123D Design – Several Ways to Skin a Basic Cat
6 December 2018
123D Design Modeling Texture

Greetings everyone,

Today we’re going to talk about how there is often (though certainly not always) more than one way to achieve the same or roughly the same basic shape or design using the tools available in 123D Design and how there are often several different approaches to the same task.

These different approaches may yield exactly identical results or the results they produce may differ to a greater or lesser degree and not all of them might end up doing exactly what we had planned or intended to achieve. Also, the different ways you can go about modelling the same thing may pose varying degrees of difficulty or involve different numbers of steps and each require a different amount of work and time input.

Sometimes, the best and most accurate results require the most elbow grease and time and can be the most tedious to achieve. For example, such is the case if one wants to accurately model a spiral or threading in 123D Design, which can only be done accurately using the Loft tool.

When there really is only one way to achieve exactly the desired outcome, you can generally expect to have to rely on some of the more exotic tools in the 123D Design tool set, such as the Loft or Sweep operations. For instance, the only way one can go about producing truly accurate spirals or threading (as on a bolt or nut) inside of 123D Design really only is by using the Loft operation with a high enough number of profiles as guides.

The following is a short video showing several different ways to model the same basic shape: two straight walls at a 90° angle from one another joined through a curved corner.

With the first approach, we initially sketch and extrude the wall section as a straight, 90° corner but then use the Fillet tool on both the inside and outside corner seams to introduce a curvature with a radius of 10 units to the inside surface of the wall and one with a curvature of 15 units to its outside surface. The reason for the difference of 5 units between the inside and outside surfaces’ respective corner curvature radii is to account for the thickness of the wall itself, which we chose to be 5 units in this particular example and which constitutes the distance between the inside and outside surfaces of the wall and is thus necessarily also the difference between the inside and outside curvature radii of the swept corner.

Of course, there are multiple different ways of even sketching the basic wall corner section footprint. In this example I opt for the one which seems the easiest one to me, which is to create two squares using the ‘Sketch Rectangle’ tool from the Sketch menu and then use the Trim tool (from the Sketch menu) to cut away the extraneous sections of bottom and right edges of the two squares – although this is actually redundant for our purposes. One important observation to make here is that, after I’ve sketched the first square, when I again pick the ‘Sketch Rectangle’ tool from the Sketch menu, I click on the previously drawn square.

The reason I do this is because of a very important aspect of sketching in 123D design which one has to be aware and mindful of when using this software, which is the concept of sketch spaces. Different sketches you create in this software will not interact with one another or be regarded as the same sketch or pertaining to the same 2D shape and as such cannot be used together to define the projected 2D outline of the one and same 3D shape so that that solid can be created through a single instance of using the Extrusion tool rather than multiple separate instances of using the ‘Split Solids’, Boolean Merge, Boolean Subtract or Extrude tools.

By clicking on the first square I drew with the ‘Sketch Rectangle’ tool rather than on the default sketch plane, I’m telling the software I intend to further add to and extend the previously created sketch and wish to draw the second rectangle in the same ‘sketch space’ as the first rectangle I drew. This enables me to then use the Trim operation (which you can use to tell the software to trim away that segment or arc of a straight or curved line which continues past the intersection point or what would be the intersection point with another curved or straight line or between imaginary extensions of existing curved or straight lines in the same sketch space) whereas I would not be able to do that if the software regarded the second rectangle I sketched as being a separate sketch / being in a separate sketch space than the first one.

Nor would I be able to then extrude a corner wall section in one instance of the Extrude operation instead of two different square prisms created in two different instances of the Extrude operation which I would subsequently need to subtract the smaller from the larger of (using Boolean Subtract) to obtain the wall corner shape I’m going for.

Here’s another video to illustrate the above and also show the different behaviour exhibited by 123D Design when we have the two square outlines in the same sketch space versus in separate sketch spaces. It can also be seen that taking the time to trim the excess from the sketch isn’t actually required when trying to achieve this particular shape and this step can be skipped entirely.

Back to the first video, with the second approach shown therein, the only difference between it and the first approach is that we took the time to perform the Fillet operation at the sketch stage rather than after extruding the 2D sketch into a 3D solid and performing the Fillet operation on the 3D solid. We again choose a radius of 10 units for the inside surface’s curvature and a radius of 15 for the outside surface’s curvature and the sketch confirms that the two circle arcs are concentric.

The third approach is the clunkiest and most unwieldy so far (but we’ll certainly find worse, more complicated ways of achieving the same thing later on). Instead of relying on sketching to create the shape, I mostly use the primitives menu to create the straight wall sections. The first one of which I find I’m having trouble placing precisely where I’d like to because the snap point of the rectangular 3D shape’s bottom surface is its center rather than any of its corners. So I accept I’ll have to place the shape close to rather than exactly where I intended then rectangle select the 3D shape I just created before selecting its ‘southern’ facing surface and pulling it out 5 units. I do this because I plan to afterwards use the ‘Split Solid’ tool to trim it down to size but, being my usual lazy self, I initially hope to be able to get away with splitting my newly created 3D shape using all of the surfaces of the corner wall section built previously using a different approach.

But the surfaces of the previously created wall section aren’t extended to meet the wall section I’m building at the moment and which I want to use the ‘Split Solid’ tool to cut away at as this would only work if the two solids were actually intersecting and they don’t even actually touch. So I instead need to sequentially cut away at the newly created paralelipiped in several distinct steps, each time choosing a specific surface of the previously created wall corner section’s faces. This way, the planes defined by those respective faces are each indeed extended as necessary to define cutting planes for the newly created paralelipiped. I then delete the unnecessary shapes.

With the other straight wall I prefer to initially make it twice as thick so the fact that its snapping point is actually the center of its bottom surface rather than any of its corners won’t give me trouble placing it into position such that all but one of its surfaces are where I need them to be. I then select the newly created solid and subsequently its single out of place face to apply the ‘Press/Pull’ modifier to and push it in 5 units. I then use the Boolean Merge operation to merge the two solids into one wall corner section with a straight 90° join. I then create a cylinder of radius 15 units and, using the Shell tool, hollow it out from the top to a wall thickness of 5 units then select the inside surface of the bottom of the hollowed out ‘cup’ resulting from using the Shell tool and press the Delete key to delete that surface. This turns the cup shape into a hollow pipe shape.

I then use the ‘Split Solid’ tool to trim away the excess height of the now hollowed-out cylinder using the top face of the wall corner section as cutting plane. This ensures that the two surfaces will be exactly coplanar. I also extend the cylinder down 5 units before repeating the ‘Split Solid’ operation but this time using the bottom surface of the wall corner section as cutting plane to trim the excess bottom length of the cylindrical shape. This ensures that the bottom surface of the cylinder wall and the bottom surface of the straight wall sections match exactly so that they are regarded as a single unitary surface.

While not actually needed in this particular instance, this is a good habit to get into when planning to apply Boolean Merge to complex solids or 3D shapes which were created separately using sketches created using the Project tool and/or which were subsequently translated, rotated, snapped, arrayed aligned or otherwise relocated from their original positions and whose faces might, as a result, not be exactly aligned or coplanar with one another.

Finally, I sketch a square on the default sketch plane then use the Trim tool to delete two of the edges of the square (which is actually redundant / unnecessary in this application but is still something you’ll want to get into the habit of doing) and I then use the remaining two edges with the ‘Split Solid’ tool to split both the pipe / hollow cylinder I created and the straight wall corner shape so I can delete the straight corner and also delete the other 3 quarters of the hollow cylinder before using the Boolean Merge tool to merge the straight wall sections with the curved corner section.

This kind of approach (and, generally, creating things simply using Boolean Merge or Subtract from 3D solids created using the Primitives menu) isn’t something you’ll want to use as your main modelling technique since, if you aren’t very careful with it, this technique can produce less accurate or topologically ambiguous results where, for instance, apparently adjacent surfaces (and which indeed ought to be adjacent) aren’t actually neighbouring surfaces due to very minute interceding surfaces resulting from modelling inaccuracies or topological ambiguity from using this kind of modelling approach.

The last approach in the first video shown above is simply a variation of the second approach, which we already went over. It differs from that simply in the way the sketch is created. I use the ‘Sketch Rectangle’ tool to draw the inside outline of the wall and can afterwards use the Offset tool, likewise from the Sketch tools menu, to derive the outline of the outside surface of the wall from the outline of the inside surface. I can choose to use the ‘Sketch Fillet’ tool to introduce curvature to the inside outline either before or after using the Offset tool.

If I use the Offset tool first, I’ll then have to use the ‘Sketch Fillet’ tool twice, once for the outline of the inside surface of the wall and once for the outline of its outside surface. Whereas if I use the ‘Sketch Fillet’ tool first, on the outline of the inside surface of the wall and before using the Offset tool to create the outline of the outside surface of the wall, I won’t then need to use the ‘Sketch Fillet’ tool again on the outer outline of the wall as well. Since the outline of the curved corner of the inside surface will be offset along with the straight sections.

123D Design only allows closed 2D shapes to be extruded into 3D shapes so we need to complete the sketch by closing the wall footprint perimeter using the ‘Polyline’ tool to link the outlines of the inside and outside surfaces of the wall before we can apply the Extrude tool to the now enclosed perimeter to get our curved corner wall.

In the following video we see how it is possible to achieve the same simple result we ended up with through the methods we’ve already gone over but this time without using any sketches at all :

We use the Box and Cylinder tools from the Primitives menu to create what will be the curved corner as well as the straight sections of the wall. We then use the Shell tool to turn the cylinder we just created into a pipe as we’ve already done before. After that we use the ‘Split Solid’ tool to cut the cylinder in length until it is exactly the height of the wall.

Which is actually a redundant operation and a mistake, as I realise immediately afterwards, as we need to merge the pipe and the wall first, while the two are of different heights and the pipe extends further above as well as below the wall, so that portions of the wall’s top and bottom faces will be separated from the rest of their respective horizontal surfaces by the curved vertical exterior surface of the cylinder. This will enable us to afterwards actually delete the sharp 90° corner and extend the cylinder’s curved outer surface in continuation of the the flat, vertical outside surfaces of the wall.

In this way we make use of one of of 123D most handy and powerful features. Which is that of allowing the user to select faces of a 3D solid they’d like to delete and, as they do that, automatically ensure that the 3D solid in question remains a watertight hull by extending surfaces neighbouring the faces or surfaces which have been deleted as necessary to cover the holes which the deleted faces or surfaces would otherwise have left behind in what would have formerly been the closed watertight hull of the 3D shape. We used this same feature earlier to convert a cup shape to a hollow pipe by deleting the bottom of the cup.

When it is not possible to cover or plug the hole resulting from deleting a face or a surface of a 3D solid by extending neighbouring faces or surfaces, 123D Design will either show an error message stating ‘Invalid Operation’ and not allow us to proceed or will otherwise simply delete the entire 3D shape in question altogether.

Finally, we complete the sought after shape by pushing in the external surface of the part of the hollow cylinder which protrudes out on the inside of the wall until we get our correctly rounded corner.

In this next video we start with a cylinder which we then hollow out into a cup using the Shell tool before deleting its bottom wall to turn it into a hollow pipe, as we also did before. We then sketch a rectangle (we could just as well use the Polyline tool for the same purpose instead) to use the ‘Split Solid’ tool to cut out a quarter of the hollow pipe before deleting the rest (as well as the sketch itself). We then use the Project tool from the Sketch menu to project the end faces of the curved corner section unto themselves to then be able to use the Extrude tool to extrude the straight wall sections from each of the projected sketches respectively.

Another reasonable way to model a wall with a swept corner is using the Sweep tool on a sketch of the outline of either the inside surface or the outside surface of the wall, with the fillet included in the outline of either the inner or outer surface of the wall at the sketching phase :

And finally, we’re going to bring this article to a close with this last video :

Wherein we explore modelling the same familiar curved corner wall this time using the Loft tool, both in a very ill-conceived and inadvisable manner and in the best way the Loft tool can be employed in this specific use case.

First, we create a rectangular prism which is 10 x 10 x 15 units in size (as before, because the snapping point of the prism’s bottom surface will be the center point of that face) which we then resize to 5 x 5 x 15 when in position. Next, as before, we create a sketch of what we intend the wall’s external surface outline to be, as projected on the ground, taking the time to also apply Sketch Fillet to round off the corner. Then we use the ‘Path Pattern’ tool from the Pattern menu to create an array of rectangular prisms, similar to the one we started with, along the path defined by the sketched outline of the wall’s external surface. Then we use the Project tool from the Sketch menu to project one vertical face of each prism unto itself. With each prism, the face we project is the one which faces in the direction of the path.

Finally, we employ the Loft tool on the resulting collection of sketches and we get a… less than stellar result. Showing the limitations and caveats of this tool and why it’s not advisable to be used when any other, simpler tools suffice. And, when we do use the Loft tool, it’s best to do so in a considerate and thoughtful manner, making sure we apply the tool in a way most amenable and conducive to what we are trying to achieve and where the results it produces will be the most accurate and consistent.

For example, towards the end of the video, we can see the best way to apply the Loft tool to this particular use case is to do so using just two vertically stacked horizontal profiles of the entire wall, which are horizontal cross-sections of it. Even though this produces arguably good results, many of the other approaches described in this article are arguably superior because models they are used to create will be topologically simpler and be comprised of primitive surfaces (flat surfaces, cylindrical, spherical or rotational surfaces) which are easier for the software to keep track of and solve various operations against and with.

By using more basic modelling tools to achieve the same shape you’ll be able to afterwards employ some techniques which aren’t applicable (or not applicable in the same way or to the same extent) to models created using lofting, such as deleting specific faces or surfaces of the model. A 3D solid produced by lofting can have fewer seams or no seams at all where the same basic shape modelled using some of the more basic tools would indeed have seams between the various neighbouring primitive surfaces it will probably be composed of. For instance, it is possible to convert the swept corner wall shapes we created back to straight, 90° corners by selecting the inside as well as the outside curved surfaces of the wall and deleting them. Which results in the adjacent flat faces of the wall getting extended until they again meet in a square, 90° corner.

And to round off this article, here are a few takeaways from the information presented above:

  1. For simple shapes and models, there are often several different ways of achieving exactly the intended result. For flexibility and even as practice for other, more complex shapes with fewer modelling approaches, it may be useful to consider more than just the first one which comes to mind.
  2. When there are two or more different possible approaches or modelling techniques which will yield the same shape in the end (which will generally be the case with basic shapes), there will probably be some ways to go about achieving that shape which are better than others, either from an accuracy, topological precision and consistency point of view or from the perspective of the amount of work and time required.
  3. Even when there are two or more different ways of achieving exactly the intended result, some of them will be more cumbersome, time consuming or labour intensive than others. It may pay to think of different ways and new techniques of doing things you find most cumbersome or time consuming to do using your current repertoire of techniques.
  4. It’s generally advisable to do as much work as possible at the sketching phase.
  5. It’s generally preferable to use sketching tools and other tools besides the ones in the Primitives menu.
  6. It’s best to familiarise yourself with all of the tools available in 123D Design, their respective strengths and weaknesses, advantages and drawbacks, what they’re each good for / advisable to use to achieve and how, in which context or for which purpose it’s best to use any one particular tool. That way you’ll not be closing yourself off from modelling possibilities and techniques you can devise for yourself on the basis of other tools than just the subset you may be comfortable with at present.
  7. When modelling with 123D Design you’ll want to plan ahead and plot out in your mind how, exactly, you’ll be going about achieving a given shape and what tools and in what sequence you’ll be using them to that end, and how.
  8. Once you are thoroughly comfortable and well versed in all of the tools available in 123D Design and what they each can and can’t do and what usages they’re each best suited towards, it’s generally best to use the most basic tool that will achieve the intended result. If you can achieve the same intended result using either Extrude or Loft, you should go for Extrude every time. This will help keep your modelling clean, topologically sound as well as precise, accurate and simple.
  9. It’s best to delete no longer required sketches to keep your scene tidy and clutter free as well as help maintain the performance and responsiveness of the software while working. If you find you again need or still require a sketch you’ve gone ahead and deleted, you can usually easily re-create the deleted sketch by simply using the Project tool from the Sketch menu.
  10. You can also use the Project tool to get two disjointed sketches / two different sketches into the same sketch space so that they can both be used to describe the same unitary, singular 3D shape.
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