runs right at the limit of precision flat sheet metal in a job shop environment. Precision brakes typically repeat within ☐.004 in.Īs a general recommendation, ☐.005 in. Flat parts coming off of lasers or nibblers are usually consistent within ☐.004 in. Don’t Tolerate Crazy TolerancesĪs good design practice, set the tolerances in the finished/folded design using your understanding of where variation is likely to occur as the workpiece goes through various manufacturing stages.Įach piece of equipment used-to cut or to bend, as examples-contributes variation in the workpiece. Applying the same bend deduction/allowance as the fab shop is good practice but is not required for good design. In the majority of projects, it is sufficient for the CAD jockey to simply verify that the design will unfold. This example part has interior tabs, overlapping corners, and bend relief cuts. Here’s another CAD tip: In the unusual situation where the accuracy of the flat layout is critical to the design and function of the product, the CAD jockey must be well-informed regarding the specific fabrication process. Variations in the material thickness and speed of the machinery change how the workpiece will react with tooling.
The tooling used in the brake has a large impact on the way the material stretches as it is bent. On the other hand, accurate flat layouts are more challenging to produce. On one hand, flat layouts are usually a button-click in 3-D CAD-very easy.
An example nest of flat parts is shown in Figure 1c. Material planning allows for the prediction of the economic order quantity (EOQ) as described in Part II of this series. Smaller kerf widths, as small as the cutting orifice, are practical for laser- cut or waterjet-cut parts.Īn accurate flat layout helps with material planning and cost estimating. Here’s a CAD tip: The default kerf width is equal to the material thickness. The designed width of the kerf cut is likely to dictate whether the part can be stamped, laser-cut, or punched/nibbled from the sheet stock. At the same time, designers should understand that fabricators will adjust the flat layout to satisfy immediate circumstances, such as available tooling, machinery, and material.įlat layout planning on the part of the designer can lead to better design of kerf cuts when planning interior tabs and flanges. Designers of sheet metal parts should care greatly about their flat layouts. Sheet metal has memory and must be over-bent to achieve the desired-unrestrained-bend angle.Ī flat layout, shown in Figure 1a, is the prediction of what the finished part, shown in Figure 1b, looks like before it is bent.It often is adjusted to compensate for variations in the flat blank. Unless dedicated tooling is used, the inside radius is likely to vary from batch to batch or from shop to shop.
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It can crack instead of stretch depending upon several variables, including the tooling applied and the direction of the micrograin of the material. Regardless of the machinery used, sheet metal has characteristics during bending that will be evident in the final product: The story can be found on at Here is a CAD tip: Check with the fab shop to verify that machinery is available to produce the intended design. (“Bending up and down, no flipping required,” The FABRICATOR, July 2008, has additional information on folding technology. Folders are more common in the architectural, ornamental, and ducting trades.
To summarize this month’s recommendation: Understand how the sheet metal gets bent. Punching requires a kerf width that is equal to the material thickness.Ī reader recently asked for guidance in using 3-D CAD for designing sheet metal parts. Kerf is the air gap left behind by the cutting tool. The flat pattern requires prediction of how the material will behave as it is bent.