Planning reveals design intent which, in turn, helps to determine modeling technique
January 10, 2012
Columnist Gerald Davis focuses his 3-D CAD skill on designing a welding table. He invites readers to participate in the creation of this support structure.
The goal of the next few columns is to model a welding table. We will use our 3-D CAD model to produce a bill of materials, a set of manufacturing drawings, and some illustrations for customer support.
Of course, a “welding table” covers a lot of possibilities. We are going to ask you to tell us which features you want in a welding table. Just e-mail your suggestions to the address at the end of this article.
From past experience I know that a full three months pass from the time I write a column to the time most of you read it. So I’m planning on writing the final weld table project definition in February for publication in the April issue.
One more quick note: Some of the verbiage used in the discussion of CAD modeling in this column is unique to the software I’m using. However, you will find that other software offers similar functionality in most instances, even if wording and commands may not be similar to these mentioned here.
While we wait for the fabricating world to comment on what they want in a welding table, we’ll have plenty of preparatory work to do. From the start we’ll build a design library. We’ll model a simple cutting table to both develop our CAD skills and to add content to our design library. The design library will contain templates for parts, assemblies, tables, and drawings. You’ll also want to include Property Tab definition files to make the data entry easier for our custom properties. We’ll try to cover all of that while we wait.
Here is your homework outline:
Step 1 is due in my inbox by Feb. 3.
As fabricators, we focus frequently on manufacturing to specifications provided to us. We don’t often delve into the process of defining and inventing. Let’s start by describing the absolutely ideal welding table. From there we can match the ideal to the practical and winnow a good design from all of the possibilities that were considered.
An ideal welding table would give perfect access for the welding process; hold without marring, shifting, or otherwise impeding production; operate quickly if not instantaneously; be sufficiently strong for reliable production; not waste any asset in its method of construction; and be perfectly suited to the intended product and process.
Now we don’t know exactly what you’ll instruct us to include, but we expect questions like these:
That is hardly a comprehensive list, but you should get the idea. We need to know what this table will be used for. Once we know that, then we’ll dig into the general method of construction—materials, fasteners, finishes, and sizes. Armed with that information, we can reasonably establish our design intent and proceed with modeling and invention.
As a demonstration of the modeling process, let’s build a light-duty cutting table like the one shown in Figure 1. Pretend that you didn’t see that picture, but it is worth a thousand words.
Our starting point is really to write a statement of which features this table should have:
Our features list does not indicate whether the 30- by 30-in. size is likely to change. Just in case it might, let’s model the parts accordingly. This should be a breeze as long as the tabletop remains a square. We want this short demo to be short.
The modeling technique I’m using is to create an empty assembly and then insert parts into that assembly until it looks like a cutting table. The software I’m using allows me to create “virtual parts” that are easily renamed. I like that, particularly on a project where I don’t know what part numbers for file names will be appropriate until the model is pretty much complete.
The first part that I’m going to model is the subpan. I’ll make it 30 by 30 in. Figure 2a shows the starting sketch—a rectangle with dimensions. It might have been wiser for me to make two of the perpendicular line segments equal; then one 30-in. dimension would control both sides.
Note that the rectangle is centered about the origin. That comes in handy later.
Using the Insert>Sheet Metal>Base Flange tool converts the sketch into a sheet metal part. It could just as easily have been made using an ordinary extrude, but the sheet metal functionality makes it easy to select a standard sheet metal gauge thickness. I opted for 14-gauge cold-roll steel for no particular reason.
To minimize the blowback during flame cutting, I’ve perforated this subpan using the Fill Pattern tool. I set the hole size at 1 in. and the spacing at 1.5 in. on 60-degree staggered centers. Figure 2b shows the result—a very simple part. It would probably oil can in real life. Only the truly obsessed would try to model that.
In Figure 3a we see a sketch for the side frame. The lower line segment for the leg at 1.500 in. supports the subpan. The shorter leg at 1.000 in. will support the egg-crate top. We’ve added 0.500 in. between the edge of the subpan and the start of the bend for tolerance, which also allows for the removal and installation of the subpan. The 0.750-in. vertical flange will become fingers to locate the egg-crate.
Again using the sheet metal Base Flange tool, we extrude the sketch to create a sheet metal part. In Figure 3b you can see that the Offset From Surface option was used to keep the ends of the side frame 0.750 in. short of the edges of the subpan. I opted for 10 gauge, which seems pretty stout, for the sheet metal thickness.
To make the finger slots for the top slats, a cut-extrude is made as shown in Figure 3c. Once that cut is completed, it is patterned on an interval of 3 in. for a total of 10 slots to complete the design.
Holes for bolts are created in Figure 3d. I sketched two construction lines. The horizontal line centers the pair of holes vertically. The vertical centerline is used to mirror the pair of holes to the other side. The model for the side frame is complete. We modeled it in the context of the top-level assembly, as shown in Figure 3e.
I like to take shortcuts. One shortcut I can take with a square tabletop is to make a circular pattern of the side frame to create the other three sides. I created an axis using the right and front planes. Because the sketch for the subpan is centered on the origin, the axis also will pass through the origin and the center of the subpan. The result of the circular pattern is shown in Figure 4.
To model the leg, I started with a simple two-line sketch, made both legs equal, and dimensioned one of the legs (see Figure 5a). This sketch is on the same plane that the subpan is on, so I extruded the leg up 5.750 in. and down 24 in. to achieve the desired table leg length as shown in Figure 5b.
The holes for the bolts must match the bolt holes in the side rails. While sketching the holes in the leg, I simply used the Convert Entity tool on the edges of the holes in the side rails. I then used the resulting sketch to cut-extrude with the depth linked to the thickness of the leg—also 10-gauge cold-roll steel for no particular reason. This process was repeated to cut the other two holes on the other side of the leg.
We now have one leg but need four legs. We could create another circular pattern as we did for the side rails. But, as shown in Figure 5c, we could more easily edit the circular pattern we already have so that it patterns both the side rails and the legs.
In Figure 6a we have sketched the profile of one of the top slats. I made it symmetrical about the origin and coincident with the supporting flange on the side rail.
Figure 6b shows the sketch and settings for cutting a slot for the egg-crate. To allow for manufacturing tolerance, I made the slots a little wider than the 10-gauge thickness of the sheet metal slats—0.135 in. versus 0.150 in. I also extended the slots 0.010 in. extra deep for good luck. You might want to adjust those dimensions for a tighter fit.
To create the other slots, we can use the Linear Pattern tool. With the slot patterning completed, I used the Sheet Metal Break Corner tool to add that extra elegance and safety that you might have noted in Figure 1.
The perpendicular slats were modeled in the same way as the previous slats with two minor exceptions: The slots for egg-crating go the other direction, and the sketch plane is a face of a slat slot. For the first slat, we used a finger slot face for a sketch plane. I probably should have told you that earlier, but I wanted to write a paragraph that you’d want to read aloud to your spouse.
A pattern of slats is created in Figure 7a. The perpendicular pattern is shown in Figure 7b. This completes the tabletop. It wouldn’t be difficult to embellish this egg-crate top with points to further suspend the workpiece over the edges of the slats. Actually, it might make them last a bit longer.
The legs bolt to the side rails to create the structure upon which the subpan and top grille sit. I obtained the models for the bolt and nut—Figure 8—from www.mcmaster. com. Also, www.3d con tentcentral.com and www.grabcad.com are great resources for ready-to-use 3-D CAD models.
In preparation for the assembly illustration, let’s add an exploded view. We’ll also need to set some custom properties so the title blocks and BOMs fill in correctly. Those seem like good topics for next month.
In several places in this column I’ve claimed that I selected a material thickness for “no particular reason.” We could explore simulation tools to try to predict how well I did in selecting materials for a structure that must support a 300-lb. vertical static load. Let me know if you think that’s a good topic, please.
I would sincerely appreciate your ideas for modeling projects that we can discuss. If shop carts and welding tables aren’t inspiring, tell us what is. Perhaps we could make tool chests, vaults/safes, or go-karts. Remember, we are at work doing professional things.
But, I do have to admit that as I watched some television over the Thanksgiving holiday, machinery used to toss pumpkins more than 4,000 feet really caught my attention as a design challenge. Think about it: Precision Pumpkin Matters.
Gerald would love to have you send him your comments and questions. You are not alone, and the problems you face often are shared by others. Share the grief, and perhaps we will all share in the joy of finding answers. Please send your questions and comments to firstname.lastname@example.org.