Designing Sheet Metal Assemblies that Stand Up to Manufacturing Tolerances

Christian Bourgeois . March 2, 2019

Sheet metal assemblies are frequently seen when producing large scale products as it’s a very cost effective way to create big structures, as the processing equipment is well established and there is no need to create large expensive tools that would be needed making a plastic part. However because parts are made from an assembly of parts rather than formed together there are certain tolerance concerns that need to be considered that could otherwise be avoided using more of a one-shot process such as hard injection, RIM, or Structural foam.

To understand why this is, one needs to better understand the process in which sheet metal parts are made; there is a diverse range of processing technology from drawing, to shearing, stamping and bending. However for designers working on large parts with relatively low quantities (<10k) most sheet metal parts will be stamped or cut using CNC presses or lasers then formed on semi-automated brakes. This is because the tooling investment for large stamping dies and forming tools would never be amortized in relatively low quantities.

However because these machines process bends one step at a time the tolerance considerations are not the same as parts that have all features formed at once (RIM/Structural Foam, Progressive Die formed parts). As such the way parts are toleranced and how that tolerance is accounted for is very important; the more bends or features you add, the worse the tolerance between them will be. As a design consultancy we’ve seen multiple cases of parts that were designed to normal manufacturing tolerances but with too many bends stacked up so the parts ended up severely bowed or not fitting at all.

Figure 1: Bowed sheet metal parts caused by the stackup of 4 bend tolerances (Photo by Selmach)

When designing sheet metal parts the most important thing to remember is that the initial form is usually cut out of a flat sheet in one setup, this means that features that remain in one plane will have the highest tolerance as bends won’t have been introduced. As such when designing parts that need to key together it’s a good idea to use features that all exist in a single plane.

For instance when creating the vertical divisions for something like a drive rack it may be tempting to try to position the dividers using the same bolts that will secure them in place. However there will be at least two bends in between the two bolt holes; therefore the tolerance between them will be fairly loose leading to canted dividers.  Instead, tabs in the vertical dividers could fit into slots in the top and bottom, this would provide a tighter tolerance because feature that provide alignment were all formed as a flat sheet. To fasten the divider a flange with oversized holes could be formed, the over sized holes would account for the tolerance caused by the bent flanges.

Figure 2: Theoretical Drive Rack

This works better than folding down tabs from the horizontal members as each fold will have a tolerance relative to each other, which is going to be greater than the tolerance from slot to slot. The shoulder of the tabs should always be designed to be above the flange so that the more accurate feature is what provides alignment.

Figure 3: The shoulders are used to provide the vertical height of a divider as they will be more accurate than the folded tab

This same logic applies to locating two planar pieces together, if you can use all features that are formed flat then you will get better alignment. Items like Cleco clips (temporary) or rivets (permanent) are very effective at using punched holes for locating sheets relative to each other.

Figure 4: “Cleco” Fastener used for temporary alignment of panels prior to welding or riveting

The mechanically inclined may notice a glaring issue with the proposed tab solution however; the assembly order is completely wrong. All the dividers would have to assembled to upper and lower plates before attaching the side plates, and to remove any one of them you have to remove the side plates and the top plate. This makes for a unit that is terrible to assemble and service if needed. But as we’ve discussed simply using holes does not provide enough alignment due to the bend tolerance, so what should we do? In cases like these half-shears or slits can be very effective, by creating a protrusion in the top and bottom plates  you can get tight alignment as the shears are made when the sheets are flat so you don’t lose any tolerance to bending.

Figure 5: Slit used for alignment, during assembly the divider would be pressed up against the shear prior to fastening

These tolerance constraints apply not only when attaching two sheet metal parts together, but also when attaching sheet metal to other components such as weldments, or plastic/structural foam skins. Therefore when possible you should have any high tolerance alignment occur within one plane of sheet metal, strength can come from bends and flanges with slotted/oversized holes so they don’t over constrain the assembly.

By keeping the constraints of low/medium volume sheet metal manufacturing in mind you will be well on your way to designing parts that fit together regardless of where parts fall in the tolerance range. By working closely with vendors you can understand the manufacturing realities and design accordingly.