Smoother Transitions To Flat Surfaces in Fusion 360

[Music] all right this is something that's popped up a few times in discussions at work essentially we're making g3 connections to flat surfaces what are we talking about with this well let's just take a look at this toolbox real quick if we look at these rounded corners we can notice a line where the flat surface transitions to the rounded surface this is because the transition between these surfaces is tangent or g1 continuity continuity is all about how surface transitions interact with light shadow and reflections since this is a g1 or tangent transition we are instantly changing from zero curvature on the flat surface to a constant curvature on the fillet which is why we notice this break curvature continuity softens this transition by having the amount of curvature at the ends of the curve match the curvature at the end of what it's connected to for transitioning to flat surfaces like this it would mean the curve would start at zero curvature and ramp up from there this is a g2 curvature continuous transition where the curve velocity matches what it's connected to in this case it would transition from zero now we could take this one step further and go to g3 where we are not only matching the curve velocity but also the acceleration of the curve at this point is really hard to tell where one surface starts and the other begins unlike with automotive design where they sometimes even go to g4 g3 is usually not a prerequisite in project design to call the surfaces on a model class a or production quality the surface transitions on this controller for example aren't g3 but at this scale it doesn't grab enough attention for that to really matter the one place we do see g3 fairly often in product design though is when transitioning to flat surfaces or linear sections mainly because this is where the transitions tend to be the most noticeable this can be seen on nearly every apple product there was a joke back when i was in college that apple invented the fill it but honestly they did seem to bring attention to this type of transition for a lot of designers looking at the reflections on this airpod case we can see that there is actually a small flat section in the middle but the transition is so gradual that it's nearly impossible to tell where that patch stops on the model this is what we're looking to achieve but here's a challenge the tools within fusion allow us to define g2 connections fairly easily because it's built into many tools like loft fillet and patch there's also a sketch constraint for defining g2 connections on a spline but there's no option for us to explicitly define g3 so we'll be looking at defining the transition manually using a control point spline so if we take a control point spline let's look at something and this condition is only true when transitioning to flat linear profiles if we have one control point collinear with a flat profile that's tangent g1 if we have two that's curvature continuous g2 and if we have three that's curvature continuous with matched acceleration g3 so that's how we're going to define the transition but we also need to get this into the shape we want for our design and make it easy to edit parametrically oftentimes people are looking for it to appear like the curve has a constant curvature throughout most of the profile like you might have with a standard fillet but have it smoothly transition at the ends so let's use a simple affiliated corner as a reference and see what we need to do to make a g3 version of it there are a few basic instructions that we'll look at for defining these curves ranging from 7 to nine control points think of these as just places to start because you may need to change them up a bit for your specific condition for all of these spline constructions the first thing we'll need to know is that the start point of this curve has to begin earlier than the tangent version because we'll need space for us to ramp up the curvature from zero and we can fine tune this position later next if we want this to be g3 we know that we need three collinear control points for the transition but one thing to note is that the constraints on these control points are not created automatically so we have to define them after we make the curve that means we don't have to be too precise with the initial definition of the curve just make sure we have the right number of control points let's start with a simpler one with seven control points so we'll start at the endpoint of one line roughly drop in one two and three control points then continue to define the same for the other line so we already have 1 which is the middle then 2 3 and end at the end point now we use our sketch constraints to define everything so we need to make these segments co-linear to our lines we can do that a few different ways we can select these segments between our control points along the line we started with and make them collinear another option would be to add a coincident constraint between our control points in our original line or since these segments are horizontal and vertical we can apply horizontal and vertical constraints to the segments between our control points all will give the same result in this case so we have everything lined up now we need to worry about spacing this is the approach i use it may not be what you're looking for for your specific transition but this works for me as a good starting point now i don't want to have six dimensions driving this feature so i use the equal constraint to help simplify this and i make the first two segments connected each line equal and i make the middle two segments equal now we only have a single free parameter to define on this curve for most transitions simply defining this dimension as half of the overall curve dimension works for me so click on the driving dimension we want to reference then divide by two we can make sure these two dimensions are always equal as well now this curve is fully defined and being driven by a single dimension so if we view the curvature of the spline this is a nice gradually changing curve transition that works for a lot of surface blends but it doesn't really get us to something that looks like a constant radius fillet to do that we need to add another control point or two now we could right click this curve and add another spline control point but just note that when we do that it'll break all the constraints and dimensions we've added to the spline and we'll have to redefine them so i'm just going to make a new curve again we have one two and three points for the transition but instead of going straight down to the other line we create an extra point and go down from there so we can start defining this similarly to the last one make the first two segments in each end equal make the third segment on each side equal now we can also drop in a 45 degree angle dimension on the middle section which will make the curve symmetric now we have two free parameters to define one strategy we can take here is similar to what we saw with the first curve and make this segment the same as the first two combined segments that would leave us with just one parameter to define but let's simplify this for this example and just break this into thirds so the first two segments will be one-third the next segment will be one-third and the horizontal dimension of the middle segment will be one-third there are a couple ways we can define this probably the easiest is with dimensions so we dimension two of the three distances and make them both equal to the overall dimension divided by three also possible to do this only using sketch constraints if you add a couple line segments and make the three segments equal so this one gets us a lot closer to our constant radius fillet and still has a smooth transition to the line segments we're transitioning to we won't go into this last one too much but if needed we can add a ninth control point in the middle i don't use this one too often since it takes a bit more effort to define for this one i usually build some construction geometry and use sketch constraints to help define most of it then we are left with the single parameter to define in the middle this one i usually just adjust by eye then drop in the dimension to lock it in place this is fine if the driving dimension doesn't need to change but if it does we need to find what this is relative to the driving dimension so we take this divided by the overall dimension as it is now multiplied by the parameter of the driving dimension so this is now fully defined by a single dimension so we've got some sketch curves but how does this apply to an actual model certainly we can extrude these as surfaces make lofts and whatever else but making surfaces isn't really the hard part of surface modeling it's defining the transition between surfaces that's challenging so let's just try building the airpod case we looked at before and see what happens first a bit of strategy so the main surface on this thing is symmetric in all directions so we only need to define an eighth of this and we'll just mirror everything for this we don't want any surfacing tool like loft or patch to work too hard because that makes it hard to get good surface quality so we'll build this with a few smaller surfaces and construct what is often referred to as a patch layout this is not to be confused with the patch tool which we'll also look at for this patch layout we generally want to define large surfaces first sweeps and extrusions then lofted connections and work our way down to defining the smaller transition surfaces in terms of what tools we should use there's an order for that too in general if it's possible for us to define something using extrude revolve sweep or a planar patch we should start with that first then second is lofts third is non-planar patches for when loft doesn't work and last is any kind of offset why this order because each tool has different surface quality and we want to make sure that the foundational surfaces on our model are as robust as possible so that the other surfaces that reference that geometry will build properly and not have any problems as an example here we have two surfaces built with a sweep two with a loft and two of the patch they all look very similar but if you're trying to make a lofted connection between each of them this is just a simple g2 loft and we look at the curvature map analysis we can see that the quality diminishes as we move to the right sweeps extrusions revolves and planar patches are very robust you can trim and build from them all you want at the other end non-planar patches should always be at the end of the model chain when defining geometry we don't want to try building other surfaces from patches if we can help it so a typical patch layout could be something like this where we might combine revolves or extrusions then lofts then patches at the end if we can't get it to work for the loft and there are usually ways to avoid non-planar patches altogether if we plan the patch layout properly to put this simply when we were building a feature by referencing other geometry like we often do with surface modeling every imperfection is compounded with each operation so we need to make sure the geometry or sketches we start with are as clean and robust as possible so if we take this thinking and apply it to our model we can start with a planar patch then the straight sections on the top and side can be extrusions then a single loft for the corner depending on how the loft interacts with the tight corner of the initial patch we may decide to break this up into two separate surfaces because again we don't want to make the loft tool work that hard now let's make it so we can start by making a reference master sketch starting with a couple rectangles we'll use for construction this is going to be 54 by 44 millimeters and the center patch i would estimate that is opposite from the sides about 14 millimeters we could put in a couple lines on top and bottom i'm just going to estimate that our curve needs to be about 15 millimeters in each direction so drop in a couple dimensions now we can define the eight control point version number spline like we looked at before and i'm just keeping the same approach for now and dividing the sections into thirds relative to the driving dimension and we can adjust the size of this feature if needed the patch in the center is a little more tricky to figure out sizing but i'm going to start by making this four millimeters and we're going to find this curve the same way so this will be our driving sketch for the center patch we need to bring this sketch to the right position so this looks like 21.3 millimeters thick so if we create an offset plane at 21.3 divided by 2 then we can sketch on that plane project our master sketch close it off with a couple lines then in the surface tools we can create a patch now for the outside surfaces we can start by simply extruding the outside profile down the distance doesn't really matter since we'll end up using this only to create a tangent reference for mirroring so i'm just going to make it two millimeters so now we need to make the extruded profiles for the top and side we can sketch on one of the origin planes and pull on these lines we want to reference using the project tool and ideally these are set as construction lines this one will be a little different than what we saw before because we don't actually want a curvature relationship to this extruded profile we want curvature continuity to the mirror of this curve if we make the relationship tangent to the straight line and mirror that it will always be at least g2 continuous and we can adjust the control points to get to g3 or as mirrors make some difference anyway so we'll want three collinear control points to the center patch and just one collinear control point to the extruded profile we can start defining this similarly to how we did before make these two segments equal make these two equal make this 45 degrees i'm going to drop in a construction line between the first and third control point so that we can make an equal constraint between these two if we mirror this to the other side and view the curvature we can fine tune this by i and get it essentially to g3 then drop in a dimension to make it fully defined with this profile we're going to extrude this past where it needs to go because we're going to trim it back now we want to make sure that the other extruded profile is the same but since this isn't axially symmetric we can't just mirror it one trick that can work here is by using pattern on path so we'll pattern this as a body select our object and our path then drag this arrow until it snaps to the endpoint of the curve we only need two for the quantity and make the orientation follow the path direction now we can take this surface and mirror it on the origin then remove the original we can sketch on the top view and we're going to want to project these four points then connect them with lines now we'll do a trim surface and use this sketch as the trim tool and trim away these two sections now we can deal with the corner i suspect we'll need to add more curves to define this loft perhaps even break it up into two lofts but let's see what happens with what we already have we can make a surface loft the continuity to the top batch will be curvature then tangent to the extruded reference and both of the rails will be curvature although the selected continuity says g2 since the curve defining the transition are g3 the continuity here is essentially g3 so at first glance if we rotate the model it seems to flatten out a bit in the middle i've gotten several comments as to why i keep rotating the model so much and this is why i'm constantly evaluating what surfaces look like from all directions to see what needs to be refined there are a few tools that can help us evaluate this one is to simply put a high gloss material on it and render it but there are also specific evaluation tools for looking at surfaces the first is zebra analysis which is a great tool for quickly evaluating continuity between surfaces as well as surface shape to a certain degree another evaluation tool is curvature map analysis this is helpful for looking at overall surface quality this next one and this is a fairly recent addition to fusion is probably my favorite which is iso curve analysis in its default mode it doesn't really tell us much but we're also able to view curvature combs on these iso curves which tells us very clearly what the surface is doing another trick is to temporarily split a body or face and use curvature comb analysis on the resulting edge i use this one less often now that we have the iso curve analysis to get a better surface out of this we'll need to add another profile probably two so let's move back before the loft in the timeline and add another sketch profile i think i'll start by adding the offset plane let's say eight millimeters from the origin and sketch on this plane we'll need to pull in our reference curves but since we can't use project here we'll need to use intersect we can make our corner spline yet again i'm using equal constraints here instead of dimensions since there's no easy dimension to reference now we could try building our loft again and add this profile if you forget to make the reference lines construction you can just make sure chain selection is unchecked and it will grab individual curves instead of grabbing everything reorder this and put this as profile 2 we'll have to reapply our continuity and this is slightly better doesn't flatten out as much but could really benefit from an additional curve in the other direction if we reference this sketch we did before and create a bisecting line we can make a plane at angle perpendicular to the sketch plane and we can create another sketch pull some references using intersect then create another curve for this one we're going to make the intersection point of the other curve coincident with this one now we can rebuild this surface with this additional rail once this is made we can continue to refine the shape until we get the shape we want the shape is looking a bit better now and we can look at the iso curves now it is possible for us to get even better surface quality by breaking this loft into two separate surfaces the general rule for lofts is to think of it like a stretchy rectangular piece of fabric it can conform to a lot of different shapes but if you have a tight corner or something like we have here that fabric might either bunch up or stretch too much and we're starting to see that near this corner if you do feel like breaking this up into multiple lofts we can use all the sketches and reference geometry we already have if we want to make a symmetric feature we can extrude this middle curve as a tangent reference we can then build this in half and then mirror it for example we can already see that the curvature combs are a little cleaner we can continue to subdivide this as necessary if we need further surface refinement but this is probably enough considering we're looking at this at about 10x scale so the last step is to remove our reference geometry mirror everything about the origin and stitch everything together we can add a few details and there we have the outside of an airpod case with g3 transitions before we go i'm sure there's a question about what to do when transitioning to something other than flat linear profiles that's a big topic in and of itself but the hyper abridged version of it is that i tend to not use control point splines in that case because they don't seem to be as robust as fit point splines when using curvature and tangent sketch constraints so in those cases i use fitpoint splines to find the connections to be curvature and adjust the handles as necessary to get somewhat close to g3 although i'm usually not as concerned about getting to g3 on these types of transitions occasionally i'll add a single extra fit point if i need a little more control but that's it unless there's a really specific reason to add more anyway that's about it for now hope it was helpful

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Length: 20min 34sec (1234 seconds)

Published: Sat Sep 11 2021