sales@bonporaparts.com
0086-10-64988621
English
A washer is a flat part with a hole, and it is easy to assume that the process behind it does not matter. In practice almost every washer complaint that reaches a production line, a burr that stops a joint seating, a dish that distorts the stack, a coating that will not fit over a stud, comes from a process detail that was never specified. This guide follows the manufacturing route for metal and non metal washers, explains which process step controls which characteristic, and sets out the inspection plan and defect controls we apply to washer and spacer production.
The route depends on the material and the quantity, but the sequence below covers most flat washers and spacers produced from sheet or strip.
| Step | Process | Characteristic controlled |
|---|---|---|
| 1 | Incoming strip or sheet | Grade, thickness, flatness, hardness |
| 2 | Blanking and piercing | Inner and outer diameter, concentricity |
| 3 | Deburring or tumbling | Burr height, edge condition, surface finish |
| 4 | Levelling or flattening | Flatness and thickness consistency |
| 5 | Heat treatment | Hardness, spring properties where required |
| 6 | Surface preparation and plating | Corrosion resistance, coating thickness, appearance |
| 7 | Inspection and sorting | Dimensions, flatness, coating, function |
| 8 | Packing and identification | Batch traceability and protection in transit |
Low carbon steel strip is the standard input for flat washers and spacers because it is cheap, predictable to stamp, and available in consistent thickness. Where corrosion resistance is required without plating, stainless steel strip is used. Copper and aluminium strip are used for sealing, electrical, and weight driven applications. Non metallic washers are produced from PTFE and nylon sheet, or moulded where the geometry justifies it.
The incoming check covers the mill or supplier certificate, the thickness measured at several points across the coil, the flatness of the strip, and, where the part is heat treated afterwards, the hardenability of the grade. Thickness variation in the strip becomes thickness variation in the finished washer, so a strip that is at the edge of its band will produce parts that are at the edge of the washer tolerance. This is worth checking at goods in, because the defect appears later and looks like a stamping problem.
For volume production, a progressive die carries the strip through a sequence of stations that pierce the hole, blank the outer profile, and eject the finished part. The advantages are speed, dimensional consistency, and a low piece cost once the tool is made. For low volume, prototype, or large diameter parts, a single hit die or laser cutting is used, with a correspondingly higher piece cost and, often, a wider tolerance.
Washers are nested on the strip, and the layout determines how much material is scrap. The layout also determines which direction the rolling direction of the strip runs relative to the part, which matters for ears and for any subsequent forming. When a request specifies an unusual outer diameter purely to suit an existing stock size, the tooling and material consequences should be reviewed, because a small dimensional change can alter the nesting efficiency significantly.
Punching shears the material, and the shear leaves a burr on one side and a rounded edge on the other. A burr under a bolt head prevents the joint from seating over its full area, which changes the clamp behaviour and leaves a gap that traps water. A burr inside the hole reduces the effective clearance for the fastener, and where a pin or stud has to pass through, the burr can prevent assembly altogether. Burr height is therefore a functional characteristic, not a cosmetic one, and it should be stated on the drawing with a maximum value.
Small stamped washers are usually deburred in a vibratory tumbler, which rounds the edges uniformly and improves the surface finish at the same time. Larger or thicker parts may be deburred by a secondary operation or by a chamfering station in the die. Whatever the method, the result needs to be verified on a sample with a burr gauge or a profile measurement, because a tumbler that is overloaded or worn will leave parts with the burr intact and the operator may not see the difference.
Flatness is the characteristic that most often causes trouble downstream. A washer that is dished rather than flat changes the effective thickness, distorts the mating surface, and, on a stack of shims, produces a gap that cannot be closed by tightening. Dishing comes from three sources: residual stress in the strip from rolling, stress introduced by the punching operation, and thermal distortion during heat treatment or coating. The material around a punched hole is heavily worked, and releasing that stress on one side of the part more than the other causes the plate to bow.
The countermeasures are to specify flat, stress relieved strip where flatness is critical, to use a die design that balances the punch forces, to level or flatten the parts after blanking, and to support the parts properly through any heat treatment. For thin shims used to set a precise gap, a final flattening operation is standard, and the flatness value is inspected rather than assumed.
A flat protective washer in a lightly loaded joint can be supplied in the as rolled condition. Where the washer is part of a loaded joint, where it must resist deformation under a bolt head, or where it acts as a spring washer, heat treatment is needed to give the required hardness and elastic range. Stainless washers may be supplied in a soft condition for sealing duty or a work hardened condition for load, so the condition should be stated explicitly.
Hardness is verified on a sample from each heat treatment batch and recorded. The relevant values are the surface hardness, the consistency across the batch, and, for case hardened parts, the effective case depth. Over hardened parts crack rather than deform, which is a worse failure mode because it removes the warning that a soft part gives by losing thickness slowly.
Heat treatment also affects flatness, so the sequence matters. Parts that are heat treated and then flattened do not hold their flatness as well as parts that are flattened last, and for tight flatness requirements the process sheet should be arranged accordingly.
Zinc plating is the common choice for carbon steel washers, with trivalent passivation for improved corrosion resistance. Zinc flake coatings are used where higher salt spray performance is needed, and stainless steel parts are usually passivated rather than plated. Copper and aluminium parts may be supplied bare or with a specific finish agreed with the customer.
Plating builds on every surface, including the inside of the hole. On a washer with a small inner diameter, that build up reduces the clearance and can make the part impossible to fit over its stud. The drawing should either specify the inner diameter before plating with a plating allowance added, or specify the finished diameter with the coating included. This is a frequent cause of a technically correct washer that will not assemble, and it is avoided by stating which condition the tolerance applies to.
PTFE and nylon washers are produced by stamping or die cutting from sheet for simple flat geometries, by machining for small or thick quantities, and by injection moulding where the volume and the shape justify a tool. The important difference from metal is that these materials have much lower stiffness and they creep under sustained load, so the thickness tolerance and the flatness requirement have to be considered together with the application. A non metal washer that is used for low friction, insulation, or sealing is well within its capability. A non metal washer used to hold a defined gap will lose thickness over time and close the gap, regardless of how well it was made.
Where low friction is needed in a joint that must also hold a dimension, a PTFE faced metal washer gives both properties, with the metal core carrying the load and the facing reducing friction.
The table below is the default inspection plan for a production washer or spacer. Frequency can be tightened where the characteristic is critical to the assembly.
| Characteristic | Method | Frequency |
|---|---|---|
| Material grade and condition | Certificate review | Every batch |
| Thickness | Micrometer at several points | First article, hourly, last part |
| Inner diameter | Plug gauge | First article and hourly |
| Outer diameter | Caliper or gauge | First article and hourly |
| Concentricity | Gauge or coordinate measurement | First article and per shift |
| Flatness | Surface plate with feeler gauge | First article and per shift |
| Burr height | Burr gauge or profile measurement | Per batch sample |
| Hardness | Hardness tester | Per heat treatment batch |
| Coating thickness | Magnetic gauge, plus sectioning on first article | Per plating batch |
| Salt spray resistance | Neutral salt spray | Per agreed period |
| Defect | Usual source | Control that prevents it |
|---|---|---|
| Dished or bowed part | Blanking stress, heat treatment, unsupported cooling | Stress relieved strip, balanced die, flattening operation |
| Burr remaining on the face | Worn die, overloaded tumbler | Die maintenance schedule, burr inspection on samples |
| Eccentric hole | Die alignment drift | Concentricity check on the first article and per shift |
| Thickness out of tolerance | Strip thickness variation | Incoming thickness check, in process micrometer checks |
| Washer will not fit the stud | Plating build up in the hole | State pre plate or post plate dimension on the drawing |
| Cracked after hardening | Over hardening or sharp edges | Hardness range control, edge preparation |
| Rust in storage | Insufficient coating, humid packing | Coating thickness control, moisture barrier packing |
Washers are supplied in bulk, so the packing has to prevent the two things that damage them in transit: abrasion between parts and moisture. Parts are packed in bags or cartons of a defined quantity, with a moisture barrier where the coating is thin, and the carton label carries the part number, the batch number, the quantity, and the material. Batch identification links back to the strip certificate, the stamping record, the heat treatment record if applicable, and the plating record. That chain is what allows a fitting problem on the line to be traced to the specific production lot rather than to the supplier as a whole.
The washer selection guide explains how the type, material, and dimensions are chosen before production, including the difference between a flat washer, a spacer, and a locking washer.
By combining three controls: stress relieved or flat incoming strip, a die design that balances the punch forces around the part, and a flattening or levelling operation after blanking and again after any heat treatment. For shims that set a precise gap, the flatness value is inspected on samples rather than assumed.
Because plating builds up inside the hole as well as on the faces. The drawing needs to state whether the inner diameter tolerance applies before or after plating, and a plating allowance has to be added if the finished size is the controlling dimension. Masking is an alternative where the fit is very tight.
No. A flat protective washer in a lightly loaded joint can be used in the as rolled condition. Heat treatment is specified where the washer is part of a loaded joint, where it must resist deformation under a bolt head, or where it acts as a spring element.
Usually a combination of residual stress in the strip and the stress introduced by punching, sometimes compounded by distortion during heat treatment. The fix is stress relieved material, a balanced die design, and a supported process through any thermal operation.
Send a drawing or sample with the dimensions and tolerances, the material and finish, the quantity, and a note on what the washer does in the assembly. Our engineers will confirm the specification, propose the process route, and flag any requirement that the chosen material cannot meet. See the washer and spacer range, review the wider shock absorber parts catalogue, look at the custom manufacturing service, or contact our sales engineers to begin.
