Spring End Grinding: Flatness, Squareness and Bearing Surfaces
Short answer: end grinding turns the closed ends of a compression spring into flat bearing surfaces that sit square to the spring axis, so the spring stands upright and delivers load evenly instead of tilting under force. Grinding typically removes part of the end coil to create a bearing face covering most of the wire circumference, and it controls three measurable qualities: flatness of the face, squareness of the face to the axis, and parallelism of the two end faces. A compression spring that seats in a pocket or against a flat plate almost always needs closed-ground ends; a spring that runs on a rod or in a tube can often skip grinding and save cost.
A compression spring transmits force through its ends. If the ends are plain cut wire, the force enters along a narrow edge, the spring tilts as it compresses, load goes in sideways, and the effective axis wanders. Closed-ground ends fix that: the end coils are closed (pressed flat against the neighbor) and then ground to create a flat annulus that bears on the seat. End grinding is not cosmetic — it is what makes a compression spring predictable in a mechanism, and it directly sets free-height and squareness tolerances that downstream assembly depends on. This guide covers the end types, the grinding process, the tolerances you should specify, and when grinding is genuinely necessary.
End Types: What Grinding Adds to Each Configuration
Compression springs are specified by their end condition, and grinding applies only to some of them. The standard families are plain ends, plain-ground ends, closed ends, and closed-ground ends. Plain ends are simply cut wire; closed ends have the last coils pressed flat without grinding; closed-ground ends are closed and then ground. Plain-ground and closed-ground variants add grinding to improve seating.
| End type | End coil | Ground? | Typical use |
|---|---|---|---|
| Plain (open) | Not closed | No | Cheapest; seats in pockets or on rods |
| Plain-ground | Not closed | Yes | Better seating where open ends are acceptable |
| Closed | Closed flat | No | Most machine-made springs; stands fairly square |
| Closed-ground | Closed flat | Yes | Precision seating, most load-bearing applications |
As a rule, closed ends alone give acceptable squareness for many light duties because the closing operation itself squares the end reasonably well. Grinding is specified when the spring must stand on a flat plate, when free length and load tolerances are tight, or when the spring is long enough that end tilt would cause buckling. The full comparison of end styles, including when plain ends are acceptable, is covered in our spring end types guide; the rest of this guide focuses on the grinding operation itself.
How Compression Spring Ends Are Ground
End grinding is done on double-disc grinders built for springs. The spring is held in a rotating cage or between tooling that presents both ends to two opposing grinding wheels, so both bearing faces are ground in one operation with the spring free to rotate against the wheels. Grinding stock is removed from the closed end coil until a flat annulus of the required width is produced, and the amount removed is controlled because it directly changes the spring's free height and, marginally, its active coils. After grinding, the faces are deburred and edge-broken, because a sharp ground edge is a stress raiser where the bearing face meets the wire.
The process order matters. Cold-wound springs are coiled, stress-relieved, then ground, so the wire is stable before material is removed; hot-wound or hardened springs may be ground after heat treatment depending on the tolerance target. Grinding generates heat, so coolant and feed control are essential — overheating the end coil locally can burn the surface, soften the wire, and start a fatigue crack exactly where the spring bears its load. The tolerance chain that grinding must satisfy runs through the whole manufacturing route described in our spring manufacturing process guide, from coiling accuracy to the final set operation.
Flatness, Squareness, and Bearing Surface: The Numbers
Three qualities define a well-ground spring end, and they are measured, not assumed. Flatness is the deviation of the ground face from a true plane; squareness is the angle between the ground face and the spring axis; parallelism is the relationship between the two end faces. The bearing surface itself is expressed as the percentage of the wire circumference that is ground flat — industry practice targets a ground annulus covering most of the end, with 75% or better as a common specification for load-bearing springs.
| Parameter | Typical production value | Precision value | Notes |
|---|---|---|---|
| Squareness to axis | Within 2° | Within 1° or better | Set by grinder alignment |
| End face parallelism | ~0.1-0.25 mm | ~0.05 mm | Affects free-height consistency |
| Bearing surface coverage | ~75% of wire circumference | 80-90% | More area, better seating |
| Ground face roughness Ra | ~0.8-3.2 µm | ~0.4-0.8 µm | Fine finish resists fretting |
| Free-height shift | Controlled per order | Held to tolerance | Grinding removes material |
Squareness is the number buyers care about most, because an out-of-square spring tilts under load, adds side force to the mechanism, and can buckle early. Typical commercial compression springs hold end squareness within about 2°, and precision springs are specified at 1° or tighter; the achievable value depends on spring length and index, so the tolerance should be set against the spring tolerance standards rather than pulled from the air. Remember that grinding removes material from the free height, so the closed-ground free length is deliberately targeted during coiling with grinding stock allowed — an unground spring cannot simply be ground down later without re-checking its rate and load.
When You Can Skip Grinding
Grinding adds a process step, so it adds cost and lead time, and good design avoids it when the spring does not need it. A compression spring guided on a rod or inside a tube does not stand on its own, so plain or closed ends are often perfectly serviceable and cheaper. Short, stiff springs with a low slenderness ratio tolerate end tilt far better than long slender ones, and springs seated in a counterbored pocket are located by the pocket rather than by their own squareness. Extension springs and torsion springs are not ground at all — they transmit load through hooks or legs, not through flat bearing ends. The decision rule is simple: if the spring bears on a flat surface and must stand square while carrying load, specify closed-ground ends; if it is guided or pocketed, question the grinding callout on the drawing and let the factory quote both versions. Sending the drawing with the end condition marked is the fastest route to a realistic comparison — a quote request to sc@bquq.com within 12 working hours will come back with both options priced if you ask.
Measuring Ground Ends in Inspection
Ground end quality is measured with simple, reliable tools, and the inspection routine is worth defining before production rather than after a field failure. Free height and end flatness are checked on a surface plate with a height stand: the spring stands on one ground face and the height variation around the other face shows both flatness and parallelism at once. Squareness is measured against the axis with a squareness gage or on an optical comparator for small springs, and the ground annulus width is verified against the drawing — a bearing face that covers only half the wire circumference seats poorly no matter how flat it is. The face finish is compared against the specified roughness, and the ends are visually checked for grinding burn, which appears as localized discoloration and indicates the wheel or feed ran too hot.
Two process points protect the inspection results. Grinding is scheduled after stress relief so the spring is dimensionally stable when material is removed, and the grinding wheels are dressed and cooled so the end coils are not overheated — a burn there is a fatigue crack waiting for a cycle count. On production, first-article and periodic checks track the same parameters, and the batch record ties the measured free height, squareness, and bearing width to the load test at the working height. That record is what lets a buyer trust a closed-ground compression spring in a valve, a latch, or a precision mechanism without re-measuring every piece on arrival; it is also exactly what our inspection documentation covers on every spring batch we ship.
Frequently Asked Questions
Q: When does a compression spring need ground ends?
A: When it stands on a flat surface and must carry load without tilting — such as on a valve seat, under a plate, or inside a mechanism with tight axial alignment. Guided springs running on a rod or in a tube, and short stiff springs in pockets, can usually use closed or plain ends and avoid the grinding cost.
Q: What is the difference between closed ends and closed-ground ends?
A: Closed ends have the end coils pressed flat against the neighboring coil but are not machined, giving approximate squareness. Closed-ground ends are additionally ground to create a flat bearing annulus, which delivers much better squareness, even load distribution, and tighter free-height control for precision applications.
Q: How square can a ground spring end be?
A: Commercial compression springs typically hold end squareness within about 2° of the axis, and precision springs are specified within 1° or better. The achievable figure depends on spring length, wire size, and index, so tolerances should be set realistically against standard spring tolerance tables.
Q: Does end grinding change the spring rate?
A: Grinding removes material from the closed end coils, which are inactive for deflection anyway, so the spring rate changes little; what does change is free height, because material is removed from the overall length. That is why coiling targets a length that includes grinding stock when closed-ground ends are specified.
Q: Are extension and torsion springs end ground?
A: No. Extension springs transmit load through hooks or loops and torsion springs through legs, so neither has flat bearing ends to grind. End grinding applies to compression springs that seat against flat surfaces, and only to the closed-ground and plain-ground end configurations.
Related Resources
- Spring End Types Explained — plain, closed, and ground end configurations and when each is specified.
- Custom compression springs — compression springs with closed-ground ends from a Dongguan source factory.
- About BQUQ — an ISO9001-certified factory running spring, stamping, CNC, and heat sink lines in Dongguan.
- Contact us — send your drawing and get a spring quote with the right end treatment within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
