Stamped washers, shims and spacers: flatness, thickness and burr direction

Three parts that look alike and do different jobs

A washer, a shim and a spacer are all flat metal parts with a hole. They are often quoted together, made on the same press and packed in the same box. Their functions are not the same, and treating them as one commodity is how a programme ends up with a joint that loosens or a bearing that preloads incorrectly.

A washer spreads a fastener load over a larger area and protects the surface underneath. A shim fills a gap to set a height, a clearance or a preload. A spacer holds two bodies a fixed distance apart and may also carry a compressive load. Each one has a different critical dimension, and that dimension should drive the drawing, the tolerance and the inspection method.

Thickness tolerance and flatness

Stamped parts are made from sheet or strip with its own thickness tolerance. The stamping process does not normally improve that tolerance unless the part is coined, ground or lapped. If a shim has to set a gap to 0.02 mm, a standard cold rolled sheet with a wider thickness range will not do it, no matter how well the outline is cut.

This is the single most common misunderstanding in shim procurement. The drawing shows a nominal thickness, the supplier quotes a standard gauge, and the first article arrives at the edge of the sheet tolerance. For a shim, the thickness is the function. Specify the thickness with the tolerance that the gap actually needs, and state whether it is measured at the centre, at the edge or as an average.

Flatness is the second call. A washer that is dished will contact a joint face at one edge and leave a gap at the other. Under a bolt, it can flatten, but the load distribution is uneven and the joint may relax. A shim that is bowed changes the gap it was meant to set. A spacer that is not flat rocks between the two bodies it separates.

For thin parts, flatness and thickness interact. A part can be within thickness tolerance at every point and still be bowed. Specify flatness over the full face, or over the annulus between the hole and the outside diameter, and say which. Free state and restrained state also matter, because a thin washer can be flattened by the fixture during measurement.

Burr direction and edge quality

Every sheared edge has a burr on one side. In a simple blanking tool, the burr forms on the side away from the punch, so the direction depends on how the strip enters the tool. In a progressive die, the burr direction can change between stations. The drawing should say which face must be burr free, not just that the part must be deburred.

Burr direction matters in three places. On a washer under a bolt head, a burr against the clamped surface can dig in and relax the joint. On a shim in a precision stack, a burr adds a local high point and changes the gap. On a spacer between two machined faces, a burr prevents full contact and can mark the mating surface.

The usual solution is to specify the burr height and the face that must be clean, then agree how the part is oriented in the press or how it is tumbled. Tumbling rounds the edge and reduces the burr, but it also removes material from the edge and can change the thickness at the rim. It does not flatten a dished part. For a critical shim, a coined or ground edge may be needed.

Material and finish

Low carbon steel is the default for general washers and spacers. It is cheap, strong enough for most clamped joints and takes a zinc or zinc flake finish. Stainless steel is used where corrosion matters or where the part is exposed, and it avoids the plating thickness problem at the bore.

Brass and copper are used where conductivity or a non magnetic part is needed. Aluminium is used for weight, but it galls and it is soft under a rotating fastener, so a washer in aluminium should be sized generously. Spring steel is used for spring washers, where the part has to deflect and recover rather than simply spread load.

Plating adds thickness. On a thin shim, a 10 micrometre coating on each face is a measurable change, and it can close a hole or change the fit on a shaft. If the thickness is critical, specify the finish and the thickness after plating, or choose a material that does not need a coating. For stainless parts, passivation is the usual finish and it does not add a meaningful thickness.

The calls that separate the three parts

The table sets out the design calls for each part type. It assumes a stamped part in the 0.2 mm to 4 mm range and a drawing that already defines the material.

Sorting, tumbling and packing

Small flat parts are hard to handle. They nest, stick together and bridge in a feeder. A stack of thin shims can appear to be one part, which is a serious problem if the assembly counts them. The supplier needs to know whether the parts are supplied loose, in a tube, on a tape or in a counted pack.

Tumbling is common for deburring, but it is not a precision operation. It rounds edges and can bend thin parts. If the part has a tight flatness call, tumbling may make it worse. A better route is to control the burr in the tool, then use a light tumble only if the edge needs it.

Packaging should protect the faces. A shim with a critical flatness call should not be packed in a way that bows it. A washer with a plated finish should not rub against another part in transit. Specify the pack quantity and the protection, especially for parts that are too small for the assembly operator to count reliably by eye.

Inspection that matches the function

For a washer, measure the outside diameter, the hole, the thickness and the burr. Check the flatness if the washer sits under a bolt head that has to stay tight. For a shim, measure the thickness at several points across the face and check flatness in the free state and in the restrained state. For a spacer, measure the height at several points and check that the two faces are parallel.

The measurement method matters as much as the tolerance. A micrometer on a thin, flexible shim can flatten it and hide a bow. An optical or a supported measurement may be more appropriate. Agree the method with the supplier before the first article, because a disagreement about method is a disagreement about whether the part is good.

Sampling should reflect the process. A stamping tool wears, and the burr grows as the edge wears. Thickness variation follows the incoming strip. A first article, a mid run check and a last off check give a better picture than a single sample at the start.

What to send for a quote

Send the drawing with the function stated. A supplier who knows the part is a shim will quote a different process from one who assumes it is a washer. Include the material, the thickness and its tolerance, the flatness call, the burr requirement, the finish and the annual volume.

If the part is part of a stack, send the stack drawing. The shim tolerance only makes sense in the context of the parts around it. If the part is supplied in a kit, say so, because counting, packing and labelling become part of the scope.

• Function: washer, shim, spacer or a combination
• Material, thickness and thickness tolerance
• Flatness call and the face that matters
• Burr height and permitted direction
• Finish and any plating thickness limit
• Annual volume and pack format