Spring tolerances explained: what ±0.05mm actually means for your design
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Spring catalogs throw tolerance numbers at you like everyone reads them the same way. ±0.05mm wire. ±1% free length. ±10% load. What do those actually mean for the assembly you’re building?
If you’re designing a spring into a critical mechanism, tolerance is the difference between a part that fits every time and one that fits four times out of five. Here’s a practical breakdown of what each tolerance means and when to ask for tighter.
Wire diameter tolerance
Spring wire is drawn through progressive dies to its final diameter. The diameter you order — 1.0mm, 1.5mm, 2.0mm — is a target. Real wire varies slightly along its length and between batches.
Daichi standard: ±0.05mm on most wire diameters. So a 1.0mm wire might measure anywhere from 0.95mm to 1.05mm at any given point.
What this means for your design: - The spring rate (force per unit deflection) varies as the fourth power of wire diameter. A ±5% wire variance translates to roughly ±20% spring rate variance. If you need the rate dialed in, the wire tolerance matters. - For most industrial applications — door springs, latch returns, press dies — this variance is invisible. The mechanism has enough margin. - For precision applications — load cells, force gauges, calibrated mechanisms — tighter wire tolerance matters. We can source ±0.02mm wire on premium runs.
When to ask for tighter: when the spring rate (force vs. compression) needs to be repeatable to within ±5% across pieces, or when the assembly has tight envelope constraints that don’t tolerate diameter variance.
Free length tolerance
The free length is the spring’s height (or body length, for extension and torsion springs) when no force is applied. It’s controlled by where the CNC coiler cuts the wire after the last coil.
Daichi standard: ±1% of the free length, or ±0.5mm minimum, whichever is larger. So a 100mm spring is ±1mm; a 30mm spring is ±0.5mm (because 1% would be smaller than the minimum).
What this means for your design: - If the spring sits in a fixed cavity, ±1% free length usually fits without preload variance issues. - If the spring is preloaded to a specific working position, free length variance translates to preload-force variance. The shorter the spring’s working travel, the more sensitive. - For valve springs, regulator springs, and other applications where preload matters, tighter free length tolerance is worth specifying.
When to ask for tighter: when the spring must hit a specific load at a specific length (preloaded applications), or when total assembly height has tight constraints.
Load tolerance
Load tolerance is the variation in force at a specified compression. Typically expressed as a percentage of the target load.
Daichi standard: ±10% load, measured at the working compression specified in the drawing.
What this means for your design: - ±10% sounds large but for most applications it’s fine. A door latch return spring loaded to 5N might actually deliver 4.5–5.5N. The latch still works. - For applications where consistent feel matters (consumer products, high-cadence mechanisms), ±10% can be perceptible. - For applications where load directly drives a calibrated outcome (force gauges, scales, regulator pressure), ±10% is too loose. We can hold ±5% or tighter on premium runs with load-test verification.
When to ask for tighter: when load consistency drives perceived quality, or when a downstream mechanism is calibrated to a specific spring force.
Squareness
Squareness is how perpendicular the ends of a compression spring are to its axis. A spring with poor squareness “leans” — pushes off-axis when compressed.
Daichi standard: 3° or less off-square.
What this means for your design: - For springs in a sleeve or guide, squareness rarely matters — the guide controls alignment. - For free-standing compression springs or springs in shallow pockets, off-square springs can buckle or wear unevenly. - Closed-and-ground ends (where the end coil is machined flat) generally produce the squarest springs. Open ends produce the least square.
When to ask for tighter: when the spring is free-standing or in a shallow pocket, or when sliding wear on the end face is a concern.
Coil count and pitch
Active coil count determines the spring rate. Pitch (distance between adjacent coils) determines the solid height — how short the spring gets when fully compressed.
These are usually held tight (±0.25 coils) without specifying. The variation only matters if you’re designing to fit between exact assembly heights or if you need exact pitch for a sliding mechanism.
When does tolerance actually matter
Most spring orders don’t need tighter than our standard tolerances. The standard ranges are tight enough that the assembly tolerates the variation without anyone noticing.
Tighter tolerances are worth specifying when:
- The spring rate matters precisely — calibrated mechanisms, force-sensing applications, regulators
- The spring is preloaded to a specific length — valve springs, certain return mechanisms
- The downstream mechanism has tight envelope constraints — small assemblies where every millimeter counts
- The spring is part of a calibrated product — anything that’s inspected against a load curve at finished-goods QC
- The application demands consistent feel — consumer products, controls, anything operated by a person
For everything else — door latches, press dies, suspension springs, cabinet hardware — standard tolerances are fine.
How we control tolerances
CNC coiling on calibrated machines, in-process measurement of wire diameter and free length, sample load testing on a calibrated load cell for premium runs. For production runs above 500 pieces we provide dimensional reports on request at no extra cost.
If you specify a tighter tolerance, we’ll quote slightly higher per piece (to reflect the extra QC and yield loss) and confirm feasibility before starting production. If we can’t hold what you’ve asked for, we’ll tell you up front.
A practical rule
Specify tolerance one band tighter than your application actually needs. Not three bands tighter.
Over-specifying tolerance is a common mistake — engineers, understandably risk-averse, ask for ±0.02mm when ±0.05mm would have worked, paying 30% more for tolerance the assembly can’t even use. Under-specify and you risk yield issues at final assembly.
If you’re not sure what tolerance your assembly tolerates, send us the drawing and the application context. We’ll suggest a tolerance band that matches.
Daichi Springs manufactures custom and standard springs in Valenzuela, Metro Manila. Standard tolerances on most spring work; tighter tolerances available on premium runs with load-test verification. Get a quote →