Following on our discussion in Part 1, I used a Gear Assembly to show the creation of size configurations for the Gear. In Fig. 1 below, you can see in the Configurations Tab that there are currently two configurations: the Default, and Medium. (I’m using screen shots because some of the feedback I received expressed concerns that the video went too quickly.)
In Fig. 2, I am creating a third configuration, Large. Note at the bottom there are two check boxes, the first of which Suppress Features, is very important. It provides a setting so that any features I create for this configuration are automatically suppressed in the other configurations, instead of me having to do it manually. You can also designate acolor for each configuration, which makes it simpler to recognize different configurations at the assembly level, which is particularly handy when the difference(s) between configurations is not immediately obvious. This applies at the part & assembly or subassembly level.
Fig. 3, the sketch for the primary extrusion of the wheel disk is open, showing me modifying the diameter dimension. You will note that on the dimension dialog, there is a tab selected, opening a flyout allowing me to specify which configuration this diameter is to apply to.
I went back and reviewed where I was going with this post, and more critically, what was needed to effectively get across the comprehensive grasp of these principles, and I came to the conclusion that this needs more illustrations, and it needs to be in at least three parts. What follows is Part 1, with some fairly significant revisions…
Principles & Concepts
One of the challenges many CAD newbies complain about is that most high end CAD packages start with the assumption that the newbie is coming from a CAD background of some sort, most likely from a competittor program. With the growing confluence of design tools I have been commenting on here, this is more and more often not the case. That’s why I’m going to start this discussion with the basic concepts and ‘build’ from there.
While there are various tutorials & guides online about configurations, a discussion of the ‘when’ & ‘why’ — even the what — is not often covered. Configurations are one of those features which, if you don’t know about it, you might not even think to wonder about it, especially if you don’t come from an engineering background. However, in many ways, the premise behind configurations is intimately linked to foundation engineering principles, including modularity, or modular design.
Let’s use an example we’re all familiar with — computers. When IBM opened its PC architecture to other vendors, triggering the ‘flood of the clones’, this was a grand lesson in the +’s & -’s of modular design. The PC is an example of modular design:
- The core of a personal computer is the motherboard, which mounts the CPU, among other things. It is the nexus point coordinating all of the peripherals to function in unison, including
- Video card
- Network card
- Modem card(for some)
- Sound card(most newer computers have integrated this into the mother board), etc.
As the indentation shows, the peripheral cards are subsidiary to the motherboard. Each type of peripheral is a component which fulfills a specific function needed by the systmem, and facilitated by the motherboard. It is worth noting as an aside that this “indentation” is a mode of representation which carries a specific meaning in engineering — it is used in what we call an indentured parts list. It is a valuable documentation tool.
The PC is a good example of modularity because once IBM set the interconnect standards — what sort of edge connectors did each card need to interface with the motherboard — vendors then based their designs around this constraint. Parameters of size, including net component height were also important. What this meant was that it created a broad spectrum of vendors who chose a niche, an area of expertise, which became the basis of a subsidiary industry. There are now multimillion $ corporations who only build one type of peripheral, like ATI or NVIDIA who make video adapter cards. Each card is its own module. Because there are many types of video cards, even from the same manufacturer, each model corresponds to a ‘configuration’ in the context of our discussion. Obviously, you can have configuraitons within configurations, but let’s look at some examples of this.
This simple drive assy can have a series of various configurations. For instance, there might be several configurations of the gear wheel alone, based on the number &/or size of teeth, the diameter of the wheel, and combinations of these. Each variation in a parameter of the wheel would constitute a configuration of the wheel AND the assembly. So, for example, a version of the assembly where the two wheels are of different sizes would also constitute a different set of configurations. This might be done to facilitate the use of the assembly in an application where the gear ratio needed to step down or step up in order to effectively engage the systems involved.
Seeing how this could impact this assembly, we can look at how configurations in the model facilitate these variations in the assembly.
In the above video, you can see that I already have created secondary configurations in the Gear Wheel part, the Gear-Shaft assembly, as well as the top assembly, Axial Example. In the video, follow as I switch between configurations in each model, and the resulting effect. This gives some insight into configurations. As stated, part of what this enables is a reduction in the number of models, and a simplification in the feature & assembly trees at the left.
One of the key aspects of this, which I will show in Part 2 later this week, is how you can control configurations as you create the part through the specification of which configuration a given sketch, sketch dimension or feature is intended to apply to. I will also discuss the use of design tables in creating & managing configurations.
The role of fabrication process & choice of materials is one of significant impact on any given design. With the advent of 3D printing, a rapid tooling or prototyping process, it is sometimes even more critical since this approach is an additive process — building up material instead of removing it, as in a machining process. It is also significantly different from molding or casting since there is no die or ‘container’ present for support in critical areas. For those coming into CAD from rendering tools like 3D Studio, Maya, etc., or from virtual world environments, the significance — and subtleties of these issues may not be readily apparent.
The use & application of concentric or coaxial features in mechanical design is a common component of the CAD skill set, though it is often misunderstood. As mentioned in an earlier post, GD&T(geometric dimensioning & tolerancing— ASME Y14.5) is a baseline tool and industry standard. Many of the principles of GD&T come into use in the design of coaxial features & parts. Further, one can gain a more direct understanding of their applicability through corresponding aspects within the CAD package itself. In SolidWorks, this is most evident in Relations and Mates. Parenthetical references to sections below refer to the text of the ASME Y14.5 standard.
(In this discussion,
there is the presumption that most readers
have a certain basic grasp of fundamental geometric terms & principles.
Links have been provided in certain instances to augment this.)
Relations are exactly what the word suggests — the establishment of constraints on how two or more elements of a sketch or feature relate to one another. These include such things as concentricity, parallelism, coincidence, tangency, perpendicularity, etc. Relations are the geometrical laws which enforce constraints on a design, thereby controlling certain variables so that your model ends up being fully defined. It is not always necessary for a sketch to be fully defined to move forward with the creation of a feature, but for newbies, it is the safest path.
In GD&T, these principles appear in essentially two contexts: Limits(2.4) & Geometrical characteristics(3.3.1). Limits are familiar to us in many contexts, most commonly through the use of dimensioning, which establishes limits on size and relative position. Geometrical characterics are represented via symbols which are a language independent means of conveying this information, which was one of the key bases for establishing this standard.
In engineering design of machined parts, there is often a misunderstanding of meaning when someone uses terms like ‘economical or low cost design’. Conditioned to think of cost in terms $$, many often overlook the factor of time, most significantly in the context of machining steps — operations, iterations, repeated passes, etc. Whatever we can do as designers to reduce the number of operations & iterations in the machining process saves $$$. This is what time study engineers do — analyze ways to streamline a task process. For instance making a series of circular features concentric — even when this may not be crucial to the design — can significantly assist those in the machine shop & inspection area to more swiftly and effectively complete their tasks.
One of the features which sometimes befuddles folks is the Sweep(in SolidWorks), whether an extrusion or cut. I will address some tips in this area. However, it is important to keep in mind that it is a mistake to presume that just because you can design a 3D model, it can be machined. It is also true that a feature you apply in a CAD design may not be able to be machined in an analogous process. For instance, a sweep along an edge of a circular part is a fairly simple lathe operation. However, a sweep along a polygonal part is more complex. For some machining systems, working their way around a 4, 5 or 6-sided part is not so challenging. For others, it is a harrowing issue. Allow me to reiterate: CAD is not a stand alone tool/premise/concept — it was always conceived as a partner in CAD/CAM = “Computer-Aided Design” & “Computer-Aided Manufacturing(or Machining)”. Once you have completed your model and submitted it to the shop, they must go through a process of conversion to their CAM software to program their CNC machines to actually do the work.
Some users and others are coming to CAD fairly recently, and may be unfamiliar with its origins, how it got to be what it is today. Having been in the design field since well before its introduction, I thought I might be able to share some insights. This is pertinent because in many instances, the actual process and mindset one needs to bring to the CAD process is ensconced within the process of its evolution.
For an iteration of the technological/corporate developments is CAD, I have found The History of CAD to be an exceptional resource. One of the keys I found intriguing was in his section on 1970-1980, where Bozdoc asserts:
“MCS was founded in 1971 by Dr. Patrick J. Hanratty. Since the day it was founded in 1971, MCS has enjoyed an enviable reputation for technological leadership in mechanical CADD/CAM software. In addition to selling products under its own name, in its early years MCS also supplied the CADD/CAM software used by such companies as McDonnell Douglas (Unigraphics), Computervision (CADDS), AUTOTROL (AD380), and Control Data (CD-2000) as the core of their own products. In fact, industry analysts have estimated that 70% of all the 3-D mechanical CADD/CAM systems available today trace their roots back to MCS’s original code.”
I consider this to be important because, as many CAD polyglots will tell you, there are certain challenges which seem largely identical across certain types of CAD apps. This is also pertinent in that it further underscores the veracity of the premise that what you learn — in terms of technique/process — on one system is generally mappable to others.
This close up shows the type of thread spool arrangement I spoke of in the previous post. Of course, a mounting arrangement must be configured to hold the spool steady, allowing the thread to pull off, instead of the usual where the spool spins as the thread pulls off. However, you can see this arrangement minimizes potential snags, etc. It also allows for the entire needle cartridge to be lifted up through the top — and threaded at eye level(!) — before being reinserted, and the bobbin/spool being replaced.