Complex Large-Size Deep Drawn Parts

A large deep-drawn part is a different process from a small one. The tooling is different, the press is different, the lubricant is different, and the acceptance criteria are different. Describing it with the same words as a small draw is how large parts end up with wall thickness variations that nobody can explain afterwards.

Draw ratio decides which failure you get

The two ratios that govern everything are the blank-to-die ratio and the cup depth-to-diameter ratio. Each one produces a specific failure.

• Low blank ratio, deep cup. The blank is only slightly larger than the die opening and the cup is deep. Here the limiting factor is the corner radius at the die entry and the friction of the material sliding over the die shoulder. Wall thinning at the corner is severe and the bottom is the stiffest part of the part, because it received the least deformation.
• Large blank ratio, shallow cup. The blank is much larger than the opening and the depth is modest. Here the limiting factor is material draw-in. Excess blank area wrinkles in the flange before it can be pulled in, and the flange has to be trimmed away afterwards anyway.
• Both high. This is the large-size trap. The part is big in every direction and deep, and the press has to supply the force through a stroke long enough to get the draw ring over the whole flange.

On a large part, the practical consequence is that the blank is sized from the process, not from the finished outline. A blank that is correctly proportioned for a small part will wrinkle on a large one, and the wrinkling marks the flange permanently because the material has yielded in the wrong direction.

Why friction behaves differently when the part is big

Draw-in force is proportional to the perimeter being drawn, because the draw ring contacts the entire flange at once. The normal force from the blankholder grows with the flange area, which grows with the square of the size, while the useful draw force grows only linearly with the perimeter.

So on a part twice the linear size:

• Blankholder force grows roughly fourfold.
• Draw-in length grows twofold.
• The material has to travel twice as far over the die shoulder.
• The same lubricant that worked on the small part is now spread over four times the area and has to last twice as long.

This is why a large part frequently cannot be drawn in one operation and has to be split into a shallow draw followed by a restrike, or a draw followed by a draw ring reduction. The first operation gets the shape and the wall distribution approximately right at low force; the second brings it to final size with the material already in a favourable condition.

The other half of the problem is lubrication. Large deep draws are among the hardest operations to lubricate well because the lubricant has to survive a long sliding distance at high pressure. The failure shows up as galling of the draw ring and a torn surface on the flange sidewall, and it is the single most common reason a large draw is rejected.

The corner radius is the dimension that sets the wall

At the die entry, the sheet wraps from the flange around into the wall. That bend is where the material is stretched most, and the radius it wraps over controls the strain.

A sharp entry radius produces:

• A localised stretch that thins the wall immediately below the corner.
• Tensile stress on the outside of the bend, which on a large part is a large absolute area and therefore a high probability of a visible surface crack.
• Reduced draw force, because a sharp corner allows more material to feed — which sounds like an advantage until the thinning becomes out of tolerance.

A generous entry radius does the opposite: less thinning, higher draw force, and a smoother transition into the wall. The correct radius scales with material thickness and tensile strength, not with part size, so on a large part it is the same dimension but it matters much more because the affected area is much larger.

Where a part has a square external corner as well — which most sheet metal parts do — the external corner radius is a separate decision, and it interacts with the internal one. A tight external radius pulls material out of the wall and thins it further. A common drawing error is specifying external radii without checking the wall at the corresponding internal corner.

Piercing and the hole that is not in the flat blank

On a large part, holes in the flange are pierced in the same stroke, while holes in the wall or the corner are a separate problem. A hole in the sidewall has to be pierced after the cup is formed, because before forming it is not in the flat blank.

The tooling implication is a pilot hole or a piercing station positioned after the draw, and this is where a large part’s tooling cost lives: a progressive die for a large part is large, and adding a post-draw piercing station means adding die height, which means a taller press and more machine cost.

There is also a hole-quality consideration that is specific to large parts. The piercing force is proportional to the shear strength of the material times the hole perimeter times the thickness, and the burr direction and the break edge are determined by the clearance between punch and die. On a large part the punch is long and slender, and a long punch under a large side load will deflect, producing a hole that is not round and a burr that varies around its circumference.

Springback and residual stress on a part this size

A large part has a long lever arm from the press. Any out-of-flatness developed during forming is retained, and on a part this size it is measurable without instruments.

The mechanisms that produce it:

• Bending and unbending through the wall. The sheet is bent at the corner and unbent as the wall forms. The unbending leaves a residual stress, and the part relaxes a little after the tool releases.
• Non-uniform draw-in. If the flange feeds more on one side than the other, the part is not symmetric and it will distort. This is usually a blankholder alignment or lubrication problem and it shows up as a twisted rather than merely a flat part.
• Ears. At a low blank ratio, the corners feed less than the sides, so the part comes out with four ears. On a small part ears are a cosmetic issue; on a large part they can be several millimetres and they change the fit of every downstream assembly.

The correction for ears is a blank shape that is not round — a compensated blank with material removed at the corners — and this blank shape is itself a designed item. On a large part it should be a stated, revisable drawing component with its own tolerance, because the compensation is empirical and will need adjusting between first article and production.

Flatness is a specification, not an outcome

For a large part, flatness needs a stated tolerance and a stated measurement method, and the method matters because a large part is never perfectly flat.

• On a granite plate with a straightedge and feeler. Correct and slow. On a part with a 300mm diagonal it needs a surface plate big enough and a method that defines the reference plane.
• On a measuring machine. Accurate and expensive, and it defines flatness as deviation from a best-fit plane rather than from a supported plane — which is a different and usually more forgiving definition.
• Against a functional fixture. Often the most meaningful: does the part sit flat on the assembly it goes into. This is the requirement that actually matters and it is frequently different from a general flatness figure.

Whichever method is used, the drawing should name it, because “flat within 2mm” is not measurable without a method and will be interpreted differently by the press shop and by incoming inspection.

Questions worth asking on a large deep-drawn drawing

• Is the blank a specified drawing item with its own tolerance, or is it treated as a consumable detail?
• Is the draw planned as one operation or a draw plus a restrike, and if a restrike, what wall distribution does it target?
• What draw radius and blankholder force are assumed, and are they reviewed against the actual press tonnage rather than a generic table?
• Is ear compensation applied, and is the blank shape revisable between first article and production?
• Are the corner and flange hole positions achievable in the planned stroke, or do they need a post-draw station — and is the press tall enough for it?
• Is flatness specified with a method and a reference, and does it match the assembly requirement?
• For a stainless steel part, is the lubricant specified and is the galling risk on the draw ring addressed?
• Is the wall thickness distribution specified at the corner, or only as a nominal?

The last question is the one that catches the most expensive problems on these parts. A nominal wall with no distribution is satisfiable at the flange and unachievable at the corner, so the drawing ends up either rejecting good parts or accepting parts that fail in service where the wall is thin.

This part belongs to our Custom Metal Stamping Parts range. The custom metal stamping parts pages collect the process notes, common materials and inspection points behind every product in the family.

Specifications

Item	Complex and Large Size Deep Drawing Part Stamping Custom Stainless Steel
Brand	AUNOV
Designing	Your drawing sets the part – unusual walls, threads or tolerances are feasibility-checked first; the agreed version governs every later batch.
Tooling	Die design and toolmaking stay in house, so tooling lead time stays under our control, and the charge is refundable against agreed volumes.
Samples	Pre-production samples come with a full inspection report, so any deviation is corrected at sample stage, and later batches are gauged back to the signed sample.
Materials	Carbon steel (SPCC / DC01), 304 / 316 stainless steel, aluminium and brass are held on site; the final grade is fixed with the drawing review.
Surface finish	Natural, plated or coated as the print decides; the agreed finish is logged, and thickness and adhesion are checked on the first article.
Processing	Progressive die stamping, trimmed and formed in one pass; first articles verify the whole flow before the run is released.
Equipment	Mechanical presses 5-250 T, in-press forming and piercing, backed by CNC turning centres and a climate-controlled inspection room.
Tolerance	+/-0.05 mm to +/-0.1 mm across the part; capability is checked on the first article before the run continues.
Delivery terms	FOB with DDP offered for EU and US addresses; payment T/T or L/C, freight quoted to destination.