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Thread: Post SOLIDWORKS Designs Here

  1. #61
    Mark Meredith's Avatar
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    Bill, okay, got it. I went to the next sketch in the Fork and fully defined it. Man, a LOT of dimensioning. That may cause me to sketch in a different way with fewer/simpler curves (already figured that out anyway - can't make a smooth part with lots of curve points).

    But all this dimensioning also means it's hard to make fine changes. No going in and just grabbing something to adjust, then close the drawing. I've done plenty of that. Gotta change the dimension instead of dragging the point. Or delete dimensions, drag, then put them back. I see the wisdom of this discipline though.

    My Chipmunk model is a mess of interrelationships (many external refs) that would require major rebuilding to modify anything. For example I know the wing incidence is a little off. Even though it will be used by Horizon Hobby to make a flying model, I ain't gonna fix it. I already rebuilt it once to change the airfoil a little and it was a disaster - started over instead of fixing. Guess this is what you mean by blowing up! And of course building assemblies would have been much better too. I wonder if Horizon is even going to be able to use this thing. They may pull out a drawing or two, reference a few curves and start over with their own 3D model.

  2. #62
    cwilliamrose's Avatar
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    It's a different world when you're trying to re-create an existing part. You would likely not use splines to define your geometry in a new design. If it had to be shaped like your fork you could use arcs instead;

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    Still lots of dimensions but a bit more straight forward than dimensioning splines. As you can see, this sketch closely follows the shape of your fork without using splines. A simpler design with less arcs and a straight line or two would look almost as nice and be much more producible.

    Remember to use the 'Equal' constraint for line lengths, radii, etc. Makes for a less cluttered and easier to modify sketch. I only used one 'Equal' dimension here -- the R.43 is equal to the arc's radius in the upper left corner of the sketch which is why that one isn't dimensioned.

  3. #63
    Mark Meredith's Avatar
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    Defining a Part

    Yes, I see that is much simpler. And no need to fight to find the problem when a spline won't go black.

    Did you set the arc joins to be tangent?

    Okay, what if all the drawings in a part are fully defined but the part still shows (-)?
    Last edited by Mark Meredith; 01-03-2017 at 03:54 PM.

  4. #64
    cwilliamrose's Avatar
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    Yes, the arcs in my sketch are all tangent which allows me to fully define them with only one location dimension.

    >>Okay, what if all the drawings in a part are fully defined but the part still shows (-)? <<

    I assume you mean in the context of an assembly? If the part file or sub-assembly has a "-" that means it's not fully constrained within the assembly and it has at least some freedom of motion. Sometimes you don't really need or want to fully constrain a part but most times you do. If you have a bolt you constrain the shank to be concentric to the bore and the base of the head to be coincident to the surface the head draws up to but you don't really need to constrain it in rotation. If your fork is free to rotate you can move it and all the parts attached to it in rotation (steering) if that has value to you. If not you'll want to fix the rotation so it doesn't just move because you grabbed it with the mouse or Spaceball by accident.
    Last edited by cwilliamrose; 01-03-2017 at 05:21 PM.

  5. #65
    SOLIDWORKS Support Volunteer Jeffrey Meyer's Avatar
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    Hey guys, I like/love using fully defined "black" splines. Here's an example:

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    I like splines for two reasons:

    1. They are aesthetically pleasing.
    2. They can be constrained to have curvature continuity, unlike circular arcs that can only be constrained to have tangency continuity.

    In a way these two characteristics are linked together by nature. There is only one thing in nature that is (almost) perfectly round - your eyeball/lens. Apart from the stalks of some plants practically all other curved objects in nature have curvature continuity - that's what makes them aesthetically pleasing.
    This is true in engineering as well - you do not want to have your airfoil made up of a series of tangential circular arcs - it will be inefficient and have terrible stall characteristics.

    Incidentally, the tail-wheel fork (spline version) shown would be somewhat lighter because it has strength where needed and less material elsewhere. (IMHO it looks nicer too!)

  6. #66
    cwilliamrose's Avatar
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    Tell me about the blue vectors, are they subject to change because they are not fully defined?

    There are places where only splines will do and airfoils is a good example. I still use arcs as a first choice for most things because I can create the part with simple tools and a drawing. Wood ribs have splines for their shape but the wood isn't fully supported along its length if it's a stick rib so the accuracy is not going to be great in the finished part. If it's fabric covered there are a whole list of other deviations that happen. Still, you have to start somewhere.

    The tangent arc version of the fork I drew would be indistinguishable from one defined by splines unless you could measure it very closely. Visually there would be no difference.

  7. #67
    Mark Meredith's Avatar
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    Arcs and simple tools: Bill, do you know an example of where an arc would be easier? Most of my aircraft part making has been sheet metal shaping, forming with hammers and wheels, and fiberglass/molds. A lot of stuff with big rolling/sanding motions that do complex curves nicely. I did a little aluminum work using a metal lathe for the Chipmunk - in that case only an "arc" was possible of course. As I think about a clean sheet design I'm very interested in materials and structures, and how to make them!

  8. #68
    cwilliamrose's Avatar
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    Mark,

    When I do metal shaping I usually don't have much of a drawing, just some basic targets to hit. The inner panel here is hand formed to fit over the brake caliper, fit the existing mount bracket and the pant. No drawing at all, just keep making the bump bigger until it clears the caliper and then smooth it out;

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    For something like the turtle deck on the Pitts I have drawings for the bulkheads which I can use to make the form blocks. If I made the curve a spline I would have to make the drawing full size, go the the print shop and have it printed on bond paper, glue the drawing to poster board, cut it out and use that as a template for the wood block. As a curve made up of arcs I can lay it out directly on the wood referencing a small drawing I can print myself and start cutting;

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    The front bulkhead is reclined and was done by drawing an intersection curve on an angled plane. That bulkhead is described by splines but I'll use arcs to make the drawing so I don't have to do the print shop routine.

  9. #69
    Mark Meredith's Avatar
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    Bill,
    Okay, your turtledeck example makes it clear why curves instead of splines. Thanks!

    Some BS (Before SolidWorks): I avoided the problem of having to draw much of anything on my Chipmunk because I had no detailed airframe drawings. All the metal forming was with paper templates and lots of fitting until it looked right, kinda like in your pics (very nice forming, Bill!) On the turtledeck I figured out the fore and aft widths to get the curved height I needed to cover my head and join with the fin (trial and error mostly), using the full 8' length of my sheet. I only made forward and aft bulkheads, nothing in the middle to deform the shape, and made those AFTER the turtledeck was mounted via stressed skin to naturally hold a smooth curve. Cut the turtledeck sheet out, bent the flanges where they rivet to the fuselage on an 8' brake, and drilled the rivet holes in the flanges. Snapped lines on the fuselage then drilled and clecoed one side so the sheet pointed straight up. Then bent it down to the opposite snap line using battons and friends...drilled and clecoed...riveted...done. Everything here and elsewhere was trial and error with lots of scrap, wood, cardboard, etc. I'm looking forward to trying to design with SWx instead of "that looks about right" directly on the airplane! Name:  IMG_20130613_190811_099.jpg
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  10. #70
    cwilliamrose's Avatar
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    There are times when doing it on the airplane works best but SWx gives you options you never had before so I try to use that tool when I can.

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    This confusing view shows my layout for the T-deck and how the upper side stringer needs to be made to give me a positive break angle for the fabric coming off the bottom of the T-deck. This was an iterative process that defined the bulkhead shapes as I was paying attention to where the top stringer would have to be to make the fabric look right. The lines you see on the T-deck skin are intersection curves created from an array of planes that cut through the T-deck. The construction lines are tangent to those intersection curves and extend down past the top stringer. The highlighted green line is a curve defined by the points of intersection between the construction lines and the plane of the stringer. In order for the fabric to have a positive break angle at the bottom of the T-deck my stringer has to be inside of that green line. I'll offset that line some distance inboard as a first shot at defining the stringer's curve.

    Before I did this layout and made the needed changes to the bulkhead shapes my T-deck would not have had a break angle at the bottom and the fabric would have come out of contact with the T-deck skin before it got to the bottom. I have seen airplanes with this problem and it is ugly. In the air the fabric sucks in due to the interior of the fuselage being a low pressure area but on the ground the fabric would not neatly flow off the skin to the stringer. Trying to glue the fabric to the T-deck to force contact makes things look even worse. You could do this on the airplane but using SWx makes the process much quicker since you don't have to make multiple versions of temporary parts to define this area.

    Thanks for the kind words about my little panel. That was supposed to be a practice panel so I made it out of a piece of a jon boat seat we had in the scrap pile. Turned out to be a keeper which probably shows that taking the pressure off by calling it a practice part has some real benefits........Bill

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