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Shop technology and 3-D CAD: Making CAD magnets

Fabricating technology and tooling that are unique to the shop can be shared as CAD resources

Figure 1a
This CAD model resembles thin-turret tooling for a formup extruded hole.

Editor's Note: If you would like to download the 3-D CAD files associated with this column, click here.

In this CAD modeling scenario, our fabrication shop owns excellent tooling that is underutilized. We’d like to generate more demand for it. A specific example is shown in Figure 1a.

This tooling creates a flanged hole in sheet metal. Also known as an extruded hole, this embossed feature provides better thread engagement with a screw when compared to flat sheet metal. Because it functions like a nut but doesn’t require labor to “install,” this sheet metal feature could be a real cost benefit to our customers.

When considering how to impress designers with our shop’s capability, we could distribute the model shown in Figure 1a by posting it on the web for the public to download and admire.

Other CAD jockeys will admire the variable-pitch stripper spring in the punch head. A few might be impressed with the multibody modeling used in the die core. The pitch and thread engagement is well-modeled.

(Here is a prediction: Any downloads and demand for this tooling model will be by people who want to make tooling. That’s not our goal. Nonetheless, we want to be faithful throughout the promotion of our capability. If we could download most of this model from the web, that would be ideal. Otherwise, we’ll have to reverse-model the physical item.)

Tooling Is Not a Forming Tool

Figure 1b shows the internal components in the die set. The top stripper plate is captured with two screws. They adjust the overall height and the preloaded spring compression of the finished assembly. The screws travel up and down with the stripper plate. Sleeves provide friction management for those screws. The die springs are arranged in four pairs to create a balanced lifting force.

As the stripper plate is pushed down, the die springs compress while the screws slide in the sleeves. As the stripper plate descends, the stationary pin in the center of the die will protrude above the stripper plate.

When the pressure on the stripper plate is released, the springs lift to hide the stationary pin. Hiding the die pin except when punching the hole keeps it safe and sharp.

To illustrate the distinction between tooling and the designed product, Figure 1c shows sheet metal designed with flanged holes.

Figure 1b
The CAD model of the tooling has all of the detail, springs and screws included. The CAD model also moves to show how the stripper plates move.

Even with those illustrations and an explanation of form-up tooling, a CAD designer who is not expert in the sheet metal trade might not yet see how this tooling matters to their design.

Tool Rules

We could offer a few more illustrations to demonstrate the fabricating process. Figure 2a shows a cross-section view of a piece of sheet metal that has been positioned with a pilot hole over the die pin. We could explain to our customer that a pilot hole is required for this process, but they don’t really need to know that.

Figure 2b shows what happens as the punch is pressed down to sandwich the sheet metal against the die. The sheet metal is bent past the stationary pin in the die. The pocket in the face of the punch tip forces the sheet metal into a ring flange around the hole.

The pressure from the punch not only causes the sheet metal flange to form, it also causes the spring in the punch head—Figure 1a—to compress, bringing the punch face and the punch stripper to be in full (hard) contact with the sheet metal.

As described earlier, the die set with its crunched springs is loaded to strip the center pin from the sheet metal. Figure 2c shows the sheet metal with the newly formed flange after it has been ejected from the tooling by the springs.

It is likely that you’re reading this article because

you have tooling expertise. You haven’t really learned anything yet. But keep in mind that this story is for a job shop’s customers as well. We want something that designers can use, not just fabricators.

So far we’ve described the function of the tooling fairly well with some nice illustrations. We also have 3-D models that we can share. Will our customers be ready to start requiring the tooling for their design? Maybe, but there are rules to using this tool.

If the customer has been paying close attention to Figure 2b, they might note that the punch stripper creates a restriction on the spacing between these extruded holes. For this tooling, the distance is 0.90 inch. This particular tool also creates a 0.125-in. through-hole and works only with 16-gauge mild steel. And it forms only in one direction, so part marking and other formed features either have to cooperate or be fabricated as downstream processes.

These rules and restrictions, along with our explanation of how the tooling works, could be enough for most designers to competently adopt our extruded hole feature into their design—something perhaps like Figure 1c.

For those of our designers who use compatible CAD software, we could go an additional step and make their modeling of our extruded hole easier by distributing a Forming Tool model for inclusion in their design library.

Figure 1c
Here’s a CAD model of sheet metal with extruded flange holes.

Finding the Right Form

In Figure 3a, the example tool model is specifically for a flanged hole for a No. 4 screw thread. To make extraction from the punch easier, the pocket in the punch face is tapered. This results in the extruded flange having the same angle.

The tapered flange along with a slightly reduced outside diameter allows the stationary pin to extrude taller flanges into the sheet metal. It takes more force to accomplish this, but it does give better thread engagement. The wall thickness of the extruded flange is less than the base sheet metal thickness.

Here’s a CAD Tip: To model a thin-wall feature in a sheet metal design, use a two-step process: Model a bump and add a hole. If the flange thickness is equal to the sheet metal thickness, then the Forming Tool can make the through-hole in one step.

Forming Tools are created as an ordinary part that looks like the interior of the extruded hole. Forming Tools require a “stopping face.” The drag-and-drop insertion of the Forming Tool into the model will be onto a face. The tool aligns its stopping face with the face on the sheet metal part. The Forming Tool also can remove sheet metal.

These settings for the behavior of the Forming Tool are saved in its Feature Manager as a FormTool1 feature (see Figure 3b).

The dimensions used in this Forming Tool model roughly match a No. 4 pilot hole.

Note the following CAD tips:

  • Use the shape of the stopping face as a visual aid during drag-and-drop of the Forming Tool into a sheet metal part. In Figure 3b, we see that the keep-away zone is a circle with a radius of 0.900 in.
  • An instructional note modeled in the stopping face of the Forming Tool is visible during drag-and-drop as a reminder of the keep-away zone. Figure 3c is a screen shot of the Forming Tool being positioned with the tool’s rules visible in yellow. Thanks to tech support from MCAD of Colorado for that tip.
  • Give your Forming Tool a good file name to reveal purpose as well as the shop that can manufacture this part.

Sheet Metal Rules

CAD sheet metal isn’t exactly like real sheet metal. CAD sheet metal requires all walls to be equal thickness. Real sheet metal is malleable. Extruded flanges can be thinner than the rest of the sheet metal part.

For an example of three variations in extruded hole models, see Figure 4a. The wall thickness of the extruded flange at the left is the same as the base sheet metal’s. It also has zero radius on the inside and outside of the flange. While actual tooling can almost do this, it limits the length of the extruded flange.

It is more likely that there will be some radius in the final sheet metal feature.

Figure 2a
Step 1: The pilot hole is aligned with stationary pin in die.

We want to make it as easy as possible for our customer to design products that require our tooling. If the Forming Tool model provided to the customer can cut the through-hole too, we will have enabled the model to do it all. The leftmost extruded holes were done in one step with the CAD Forming Tool.

When the wall thickness of the flanges is less than material thickness—as with the rightmost extruded hole—we have to ask the CAD jockey to model the through-hole separately from the Forming Tool.

For designing Forming Tools, the main consideration is the CAD requirement for uniform wall thickness. The inside bend radius of sheet metal has to be a positive value. This means that the outside bend radius minimum is equal to the sheet metal thickness.

Figure 4b shows the Forming Tool used to create the middle extrude in Figure 4a. The radius is modeled at 0.064 in. When that is pressed into sheet metal that is 0.059 in. thick, the inside bend radius will be 0.005 in. (0.064 – 0.059 = 0.005). This Forming Tool works with sheet metal up to 0.064 in. thick.

If this Forming Tool is used incorrectly, it will fail in the CAD model. For example, 14-gauge steel is about 0.074 in. thick. The Forming Tool would make a negative 0.010-in. radius, but a negative radius cannot exist.

Making CAD Magnets

Caution: This article is not about engineering tooling. Rather, this is a demonstration of how a shop might document and promote existing tooling capabilities. Punching tooling manufacturers will start with the desired shape you need in sheet metal and create the tooling to produce that shape. The tooling in Figure 1 would be designed by such a firm. The Forming Tool shown in Figure 1a is simply reverse-engineered to achieve the desired result in CAD.

When communicating with our customer to promote this tooling, we offer:

  • A CAD Forming Tool design feature for drag-and-drop modeling of extruded flanges.
  • Recommendations for spacing, materials, and screws.
  • An example of a sheet metal part made with our CAD Forming Tool.
  • Illustrations and models explaining the operation of the tooling.

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 dand@thefabricator.com.