CAD Craft

A resource to CAD Craft specialists, and a showcase of my work — in both material and virtual worlds

CAD Tips: Sweeps

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.

While some may not be aware of CNC machines, be advised that this is where the forerunners of robot technology were bred. CNC = Computer Numerical Control, and is the basis for the control of the machining systems which make many precision complex parts possible. Think of it this way: CNC is to the machine shop what your PC is to such tasks as word processing, spreadsheets, etc. Some of you have no idea what life before ‘cut & paste’ was like. Imagine you have been working on a term paper on a typewriter for hours. You finally have the finished version. As you do your last review, you realize that you left out a paragraph — or a critical sentence — on the third page of a forty page paper. That means you now have to retype 38 pages in order to put in the missing text and move the rest down while maintaining your margins. Maddening, right? CNC provides an analogous freedom & flexibility for machinists. In fact, there are CAM jockeys just like there are CAD jockeys, though this is diminishing as the software becomes more transparaent, one to the other. As this occurs, it will increase some aspects of our responsibilities — and the need for insight, information & experience @ machine shop practices.

But, back to sweeps….  Below, in Fig. 1, is a circular part with a sweep around one circular edge. Fig. 2 shows a square part. The simplest way to create a sweep is to create its profile on one of the planes which intersects the path. For a polygonal section of a part, that plane needs to be perpendicular to the path, so you may need to create a plane for this purpose. The selection/creation of the path is the simplest aspect as it usually entail just selecting an existing edge which goes the full range of the desired sweep. Keep in mind, as shown in the second part, a sweep need not lie all on the same plane. However, when that is not the case, the aforementioned machining considerations must be factored into your choices(note details of the keyway groove in section view, Fig. 3).

Figure 1

Fig. 2 shows a square part. The simplest way to create a sweep is to create its profile on one of the planes which intersects the path. For a polygonal part, that plane needs to be perpendicular to the path, so you may need to create a plane for this purpose. The selection/creation of the path is the simplest aspect as it usually entail just selecting an existing edge which goes the full range of the desired sweep. Keep in mind, as shown in the second part, a sweep need not lie all on the same plane. However, when that is not the case, the aforementioned machining considerations must be factored into your choices(note details of the keyway groove in section view, Fig. 3).

Figure 2

Figure 3

 

Keep in mind that the creation of a multi-plane(or 3D) path for the sweep requires a 3D sketch for most cases. Had I not put the step in the groove on one face, a simple rectangle in a sketch on a plane perpendicular to the plane of the profile sketch would have been sufficient. However, in the case of this example, it would not have made these other points quite so readily clear & visible.

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