How EFS coordinated forging, heat treatment, precision machining and thread rolling to produce a 560 mm structural bolt within tolerance.
A structural steelwork contractor needed something no catalogue could offer: a square head bolt in M20 × 560, property class 8.8, with a wide 45 × 29mm head and a fully machined shank running for most of its length. Achieving that geometry required a carefully coordinated manufacturing process rather than adapting a standard product.
When the first attempt fell short
The first attempt failed before the bolts even reached site. The supplier forged the head correctly, then heat-treated the bolts lying down, which is standard practice for shorter fasteners. At 560 mm long, however, that approach created a problem. Several shanks emerged from quenching and tempering with a visible bow, placing them outside the straightness tolerance specified on the drawing. The client was left with bolts that could not be relied upon to sit correctly within a structural connection. That is when the project came to EFS.
How EFS got the sequence right
EFS’s solution began with the heat- treatment process. The bolts were quenched and tempered while standing vertically rather than lying down, specifically to control distortion across the full 560 mm length. This addressed the same defect that had caused the original production attempt to fail. Straightness, however, was only the starting point. The drawing specified very tight tolerances on the shank because the bolt needed to seat accurately in its housing. The surface left by hot forging, with its characteristic rough finish, was not suitable for that level of fit. To achieve the required dimensions and surface quality, the entire sub-head shank was machined by turning. This process restored the precise dimensions needed after forging and heat treatment while producing the surface finish necessary for correct positioning within the seat.
Once this stage was complete, the bolts moved on to thread rolling, which formed the 115 mm M20 thread through plastic deformation rather than by removing material.
Because thread rolling preserves continuous grain flow instead of cutting through it, rolled threads offer improved mechanical properties compared with cut threads, provided the diameter has been prepared accurately beforehand to ensure the profile forms correctly.
Four processes, one coordinated solution
None of these four stages, hot forging, quenching and tempering, turning, and thread rolling, is particularly difficult in isolation. What makes a component like this successful is ensuring that every stage works together in the correct sequence on the same part.
Coordination was EFS’s role throughout the project. Rather than assigning the entire job to a single workshop, EFS selected and managed each specialist supplier in the chain, covering forging, heat treatment, turning and thread rolling. Acting as the main contractor, the company checked compliance with the drawing at every stage before authorising progression to the next operation. EFS is a TÜV-certified Italian company,
and every stage of this order was completed in Italian workshops using European raw materials. The finished bolts arrived with straight shanks over the full 560 mm length, correct tolerances and the required seating performance, delivering the result that the original manufacturing attempt failed to achieve.
Source: Let’s Fasten.it https://bit.ly/letsfastenitautumn2026_IndustrialNow
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