Transfer Skirt Modeling: Part 2
9. The lower flange is finished the same way. The final point spread for the lower skirt looks like this:
Attachment:
Lower Skirt Point Profile.jpg [ 197.6 KiB | Viewed 1390 times ]
10. The points are selected sequentially and when “P” is pressed, a polygon is created matching the lines on the reference profile.
For a sanity check, the positions of the two bottom inner flange points are verified to be in alignment with the bounding box edges at the correct position, (X=134.62cm / 2) since the bounding box is centered and only half are needed to establish the proper radius (r=1/2 the diameter.) There isn’t much math required for 3D modeling as opposed to CAD work, but a little basic geometry knowledge saves a lot of headaches later and ensures accuracy.
Attachment:
Lower Skirt Poly Profile - Annotated.jpg [ 217.42 KiB | Viewed 1390 times ]
11. Building the skirt mesh is easy. Every 3D app has a different name for the next tool, but they all perform the same function.
A profile is used to generate a symmetrical mesh around an axis. In this case, the tool is called Lathe, because that’s what it does – it lathes a shape around (in this example) the Y axis. Creating a symmetrical shape around the Y axis is the reason for centering the bounding box in step 1. Other apps have descriptive names like Spin, etc.
Attachment:
Lathed Lower Skirt Views.jpg [ 382.82 KiB | Viewed 1390 times ]
12. This is a good spot to point out the location of the intersection of
Potential and
Reality. With a drawing that contains some, (but not all) measurements, we have a
potentially accurate depiction of the lower skirt dimensions. But in
reality, the upper and lower flanges need a little more “meat” on them to attach fasteners. This is why it pays to eyeball reference material in minute detail. I ended up extending the upper flange in each section of the skirt outwards just a little more than the reference drawing to allow placement of the mating flange fasteners later on.
This drawing – as helpful as it is, is only a guide, but it's prudent to avoid sole-source tunnel vision by referring to other sources of reference while modeling. This helps to minimize bigger problems that are discovered further along and require extensive, time-consuming repairs. When it comes to drawings, (that aren't blueprints)
Trust - but verify!Attachment:
Revised Lower Skirt Upper Flange - Annotated.jpg [ 186.69 KiB | Viewed 1390 times ]
13. With the lower skirt shape complete, the upper skirt is built the same way.
Attachment:
Upper and Lower Skirt Shapes Complete.jpg [ 383.29 KiB | Viewed 1390 times ]
14. Using my keen powers of observation honed to a narrow focal point by years of mistakes, several reference photos were used to determine the mating flange has 18 circular cut-outs. The flange polys of the upper section of the skirt are selected, copied, and pasted into a blank layer (or workspace) as a reference point to use in the background so an extruded disk of the proper size can be positioned, before creating a radial array of 18 of them. The disks will be used as “cutting tools” to subtract their shape from the upper and lower mating flanges on the top portion of the skirt. The cutting tools must penetrate all the way through the geometry in order to be subtracted properly. When the fastener seating surfaces of the flange polys are cut and pasted in a blank layer (or workspace) they retain their original position, so they make an excellent reference template. By selecting just the polys I need for reference purposes, it's easier to see what's going on, rather than look down in a top view at a mesh with hundreds, (or thousands!) of polys.
To recap, the basic workflow is:
a) Copy the flange geometry for use as a reference into an empty layer.
Attachment:
Step 14A.jpg [ 359.75 KiB | Viewed 1390 times ]
b) Determine the size of the disk required to get the right spacing when 18 are needed, by making the array and going backwards to re-size and re-position the base disk. Trial-and-error are used until everything falls into place. That's one of the cool aspects of 3D modeling: you can go forward or backwards without trashing the base mesh when working in several layers to create cutting tools.
Attachment:
Step 14B.jpg [ 247.6 KiB | Viewed 1390 times ]
c) After trial-and0error to get the positioning of the first (small disk) cutting tool correct, a final radial array of 18 equally-spaced disks is placed around the flange perimeter.
Attachment:
Step 14C.jpg [ 319.54 KiB | Viewed 1390 times ]
d) Subtract the volume of the disks from the upper and lower skirt sections.
Attachment:
Step 14D.jpg [ 361.31 KiB | Viewed 1390 times ]
15. And now . . . I’ll share one of the secrets of 3D modeling:
In the real world, very few edges, (including corners) are really flat.
Look at the edges of the objects in the room you’re in and you’ll see a slight glint where the edges catch the light. Adding a slight bevel or chamfer to an edge is the key to making photo-realistic 3D objects. While we aren’t striving for photo-realism in this tutorial, the mating surface of the upper and lower skirt flanges need to be chamfered to emphasize the joint. This small detail will pay off later when the 3D printed part is built in two pieces and bonded together.
Modeling chamfered edges to accentuate detail separates a beginner from a more experienced modeler.
Every corner/edge that is visible to an observer is chamfered to catch the light. The skirt sections started out with 120 sides (that was the Lathing sides number) before the flange cut-outs were made in Step 14D increased the polys (and their associated edges and points) even more. The chamfering command is applied to these external points manually, but with so many, it's easy to miss one and end up with a messed-up polygon. I might not see a trashed poly until later when it’s harder to repair.
The solution is to divide and conquer!
To save time and make it easier to see a missed point or edge, (especially since this part is symmetrical) the entire mesh is quartered within the Top viewport. The two open sides, (90 degrees apart) are capped with an end poly by selecting the exposed points and pressing the "P" key to generate a poly. This is repeated on the other open end to "seal" up the quartered skirt. A "watertight" mesh accepts commands easier. Some commands will not work at all on an open mesh.
After the edges are chamfered, the two end faces are deleted, (so "junk" polys aren't left inside the mesh) and the upper and lower skirt sections are mirrored in the X and then the Z axis, restoring them to their original form as shown below.
Attachment:
Step 15.jpg [ 146.19 KiB | Viewed 1390 times ]
End of Part 2.