Die Making and Machining

A new die set is cut in 3–4 weeks from a released layout — rough-milled, vacuum heat treated to 47–52 HRC, finished on 3-axis and 5-axis machining centres, EDM for closed cavities, then tried out on the press before the first article is measured. Inserts are H13, 5CrNiMo or 5CrMnMo depending on batch size, and the dies run on a hot-die line from 1,000 to 5,000 t producing parts of 0.1–500 kg to ISO 8062 class C as-forged.

The die route, step by step

Die making is not one operation, and the step that most often goes wrong is heat treatment placed after the last finish cut. A block that is finish-machined and then hardened moves, and the cavity that measured 0.02 mm oversize in the soft state comes back distorted with no material left to correct it. The route below hardens between roughing and finishing, which costs one extra setup and removes the risk.

EDM follows heat treatment for closed cavities and for sharp internal corners that a cutter cannot reach. The electrode is cut from the same 3D model that drove the finishing pass, so sinker and milling agree to the same nominal.

Which die steel for which batch

The three grades below cover almost every hot-die job in the 0.1–500 kg range, and the choice is driven by batch size and by how much thermal cycling the die sees, not by price per kilogram. H13 costs more per block and less per thousand parts on any run above roughly 15,000 pieces.

5CrNiMo is the workhorse for medium batches: cheaper, tougher, and more forgiving of a marginal cooling setup. It softens earlier, so it is the wrong choice for a die that runs two shifts at high temperature. 5CrMnMo sits below it on hardenability, which matters on thick blocks where the core does not reach full hardness.

Cutting parameters that decide die life

Roughing removes volume and finishing decides surface integrity, and the two want opposite parameters. A finishing pass run at roughing speed leaves a damaged layer that becomes a crack initiation site on the first thermal cycle, and the die fails at a fillet that measured perfectly on the CMM.

The values below are for coated carbide in H13 at 46–52 HRC on a five-axis centre with through-spindle coolant. They are starting points; the finishing pass is adjusted after the first part is measured.

What "as-forged" and "machined" mean on the same drawing

A drawing that mixes both states without labelling them is the single most common reason a first article is rejected on paper while every measured value is inside its own tolerance. The convention that avoids it: state the as-forged tolerance against ISO 8062 class C for every surface that leaves the die, and put a separate machining callout with its own datum on every surface that is cut afterwards.

In practice the forging is quoted at ISO 8062 class C with 1.5–3 mm of stock per machined face; the machined features are then held to ±0.2 mm on critical dimensions and to ISO 2768-m as the default general tolerance. The machining datum has to be a forged feature that is stable, which usually means the die face rather than a draft wall.

Die maintenance between batches

A die is not consumed at a fixed number of strokes; it is consumed by thermal fatigue at the flash land and by wear on the walls that see the most metal flow. Both are inspectable, and both are cheaper to correct early.

The refurbishment cycle that works is tied to the wear pattern rather than to a fixed count: polish the flash land, measure the wall at three heights, and re-sink or weld only the faces that have moved. A die that is re-sunk on schedule typically returns to nominal for two or three cycles before the block is retired.

• Every 5,000–10,000 strokes: flash land polish, vent depth check, water channel flow check
• Every batch: wall measurement at three heights on the faces with the longest metal path
• On any visible heat-check crack: grind out before it propagates into the insert body
• On a dimensional drift above half the tolerance: re-sink rather than compensate on the part drawing
• Store with the cavity oiled and the die faces closed, never with the cavity open to the shop air

Frequently asked questions

How long does a new die take?

Three to four weeks from a released layout for a single-impression die set on a part under 20 kg. The heat-treatment step is the one that cannot be compressed: austenitising and double tempering for H13 runs three to four days, and skipping the second temper is what produces a die that cracks in the first week.

Can you cut a die from a 3D model only, without a 2D drawing?

Yes, provided the model carries the material grade and the surfaces that will be machined afterwards. The layout is generated from the solid, and the 2D drawing is then produced from the layout so that both the die and the inspection plan reference the same geometry.

Which die steel should I specify?

For runs above roughly 15,000 pieces on steel, H13 pays for itself: it holds hardness at temperature and resists heat checking. For 20,000–60,000 pieces, 5CrNiMo is tougher and more forgiving of a marginal cooling setup. Below 30,000 pieces on small blocks, 5CrMnMo is adequate. The quotation states the grade and the heat-treatment route rather than leaving it to the die shop.

Who owns the die, and can I move it?

The customer owns the die once the first article is approved. The quotation states the die ownership, the condition it is returned in, and the fact that the die drawings and electrode set travel with it. Moving a die to a second source is possible; what does not travel is the process card that made it run, which is why the process card is also handed over.

What is the difference between die try-out and the first article?

Try-out is a fill and release check on the press: whether the cavity fills, whether the part strips cleanly, and where the die shows its first wear. The first article is the dimensional measurement against the drawing, from a CMM, in a controlled room. Try-out happens first, and a die that fails try-out is corrected before it is measured.

How much machining stock is left on a forging?

One and a half to three millimetres per machined face, fixed on the die layout. The lower figure applies to faces that will be skimmed for flatness only; the higher figure applies where a decarburised skin or a scale pocket has to be removed completely. Stock that is too thin is worse than stock that is too thick, because it puts the decarburised layer into the finished part.

All figures are typical reference values for the materials and processes named. Final acceptance criteria are fixed by the drawing, the applicable standard (ISO 8062, ISO 2768, ASTM E140, EN 10204, ASTM A388) and the agreed inspection plan.

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