Press Tooling Data

Die Clearance and Cutting Force in Press Tooling

The gap between punch and die decides the cut edge, the burr, the die life and the tonnage the press has to deliver. This page collects the clearance ranges the source gives for common sheet materials, the formulas behind cutting, stripping and lateral force, and the design moves that bring the peak tonnage down.

Clearance as a percentage of stock thickness Clearance is normally expressed as a percentage of stock thickness per side, and once it has been determined it has to be held when the punch and die are made.

Work material	Clearance per side	What to expect
Mild steel	5 to 12 percent	The widest band of the common materials. Raising the clearance inside the band lowers the cutting force and lengthens the run between resharpenings, with burr height and edge taper as the limits.
Aluminum, brass, draw quality cold rolled steels	9 to 11 percent	Soft stock runs best in the middle of its band.
Low carbon cold rolled, hot rolled pickled and oiled, CDA 110 copper, hardened brass	12 to 13 percent	These materials run best at the upper end of the mild steel range.
Higher carbon steel and annealed stainless steel	14 percent	Harder stock needs more room for the fracture path to run.
Hardened material	Above the values listed	Hardened stock needs additional punch to die clearance.

Source: Fundamentals of Tool Design, 6th edition (Society of Manufacturing Engineers, 2010), Chapter 8. Treat the figures as starting points. The source notes that bringing the clearance to its best value for a given material may take some experimentation.

The clearance values are per side. The physical gap between punch and die is twice the figure listed.
A cut edge made at a normal clearance carries about one third sheared edge and two thirds fractured edge, on the part and on the slug alike.

What a tight clearance and a generous clearance each give The two ends of the range trade edge quality against die life and cutting force, and most jobs sit somewhere between them.

Clearance band	What it gives	What it costs
Tight, 3 to 5 percent per side	Less taper on the cut edge, and fewer slugs pulled up out of the die opening	Higher cutting forces, and a tendency toward double breakage, particularly in thick material
Generous, 7 to 25 percent per side	Longer punch and die life between resharpenings, lower cutting forces, and no double breakage	A slug puller becomes necessary, and the cut edge carries more taper and more burr height

Double breakage and how it is solved

Double breakage appears when the clearance is too tight. The fracture starts from each side, the paths fail to meet evenly, and the edge is left ragged with secondary shiny areas inside the hole or on the slug. The usual answer is more clearance, often obtained by making the punch smaller, which holds both the hole size and the part size. Soft steels may need 12 to 15 percent per side, and thick blanks occasionally 25 percent. The fractured portion of the cut edge then carries a pronounced taper.

A generous clearance also raises the lateral force on the punch and the die, which can shorten tool life. The window between a clean edge and a long-lived tool is usually found on the press rather than on paper.

Which member is made to the drawing The size that has to be held is made to the drawing, and the clearance is taken from the other member. The rule covers irregular contours the same way, with the clearance applied around the whole outline.

The part that matters	Member made to size	Where the clearance goes
The punched slug becomes the part	Die opening, made to the part or blank size	Subtracted from the punch
The punched opening is functional	Punch, made to the hole size	Added to the die opening

Where this lands in the shop

For a washer the two cases sit in the same part. The outside diameter is a blank, so the die is made to size and the punch follows. The hole is functional, so the punch is made to size and the die opening follows. A drawing that does not say which feature is the product leaves the decision to the shop.

The grade a shop reaches for on this kind of tooling is discussed on the selection page for blanking and piercing dies.

The forces behind press tonnage Press tonnage is the sum of every force the die produces in one stroke, and the cutting force is the largest part of it.

수량	Formula	Working notes
Peak cutting force	F_s = L × t × S_s	L is the total length of cut, t the stock thickness, and S_s the shear strength. Shear strength is typically 60 to 80 percent of ultimate tensile strength.
Stripping force	F = L × T × 1.5, F in tons, L and T in inches	A rough approximation. The real figure moves with the area of metal in contact with the punch, the clearance, the sharpness of the punch, and the position of the stripper springs.
Stripping force, metric	F = L × T × 20,600, F in kN, L and T in metres	The source prints L and T in millimetres against this coefficient. The coefficient reconciles with the imperial rule at 20,600 kN per square metre, which is about 20.6 MPa of cut area, so the metric form reads L and T in metres.
Lateral force	F_H = F_V × C / (T − P)	C is the clearance, T the material thickness, and F_V the cutting force. P is the penetration, typically 0.33 × T.
Press tonnage	Sum of every force in the stroke	Add the stripping force where a spring loaded stripper is used, because its springs compress during the cut, and add spring pressure for forming, draw pads and similar elements.

The cutting force calculation takes no allowance for shear angles or for the timing of punch entry, which leaves a safety factor inside the result. AISI SAE 1010 cold rolled steel carries an approximate ultimate tensile strength of 56,000 psi (386 MPa) and a shear strength of 42,000 psi (290 MPa). Shear strength rises at the fast strain rates of pressworking, and the ultimate tensile strength is the safer basis for the estimate.
In progressive die work the length of cut has to include every pilot hole and all the cutting done on the carrier strip. The tool steel for that class of die is set out on the progressive die selection page.

Why alignment matters as much as tonnage

For round and symmetrical holes the lateral forces balance out, though the die still has to withstand the spreading load. Notching, shearing and other unbalanced cuts put the whole lateral load into the alignment system, and the pressure can exceed the press force by a factor of three or more. As punch deflection grows, the clearance grows with it, and the lateral force grows again. Guide pins and heel blocks exist to hold that deflection inside acceptable limits.

Reducing the tonnage without changing the part Two design moves spread the cut along the stroke. Both are estimated from one third of the stock thickness, the depth at which complete fracture is expected at normal clearance.

Move	What it does	Detail worth knowing
Step the punches in length	Punches that differ in length by one third of the stock thickness cut in sequence. Three identical punches stepped this way need about one third of the tonnage of a simultaneous cut.	The stepping also spreads the load along the stroke, which softens the shock at breakthrough.
Grind shear on the punch or the die	Shear equal to one third of the stock thickness cuts the tonnage by about 50 percent over the area that carries it.	Put the shear on the member that contacts the scrap, so the deformation stays away from the part. In piercing, the cut runs from the outer extremities of the contour toward the center.

Snap-through

The loud boom at breakthrough is stored energy in the press and die releasing at once. Presses are not designed to take reverse loading above about 10 percent of capacity, and the shock can work die components loose. The aim of punch timing and die shear is a gradual release of the tonnage. The limits are the tonnage the press can carry above the bottom of its stroke, and the flywheel energy available per stroke.

Center of pressure An irregular blank loads one side of the ram more than the other, and the layout of the tool has to keep that bending moment out of the press.

Finding the point

The center of pressure is the point at which the shearing forces balance. It is the center of gravity of the line that forms the perimeter of the blank, not the center of gravity of the area inside it. It is found from the length of each cutting edge element and the distance of each element from two axes laid out on the drawing, and the same procedure covers any contour.

The full calculation is tedious, and a result within about 0.5 in. (12.7 mm) of the true point is normally close enough. One quick method is to bend a soft wire to the blank contour and balance the frame across a pencil in two directions. CAD software returns the same information. The tool is then mounted so that the center of pressure sits on the central axis of the ram. The source names fine blanking among the work where this matters most, and the tool steel for those dies is a separate discussion on the fine blanking die selection page.

What the die steel carries

Press tooling wears at the punch edges and the die opening, and it takes the lateral load that grows with the clearance. Wear resistance decides most long-run dies, and toughness decides the tools that chip before they wear. 디2 and its family carry the wear side, and 오1 covers general press work at lower cost.

Where breakage is the limit

Thick stock, unbalanced cuts and tight presses move the decision from wear to impact strength. The shock resisting grades give up some wear resistance to buy toughness, and S7 is the common name in that group. The failure modes behind the choice are collected on the chipping resistance page.

The clearance has to survive heat treatment

A clearance ground into a die only holds if the tool keeps its size through hardening and tempering. The movement each grade makes during treatment, and the ways to plan around it, are set out on the size change in heat treatment page.

The material side of the same job

Clearance and tonnage sit on top of a grade decision. Production volume, stock thickness, impact load and dimensional stability are the inputs, and the way they map onto grades is worked through on the stamping, blanking and piercing die page.

Confirm before quoting

These figures are for general reference only. Clearance, tonnage and edge condition move with the material, the lot, the tool condition and the press. Confirm against your own trial or contact Aobo Steel.