Error or Artifact?

[Anonymous]

I'm currently navigating the wonderful world of lofts, and being perpetually new at this I'm not sure what to do with an all-to-frequent error message that the rails don't intersect the profiles on my 3D sketch. I don't know what to do to find out exactly where the disconnect is, and this has led me on many a not-so-merry chase in the last week or so.

I have tried to use approaches such as magnifying the intersection to the maximum to see if a disconnect is visually obvious. In doing so, I have found on many occasions that a line I snapped to an arc (for example) goes past the arc, which makes me think, "AHA! There you are you little rascal!!" But the overshoot can't be trimmed, and if I try to place a point on what appears to be the intersection, it won't go anywhere except at the top of the overshot line.

I'm beginning to think that this isn't really an error, but a display artifact. Is this a known issue? Just need a little clarification on this so I can go sniffing around somewhere else.

[TrippyLighting]

I had considered creating a screencast or two, starting with your sub-optimal sketching practices continuing through your sub-optimal lofting practices, but this post has me reconsider that!

Have you actually taken a look at any of the models I attached to the last few posts in the threads you've created and gone through the timeline to understand the process ?

If so, how can you still mirror all that geometry in a sketch ?

How can you still cling to the lofting techniques you are using in that model ? That lofted surface is terrible!

[Anonymous]

I was actually waiting for the screencast! My model might be
substandard, but do you remember the first one I posted where the
browser content went down off the screen and halfway to China and the
timeline went so far off to the right that even Fusion couldn't see
where it ended? I hope you can see the difference because I intend to
blame you for any progress! Perhaps it slipped by in this rather long
thread, but I was not able to download your corrected files and learn by
stepping back through the timeline because Fusion will not let me load
the files. This is a problem I've had with every file that anyone's
posted here. I don't know why these obstructions seem to happen to me a
lot, but they do. I can tell you're ready to bang your head against a
wall (or maybe /my /head would be your choice), but if it's any
consolation, you'll have company.

Just a little more patience, please. Creating a beautiful, elegant, and
perfect model is high on my list of priorities, but I need to get
/something /into CAM and on the spoilboard before the sands of time run
out. It's this damned need we all have to make things we can sell in
order to afford life's luxuries, the little things like food, clothing,
shelter, and so forth.

I have a number of other, similar projects that are on hold while I wait
to learn how to project sketches in the way you have suggested and do
everything else best practice right from the start. It would be a shame
to quit now.

[TrippyLighting]

Im apologize for my lack of patience. That's never be a strong suit for me 😉

Below is a screencast explaining some of the problems with your sketching:

I'll do a lofting one later this weekend.

[Anonymous]

No apologies needed. When I began my apprenticeship under a German master many years ago, he would say such helpful and encouraging things as "Bob, your ff-holes are horrible!" So I am used to blunt criticism. I always know where I stand, and if a compliment should ever sneak into the discourse, I know I earned it.

Your screencast was wonderful. It gave me the new perspective I needed, although there are still a few(?) questions. But I thought first I should answer the questions you asked, especially since we are getting a blizzard in a few hours and I expect to lose power at some point!

1) In the older drawing you worked on there was a departure from the center line. That was there to allow extra material on an aluminum template to drill two holes on the center line for locating pins. The template is used to draw a half-outline on plywood sheet stock, then flipped over and pinned again so that the full outline drawing is symmetrical. Very old-school, but time-tested if you are careful in drilling the locating holes. It occurred to me that sketching in Fusion and cutting the mould on a CNC machine allows me to eliminate the template, so I deleted that little bump-out from subsequent drawings.

2) You asked if the straight lines at the bottom and top of the inner profile were needed. The answer is a definite yes . . . and no. The approach I used was done with one eye looking ahead at subsequent steps on the CNC machine. The flat spots are needed on the underside for gluing platforms for the instrument's six structural blocks, but on the outside the convex arch dips below the origin plane and then rises again to the purfling groove. My idea was that the flat "shelf" would be partly corrected  when routing the purfling groove, and the rest would be done by hand when cutting the purfling down to the contour.  I had considered making an offset copy of the drawing after it was lofted, and then working on the outside face separately from the inside face. My two problems are a) that I'm not sure how to connect the two drawings to give a consistent edge height of 8 mm; and b) that the reversing curve goes below the plane of the sketch and then comes back up again at the outside edge. The lowest part of the convex curve has to be at a consistent dimension all around, and I didn't know how to do that, either, or how to tell the CAM program to cut it.

3) The arcs on the basic "template" drawing are made up of different tangent curves, and each arc has three of them until it reverses or goes past the center line. When doing the profile for lofting, these are the lines that get offset inward (and modified at the corner points). A long spline should work, but I find constraining splines to be difficult. I'd really like to know how to do splines accurately over the offset arcs and then delete the arcs afterward. I can tell from your lofts that you do the entire arc all at once. Very cool.

I think of the many things I took away from your screencast, doing half-sketches and mirroring 3D objects instead made the most impact. There's a step in my process where I have to cut counterforms to construct the rib outline. Since there are almost two dozen of them, and I was sketching each one (or sketching half and then mirroring the other half), the clutter on my drawing was, well, horrible. This should make things a lot easier.

[TrippyLighting]

Ha, that is interesting. I am indeed German, but I wouldn't call myself a master 😉

Your post was very helpful for me to understand that what needs to be modeled is not actually the instrument, but the tooling to help you build it. Please correct me if I am wrong. 

 I guess in general it would help me to understand the process you have in mind for building the instrument.

To your point #2 

The CAM software will cut whatever geometry you will design. But keep in mind that such 3D structures as the arched top will usually be cut with a ball end mill. As such the sharp edge that is part of the model is actually going to have a fillet as big as the radius of the ball end mill. 

To your point #3

You can design this with offset arc segments to maintain a consistent offset from that outline, but the key here is that all these segments need to be tangent and the end points need to be coincident.

This is a single loft and was created with offset arcs connected with 2 splines :

The curvature isn't ideal as some irregularities can be seen in the Zebrastripe Analysis, but assuming that this surface is going to be scraped and/or sanded after machining that is pretty irrelevant.

Also, I believe that this can be improved by shortening these arcs and lengthening the splines just a little bit.

I'll experiment with that when I have some time this week.

[Anonymous]

You'd asked about the construction process the design will be used for, so I've been working on the attached (when I haven't been shoveling snow). The sketch shows the next step after the basic template, which is to create the construction mould. You can see that the template has to be significantly altered to do this, which is why I work from a copy.

The drawing shows a half template and the essential counterforms that I use. The sketch is cluttered, but I wanted to show how the curves of the counterforms are taken from the original sketch. The number on the counterforms indicates how many of each will be needed. All of these parts will be cut from plywood or similar sheet stock 12 to 19 mm thick, depending. The ones with the number 4 will be doubled in height, but that's a detail for wood rats like me.

I plan to extrude the parts into solids and then mirror all of them. Once I have all the pieces, I can make copies of any additional ones I need. The final step will be to nest them for the most economical use of material.

When that's done, I'll sketch the back arching. I'm planning to sketch a half-body, add the arching lofts, etc., the mirror it and thicken it. If I am heading toward a disaster here, warnings would be appreciated.

I know the sketch is not fully constrained yet. I find that constraining this particular shape is more difficult than one might think. Not sure what Fusion is looking for, but when I try to turn the remaining blues likes black  it says that the drawing will be over-constrained.

Last of all, you are correct that this is an important step that comes before the final model. But something tells me that the better the model, the better everything else will be. And with regard to your comments about the form analysis in Fusion, it's something that most instrument makers have never worked with. That's not to say it lacks value. We just need to learn how to use it.

[TrippyLighting]

I had planned to make a screencast using one of your models, but ended up making one for another user in another thread using the same techniques I used to create your models.

[Anonymous]

This was a wonderful screencast. It was not only a pleasure to watch you work (and deal with Fusion's spline inconsistencies 🙄), but to see how to reduce things to their simplest in a very complicated program. What's even better, thanks to all the help I've received here over the past few months, I was actually able to understand everything you did and why it worked. Amazing! Now I just have to put what I've learned into action, but with some practice I will improve a few degrees each time. I don't know where else I could have gotten this in such a concentrated form.

Your tutorial helped me understand how to get more sophisticated curves with splines, which will ultimately mean better results on the CNC machine, and that will possibly eliminate some hand work. Until my shoulder is fully healed, that will be a very good thing. You dealt with some problems and error messages that were secondary to the topic, but some of them have been bugging me since day one with Fusion. I learned how to fix some things, and that has given me hope that one day I will be able to deal with the dreaded yellow squares in the Timeline no matter what has caused it.

So, Bravo, TrippyLighting! Bravo! You might not consider yourself a Master, so I will have to apply another term from the musical world. Virtuoso! I do not know the guitar maker for whom you created this tutorial, but if he is as happy about it as I am, he's a pretty happy camper!

So best wishes for he holiday season, and if you ever get the urge to do more of these, please don't resist too hard!

[TrippyLighting]

Thank you for the kind words!

[Anonymous]

I am back again, working on yet another iteration of my violin model with the goal of getting it into the CAM portion of Fusion as soon as I can. I have tried to use as many of @TrippyLighting 's surface modeling suggestions as I could, and I must say they have all been very helpful.

Before I get myself into more problems, I thought I'd show my progress to date and ask for some suggestions on how best to proceed. You see in my model that the corners of the outline are truncated and can't be drawn entirely with splines. The corners of middle line, however, are not truncated, and the innermost (construction) line does not have corner points at all. It also does not exactly follow the contours of the other lines. So . . .

My goal is to loft a surface on this model using as few rails, etc. as possible. If you look at the upper and lower end of the sketch, you will see that the spline drops below the drawing plane and then returns near the edge. The construction line serves to ensure that the spline curve reverses on the drawing plain all the way around. I can see that lofting into the corners might prove problematic, so any advice or suggestions will be appreciated.

[davebYYPCU]

The outside profile, - flat,

but your centreline spline is not tangent to those flat sections, curve comb is spiked.

Might cause trouble.

are you asking, where to put the intermediate rails, or are you just getting thoughts so far?

[TrippyLighting]

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@Anonymous wrote:

I can see that lofting into the corners might prove problematic, so any advice or suggestions will be appreciated.

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It absolutely is!

What you are creating when lofting is NURBS surfaces. Mathematically a NURBS surface is always a 4-sided surface. 

Or in other words, the shape of a NURBS surface is controlled by a quadrilateral mesh of control points. Any other shape is either created by trimming a NURBS surface or by lofting into a point, which is really not a great idea unless you intend to trim that area off.

The problem, in this case, is that most of the surface has relatively gentle curvature, except in those corners there is high curvature. A multi-step approach is needed. Not sure yet how ...

[Anonymous]

Hi davebYYPCU--

The answer to both your questions is YES! If I'm happily shoveling myself into a hole, I'd like it to be as shallow as possible when I have to fill it in again later!

The termination of the spline could be problematic from a software modeling perspective, but it is faithful to the real-world model, which has a slight ridge around the perimeter. I figured I would cross that bridge when I came to it. But, hmmm . . . it looks like I've come to it, doesn't it?

An off-topic question next, but if I want to take a screen shot of a model so I can get close to certain features, Windows will produce the results as a .jpg or similar file. Is there a way to embed this in the text of a message rather than send it as an attachment?

For more head-swelling information, check my response to Trippy. It deals with a similar problem.

[TheCADWhisperer]

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[Anonymous]

Don't lose too much sleep over this one, TrippyLighting!

From the perspective of surface modeling, it would be nice to have a model that both solves the problem and is true to the original object. But since one iteration of the drawing will go to the CAM module next, we might need to have a dialog with the cutting bit first. Since there is no tool I know of that could cut into a point, it's likely that the cutter toolpath will have to be held back from the corner point by at least bit's radius dimension, so we could be looking at an arched or even a square termination of 3 or 4 mm in this drawing. If it makes the loft possible, then the router bit becomes our friend. If @laughingcreek is following, perhaps he will be able to use his experience to get  me past this point (yeah, I know. Very bad play on words. Sorry . . .  😐)

[Anonymous]

Duh . . .

That was fast!

[Anonymous]

OK, now that I know where to find the camera icon, next thing is to learn how you made those red underlines and nifty red stars! 😀

[davebYYPCU]

Don’t put the cart in front of the horse.  Fusion Cam is powerful, with a nominated cutter, it may leave a slight amount of material to clean up in a creased edge, even so that bump is not so bad that a Vee cutter bit will handle it.  You cant do much in there without a model, testing pieces will help....

So you have a flat section drawn, you have a creased bump, a valley then a smooth transition away from those - to the arch.  Sounds like a job for Tsplines.

[TrippyLighting]

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@davebYYPCU wrote:

Don’t put the cart in front of the horse...

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Yep!

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@davebYYPCU wrote:

So you have a flat section drawn, you have a creased bump, a valley then a smooth transition away from those - to the arch.  Sounds like a job for Tsplines.

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If you re familiar with T-Spline modeling and have some experience under your belt, maybe.

You still, however, have to solve exactly the same problem of having a  small, high curvature area adjacent to a low curvature area. 

[davebYYPCU]

I was thinking, a ribbon Tspline to bottom dead centre of the valley, presuming a consistent depth, but that ain’t a given, either.  

I have not much time in Tspline, was hoping you were here at sometime.  

I’m gunna fiddle a bit.