A drawing die consists of several functional zones: the entrance radius centres the wire, the lubrication angle feeds the lubricant, the reduction cone (drawing angle) performs the entire deformation, the bearing length sizes the final diameter, and the back relief releases the exiting wire. The carbide core (nib) sits in a steel case. The geometry and surface finish of these zones govern wire quality, friction and die life.
Entrance radius: Guides the helically incoming wire and centres it into the die. May remain unpolished.
Lubrication angle: Feeds lubricant into the die — the precondition for a load-bearing lubricant film.
Reduction cone: The most important zone: the entire deformation and the compaction of lubricant onto the wire surface happen here.
Bearing length: Sizes the final diameter, guarantees roundness and straightness. Rule of thumb: 25–66 % of the die diameter.
Back relief: Releases the exiting wire and protects the bearing edge from chipping.
Steel case: Encloses the carbide nib and absorbs the stresses of the drawing process.
Carbide nib: The actual tool body in tungsten carbide — at Leng made from carbide by the world's leading suppliers.
Which zones does a drawing die have?
Entrance radius. It guides the incoming wire, which often runs in with a helical motion, and centres it into the die. The entrance radius can remain unpolished.
Lubrication angle. It feeds lubricant into the die — the precondition for a load-bearing lubricant film in the next zone.
Reduction cone (drawing angle). The most important area of the die: the entire deformation and the compaction of lubricant onto the incoming wire surface happen here. A precisely ground conical angle with an extremely smooth finish is required; the flanks must be straight over their full length, free of radius or form deviations, and exactly concentric to the die axis — otherwise the wire turns out oval.
Bearing length (sizing zone). It controls the diameter of the drawn wire, guarantees roundness and straightness, and produces the smooth surface. For quality wire, the bearing must be round, parallel and made to close tolerance.
Back relief. It releases the wire as it leaves the tool and protects the bearing edge from chipping.
How long should the bearing length be?
To avoid rapid, excessive tool fatigue, the bearing length should be 25 to 66 % of the die diameter. The choice depends on the hardness of the material and the required lubricant film:
| Material drawn | recommended bearing length | reason |
|---|---|---|
| hard materials | approx. 25–35 % | less heat generation, fewer lubrication defects |
| soft materials | approx. 50 % | longer tool usage; drawing rings can be removed |
Worked example: a die of 2.54 mm diameter with a 35 % bearing ratio has a bearing length of 0.889 mm.
What is the delta factor?
The delta factor (Δ) describes the ratio of reduction-cone geometry to the cross-section reduction of a pass. It is the central design parameter of die geometry: a range of about Δ = 1.5–1.8 is considered favourable; markedly higher values (from about Δ = 3) increase the risk of internal wire damage up to centre bursts (cup-cone fractures) and wire breaks. As a rule of thumb for angle selection: the harder the material, the smaller the drawing angle; the greater the reduction, the larger the angle.
Which die material for which job?
| Material | typical use | note |
|---|---|---|
| Tungsten carbide | universal for wire, tube, bar; all diameters | our standard programme; grades and grain sizes per application |
| Ceramic | special applications, high chemical resistance | part of the Leng range |
| PCD (polycrystalline diamond) | fine wire, long runs | tool life several times that of carbide — published ranges scatter widely |
| Natural diamond | finest wire sizes | for the smallest diameters |
Hard-material coatings on carbide and ceramic drawing dies are part of our range.
Why polishing co-decides
The best geometry is worth little without the right surface: only the mirror polish of reduction cone and bearing lowers friction, stabilises the lubricant film and keeps wear from accelerating itself. The grit sequence is covered in How to polish drawing dies.
Need a design? We engineer geometry and drawing schedules with dedicated calculation software — since 1948: request a quote or +49 2777 7608.
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