Shims—thin, often tapered pieces of metal or composite—come in handy when you have to level a machine tool or adjust components to fit together properly. They often perform these mechanical modifications more quickly and cheaply than grinding and machining. And shims are also useful for suppressing equipment noise and vibration. They function in much the same way as the folded pieces of paper wedged underneath table legs to stop wobbliness; only they're more versatile.

It's advisable to design shims into assemblies and devices for three basic reasons. First of all, they compensate for tolerances, removing the time and cost of setting precision tolerances on mating components. They also offset accumulated tolerances during assembly. Second, shims precisely align parallel and angular surfaces when interfacing elements must be joined. And third, shims compensate for wear, often acting as the sacrificial part to protect the basic equipment.

Laminated shims, in particular, bring a number of advantages, including reduced assembly time, dimensional accuracy, and speedy production line adjustments and field repairs. More versatile and cost-effective than machined wedges, laminated shims are composed of peelable layers of metal or composite, which can easily be removed until the shim has the appropriate thickness. Laminations can be peeled off with a knife, and in some cases, adjusting the shim requires no tool at all. Finished shims vary in thickness from 0.006 to 0.250 in.

Laminated shims are made up of layers of precision-gauge metal foils or composite films, which are surface-bonded onto rigid, solid sheets or plates. A resin adhesive is used to bond the foils, which can be aluminum, stainless steel, carbon steel, brass, etc., to the sheets. The bond is formed through heat and pressure, which sets the resin and reduces it until it's barely visible.

At temperatures exceeding about 300°F (150°C), however, the bonding agent may weaken, and the total thickness will decrease by a negligible amount. The laminated shim's performance, fortunately, is not compromised even if the resin is heated to above 446°F. Furthermore, these shims can tolerate conventional handling, including shearing and machining.

Before selecting a shim material, designers should be aware of what forces will be applied on the laminated shim. For example, they must take note that friction-causing motions could delaminate shim faces unless the shim has a PTFE treatment. Designers don't have to worry about this issue, however, if parts have location holes, in which case the only forces will be those applied by the tightening of screws. In such instances, the use of laminated shims is not constrained by any limitations.

Designers should also exercise care in choosing the laminate material, which can be aluminum, Intercomposite (a proprietary Spirol material made of glycol polyethylene terephthalate film), brass, stainless steel or carbon steel, among others. For applications with temperatures below 300°F and in which no pressure will be exerted on the shim, aluminum and Intercomposite are suitable laminates. Meanwhile, for applications in the same temperature range but with pressure, any of the aforementioned five materials will work. Finally, for applications with temperatures exceeding 300°F (with or without pressure), brass, stainless steel or carbon steel can be used.

Users should machine—rather than stamp—laminated shims. Machining produces clean edges, which facilitate the peeling of the shim. Moreover, it keeps burrs from forming when you remove the layers. Burrs are often the result of die rolls and stamping. And deburring laminated shims may make them hard to peel. Additionally, users should avoid rounding the edges of laminated shims.

Users should keep in mind that laminated shims have certain size restrictions. As a general guideline, wall thicknesses should not be less than three times the total material thickness. Furthermore, brass-laminated shims with diameters exceeding 12 in. and laminated shims with diameters over 20 in. should be constructed in sections.

One type of laminated shim is the semisolid shim, which is partially solid and partially laminated. This kind of shim can be either half-solid or three-quarter-solid, based on the ratio of solid section to total thickness. Regular thicknesses for the solid part are 0.062, 0.094 and 0.125 in. Semisolid shims are utilized for several reasons: to reduce costs, to bring rigidity to a design, to adapt to a bearing surface on one side of the shim, and to satisfy specifications for a thick shim which will need few adjustments.

In short, when properly specified, shims represent a quick and economical way to make mechanical adjustments—from leveling a machine tool to improving the way components fit together. And laminated shims are especially helpful, easily changing to the desired thickness and adding layers of versatility.

Source: Shims? Make Mine Laminated!
Russ Radant
Machine Design, June 5, 2003
http://www.machinedesign.com