Spline Curve Drawing Technique (Tips n Tricks)

[basty]

Is there any tips and tricks how to draw with spline curve?

 

Is there any drawing technique (mechanical drawing) how to draw with spline curve?

 

Please

Edited May 23, 2016 by basty

[TheCADnoob]

Its realtivley straight forward, i guess the trick would be application dependent. What are you trying to draw?

[basty]

Its realtivley straight forward, i guess the trick would be application dependent. What are you trying to draw?

 

For example, I would like to draw this body of electric guitar:

 

http://fullhdpictures.com/wp-content/uploads/2016/04/Electric-Guitar.png

[RobDraw]

I think most of that is done with arcs and a couple/few lines with maybe a couple splines. I think it would be very difficult to do that entirely with a spline.

 

You might want to take a look at this thread:

http://www.cadtutor.net/forum/showthread.php?71752-WIP-Modeling-My-Fender-Squier-Stratocaster&highlight=strat

[basty]

I think most of that is done with arcs and a couple/few lines with maybe a couple splines. I think it would be very difficult to do that entirely with a spline.

 

You might want to take a look at this thread:

http://www.cadtutor.net/forum/showthread.php?71752-WIP-Modeling-My-Fender-Squier-Stratocaster&highlight=strat

 

It's not helping and This is not as step-by-step. I need step-by-step of drawing technique, and tips and tricks how to draw with spline. Please help me.

[RobDraw]

I'm helping by suggesting that you approach this differently. You're welcome.

 

I wouldn't even try to do it with a single spline. It would be a highly complicated way of doing it. I highly doubt it was done that way originally.

 

Is it a requirement to do it with a spline?

[basty]

I'm helping by suggesting that you approach this differently. You're welcome.

 

I wouldn't even try to do it with a single spline. It would be a highly complicated way of doing it. I highly doubt it was done that way originally.

 

Is it a requirement to do it with a spline?

 

Yes.

 

Below tutorial is a good example of what I am asking about:

 

 

What is the mechanical drawing technique of a spline? I can't find about spline in my mechanical drawing textbook.

[RobDraw]

That video shows you that using a spline is just an approximation of the actual shape. There was a lot of click and drag until the shape was close. It is a less than accurate way of tracing that shape.

 

Splines have their place but this is definitely not one of them.

 

Splines are actually very complicated. This exercise is pointless as there are much more accurate ways of doing this, IMHO.

 

BTW, the software used in that video is not AutoCAD.

[basty]

That video shows you that using a spline is just an approximation of the actual shape. There was a lot of click and drag until the shape was close. It is a less than accurate way of tracing that shape.

 

Splines have their place but this is definitely not one of them.

 

Splines are actually very complicated. This exercise is pointless as there are much more accurate ways of doing this, IMHO.

 

BTW, the software used in that video is not AutoCAD.

 

It doesn't matter what software is used as AutoCAD have a spline tool too.

 

So can anyone tell me mechanical drawing technique of a spline?

[RobDraw]

I told you the technique of click and drag used in that video is a very inaccurate way of tracing a shape.

 

What do you want to with a spline? I guess the guitar was a bad example because in order to get that shape, a single spline is not appropriate.

 

If you really just want to learn about splines, why don't you do some research and come back with specific questions? They are quite complicated. That is why you are not getting any other responses.

 

Here is a good place to start:

https://knowledge.autodesk.com/support/autocad/learn-explore/caas/CloudHelp/cloudhelp/2015/ENU/AutoCAD-Core/files/GUID-5E7D51E2-1595-4E0C-85F8-2D7CBD166A08-htm.html

[Dadgad]

I told you the technique of click and drag used in that video is a very inaccurate way of tracing a shape.

 

What do you want to with a spline? I guess the guitar was a bad example because in order to get that shape, a single spline is not appropriate.

 

If you really just want to learn about splines, why don't you do some research and come back with specific questions? They are quite complicated. That is why you are not getting any other responses.

 

Here is a good place to start:

https://knowledge.autodesk.com/support/autocad/learn-explore/caas/CloudHelp/cloudhelp/2015/ENU/AutoCAD-Core/files/GUID-5E7D51E2-1595-4E0C-85F8-2D7CBD166A08-htm.html

 

 

+1

 

Rob is right on the money, all you have to do is look at how poorly the shape of the pickguard attempts to follow the contours of the lower edge of the body, which is reasonably horrific.

 

Draw this with a Closed polyline made up of mostly arcs, and it will be much more accurate, and easy to tweak to get a better copy of the image.

There would ideally be more vertices than in the spline created in the video, as there are a lot of curves which need to be represented.

 

While it is a terrible idea to use a spline, once you start the SPLINE command, just click however often you want to, along the shape you are tracing, more vertices will yield a better result, but a POLYLINE is still a better way to go. After tweaking your closed polyline, if you want to, you can turn it into a Spline. Not sure why you might want to?

Edited May 25, 2016 by Dadgad

[MillerMG]

Just draw a polyline then convert it to a spline using PEDIT. Good luck.

 

 

Edit

@Dadgad I didn't see that you already said something about this. My apologies. What in the world is a mechanical spline? This post is cracking me up with how Rob has helped more than enough and still no luck

 

Miller

[lrm]

I have a lot of respect for RobDraw but in this case I have a different opinion. I think splines are the way to go.

 

Here's what I would do.

 

Working in the top view, attach the picture then scale it to the correct size.

 

Move it slightly in the -z direction. This will make it easier to see the spline and its frame as you edit the spline.

 

Note, there are two general methods for creating a spline. You can create a spline using defining points. That is, points through which the spline passes. Or you can create a spline using Control Vertices (CV's). The CV's are the fundamental points that the CAD software uses to define the mathematics of a spline. The spline starts at the first CV point and ends at the last CV point. The second CV point of a spline sets the slope and radius of curvature at the start of the spline. In the same manner, the second to the last CV point defines the slope and radius of curvature at the end of the spline. I prefer using CV points.

 

Start the spline construction by crudely eyeballing the location of CV points. Turning the frame of spline on may help you visualize the process.

 

You can the drag the CV's to better match the shape of the guitar body.

 

Use the SPLINEDIT command then E (for Edit vertex) and A (for Add vertex) to add additional CV's if necessary as I have done here (be sure OSNAP is OFF).

 

The CVs at the back of the body should be directly in line vertically to ensure a vertical slope. Or, stop at the centerline and make sure the last two CVs are in line vertically. Start a new spline at this point.

 

There is a limit to the number of CVs in a spline so I might make the top and bottom separate spline then turn the closed shape into a region before extruding the shape.

With a little bit of practice you will get the hang for how many CVs are needed and where they should be placed.

 

Good luck!

 

~Lee

[RobDraw]

Thank you Irm, I do appreciate the comment.

 

As to the spline, do you agree that it is at best an approximation?

 

What is the value of a spline in this shape?

 

How would one build it? I'm pretty sure CNC software prefers polylines.

 

Where is Mr. Frampton when you need him? He is the one that did the 3D version I linked to earlier in the thread.

[lrm]

As to the spline, do you agree that it is at best an approximation?

 

What is the value of a spline in this shape?

 

How would one build it? I'm pretty sure CNC software prefers polylines.

 

When you come down to it most models are an approximation. Since the OP is using a photo for the source shape that will force some degree of approximation.

 

Polylines are a series of straight lines. A very crude approximation. Linking arcs end to end is not much better. You can match the slope at the junction of two arcs but cannot make the change in the change of slope smooth (the second derivative of the curve).

 

If a polyline is used and it is converted with the spline option then it is a B-spline whereas the newer AutoCAD splines are NURBS curves. B-splines have serious limitation that do not exist with NURBS. Many CNC post processors can handle both but some, I agree, cannot handle AutoCAD "new" NURBS splines. There is a recent thread about this.

 

What is the final goal of the OP? If it is to make a pretty picture then the approach that is easiest for him to do might make the most sense. If it is to fabricate the guitar body either via CNC or 3D printing (if that makes sense) I think using splines gives you the most accuracy and ease of shape control.

 

~Lee

[Dadgad]

When you come down to it most models are an approximation. Since the OP is using a photo for the source shape that will force some degree of approximation.

 

Polylines are a series of straight lines. A very crude approximation. Linking arcs end to end is not much better.

 

Clearly LRM, you are at the top of the Spline user food chain, which I definitely am not.

You have also delivered exactly what the OP asked for.

I enjoy using them occasionally, although I do greatly prefer the use of spline leaders to straight, and I have a pretty good command of them.

I don't mean to invalidate what you said, and couldn't even if I wanted to, but one thing which you said did strike me as a bit odd, as my intention was not to define a bunch of straight polyline segments, which clearly would be quite tedious and less than ideal, but a continuous combination polyline made up of both straight and arc segment polylines, as appropriate. The very great majority of those would in this case be Polyline ARC segments, rather than straight Polylines.

 

I traced this with a continuous polyline, and yes, having played the guitar for 55 years, owned a Pre CBS strat when they could still be had for $100, and managed a guitar shop on Sunset Blvd for a few years back in the early '80s, I am well aware it is not CNC accurate. I hate tracing photos or raster PDFs.

 

Either technique would yield an approximation at best, I doubt there are many users who are nearly as well versed in the use of SPLINES as you LRM.

 

I have no doubt that, in the hands of one who is well versed in the subtleties of advanced Spline techniques, the result would be better than the polyline approach, as they are fiendishly clever, but I suspect that few folks are, present company, and vehicle designers excluded.

 

I too, like Rob, appreciated the tact & diplomacy with which you chose to respectfully espouse the Spline approach!

 

We are all pulling oars in the same boat.

Edited May 26, 2016 by Dadgad

[tzframpton]

As author of the fender guitar referenced, I used Polyline Arcs. Worked way better for me. But takes practice.

[RobDraw]

Okay, I realized something that makes me a bit biased on the spline thing. My first job out of my drafting schooling was for a company that designed and built custom in-ground pools. Part of my job taking freeform hand drawn designs and making buildable plans for them. Sure I could draw a spline and get close enough but dimensioning it was a method that was tedious for me and laying it out in the field was even more so for the field guys. The diggers, who also marked out the hole, much preferred laying out and digging lines and arcs. Imagine, if you can, digging a spline with a backhoe as opposed to lines and arcs. To this day, I still finding myself picturing free forms as lines and arcs. I'm guessing, back in the day, that the original shape of this guitar started as a free form shape, spline, that eventually became lines and arcs for manufacturing.

[Dadgad]

The diggers, who also marked out the hole, much preferred laying out and digging lines and arcs. Imagine, if you can, digging a spline with a backhoe .

 

Having spent my fair share of time in the trenches, this gave me a good laugh!

I can be pretty patient stretching virtual spline grips around to get exactly what I want, on a drawing, when so inclined, but definitely NOT with a backhoe!

[lrm]

...I'm guessing, back in the day, that the original shape of this guitar started as a free form shape, spline, that eventually became lines and arcs for manufacturing.

 

 

Yes, back in the day splines were free form. Google "drafting spline weights" then click "images" and you will see thin wood and steel splines tha were used to draft smooth curves such as this:

 

As for laying out a free form pool shape it seems it could be about the same amount of work to locate the center point for a bunch of arcs as it would be to layout a grid with some defining points through which the curve passes but you made the pool so I will default to your experience.

 

As for which guitar shape is better that is a matter of taste. I know I am biased here and you all have been so kind but I feel the spline-based shape I show in post #13 is sweeter than the arc/line-based shape of post #16 (no offense intended). Examine each shape for discontinuities in smoothness. All taste is taste so I know I may be out voted.

 

BTW, is there any interest in a layman's introduction to spline technology? There seems to be a hesitancy to use splines. The fundamental concepts behind Bezier splines is very cool and elegant. Bezier splines is the foundation for B-Splines and NURBS curves and surfaces. My experience with splines goes back to the early 70's when I wrote a program to convert a smooth curve to a series of arcs and lines. At that time I needed to define a smooth curve for a program I was writing that created a smooth curve that could be rolled along a given curve to yield a desired shape. I didn't know about Bezier, B-splines or NURBS so I used arcs and lines. Perhaps that experience has soured me on them. I later wound up working for a CAD company and met the guy who invented B-splines. The development of curves in CAD from arcs to NURBS is an interesting history.