IDEMI

10.1  KIND OF TOOLING

In general, short-run or single-operation dies are used for small-volume production where the cost of tooling must be kept low because it is the major cost factor, and the cheapest tool materials are used. Compound dies are used for medium to large-volume production where intricacy is not the dominant problem and where accuracy often is. Progressive dies are used for medium-volume, and particularly for large-volume, production where the die is preferably not made as intricate as the part.

In adjacent and simultaneous blanking operations where the die sections are thin or intricate, type A2 tool steel is preferred for runs up to 100,000 parts of most materials.  M2, D2, D3 or carbide is preferred for longer runs.

10.2 DIE COMPONENTS

Piercing punches. The usual limiting slenderness ratio of punch diameter to sheet thickness for aluminum, brass and steel is 2.5 - to – 1 for unguided punches and 1 - to - 1 for guided punches. The limiting slenderness ratio of punch diameter to sheet thickness for piercing spring steel and stainless steel is from 3-to-1 to 1.5-to-1 for unguided, punches, and from 1 – to - 1 to 0.5 – to - 1 for accurately guided punches.

Where these usual limits are exceeded and breakage cannot be eliminated by stepping the punches, tool steels such as O1, A2 and M2 are used. W1 is used if the diameter is greater than10 mm. but less than 20 mm. or 25 mm.

Piercing-Punch Bushings. The recommended materials for piercing-punch bushings of all three types (quill retainer, guide or stripper, and die button), particularly for bushings of the precision type, for instance, where the outside diameter is ground to – 0, + 0.0075 mm., concentric with the inside diameter within 0.005 mm. The hardness of the W1 bushing should be Rockwell C62 to 64; that of the D2 bushing, Rockwell C 61 to 63.

Die plates and die parts that hold inserts are made of class 50 gray iron, alloy steel, or (for heavy work) tool steel, and of cast iron or low-carbon wrought steel for blanking and piercing soft and thin materials.

For blanking or piercing thick sheets or hard materials, either gray iron of 2,800 to 4,200 Kgs. / cm.² tensile strength or 4140 treated to Rockwell C 30 to 40 should be used.  Particularly on heavy-gage or hard material and on long runs for which inserts are pressed in, steels like 4340 or H11 are used; when inserts are screwed into the die plate, 4340 is nearly always used.

Die plates for blanking or piercing thin or soft sheets may be made of gray iron of 2,100 to 4,200 Kgs. / cm.² tensile strength, or mild steel.

Punch holders and die shoes for carbide dies are of high-strength gray iron or mild steel plate. Yokes retaining carbide sections are usually made of O1, hardened to Rockwell C 55 to 60. Backup plates for carbide tools are preferably made of O1, hardened to Rockwell C 48 to 52.

Stripper plates can ordinarily be made of some low-carbon or medium-carbon steel like 1020 or 1035. Where a hardened plate, is used for medium-production work, the preferred steels are flame-hardened 4140, conventionally hardened W1 or for intricate shapes, cyanided and oil-quenched W1. For carbide dies and high-production D2 or D4 dies, hardened strippers are of O1 or A2 Rockwell C 50 to 54.

Guides and locator pins can be made from W1 or W2 for most dies, or from alloy steels such as 4140 for short-run low-cost dies. Many commercial guide pins are made from 1117, carburized, hardened, and finished to 0.6 micro-mm.

Combined operations like blank-and-draw or pierce-and-extrude give rise to selection problems best solved by determining which of the operations is the more severe, and selecting for that operation. Selection of material for pierce-and-extrude sections of dies should follow the recommendations of this article.

Wear of extrusion or embossing dies can be offset by nitriding A2 and D2 materials. However, nitriding may shorten the life of blanking tools because edges are likely to chip, unless the dies are used for thin or soft sheet.

10.3 TOOL MATERIALS

Table 10 shows nominal compositions of the tool steels recommended in the selection tables. All of these steels serve best when used at maximum tempered hardness, particularly in blanking thin material and when shock will be absent. For conditions of shock, the hardness is lowered to produce a tolerable level of breakage. Whether longer die life can be achieved by tempering to a lower hardness or by using tougher steel at full hardness cannot be readily predicted.

W1 and W2 are readily available, readily machinable, wear-resistant and highly versatile water-hardening grades, furnished with various carbon contents in 10% ranges.  W1 and W2 are interchangeable in performance, but W2 is of little advantage except that coarse grain is less likely to develop in the steel as a result of overheating.

The depth of hardness of the water-hardening grades is shallow and for this reason such steels should not be used where grinding of the hard case will be needed to correct for distortion due to heat-treating, except for short-run dies. W1 may make a brittle, easily broken punch if less than 1o mm. in diameter, but a tough one if the diameter is about 20 mm.  Hardness should be the highest obtainable at a temperature of 325 to 375 F – usually Rockwell C 62 to 66.

Shock-resisting tool steels S1 and S5 are used for punches only where the probability of breakage is high.  With normal heat treatment they have unacceptable levels of wear resistance, and they are economical only if they are carburized to obtain 0.25 to 0.50 mm. case containing 0.70 to 0.75% C. S1 should be used at Rockwell C 57 to 60, and S5 at Rockwell C 59 to 62.

Oil-hardening steel O1 is safer to harden and distorts less than W1 steel. O2 is preferred to O1 for dies that are to be made by broaching. It distorts less in hardening.  Steel O6 is easier to weld, has consistently better life in blanking and piercing dies than O1 and has reduced regrinding and maintenance by about one-half in blanking 1040 and other steels up to 10 mm. thick. Although less widely available than O1, the usage of O6 steel has increased greatly during recent years. Advantages derived from the use of O6 in die applications relate to its greater resistance to sliding wear and its better machinability, as compared with other O grades; however, it may distort more during heat treatment.

A2 air-hardening medium-alloy (5% Cr) tool steel has wear resistance about halfway between that of oil-hardening steels and that of D2. A2 presents the least hazard of size change and cracking in heat treatment of the entire tool steels, followed closely by D2, air-hardened D4, and then by oil-hardened O and oil-hardened S types. LikeD2, the A2 steel can be nitrided for dies for thin or soft materials or reinforced plastics, to resist wear and heat.

D2 high-carbon high-chromium air-hardening tool steel is probably the most commonly used and may be the most satisfactory and most widely available tool steel for large-volume production of blanks. It is about the second-best steel for high accuracy and for safety in heat treatment and it through-hardness in 3-in. sections. Its highest usable hardness of Rockwell C 62 to 63 is recommended for punches and dies where breakage is not a problem, as in dies blanking steel less than 0.062 in. thick and softer than Rockwell B 90. Maximum resistance to breakage may be developed by tempering back to Rockwell C 58 to 60, but only at a sacrifice in wear resistance. For lamination dies, the hardness should not be less than Rockwell C 61 or 62.

D4 high-carbon high-chromium air hardening tool steel is somewhat more wear resistant than D2 and D3, particularly in blanking and piercing electrical sheet, where, at Rockwell C 63 to 65 it often wears about 20% less than D and D3 and about the same as M2 high speed steel.  All of the high-carbon high-chromium steels should be nitrided to extend die life only for blanking reinforced plastics or for soft or thin materials.

D5 high-carbon high-chromium tool steel has replaced D2, D3, M2 and M3 in some plants for the piercing, trimming and blanking of austenitic stainless steel. Metal pickup and scoring have been minimized in such applications by the use of D5, with an increase of 100 to 200% in die life for some stainless steel parts.

M2 high speed steel is the least costly, most used, and most readily available high-speed steel for blanking dies and punches. It is equal to or better than, D4 in wear resistance.  For blanking and piercing electrical sheet, the conventionally hardened M2 is surpassed only by carbide, cast alloys, and carburized M2.

When carburized, M2 is about 30% more resistant to punch wear in making laminations than it is with standard heat treatment. It is equaled only by D4 and three less widely used high-speed steels: M4 carburized, T1 carburized, and T15 carburized. It is recommended in Table 5 with standard heat treatment because it is less likely to break than other steels of equal wear resistance than the shock-resisting steels S1 or S5 in blanking dies.

M3 high speed steel, with its 1% carbon and high vanadium content, is more wear-resistant than M2 and the D grades. Its wear resistance can be improved by liquid nitriding. Selection of M3 depends on whether the dies can be ground economically; to reduce the amount of grinding, M3 is generally used only for inserts. M3 is more difficult to grind than M2; caution must be used to avoid “burning” and the formation of surface cracks.

Hot rolled mild steel plate with carbon content from 0.10 to 0.20% may be used for short runs of small parts after it has been surface hardened, either by carburizing to a depth of 0.25 to 0.50 mm. or by cyaniding to 0.1 to 0.2 mm. Because it distorts in heat treatment, its use is limited to small, symmetrical shapes.

4140 alloy steel is generally available in various sizes of plate of aircraft quality. It is flame hardened to about Rockwell C 50 for long blanking runs on soft materials. However, flame hardened tools that have either inside or outside corners are likely to have soft spots that will wear rapidly. For large dies, flame hardening the working edge only, instead of hardening the entire die, has the advantage of minimizing the changes in size and the warpage that occur as a result of heat treatment.

Carbide tooling is usually considered where production is four or more times the life of a D4 tool steel die, especially where close tolerances and minimum burr are required and a heavier press is available. Partial or complete inserts of carbide in tool steel may be considered for lower quantities or where the tool life between grinds needs to be extended. However, brazed sections are hazardous, and dovetailed or mechanically held sections will approach the cost of a complete carbide die.

The first material should be used where shock is appreciable. The second of the above combines toughness and wear resistance and is preferred for heavy-duty service, such as piercing silicon steel. Where close tolerances must be held in piercing silicon steel laminations, the third material is useful. The last of the carbides listed will be best for guides and guide rolls, or for applications involving very high shock.

Selection of die material for press tool depends mainly on the type of metal being cut, bend or formed and on production quantities. Following table will gives recommendations for materials for die, punches, housing, punch holder, stripper plate, shank, strip guides, stopper and die base parts are listed here.