Key takeaways
- Wire diameter arrives with its own tolerance band, and that band propagates into the finished part.
- Spring rate changes roughly with the fourth power of the diameter, so a small wire change has a large effect.
- Dimension to bend centres and centrelines, not to the wire surface, where the drawing allows it.
- State the wire standard and tolerance class, then check the incoming lot rather than assuming the nominal.
Wire is a purchased part with its own tolerance
A drawing that calls 2.0 mm wire is asking for a nominal, not an exact size. Wire is drawn to a tolerance band, and the band is defined by the wire standard and the tolerance class. The band may be expressed as a plus and minus range, as a class code, or as a table indexed by diameter. Two suppliers can both meet the drawing and still ship wire at opposite ends of the band.
This is not a small effect. On a formed part, the wire diameter appears in the outside dimensions, the bend geometry, the spring rate and the fit in any hole or clip the part engages. A buyer who compares only the nominal diameter is comparing a label, not a dimension.
The first practical step is to put the standard and the tolerance class on the drawing. The second is to decide which finished features are sensitive to the diameter, and to dimension those features in a way that does not double the error.
Where the variation lands
The most direct effect is on any outside dimension that includes the wire. A U-shaped part formed over a fixed tool has an inside width set by the tool and an outside width equal to the inside width plus two wire diameters. If the wire runs 0.03 mm large, the outside width grows by 0.06 mm. The inside width does not change, so the part still fits over the mating pin, but it may not fit inside the housing.
The second effect is on bend geometry. A larger wire needs a larger tool radius to keep the same inside radius, and it springs back differently. If the tool is fixed and the wire moves to the top of the band, the finished inside radius can grow and the bend angle can move. The part is still within the bend position tolerance, but the cumulative effect on a multi-bend part can be visible at the last leg.
The third effect is on spring rate. For a helical spring, the rate is proportional to the fourth power of the wire diameter and inversely proportional to the cube of the mean coil diameter and the number of active coils. A diameter change that looks small on a certificate can move the rate by several per cent.
| Feature | Sensitivity | What a larger wire does | Control |
|---|---|---|---|
| Outside width over a fixed tool | Direct, two diameters | Grows by twice the diameter change | Dimension the inside width if it is functional |
| Spring rate | Fourth power of diameter | Rate rises sharply | Tolerance the load, not just the diameter |
| Fit in a hole or clip | Direct | Clearance falls or interference appears | Check the worst case at the top of the band |
| Bend radius and angle | Indirect, through tooling | Radius and springback move | Set the tool on the actual lot |
| Plated diameter | Adds coating thickness | Finished size exceeds the wire band | State whether the size is before or after plating |
The fourth power effect on spring rate
The spring rate of a helical compression spring is the shear modulus times the fourth power of the wire diameter, divided by eight times the cube of the mean coil diameter times the number of active coils. The fourth power is what makes the diameter so influential.
For an illustrative example, a 2.0 mm wire that runs 2.5 per cent large is 2.05 mm. The ratio raised to the fourth power is about 1.10, so the rate rises by about 10 per cent if nothing else changes. A 2.5 per cent diameter change has become a 10 per cent rate change. That is why a spring drawing should call the load at a stated deflection, not only the wire diameter.
The same logic applies in reverse. A wire at the bottom of the band gives a lower rate, and a spring that was set up on a nominal coil may sit outside its load tolerance at one end of the wire range. The supplier can sort or select wire, but that is a cost. The cheaper route is usually to widen the load tolerance if the function allows it, or to specify a tighter wire class if it does not.
Two tolerances, one result
A tight wire tolerance and a loose load tolerance can produce the same part as a loose wire tolerance and a tight load tolerance. Decide which one the function actually needs, and pay for that one.
A dimensional stack you can check
Take an illustrative U-shaped clip in 2.0 mm wire with a tolerance of plus or minus 0.03 mm. The inside width is set by a 12.00 mm tool, so the nominal outside width is 16.00 mm. At the bottom of the wire band the outside width is 15.94 mm. At the top it is 16.06 mm. The inside width stays at 12.00 mm, because the tool sets it.
Now add a bend position tolerance of plus or minus 0.2 mm at each bend and an angle tolerance of plus or minus 1 degree. The outside width is no longer a single number; it is a band built from the wire, the tool and the bend. If the housing has a 16.10 mm slot, the part fits at nominal but is marginal at the top of the stack. If the slot is 16.20 mm, there is room.
The lesson is not that the tolerances are wrong. It is that the outside width should not be the controlled feature if the inside width is what the part needs. Dimension the functional feature, give the rest a general tolerance, and check the worst case before release.
Plating, coating and the finished diameter
A plated wire is not the same diameter as the wire that went into the plating bath. Zinc, tin, silver and nickel all add thickness, and the addition is on the radius, so the diameter grows by twice the coating thickness. A 5 micrometre coating adds about 0.01 mm to the diameter, which is a third of a typical wire band.
There are two routes. The part can be formed from uncoated wire and plated afterwards, or it can be formed from pre-plated wire. The first route puts the plating on the finished shape, including the cut ends, and can introduce hydrogen embrittlement on high strength wire. The second route protects the wire before forming, but the coating can be damaged at the bends and the cut ends are bare.
State which route the drawing assumes. If the drawing gives a finished diameter, say whether it is before or after plating. If the part is coated with powder or epoxy, the coating thickness is much larger and can bridge a tight bend, so the radius and the coating call have to be considered together.
Ovality and out of round wire
Diameter is usually reported as an average, but wire can also be out of round. A wire with an oval section presents a different radius to the tool depending on how it is oriented. In a bend, the stiff axis and the soft axis behave differently, so the springback can vary from part to part if the wire rotates in the feed.
For most parts the ovality is small enough to ignore. For a spring, a contact or a part that runs in a close bore, it can matter. If the part is sensitive, ask for the ovality limit as well as the diameter band, and check how the wire is oriented in the tool.
The same applies to surface condition. A drawn wire with a seam, a scratch or a heavy oxide layer can crack at a bend that would be fine on a clean wire. A certificate that reports only the diameter and the tensile does not cover the surface.
What to put on the drawing
Call the wire standard, the grade, the temper and the diameter with its tolerance class. If the finished part is plated, state whether the diameter is before or after plating. Dimension the functional features: the inside width of a clip, the load at a stated deflection, the bend centres, the leg angle. Give the outside dimensions a general tolerance unless the assembly truly needs them tight.
Where a dimension runs to the wire surface, expect the wire tolerance to appear in it twice. A dimension across two outside surfaces includes two wire radii. A dimension between bend centres does not include the wire diameter at all, which is why it is a more stable control for a multi-bend part.
Add a note if the part must fit a gauge. A functional gauge is often the cheapest way to control a feature that depends on the wire, the bend and the springback together, because it tests the assembly condition rather than a set of separate dimensions.
Check the lot, not the label
Incoming inspection should measure the wire diameter on a sample from each lot, not just read the certificate. A micrometer across two positions at 90 degrees gives the diameter and a first indication of ovality. Record the result against the lot number and keep it with the first-off record.
If the part is critical, run a first-off at each lot change and compare the functional dimension with the previous lot. That is how a diameter shift is caught before it becomes a batch of out-of-tolerance parts. The cost is a few minutes at the machine. The cost of not doing it is a sort, a rework or a line stop.
- Wire standard, grade, temper and tolerance class on the drawing
- Diameter measured at two orientations per lot sample
- Certificate retained with the lot number
- First-off functional check at every lot change
- Plating route and finished diameter stated
- Functional gauge for the assembly condition where possible
Frequently asked questions
How much does wire diameter tolerance affect a formed part?
It depends on the feature. An outside dimension across the wire moves by twice the diameter change. A spring rate moves roughly with the fourth power of the diameter, so a 2.5 per cent diameter change can move the rate by about 10 per cent. Bend positions are less direct but still affected through tooling and springback.
Should I specify a tighter wire tolerance class?
Only if the function needs it. A tighter class raises the material cost and may not improve the finished part if the bend process is the dominant source of variation. Start with the standard class and a functional gauge, then tighten if the data shows the wire is the cause.
Is the plated diameter the same as the wire diameter?
No. Plating adds thickness on the radius, so the diameter grows by twice the coating thickness. State whether the drawing dimension is before or after plating, and say which route the part uses.
Why does the spring rate change between lots?
The new lot may sit at a different point in the wire diameter band, or the tensile may differ. Both change the rate. Check the measured diameter and the certificate, then re-test the load at the stated deflection.
Sources and further reading
- ISO 22034-2, Steel wire and wire products, tolerances
- EN 10270-1, Steel wire for mechanical springs, patented cold drawn unalloyed spring steel wire
- Qumek incoming wire inspection and first-off records
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Check a wire tolerance against the finished part
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