From 2D drawings to nested sheets

Why the drawing is still the source of truth

Most sheet-metal work still arrives as a drawing. Customers send PDFs exported from CAD, scans of paper prints, phone photos and DXF or DWG files. Many parts have no 3D model at all. They were designed decades ago, or the model was never shared, and the released drawing is the only definition the customer controls.

That drawing is what the finished part is inspected against. It fixes the material, thickness, finish and every toleranced dimension. Everything downstream depends on reading it correctly: the model, the flat blank, the nest, the press tool and the quote. A misread thickness changes the material cost. A misread projection puts a hole on the wrong flange. Neither error shows until parts are made.

This guide follows one part from drawing to nest: a 2 mm steel angle bracket, 150 mm wide, with outside legs of 40 mm and 30 mm. A second, stamped part shows how the same thinking applies to coil.

Reading a drawing

A drawing describes a 3D part in flat views. Orthographic views are square-on projections of the part from different directions, usually a front view, a plan from above and one or more side views. Each view shows true lengths in two directions. The reader combines them into one shape.

The projection convention fixes where each view sits. In first-angle projection, common in the UK and Europe, the plan sits below the front view and the view from the left sits to its right. In third-angle projection, common in North America, the plan sits above the front view and the view from the right sits to its right. A truncated-cone symbol in the title block says which applies. Read a drawing in the wrong convention and the part comes out mirrored.

The rest of the information sits in three places.

• Dimensions and tolerances. Critical dimensions carry their own tolerance. Everything else takes the general tolerance in the title block. Hole callouts such as 2× Ø9 give size and count.
• The title block. Part number, revision, material grade, thickness, finish, units and projection. Quote and cut to the revision the customer has released.
• Notes and extra views. Bend notes give direction, angle and inside radius, such as UP 90° R2. A section view cuts through the part to show a profile the outside views hide. A detail view enlarges a small feature.

Common traps

• Views that disagree. A length reads 150 in one view and 148 in another, or a hole appears in the plan and nowhere in the side view. One of them is wrong, and the drawing alone may not say which.
• Inside or outside dimensions. A flange can be dimensioned to its outside face, its inside face or the virtual sharp, the point where the flat faces would meet if the bend had no radius. On 2 mm sheet, the same 40 mm figure describes legs 2 mm apart, depending on the face it is measured to.
• Missing thickness or radius. Thickness may appear only in a note, as a gauge number, or nowhere. The radius may say “to suit tooling”. Both change the flat blank, so both need an answer before anything is unfolded.
• Mark-ups on scans. Handwritten changes on a scanned print can override the printed dimensions. They count, and they are easy to miss.

Rebuilding the part in 3D

The model turns the views into one consistent solid. For sheet metal it should be built as sheet metal: constant thickness, with every bend defined by its angle, inside radius and direction. A solid that only looks right cannot be unfolded reliably.

The usual order is to start from the largest flat face, then add each flange, a bent-up edge of the sheet, with its bend and radius. Holes and cut-outs go in from the view where they appear at true shape. Where a bend runs into an edge, a bend relief, a small notch, stops the material tearing. Then every dimension on the drawing is checked against the model, view by view.

The finished model is usually exported as STEP, a neutral 3D format that CAD systems read. The customer can check it, and the shop can use it for tooling, programming and later revisions.

Unfolding: the flat pattern

The flat pattern is the shape cut from the sheet before bending. Its length is shorter than the sum of the outside dimensions. When sheet bends, the outside of the bend stretches and the inside compresses. Between them lies the neutral axis, a layer that keeps its original length. The flat length through a bend is the length of that layer.

The K-factor locates the neutral axis. It is the neutral axis’s distance from the inside surface divided by the thickness. Typical values run from about 0.3 to 0.5, depending on the material, the ratio of bend radius to thickness and the bending method. Bend radius here means the inside radius.

Two quantities follow from it. The bend allowance is the length of the neutral axis through the bend: BA = θ × (R + K × T), with θ the bend angle in radians, R the inside radius and T the thickness. The bend deduction is how much shorter the flat blank is than the sum of the outside leg lengths.

For the bracket, each leg’s straight portion is its outside length minus the outside setback, the distance from the virtual sharp to where the bend begins. For a 90° bend that is R + T. The flat length is the two straight portions plus the bend allowance.

The flat blank is 66.52 mm, against 70 mm for the outside legs added together. The difference, 40 + 30 − 66.52 = 3.48 mm, is the bend deduction. The inside dimensions, 38 mm and 28 mm, add to 66 mm, so they miss as well.

The K-factor matters. At K = 0.33 instead of 0.44, the bend allowance falls to 4.18 mm and the blank to 66.18 mm, 0.35 mm shorter. On a flange toleranced at ±0.2 mm, that error alone puts the part out of tolerance, and it repeats at every bend.

Grain and bend direction

Rolling gives sheet a direction. The grain direction is the rolling direction of the sheet or coil, and the metal behaves slightly differently along it and across it. A bend whose line runs across the grain, at right angles to the rolling direction, is less likely to crack on the outside than one that runs along it. The difference matters most at tight radii, in high-strength steels and in harder aluminium tempers. In mild steel at a generous radius it is often small.

Grain has a visible meaning too. Brushed stainless steel and aluminium have a finish direction, and customers usually expect it to run the same way on every part of an assembly.

On a sheet cut to length from coil, the rolling direction usually runs along the long side. So the grain rule becomes a nesting rule. A blank whose bends must run across the grain can sit at 0° or 180° on the sheet, and never at 90°. A part with bends in two directions has no orientation that suits both. Some shops nest such parts at 45°, so that neither bend runs along the grain, at a cost in material.

Bend direction adds a second constraint. The flat pattern marks each bend up or down relative to its top face. If one face is cosmetic or carries protective film, the blank must keep that face on the same side on the cutting machine, so the nest cannot mirror it.

Nesting on sheet

Nesting arranges flat blanks on stock material so the required parts come from as little material as possible. Shops stock a few standard sheet sizes, such as 2000 × 1000, 2500 × 1250 and 3000 × 1500 mm, and each nest starts from one of them.

Within the grain and face rules, a nest uses a few techniques.

• Rotation. Turning blanks to fit more on the sheet, where the rules allow.
• Part-in-part. Placing small parts inside the cut-outs of larger ones.
• Common-line cutting. Two neighbouring parts share one straight cut, which saves the gap between them and some cutting time. It changes the cut sequence and the heat going into the parts, so shops use it where the process and tolerances allow.
• Spacing. A gap between parts and a margin at the sheet edge, set by the cutting process and the thickness.

Utilisation measures the result: the area of the parts divided by the area of the sheet. What remains is the skeleton, the frame of scrap left once the parts are removed, and sometimes a remnant, an offcut large enough to return to stock for a later job.

The comparison below nests the bracket’s flat blank on a 2500 × 1250 mm sheet in two ways.

Nest B fits 34 more parts and uses 85.2% of the sheet against 74.5%, but every bend runs along the grain. Nest A follows the grain rule and leaves a 2500 × 150 mm strip. If the shop keeps that strip and uses it on a later job, it is not charged to this one, and A’s material per part falls to 181 g, within 1 g of B. That credit is real only if the shop actually uses its remnants.

So utilisation alone does not set the price. The grain rule, the remnant policy and the sheet sizes in stock all move the material per part. Shops that cut many jobs in the same material also mix them on one sheet and fill the gaps with small parts. Each job’s quote then needs a rule for how much of the sheet it carries.

Nesting on coil for stamping

High-volume parts are often stamped from coil rather than cut from sheet. The coil is slit to a strip, and the strip feeds through a press tool that blanks one or more parts at each stroke. The arrangement of blanks along the strip is the strip layout.

Two numbers define a strip layout. The strip width is the width of material the tool needs, including the edge margins that carry the strip through. The pitch is the length of strip used per part. A tool that blanks two parts per stroke advances the strip two pitches at a time. Between blanks, a narrow bridge of material holds the strip together.

The gross weight of each part, the material it consumes including scrap, follows directly:

Gross weight per part = strip width × pitch × thickness × density

The figure shows three layouts for a small trapezoidal blank, 56 mm wide at the base and 40 mm tall, in 2.5 mm steel. One-up places one blank per pitch and needs the simplest tool. Nested head to tail turns every second blank through 180° so the sloping edges interlock. Rotated turns the blank 90°, which widens the strip.

The nested layout uses 15 g less steel per part than one-up. Over 100,000 parts a year, that is 1.5 tonnes. It also needs a tool that blanks two parts per stroke, or a second pass with the strip turned round, so the tooling costs more. Which layout wins depends on volume, and a good quote shows the options.

This is why the strip layout drives the material line in a quote. Material cost per part is gross weight times the price per kilogram, less any credit for scrap, and for pressed parts it is often the largest single cost. An estimator who can show the strip width, pitch and utilisation behind a gross weight can defend the price. Our guide to quoting stamped and sheet-metal parts covers the rest of the quote, and CAS Quotation prices parts from these same numbers.

How CAS approaches it

2D to 3D follows the same path from drawing to nest. It shows its working at each step, so an engineer can check the result before a sheet is cut.

• Reads the drawing. PDFs, scans, photos and DXF or DWG files: views, dimensions, tolerances, bend notes and the title block.
• Rebuilds the model. A solid with its flanges, bends, holes and cut-outs, delivered in the format your CAD uses: STEP, IGES, Parasolid or a native part file. Where views disagree or a dimension is missing, it asks, with a suggested answer and its reason.
• Unfolds with your bend data. Bend allowances come from your own K-factors for each material, thickness and tool, and the flat pattern lists the value used at every bend.
• Nests on your stock. Blanks in metal, wood, plastics or composites go onto the sheet sizes you hold or onto coil strip, within your grain and face rules, with part-in-part and common-line cuts where they fit. Each nest reports utilisation and remnants. Each strip layout reports strip width, pitch and gross weight.
• Exports what the shop uses. 3D models in neutral or native CAD formats, DXF flat patterns and nests, PDF nest reports and cutting programs for your laser, punch, plasma, waterjet or router.
• Feeds the quote. Gross weight passes to CAS Quotation with the nest or strip layout behind it.

2D to 3D is in early access. If turning drawings into blanks and nests takes too much of your engineers’ time, we would like to try it on your drawings with you.