1. Using Completed 3D Models of a Part with no Bends

This is probably the most basic issue we see. Sheet metal is flat and must be bent, formed, cut, lased, and sometimes cajoled into its final shape. It’s a very hands-on process. If you design your sheet metal part as a solid object, it’s important that you send us a CAD file that appears folded, but shows where bends should go. Related to this, because the raw materials are single sheets of metal, the entire part must have the same material thickness throughout. For example, if you create a part that uses 0.125 in. (3.175mm) thick aluminum, your entire part will need to be that same thickness.

If you want to learn more, please visit our website.

2. Placing Features too Close to Bend Lines

A quick way to create difficulties during manufacturing is to place holes, tabs, or other features too close to a bend. So, how close can you get? Just follow the 4T rule. Keep all features at least 4x material thickness away from bend lines. So, if your design tells us to use 0.050 in. (1.27mm) copper, give your feature at least 0.200 in. (5.08mm) of clearance. If you don’t, the part will deform awkwardly in the press brake, and no one wants that.

3. Designing Perfectly Perpendicular Sheet Metal Corners

When you bend sheet metal in a press brake, the resulting bend doesn’t form a perfect 90-degree angle. Instead, the tool has a rounded tip that adds a radius to the bend. If you measure the length of that bent area and divide it by two, you’ll get the bend radius, a figure that is defined by the tool that made it. If the size of that curve is important to you, make sure you specify it in your model.

The most common internal bend radius (and our default) is 0.030 in. (0.762mm). An important consideration to remember is that the external bend radius—the one formed on the die side of the press brake toolset—is equal to the material thickness plus the internal bend radius.

Some designers like to get fancy and create different radii for each bend in a part. Want to save some money? Use the same radius for all of the bends. When your manufacturer doesn’t have to change tooling that saves you on labor costs.

4. Forgetting to Include Detailed Hardware Specs in your CAD File

Do you know of anyone who wants longer lead times? Probably not. Always remember to let your manufacturer know what kind of hardware you want to use by including the details in your top level assembly information. Whether it’s a self-clinching nut like CLS-440-2, flush-head stud like FHS-M5-15, or other hardware, this guarantees that you’ll get exactly what you want placed in the location you expect it. To save yourself time and trouble, download and use the PEM clinch hardware models.

5. Picking the Wrong Finish (or Not Using One When You Need One)

Generally, finishes serve two purposes. They can protect your part or they can make it look better. Some do both. Aesthetic finishes—ones that focus on looks—don’t focus on corrosion protection. Still, powder coating does offer some protection (unless a scratch reaches to the metal beneath). Silk screening, on the other hand, is used to add text and images to parts and offers no protection at all.

Chemical conversion finishes are meant to protect your parts by altering the properties of the outermost layers. For example, if you want to use steel in a corrosive environment, consider choosing galvanized or galvannealed metal, which already has a protective zinc coating. Watch out, though! We can’t weld galvannealed steel due to the dangerous toxins it would give off. Instead, we can make the parts out of steel and add a zinc coating after welding.

Chromate conversion can give your part electrical connectivity and provides a primer layer to your part if you want to paint. Anodizing can add a pop of metallic color to your parts while also protecting them. Think of that colorful, small flashlight you own.

Here’s a helpful summary of the most common finishes for sheet metal.

6. Selecting the Wrong Sheet Metal for the Job

Imagine making hundreds of parts out of unfinished steel that are destined for installation in a salty, marine environment. Amazingly, we’ve seen quotes that request just that. Save yourself the grief of the customer complaints you’ll get when your parts corrode and consider these factors when selecting the right sheet metal:

• Expected wear on your part from daily use
• Corrosion protection
• Manufacturability
• Cosmetic appearance
• Mechanical properties (tensile strength, yield strength, ductility, etc.)
• Conductivity (for electrical applications)

This handy chart can help.

7. Failing to Consider U-Channel Strength

When designing U channels, always think about the strength of the material you’re using and how easily it can bend. At Protolabs, narrower doesn’t work for us because of our tooling. If we had narrower tooling, we could manufacture narrower U channels. The bottom line is that it is best practice when working with us to maintain at least a 2:1 width-to-height ratio for your U channels. If you need a narrower channel, consider a welded or riveted assembly.

8. Designing Unrealistic Weld Requirements

Have you ever tried welding a seam inside a closed box? Nope? Neither have we. Make sure that your weld requirements are realistically achievable. It’s important to remember that if a welding torch can’t access a seam, the weld can’t happen. We strongly recommend designing so all welds are done on the outside of the part.

Metal melts under high temperature, so you need material thicknesses that can manage the extreme heat. The minimal material thickness for welding is 0.040 in. (1.016mm) to ensure that the weld doesn’t end up an unholy mess of melted metal.

Finally, in your models, always indicate the need for welds using a welding function or nomenclature. Never box corners to indicate welding.

No single design tip can cover all the sheet metal mistakes we’ve seen, but this “best of” collection is a start. Take a look at our sheet metal design guidelines to keep you moving in the right direction.   

For additional help, feel free to contact a Protolabs applications engineer at 877-479- or [ protected]. To get your next design project started today, simply upload a 3D CAD model for an interactive quote within hours.

Sheet metal parts often require multiple manufacturing processes to produce correctly. Because of this added complexity sheet metal drawings can be particularly tricky to create. This article will focus on how to prepare accurate and easy to interpret sheet metal drawings so that your parts come out in spec every time. As a bonus, the best practices included in this article can help you establish a better working relationship with your manufacturers and reduce extra workload associated with translating an imperfect drawing into a fabricated component.

Start by understanding design for manufacturability (DFM)

In this section we will be considering four foundational DFM considerations in order to create great sheet metal drawings.

1. Manufacturing process mapping
2. Flat pattern process considerations
3. Bending specific design considerations
4. Default tolerances

Manufacturing process mapping

Sheet metal parts require a sequence of manufacturing processes to transition from raw stock material to finished part. The first step in design for manufacturability is to consider this sequence of manufacturing steps and the design constraints associated with each process. Consider a low volume computer enclosure component. If the intended flat pattern is waterjet cut and then bent using a CNC press brake, what does that imply in terms of edge to bend accuracy? How will that stack up across all bends in the part? If the part is powder coated what hole diameter ranges will be acceptable for final assembly? These considerations can only take place after process steps have been clearly thought through.

Flat pattern process considerations

The first step in any sheet metal component is to transform sheet stock into a part profile suitable for bending. A variety of different processes are suitable for this process, and you’ll want to be sure you consider the DFM implications of each. Remember that bend stops and backgauge fixtures indicate off of the part profile, so this process can have impacts on bend accuracy that stack up significantly.

Consider also that certain processes favor specific cut geometries which can have large price implications. For example, cnc turret punch press machines have standard tooling that make cutting certain geometries very inexpensive. More custom shapes may warrant extra processing or the development of custom tools to manufacture at volume. On the other hand, laser cutting can cut any 2D pattern as long as laser kerf width is considered appropriately. The bottom line is you should talk to your manufacturers about their capability early in your design process in order to save time and money.

Bending specific design considerations

Here are a few of the key design considerations for formed and bent components. Note this is written primarily for manual and cnc press brake bent part designs formed from a pre-cut flat pattern.

• Think through the bend features and bending order implied by your design and determine roughly if it is feasible with standard tooling.

  US Metal Spinning supply professional and honest service.

• Consider the thickness of your material and crosscheck against allowable bend radii for each feature. This article from The Fabricator touches on material bend radius concerns and is definitely worth a read.

• Understand how grain direction must be aligned relative to bends (you will need to note this on your drawing). This is particularly important for stainless steel and some aluminum alloys with large material grain size. Large grain structure and the sheet rolling process can give the material anisotropic strength properties which can cause bends across short grains to fracture as compared to bends across long grains.

• Bend allowances should not be an afterthought. All bend processes introduce material deformations that must be compensated for in flat pattern construction. The wikipedia page on bending gives a great primer on this topic. As a designer, you should understand that the K-factor for a particular bend-material combination is a roll-up of unknown error sources. This makes it difficult to predict initially. Often, manufacturers iterate bend parameters and flat patterns until each bend falls in spec in terms of dimensional accuracy and spring back. We will touch more on this in the File preparation and manufacturer collaboration section of this article, but just know that sharing correctly formatted 2D drawing and 3D file output can help facilitate this iterative process, resulting in potentially faster and better parts.

Default tolerances for sheet metal parts

Flat pattern accuracy

As we saw in our process map, sheet metal parts leverage different processes for different part features. Remember that this can set up tolerance dependencies between features. For example, a simple 90 degree bracket with holes in each face will have hole to bend tolerances dictated by the part profile tolerances.

Bend angles

Many factors impact the accuracy of sheet metal bends, but perhaps the largest impact is material variability. To explore this lets look at the sheet thickness variation of 12 gage steel. The minimum thickness is 0. inches and the maximum thickness is 0. inches. This thickness variation will impact the springback of the part after bending. By computing springback as a function of sheet thickness, you can see the large impact of material thickness variability on bending tolerances.

Be sure to consider both material thickness and thickness variability when selecting bend tolerances.

How to make a complete sheet metal drawing

Just like any other part drawing, there are some standard items you will need to include on your sheet metal drawings in order to generate easy to interpret 2D drawings that capture your design intent. We won’t focus on the boilerplate items like title blocks, company information, revision tables etc… If you want a great primer on the basics of engineering drawings check out our Drawings 101 article and the free Five Flute drawing review checklist. The rest of this section will focus on sheet metal specific drawing considerations and best practices.

Sheet metal specific drawing items

Flat pattern views

All drawings need orthographic views to represent 3D geometry generally. In addition to these views, it can be very helpful to include a 2D flat pattern drawing with reference dimensions. This can help your manufacturer consider how the parts will be laid out and nested on sheet stock and how many profiles they can cut per square foot of sheet. Just like providing part surface area to anodizing vendors, the 2D flat pattern can speed up quote turnaround time.

But be careful because it can also cause some confusion. The exact 2D flat pattern geometry that is necessary to create an accurate formed part may differ significantly from your CAD output. Different material stretch behavior (K-factors, bend allowances & spring back), as well as equipment and forming techniques can impact the relationship between 2D and 3D forms. Manufacturers should know that your flat pattern (at best) should be used for reference geometry, and likely may not result in an accurate final part.

Complete, realistic, and properly formatted dimensioning

A fully dimensioned sheet metal drawing includes dimensions for all bends, holes, countersinks, flanges, and other formed features (such as hems and curls, ribs, dimples, etc…). It is a best practice to dimension to virtual intersection points and show included bend angles. This ensures that your drawing is universally interpret-able (with no extra math) regardless of the actual bend radius as formed.

Don’t go crazy with GD&T

We write a lot about GD&T, and sheet metal is an area where I see people go wrong consistently. Before applying GD&T to sheet metal drawings, remember that sheet metal components are relatively compliant. This means they conform to the components they are assembled with. If you are specifying tight tolerances of form (straightness and flatness) or tolerances of orientation (parallelism, perpendicularity), first ask yourself, is this even necessary? In the assembled condition, will your tolerances make a meaningful difference to the as build geometry and functionality?

This doesn’t mean you should eschew GD&T entirely however! You can use tolerances of location, like true position, and material condition modifiers (applying position at maximum material condition for example) to develop cost effective flat pattern designs that maintain design intent in the bent configuration.

Include common sheet metal specific information

Don’t forget to add these items to your sheet metal drawings.

• Sheet stock size and preferred suppliers. Stock size thickness tolerance can vary with manufacturer!
• Hardware installation details. If you or your manufacturer are installing additional hardware (PEM nuts, rivet nuts, etc…) make sure you include this on the drawing!
• Finishing information. Finishing information is a must on every drawing, but especially sheet metal parts. Manufacturers often have powder coating booths on site, so be sure to include your powder coat spec on the drawing as well. Note, the less specific you are on part finishes, the less likely you are to receive a high quality finish. It’s incumbent on you to be thorough as far as cosmetics are concerned. This means including things like surface preparation, primers, paint or powder types according to manufacturer designation numbers or other specifications like FS 595C, number of coats, and desired thickness after paint/powdercoat.

File Preparation and Manufacturer Collaboration

Because sheet metal components require multiple manufacturing processes, proper file preparation can speed up both the quoting and production processes. The first step is to speak with your manufacturers and learn what file formats they prefer for each process. This can reduce file conversion workload, which is often a source of mistakes (anyone who has received a 1:2 scaled down set of flat patterns will shudder when they read this).

As a general best practice, you should include a fully dimensioned 2D PDF drawing and a reference 3D file type (such as STEP). You may also elect to include a DXF file with the flat pattern only. It can speed up production if you remove all annotations from this view and only include an easily select-able part profile for CAM programming and quoting. The drawing should have a note that explicitly references this file (for revision control purposes). If ordering via a purchase order, you should make note of the filenames on the PO, and update the PO to include proper revision references. You don’t want to be in a position where the manufacturer fulfills the purchase order correctly by ignoring (or missing) the latest revision changes. PDF drawings, immutable CAD/CAM files, and purchase orders need to always stay in sync!

Example Drawings and Files

For the sake of completeness and clarity, we’ve included downloadable example files below that represent a typical set of complete manufacturing output files.

PDF B - size drawing

DXF file with boarders, dimensions, and all annotations removed

STEP file of 3d geometry

Create your own drawing checklist

It’s a good practice to build your own manufacturing process specific checklists for your part designs and engineering drawings. Make sure to include the items covered in this article, plus any specifications or design patterns that are common to your designs. If you want a starting point for this checklist in pdf, google sheets, or notion template forms you can check out our free drawing review checklist.

You need a review process

Despite the best intentions of design engineers and drafters designing sheet metal parts, items will be overlooked. In most cases, it helps to run a simple drawing review process or peer check for anything with a modicum of geometric and cosmetic complexity. If you want to put in place a review process that eliminates mistakes, shortens development cycles, and guarantees you’ll design and build better hardware, check out our Ultimate Guide to Drawing Reviews. You’ll learn how to institute a frictionless drawing review process no matter what stage of product development you are working through.

Five Flute - Next generation collaboration for hardware product development

If you are a design engineer or technical project manager and you want to design better products in less time, consider Five Flute. It’s the fastest way to share, review, and improve your engineering designs. From engineering drawing reviews to 3D design reviews of complex parts and assemblies, Five Flute is built for modern engineering teams that want to move faster without making mistakes.

You might also be interested in

Five Flute drawing review checklist

Drawings 101 - The basics of creating high quality engineering drawings

If you are looking for more details, kindly visit Custom Metal Shapes.