Designing, Forming And Joining Sheet Metal, A Lets Build Resource.

I want to talk about sheet metal and making sheet metal parts.  Now for a variety of reasons the maker community has  avoided using sheet metal in their project and that’s a mistake.   I see a lot of maker designed machines that use thick plastic panels where I, as designer would have used a formed sheet metal panel.

Here’s a case in point.

http://www.solidsmack.com/fabrication/botfactory-squink-one-desktop-circuit-printer/

Thick plastic panels, screwed together and making up perhaps a half dozen parts.  I would probably make three sheet metal parts and some blocks for the rails.  The mail frame would be a welded partial box.

Sheet metal is a versatile way to provide structure and form without adding weight.  Even very thin panels can be made very rigid and can be fastened in just about any way that you can think of . This blog has a great many pictures of sheet metal parts.

http://sheetmetaluk.blogspot.com/

I think that the problem is that many makers don’t understand how simple sheet metal is to work with or just how flexible using sheet metal can make your designs.  There’s also the difficulty of finding the various kinds of sheet metal in the first place.

Still, working with sheet metal is an important tool in your maker tool box and once you learn the basic tools you will be able to fabricate enclosures and chassis for your projects quickly and fairly easily.

Here’s the press brake that John was using.

http://www.grizzly.com/products/24-Pan-and-Box-Brake/G0557

You don’t even need a bending break.  You can scribe and make the bends with a pair of sheet metal pliers.

https://www.amazon.com/IRWIN-VISE-GRIP-Original-Locking-23/dp/B0000TFGFU/ref=sr_1_2

It’s fairly easy to make boxes and bent parts for prototypes. You can do a lot of fit and design validation with parts that you cut from templates, bend, fasten and add the appropriate holes in the right places.  Here’s an instructable to get you started.

http://www.instructables.com/id/Designing-for-Sheet-Metal/

And another post on basic sheet metal work.

http://www.knucklebusterinc.com/features/2012/02/17/basic-sheet-metal-work/

Here’s me hacking together toolboxes with hand tools and pop rivets.

Here’s a very good book on basic sheet metal.

http://gingerybookstore.com/SheetMetalTech.html

 

 

Of course there can be a lot more to sheet metal, especially if you haven’t worked with it very much.  Figuring out things like how much you can bend it before it cracks can be critical for a design.  Fortunately there are many references online and in books that are readily available.  Here’s some links.

http://sheetmetal.me/formulas-and-functions/k-factor/

http://www.sheetmetalguy.com/bend-allowance.htm

http://www.egr.msu.edu/~pkwon/me478/sheetforming.pdf

http://www.micro-machine-shop.com/Sheet%20Metal%20Bending.pdf

http://machinedesign.com/fasteners/methods-fastening-sheet-metal-without-fasteners#slide-3-field_images-50861

https://www3.nd.edu/~manufact/MPEM_pdf_files/Ch07.pdf

http://www.qualitytool.com/resources/Design-Handbook-Rev3.pdf

As you head to production you have to understand how to balance manufacturing methods and tooling costs.  In general practice, the more parts you need the more expensive the tooling but the cheaper each individual part gets. the more automated  and less handwork involved the cheaper the part gets, but then you have to pay closer attention to bend radii and k factors as well as internal stresses and cracking.

http://www.thefabricator.com/article/stamping/sheet-metal-stamping-101-part-i

 

 

while you may be able make your joints without fasteners, in general that requires tooling that yo may want to avoid.

http://machinedesign.com/fasteners/methods-fastening-sheet-metal-without-fasteners#slide-3-field_images-50861

most of the time, for production you are better of welding the assembly, either spot welding, which with a little practice you can do yourself some form of inert gas welding.

Inert gas welding can be tricky, especially with thin pats, but I’ve had welded parts down to .o20 in or so, perhaps even thinner.  Dealing with the thin welds requires great skill from the welder.  Still for most projects you will be using metal thicknesses of over .025 and weld burnthrough should not be a problem.

http://www.efunda.com/processes/metal_processing/welding_inertgas.cfm

If the sheets in your project are too thin for welding you can rivet. For that matter when you are building your initial prototypes, rivets are a good choice.  For one thing blind rivets are readily available at hardware and building supply stores and the tools for installation are drills for holes and an inexpensive rivet gun, though if you are installing a lot of them, you might want to buy a powered installer.  The advantage of using blind rivets is that if you need to take things apart the rivets can be easily drilled out. This can be a big help when you are prototyping.  I’ve included some links to the various vendors for blind rivets. They are all essentially the same and I would just pick them up from the hardware store or order boxes from McMaster Carr.

http://www.pemnet.com/fastening_products/pem-self-clinching-fastener_new.html

http://www.stanleyengineeredfastening.com/brands/pop

http://www.stanleyengineeredfastening.com/brands/pop/rivets/types

http://www.hansonrivet.com/avdel-speed-fastening-system.htm

https://www.arrowfastener.com/fastener-category/rivet/

https://www.arrowfastener.com/tool-category/rivet/rivet-tools/

If you need to screw parts into your sheet metal you will probably want some sort of nut attached

 

An overview of how inserts work.

For fastening and assembling stampings and sheet metal fabrications together or to other components, incorporating nut and stud inserts can provide wide design flexibility. Commonly referred to as “clinch” nuts and studs (Figure 1) or “self-clinching” nuts and studs, these versatile fasteners can be an effective choice, particularly when a large number is required and automatic equipment is used for installation. All of these fasteners can be installed economically–except concealed head studs, because they require machining operations at very high cost.


Figure 1. Examples of “clinch” nuts and studs.

Essentially, inserts are special nut or stud fasteners that are designed to be pressed into prepared holes in sheet metal parts. Inserts provide captive male or female threads in materials too thin to tap, when higher strength fastening is required, or when repeated access after assembly is anticipated. Economically, they are routinely less expensive than the conventional nut and bolt alternative.

A great number of different inserts are available in various thread sizes, lengths, classes of fit, materials and finishes to fit virtually all design requirements. When quantities justify, custom-made inserts may prove economically advantageous over standard types.

An important initial consideration in insert selection is access to the prepared hole. If both sides of the sheet are accessible, a wider range of manufacturers’ products is applicable. How-ever, when access to only one side of the sheet is available because of other design features or bends already formed, rivet nut inserts (Figure 2) should be considered since they can be installed (usually with special tools) like blind rivets. Alternatively, lack of access to the reverse side may also be solved by inserting fasteners designed for two-side installation, prior to bending or other forming operations. When such considerations arise, metalforming suppliers can offer practical advice.


Figure 2. When insertion of hardware in a prepared hole is impeded by another feature, rivet nut inserts which are installed like blind rivets from one side of the metal should be considered. Typical type of rivet nut inserts are shown.

Insert Installation

Basically, nut or stud inserts are inserted into punched or drilled holes, then clinched or crimped into the sheet metal. Depending on the type of insert selected, it is clinched against the metal surrounding the prepared hole, typically deforming and flowing the substrate metal to lock the fastener into position. Knurled flanges and similar features are often incorporated in the insert design to aid in anchoring the fastener to the sheet.

Figure 3. Inserts can be installed by standard presses with a punch and anvil set-up.

Inserts can be installed by standard presses with a punch and anvil set-up (Figure 3) and by hydraulic and pneumatic tools. With automatic feeding capabilities, specially designed presses can install inserts rapidly.

Some types of inserts, like rivet nut inserts, undergo most of the deformation via clinching, while the workpiece undergoes minimal deformation. Here, keyed or ribbed fastener heads are sometimes used to prevent rotation of the insert in the workpiece, and to resist vibration in service.

These types of inserts are usually installed via special pneumatic or hydraulic tools. For high-volume production, fully automated systems including auto feeders and robotics can be utilized.

Cost Considerations

Generally, the more functions an insert performs and the more exotic the material and finish, the higher the cost. Beyond that, the cost of nut inserts vs. extruded-and tapped holes is always a controversial issue. Often the choice depends on the designer’s preference and experience. The economy of one system vs. the other should be discussed with the supplier.

Extruded holes are ideal in stampings when the part will be tumbled and finished, then assembled with self-tapping screws. Because extruded holes are created in a punching operation, many stamping companies advise using this option when additional holes and other features will also be punched and formed to create the final stamping. For higher production volumes this approach can be the most economical choice.

Other means of installing threads–weld nuts and studs, self-locating projection weld nuts, extruded-and-tapped holes–should also be evaluated on a cost/performance basis. Metal-forming companies can advise which alternative is most suited for a particular design.

Figure 4. Example of nut inserts installed flush with the sheet.

Types of Inserts

To facilitate a particular part design, inserts can be installed flush (Figure 4) into one side of the sheet, or nonflush, with the head protruding. Flush installations usually require a special head on the insert and/or a countersunk or counterbored hole.

As depicted in Figure 5, additional fastening functions are achieved by nut inserts with self-locking features (accomplished by interrupted threads, coatings, and special nut designs, etc.). “Floating” fasteners provide for mismatch (e.g., ±0.015 in. (±0.38 mm)) between mating fasteners or holes. Blind-end types form a seal against liquids and foreign contaminants; and, special spring-loaded panel fasteners can be flush-mounted as a single unit.

Depending on the type selected (and the manufacturer), inserts can be used to join more than two components together. For example, rivet nut inserts can join or “rivet” two sheets and also provide threads to mount a third component (Figure 6).

Figure 6. Here a rivet nut insert is used to join or “rivet” two sheets and also provide threads to mount a third component, an angle bracket.

Key Design Parameters

Once the insert type is chosen, based on functional and aesthetic requirements (such as flush mount, self-locking nut or concealed head stud), other important factors remain to be addressed. Among them: strength, workpiece hardness and thickness, material compatibility, finish, distortion, clearance and tolerances.

Figure 7. Strength of inserts is measured by push-out and torque-out values.

Retention strength of inserts, as measured by push-out and torque-out values (Figure 7), is a direct result of the metal flow and interlocking that occurs during installation. Consequently, the design (and material) of the insert and the material into which it is being installed can have a significant effect on strength. As may be expected, strengths increase with larger diameter inserts and thicker sheets.

Aluminum inserts in aluminum sheet exhibit lower push-out and torque-out values than steel inserts in steel. When higher strength is needed, both carbon steel and stainless steel inserts can be used in aluminum workpieces. For applications that require optimum strength–such as when replacing weld studs–high-torque-resistant studs with heavy heads can be specified to boost pull-through values.

For rivet nut inserts, tensile strength, thread strength and shear strength, as well as torque-out values can be used in determining what insert type will resist design stresses. Highest thread strength is provided by stainless steel inserts, followed by steel, then aluminum. However, most steel self-clinching fasteners are heat treated and will then prove the strongest threads.

 

http://www.makeitmetal.com/resources/CH15_fast.htm

http://www.designworldonline.com/fasteners-hold-a-firm-fix-in-thin-sheet-metal/

http://www.pemnet.com/fastening_products/pem-self-clinching-fastener_new.html

The Pem nut has been more or less the default choice for installed nuts for sheet metal. These nuts do have one issue. you need a press to compress the nuts and deform the nut and sheet so that the nut clamps properly.  And you need to make sure that you know where the nuts are going to end up after bending because the nuts need to be inserted  before bending on things like flanges.  For production and final prototypes this isn’t an issue, but for hacks it just might be. Fortunately somebody else thought of that too and there are bunch of alternatives which I’ve linked to below.

 

http://www.sherex.com/index.html

http://www.sherex.com/pdf/Dejond%20TUBTARA%20Catalog.pdf

http://avkfasteners.com/products.AR.php

http://www.stanleyengineeredfastening.com/brands/avdel/products/threaded-inserts/thin-sheet-nutsert

http://www.stanleyengineeredfastening.com/brands/pop/jack-nut-threaded-inserts

http://www.nyltite.com/

http://www.chiefaircraft.com/aircraft/hardware-fasteners/tinnerman-nuts.html

Using sheet metal for your prototypes can be a cost saver over using extruded shapes and surprisingly easier to work with than plastic.  Most of the tools and materials you need are easily available locally and with a little work the design can be made solid and the path to manufacturing easier.  Because sheet metal is versatile and fairly rigid making changes is rarely a big issue and often just requires cutting and welding or riveting a new piece into place.  Also a sheet metal part can be lightweight yet still be very stiff where required just with the addition of a stiffening rib in the right place.

Well that’s it for now on sheet metal.  With the links I found there is enough to get started.  If you have any doubts, the best thing that I can suggest is to go down to the local hardware store and pick up a sheet and some rivets and make a box.  There’s nothing that replaces real hands on practice.

 

 

 

My general book list.

https://theartsmechanical.wordpress.com/2015/03/14/a-makers-and-engineering-bookshelf/

The Let’s Build Series.

https://theartsmechanical.wordpress.com/tag/lets-build/

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