3D CAD (Solidworks) and Knife design (Newbie question)

[Nathan the Machinist]

The 20-yr old Cincinnati laser I programmed and operated paused at each node - to clarify, I'm talking about a fraction of a second pause. If the two CAD curves in question were not tangent, the defect was even more noticeable. The Flow waterjet at the same shop was from the late 80's and used the original software/controller. I think there are plenty of shops running 20 year-old machines and controllers.

Furthermore, I have definitely received laser-cut blanks with choppy profiles which clearly indicated the spline-to-polyline conversion resulted in non-tangent geometry.

I base my comments on the experience of having over 5000 knife blanks cut over the years at various vendors, and my day-job experience programming/running tens of thousands of parts on various machines using customer CAD and native CAD.

I haven't run into this problem before, but I'll take your word for it there are shops out there running legacy equipment from a previous era. For machines in that situation I would expect them to select "arc filtering" from their CAM system, which is a standard option on the CAM systems I've used.

Expecting a person to design in lines and arcs for the benefit of a dinosaur machine tool seems like the tail wagging the dog. Machine technology has improved substantially over twenty years. If you're using them for more than just a loose roughing operation I'd think you'd want to use something current. If it were me, I'd find another supplier...

 

[P.Brewster]

Expecting a person to design in lines and arcs for the benefit of a dinosaur machine tool seems like the tail wagging the dog.

Ha I know what you mean. For years I used splines to make the nice freeform curves we all love. Honestly your posts here about the glory of splines over the years was the main reason I enthusiastically embraced them. It took some adjustment, but now drawing in tangent arcs is natural.

The reason I used Leading Edge Fab as an example is that they are not a shop with ancient equipment or software. They are highly regarded here on Bladeforums, they have modern software including Mastercam, and they have modern machines and controllers. Still, they were unable (perhaps unwilling?) to work with my native spline files, regardless of the origin (AutoCAD, Draftsight etc).

 

[Nathan the Machinist]

Without splines, CAD is kind of a crummy way to design organic shaped objects. I think this is one reason some midtech knives have a "drawn in CAD" look to them.

People who draw products for a living such as Industrial Designers, Product Designers, Stylists, Graphic Designers generally do early design in things like Illustrator or Alias Studio or similar spline based drawing and modeling tools.

Making your drawing technique and your design conform to the constraints of straight lines and constant curvature arcs (referred to as C0 and C1 continuity) is a compromise. Sure, it can be made to work, but you could also paint a portrait with crayons. It works, but it's not ideal.

Modeling in splines (and C2 curvature continuity) and exporting the splines as polyline seems like a reasonable compromise to me. While there may be some old machines that can't feed through all the little points very well it seems like a reasonable work around would be to process the data in a CAM system (such as MasterCAM) that can perform arc filtering to create G02 and G03 commands for an old control.

Perhaps I'm just being obtuse, but I don't like to compromise my work without a really good reason.

If Mark has a modern seat of MasterCAM and he uses it program his Mach4 he should be able to handle polyline or even 3D STEP. You can adjust the post processor for arc filtering. I can understand why he doesn't want to receive spline data, but I don't understand why he would have a problem with polyline derived from spline data.

 

[Kevin Wilkins]

Just about Anything you can do with splines you can do with tangent arcs. There is no compromise in the design in avoiding splines. The only time you should use splines would be because nothing else will work. Otherwise do not use splines if you want to produce the product, for all the reasons other posters have already mentioned. Splines suck.

 

[timos-]

I Just received my first batch of water jet blades from Pete Bruno today. They came out perfect and I used the almighty spline to get all my curves. The key thing that Nathan is pointing out is that you must turn them splines into polylines before you send them to the waterjet guys. I am only being persistent about this because I feel if you are not using splines you are really missing out. Splines have a weight and direction for each node. This makes getting just the right profile possible. You just gotta learn to use em proper and then export them as a high res polyline.

 

[Kevin Wilkins]

Once you are proficient with tangent arcs, you won't need splines and won't have to worry about converting anything. Splines are fine used in sketches that are just guides for other operations such as swept cuts but when they are part of the part geometry, they have a way of turning up like a bad penny and biting you in the ass.

An important part of production engineering is making sure the drawings are clean and offer the least amount of potential problems later in the production process.

I suggest the series of books called Solidworks Bible which offer a lot of very good advice in this area as well as design intent and other areas of parametric modelling that folks may not realise are very important.

Added: I should also mention that SW (and most other 3D parametric software) are not ideal for "designing" stuff. By that I mean sketching out ideas and working up concepts. For that you need quicker, more intuitive tools such as pencils and paper and scanners or real drawing programs like Illustrator of even Sketchup. SW is great for refining the design and doing the final workup and production engineering. But when you're wearing your artist outfit, a simple sketchpad and pencil are still hard to beat.

 

[Nathan the Machinist]

When is the last time we had a lively discussion about design process and file formats? This has turned into an unusual thread.

Since we're discussing the merits of splines, lets make sure we're talking about the same thing. I'm talking about NURBS, non-uniform rational B splines, which are the basis of the fundamental design tools use in the curve and surface creation in practically every design software in the design and engineering industry today. Most products in production today more geometrically complex than a cylinder or box is modeled, at least in part, using these splines. The computer mouse in your hand right now probably has very few truly straight lines or constant radius curves. Their use in industry is almost universal.



I machine splines almost every day on an ancient CNC mill that has no problems with it.

This is modeled in splines:

The spline I follow to mill my bevels is a complex 3 dimensional non-linear curve:

If the underlying geometry were made of lots of little tangent constant radius arcs the end result would be changes in the curvature of the surface at each one of those points. That is an inescapable mathematical fact, and one that a discriminating eye can see.

Manufactured spline:

The use of splines becomes more important in the creation of scales if you want to achieve smooth shapes

This is of particular importance if you want to machine those scales following three dimensional surface ISO lines. If you use tangent arcs instead of a spline you will have surface patches and it will be impossible to use that particular machining technique because the ISO lines stop and change direction with each surface patch. The only way to achieve that is to have a single continuous surface, and unless your scale is basically flat you can only get that with a spline.

The spline is one of the most important design tools we have and it's use in industry is almost ubiquitous. They're in most of the products manufactured today.

but when they are part of the part geometry, they have a way of turning up like a bad penny and biting you in the ass.

An important part of production engineering is making sure the drawings are clean and offer the least amount of potential problems later in the production process.
.

I haven't had any bad penny experiences using splines. Quite the opposite, sometimes they're the best workaround for avoiding certain manufacturing problems. Even my CAM package uses trochoidal arcs in high speed machining algorithms. But, I don't send out a lot of stuff, most of my work is done internal. From your experience, what kind of problems does the typical custom knife maker run into using splines?

 

[Kevin Wilkins]

Nathan, if something is working for you, don't change anything. Why should you?

The problems with splines have been pointed out already in this thread, they invite problem when working with outside vendors and other system.

I could draw everything you show above easily with tangent arcs, no problem. Easy as pie. You can have multiple arcs connected to each other tangentially and define their radii.

 

[P.Brewster]

We all agree that organic, freeform shapes are the way to go. Freeform shapes can be achieved with splines or tangent arcs. Both approaches have their advantages and disadvantages.

The following two statements are NOT synonymous:
(1) "I draw with arcs."
(2) "My designs are inorganic compositions of lines and constant-curvature arcs."

 

[PEU]

Let me plug again Coreldraw, it has the spline option, but I almost never use it, the node editing options give more liberty without the constant hassle of readjusting spline nodes, here are the options:

What I use constantly are the cusp/smooth/symmetrical options along with the shape adjusting arrows each node of a curve has. This gives you great flow of curves and IMHO more control than splines. Also, if you want to combine straight lines with curves thats easy too.


Pablo

 

[bndshardr]

I get around the "synthetic" look by making a prototype in wood, paint stir sticks work well for most applications. Then I take a photograph of it, import it into Pro/E and "trace" it with tangent arcs. It's not difficult to make spline-like curves with this method. Many of the arcs have short lengths and progress into adjacent arcs with longer arc lengths. This provides a sense of increasing or decreasing radii without the need for using splines. If this doesn't match to my satisfaction once the blanks are cut, it's certainly a lot closer than if I'd traced the original on blued steel and chunked it out on a band saw or with an angle grinder. Any minor tweaks that need to be made before bevel grinding only take a few light passes.

Just my method, I don't begrudge anyone with differing opinions or methods.

 

[PEU]

Curiously I do the opposite, I draw in corel, then cut with my CNC router in MDF, then if I alter the model I go your way, I use my flat scanner and then readapt the profile on the PC...

Today I was thinkering with a design:

the bottom model got its clip lowered a bit with the grinder


Pablo

 

[seionage]

I know this is a little thread necro, but, years ago I used CorelDraw, but, basically forgot how to use it.

Just throwing this out there:
Pablo:
It would be awesome to see a WIP on how you design a knife.

Thanks!

Let me plug again Coreldraw, it has the spline option, but I almost never use it, the node editing options give more liberty without the constant hassle of readjusting spline nodes, here are the options:

What I use constantly are the cusp/smooth/symmetrical options along with the shape adjusting arrows each node of a curve has. This gives you great flow of curves and IMHO more control than splines. Also, if you want to combine straight lines with curves thats easy too.


Pablo