKETCH. ASIDE

FROM THE WIDTHS OF THE VARIOUS FEATURES, NEARLY ALL THE BASIC ELEMENTS OF THE DESIGN ARE RIGHT HERE IN ONE PLACE, WHERE THEY CAN EASILY BE MANAGED, AND THE IMPACT OF ANY CHANGES ON THE OVERALL DESIGN CAN BE READILY EVAUATED. SETTING UP A SKETCH LIKE THIS REQUIRES A LITTLE PLANNING AND FORTHOUGHT, BUT ONCE IT’S DONE, REFINING THE DESIGN IS A SNAP. A FEW

THINGS TO NOTE HERE:

• THE ORIGIN GEOMETRY IS PLACED FOR MAXIMUM ADVANTAGE. THE PART IS EXTRUDED FROM THE CENTER, SO THE X-Z (BLUE-RED) PLANE IS RUNNING UP THE MIDDLE. THAT PLANE WILL BE USEFUL

FOR CONSTRAINING THE CENTERLINE OF THE PISTON ON THE ROD.

• THE 1.4OO” REQUIRED SPACING FOR THE STUD HOLES IS ESTABLISHED HERE BECAUSE IT BEARS ON THE WIDTH OF THE PART, EVEN THOUGH THIS SKETCH ISN’T USED TO ACTUALLY CREATE THE HOLES. WORKPOINTS ATTACHED TO THE BOTTOM OF THE CONSTRUCTION LINES ARE USED TO

ESTABLISH THE HOLE CENTERS.

• THE BARE MINIMUM OF DIMENSIONS HAVE BEEN USED, WITH THE REST OF THE CONFIGURATION BEING DEFINED BY 2D CONSTRAINTS. CAREFUL USE OF SKETCH CONSTRAINTS IS ONE OF THE

BEST WAYS TO CAPTURE DESIGN INTENT IN A MODEL.

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DESIGN INTENT AND THE INVENTOR SKETCH

The Inventor Way: Sketching vs. Drawing

Any designer who has ever used AutoCAD, or any other 2D drafting package, is intimately familiar with the concept of drawing. After all, that’s what drafting software does; draw highly accurate pictures of an object. Transitioning users are sometimes surprised and not a little frustrated when they attempt to apply their drawing skills to the Inventor sketch environment, and find out that what happens there really isn’t the same as drawing in AutoCAD at all. In AutoCAD, geometry is drawn with precision, but in Inventor, the technique is to define a rough approximation of the desired shape, then apply dimensions and constraints to lock it in. On the surface this seems to take a procedure that used to take a single step, and break it into two distinct operations. Why would Inventor be so different? After all, AutoCAD is the most powerful and universal software package for describing things with lines and arcs that has ever been. What’s the point in changing such a successful formula?

The answer to that question lies in the nature of what is really being accomplished in the sketch environment. In short, the act of sketching goes considerably farther in defining geometry than what occurs in a drawing environment. Let’s compare the two to see how this works.

If a simple square is drawn in AutoCAD, the result is four lines, arranged in the dictated shape. However, there is little self-awareness built into the geometry. The endpoints of

FIG 8: FULLY DEFINED. THE REASON FOR THE DIFFERENCE BETWEEN SKETCHING AND DRAWING IS THE ADDITION OF INTELLEGENCE TO THE MODEL IN THE FORM OF CONSTRAINTS AND “SMART”

DIMENSIONS. IN THE PARAMETRIC WORLD, NOTHING CAN BE LEFT AS AN ASSUMPTION, SO EXTRA STEPS ARE INCORPORATED TO ADD INFORMATION ABOUT THE SKETCH’S RELATIONSHIPS AND

DEPENDENCIES. THE PAYOFF FOR THE EXTRA EFFORT COMES IN THE CAPTURING AND COMMUNICATION OF DESIGN INTENT.

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the lines are coincident, but can be drug apart by their grips. The lines are at ninety degree angles to their neighbors, but there’s nothing to say that they should stay that way. In short, the drawing is not parametric. In parametric modeling slang, another term is used to describe the lack of definition; the geometry is said to be “dumb”. There’s nothing wrong with any of this, of course; that’s how 2D drawing works. Many things are left as assumptions, because they can safely be allowed to do so.

FIG 9: MAKE IT SMARTER. NO ONE UNDERSTANDS THE WAY A DESIGN IS SUPPOSED TO WORK LIKE THE PERSON WHO MODELED IT. USING THE PRINCIPALS OF DESIGN INTENT, THAT UNDERSTANDING CAN BE PASSED ON TO SUBSEQUENT USERS OF THE MODEL.

FIG 9a: NOT TOO BAD. THE UPPER BOSS OF THE CONNECTING ROD IS SET UP SO THAT THE THICKNESS OF THE MATERIAL AROUND THE BUSHING WILL REMAIN THE SAME IF THE BUSHING DIAMETER CHANGES.

FIG 9b: A DUMB BOSS. IF THE BUSHING SIZE CHANGES, THE MATERIAL SURROUNDING IT COULD GET NEEDLESSLY THICK, OR DANGEROUSLY THIN. THIS APPROACH IS QUICK AND DIRTY, BUT THERE’S NOT MUCH INTELLEGENCE BUILT IN HERE. BOTH DIMENSIONS WOULD HAVE TO BE CHECKED IF THE PART WAS MODIFIED.

FIG 9c: DESIGN INTENT FOR THE WIN! ENGINEERING CALCULATIONS CONFIRMED THAT THE BOSS THICKNESS WOULD ALWAYS BE ADEQUATE AT ONE-THRID THE BUSHING OD, SO THAT DIMENSION WAS DEFINED AS A FORMULA. IF THE BUSHING OD CHANGES, THE BOSS WILL AUTOMATICALLY ADJUST ITSELF. FROM HERE, IT WOULDN’T BE TOO HARD TO INSURE THAT THE PISTON ITSELF ALWAYS HAS PROPER CLEARANCE FOR THE BOSS.

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In a parametric sketching environment, where the goal is to create not a drawing of an object, but a profile for extruding, nothing can be safely left as an assumption. All the relationships can and should be carefully defined. The process of defining these relationships accounts for the differences between drawing in a drafting package, and sketching in Inventor. It is also a primary avenue for capturing design intent. In the world of parametric solid modeling, defining the relationships that embed intelligence into the model is so important that it has been separated out and made to be its own distinct step in the sketching process. This explains why drawing with precision—which is the sensible way to operate in a drafting application—is barely supported and not really encouraged in the Inventor sketch environment.

Establishing Priorities

In practice, embedding design intent into a model entails the process of evaluating the part, and answering certain questions:

What are the most important dimensions or parameters of the part? Every part will have some dimensions that are of primary importance, and others that are secondary. In the connecting rod example in Figure 7, the distance between the two bearing centerlines and the spacing of the cap studs are critical dimensions, as are the bearing diameters. The thickness of the rib has little impact on the mechanical function of the part, but needs to be structurally sufficient to bear the loads. The fillet added to round off all the hard edges is practically arbitrary. Bearing in mind our earlier discussion about feature stability and the browser tree, it’s a good general practice to deal with any part’s most important dimensions as early as possible. Those dimensions are usually important because they are the features of the part that have critical interactions with other components in an assembly, and so need to be as stable as possible.

FIG 10: SET AND FORGET. THIS iPART OF AN MS-SPEC HEX BOLT HAS A CHAMFER AT THE BOTTOM WHICH IS

BASICALLY COSMETIC, MAKING IT A FAIRLY LOW PRIORITY ITEM. SINCE THE iPART REPRESENTS A

RANGE OF BOLT SIZES, ONE VALUE FOR THE CHAMFER WOULD NOT LOOK RIGHT ON ALL THE PARTS. THE

CHAMFER COULD BE CONTROLED IN THE iPART SPREADSHEET, BUT THAT’S EXTRA WORK AND

COMPLEXITY FOR NO REAL BENEFIT, SO THE CHAMFER WAS GIVEN A FORMULA VALUE OF ONE-TENTH OF THE

BOLT DIAMETER. PROBLEM SOLVED.

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What are the most important dependencies of the part? In any component, certain features will have definite relationships to other features. One of parametric modeling’s true strengths is the ability to define those relationships accurately. This can be accomplished using dimensions, constraints or formulas. Derived parts can also be used to express dependencies. In the context of an assembly, various adaptive technologies (adaptivity, master sketch techniques, and spreadsheet-driven parts) can be employed as well. Again, these critical relationships should be defined first, and all the secondary relationships filled in around them.

Perhaps in its most elemental form, design intent is nothing more than pointing out what’s more important, and what is less important in a design. On a 2D drawing, this information is often represented in the way that dimensions are placed. The same techniques can be used to even greater advantage in parametric design, because the dimensions are intelligent, and so the sketch can be set up in such a way that the less important elements naturally adjust themselves as needed when the important parameters are changed.

Grouping Information

Often the best way to define a relationship between two elements in a model is to represent them both on the same sketch. Using dimensions and sketch constraints, the relationship can be clearly portrayed, and even set up to mimic real-world configurations, where changing one parameter can affect several others. Inventor’s ability to share sketches should be thoroughly exploited to group important information so that it can be more easily managed. Sometimes just the ability to see the relationships all laid out in a single sketch is all that’s needed to perfectly convey design intent.

FIG 11: TWO DIFFERENT IDENTICAL PARTS. HERE IS A PAIR OF DRAWING VIEWS OF THE SAME JACK SHAFT,

DIMENSIONED IN TWO DIFFERENT STYLES. GIVEN A +/-.005 TOLERANCE FOR THE DIMENSIONS, BOTH PARTS COULD

CONFORM TO THE DRAWING, BUT NOT BE INTERCHANGABLE WHEN MANUFACTURED. THE MANNER IN WHICH THE PARTS

ARE DIMENSIONED INFORMS THE MACHINEST WHAT HIS PRIORITIES ARE WHEN MACHINING THE PART. COMPARE

THIS TO FIGURE 3 TO SEE HOW SIMILAR TECHNIQUES CAN BE USED IN AN INVENTOR SKETCH.

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Grouping related information also keeps the chain of dependencies shorter, and minimizes the potential for instability by linking features at strategic places. A set of features that are all based on the same sketch are much more robust than the same features based on separate sketches, each of which are projected from the previous feature.

Dimensions or Constraints?

Because the possibilities for various combinations of dimensions and constraints in a sketch can be nearly endless, the correct application of the two can be somewhat of an art. While an experienced modeler can quickly find a good solution in most any situation, it’s very difficult to set down any guidelines for what constitutes general “best practices”. Here are a few points that can be applied to most any sketch:

Use Autoconstraints…carefully. During the sketching process, Inventor will attempt to infer the designer’s intentions, and add what constraints it can to speed the process. It does this based on the preferences set by the user in Application Options under the sketch tab. In many cases the constraints selected by Inventor will work fine, making the autoconstraint feature a powerful and helpful tool. In cases where a specific constraint is required, holding down the CTRL key while sketching will defeat the feature, leaving the way clear to add the desired constraint manually. The more complicated a sketch gets, the more individual management of constraints is needed to make sure undesirable constraints are not being added.

Use dimensions only where they are needed. Or in other words, use sketch constraints anywhere it’s possible. This will help keep the sketch uncluttered, making it easier to work with.

Use formulas to tie one dimension to another. The ability to reference one dimension within another is a powerful tool for controlling the configuration of a sketch, and also for expressing design intent by indicating a dependent relationship between the dimensions.

FIG 12: JUST A BOLT? EVEN BASIC PARTS NEED CARE IN THEIR SETUP. THIS IS THE INITIAL SKETCH FOR THE iPART HEX BOLT.

THE SHARED SKETCH WAS USED TO EXTRUDE THE HEX HEAD, THE BOLT

SHANK, AND ALSO TO CUT THE SHOULDER. THREE SEPARATE SKETCHES COULD HAVE BEEN USED FOR THE THREE FEATURES, BUT THE PART WOULD HAVE BEEN LESS ROBUST, AND HARDER TO

EDIT. LITTLE THINGS LIKE THIS ARE IMPORTANT IN PARTS THAT WILL BE USED MANY TIMES ACROSS MANY PROJECTS.

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FIG 14: STRESSED OUTRIGGER. THE FOCUS OF THIS PART IS THE HOLE AT THE END OF THE ARM, WHICH MOUNTS ONE END OF A HYDRAULIC CYLINDER. CLEARANCES WERE VERY TIGHT, AND THE ANGLES

FOR THE CYLINDER WERE TRICKY, SO THIS PART GOT A LOT OF TWEAKING BEFORE THE FINAL CONFIGURATION WAS ESTABLISHED. THE SKETCH WAS SET UP SO THAT ALL THE MAJOR ANGLES AND DIMENIONS COULD BE BUMPED WITHOUT UPSETTING THE REST; ALL THE GEOMETRY FOLLOWED ALONG,

AND THE OVERALL IMPACT OF ANY CHANGE COULD BE IMEDIATELY EVAULATED. IN THIS CASE THE DESIGN INTENT OF THE PART WAS MAXIMUM FLEXIBILITY IN THE DESIGN PROCESS ITSELF.

FIG 13: THE GOOD, THE BAD AND THE FULLY CONSTRAINED. THE MORE COMPLICATED SKETCH ON THE LEFT GETS A SMALL ATTABOY FOR THE USE OF THE FORMULA ANGLE DIMENSION, BUT IT STILL USES

MORE DIMENSIONS, MORE CONSTRAINTS AND MORE CONSTRUCTION GEOMETRY TO ACOMPLISH THE SAME THING AS THE MORE SIMPLE AND ELEGANT SOLUTION ON THE RIGHT. THE ONE ON THE LEFT WAS

MORE WORK TO CREATE, AND IT WILL BE MORE WORK TO EDIT. KEEP IT SIMPLE IF YOU CAN!

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A Few Final Notes on Part Files

Before we leave the subject of part files altogether, there are just a few more pointers to offer for expressing design intent:

Use the Parameters Dialog. If there is a value that will be used repeatedly in a part file, assigning that value a user variable in the parameters dialog will allow easier management of the part. If the value is already assigned a regular parameter name, it can be renamed to something significant. The comments field is also an excellent way to pass on design intent to the next user. Flagging a parameter for export will make it available in an assembly that uses that part. Finally, formulas can be easily set up in the parameters dialog.

Rename objects in the browser. Notice back in Figure 4 that all of the features and objects in both browsers have been given specific names. Cultivating a habit of giving browser features descriptive names is a powerful tool for expressing design intent. Anyone (including the original modeler) working in a part file that’s been treated this way will have a much easier time editing the part.

Use the Show Dimensions Tool. The Show Dimensions command is a generally underappreciated tool which can be used to easily set up formula dimensions. The help file has a good reference for how the tool is used.

A QUICK LOOK AT ASSEMBLIES

So far in the lesson, we’ve been talking about parts, with a primary focus on sketches. This is not a bad thing, since good part models tend to produce good assemblies. The overall concepts that have been expressed here, however, can be applied to assemblies as well. Just as with a part file, an assembly is built up as a matrix of dependencies which tend to get more complex the further down the browser they go. Design intent in

FIG 15: BY THE NUMBERS. FORMULAS AND CUSTOM PARAMETER NAMES IN THE PARAMETERS DIALOG BOX.

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an assembly tends to be expressed in the manner in which the parts are assembled. By working in an assembly in the same manner that a real-world project would be put together, the purpose behind the design can readily be communicated in the drawings. Working at the top of the browser as much as possible will still return gains in stability and manageability, as well as giving a clearer picture of design intent. Sean Dotson has an excellent assembly tutorial on his website. Sean deftly and concisely covers the finer points of assembly modeling in Inventor. It can be found here:

http://www.sdotson.com/freetut/tipsforassemblies.pdf

Happy modeling!