What drawings and data are needed to quote sheet-metal bending? ART-451

What is calculated during bending

Bending is not merely changing a flat sheet into a part with angles. A press brake forms the part with an upper tool and a lower die; backgauges help position the blank. This is the general process described by TRUMPF in its press-brake materials. For a quotation, it is necessary to understand how the part will sit in this process and how many times it will need to be reoriented.

In a practical request, the following are usually assessed:

• whether the required geometry can be produced on the available equipment type;
• how many bends and repositionings may be required;
• what tooling is needed for the particular thickness, radius and flange shape;
• whether already formed elements, holes or flanges interfere with one another;
• which dimensions and angles must be checked after the operation;
• how many parts are produced, what the batch is, and whether repeat setup is required;
• what the order includes: bending a prepared blank only, or material and subsequent operations as well.

Therefore, a drawing is not needed “as a formality”. It provides geometry, while the accompanying data turns that geometry into a real manufacturing task.

Minimum package for an initial quotation

For most simple parts, it is enough to assemble the following data. If a field is not yet known, it is better to state “not defined” than substitute an approximate value.

| Block | What to provide | What it affects | |---|---|---| | Drawing | PDF or another readable document with views, dimensions and bend indications | Provides the finished-part geometry and acceptance criteria | | 3D model | STEP, Parasolid or native CAD format, if available | Helps check the three-dimensional form, flange relationships and possible collisions | | Material | Grade or agreed material group, condition and coating | Affects forming behaviour and the chosen process approach | | Thickness | Nominal thickness and, where important, the actual range | Affects force, tooling, radius and dimensional changes | | Quantity | Total quantity, batch size, one-off or repeat order | Affects preparation time, changeovers and work organisation | | Requirements | Angles, critical dimensions, tolerances, surface requirements and visible side | Defines what must be checked and agreed | | Additional operations | Cutting, holes, welding, threads, painting, assembly — if included in the task | Prevents the bending price from being mixed with other operations | | Incoming blank | Who supplies the sheet or blank, its format and condition | Defines responsibility boundaries and order logistics |

This set is sufficient for an initial discussion. A final quotation may require checking the specific part, especially for complex shapes or tight tolerances.

How to prepare the finished-part drawing

The drawing should describe the exact result required after bending. It should preferably state measurement units, part number or name, document revision, material, thickness, all material dimensions, angles, radii and tolerances. If some information is held in the 3D model, state this explicitly rather than leave the supplier to guess which model revision is current.

Check separately that it is clear:

• what the finished, already bent part looks like;
• from which datum dimensions are measured;
• whether the inside radius or an allowed range is specified;
• whether angles are specified as inside, outside or relative to another surface;
• which dimensions are critical for assembly;
• which surface is the face side and whether tool marks are restricted;
• whether holes, slots, cut-outs or threads are close to a bend line;
• whether a mirrored version or a particular part orientation is required.

For general linear and angular tolerances, a drawing may refer to an adopted standard. For example, ISO 2768-1 describes general tolerances for linear and angular dimensions without individual tolerance indications and explicitly covers parts formed from sheet metal. A standard reference, however, does not replace agreement on the tolerances genuinely required for a particular part.

For geometry — for example, the relationship of surfaces, parallelism or perpendicularity — plus/minus signs alone may be insufficient. ISO 1101 defines rules for geometrical specifications, while ISO 14405-2 explains why a linear tolerance does not always unambiguously describe the characteristic being controlled. Critical geometrical requirements should therefore be placed directly on the drawing or described unambiguously in the technical conditions.

Material and thickness: why “black metal” is not enough

For a quotation, not only the thickness figure matters but also what the sheet is made of. Structural steel, stainless steel, aluminium alloys and galvanised material may behave differently in forming. Material condition, protective film, coating, rolling direction and requirements for the outer surface may also affect the result.

If the material grade is not defined, record that as a quotation limitation. The wording “approximately 2 mm steel” is suitable only for a rough estimate. For a serial part, it is better to specify the grade from the certificate or purchasing specification. If several materials are acceptable, list them separately: a tooling approach must not be automatically carried across all alloys.

Likewise, do not provide only “from 1 to 6 mm”. For assessment, a table of material, thickness, quantity and batch size is more useful. A 6 mm thickness may be an occasional item while 1.5–2 mm makes up the main volume. These are different manufacturing scenarios and different work-organisation requirements.

Geometry, bends and places often forgotten

The drawing must show more than the outside envelope. Every bend needs an angle, direction, inside radius or acceptable range, flange lengths and their mutual position. If one bend forms a datum for the next, this affects the operation sequence and tool access.

Check especially carefully:

1. Short flanges. Their minimum length is not a universal number: it depends on thickness, die opening, tooling, material and the specific machine. Feasibility cannot be confirmed from a reference table alone without checking the actual set. 2. Holes near bends. State their diameter, shape and distance from the bend line. After forming, hole geometry or location may matter for assembly. 3. Cut-outs and narrow bridges. They may change blank stiffness and affect deformation. 4. Inside corners and boxes. Deep or closed shapes may limit tool access and the bend order. 5. Visible surface. If the part is installed externally, identify where die marks, scratches or local impressions are unacceptable. 6. Mirrored parts. Left and right parts may have identical dimensions but different orientation and separate documentation requirements.

Also provide the number of bends and their logic if it has already been defined. If the order is not determined, the supplier needs to know that the sequence must be developed and agreed.

Tolerances: do not write “maximum accuracy”

The phrase “high accuracy is needed” is not a measurable requirement. Instead, mark critical angles, flange lengths, distances between datums, perpendicularity, flatness or other characteristics that affect assembly. For each requirement, state the units, tolerance and inspection method if it has already been agreed.

Not all dimensions need equally strict control. If one flange enters a matching slot, its dimension may be critical. Another outside overall dimension may have a wider allowable range. This distinction helps avoid excessive requirements across the whole part and makes the quotation more realistic.

If the drawing contains only a general tolerance, read it together with the indications on specific dimensions and geometry. A general note must not accidentally be interpreted as a guarantee of identical accuracy for all surfaces, angles and complex shapes.

Quantity, batch and order format

For a quotation, not only the total number of parts but also its distribution is needed. For example, 500 parts in one run and five batches of 100 may have different shares of preparation work. State whether it is a prototype, one-off order, repeat series or regular schedule.

Record separately:

• the required quantity of finished parts and permitted overage;
• desired delivery split, if any;
• whether you supply your own sheet or ask for material to be included in the quotation;
• who is responsible for packing and delivery;
• whether marking, batch sorting or an inspection report is required;
• which operations are performed before bending and after it.

This data does not replace a drawing, but it makes it possible to compare offers of the same content. Bending a ready blank and manufacturing a part from sheet are different scopes of work.

Which file formats to send

For an initial quotation, it is convenient to send a PDF drawing and, where possible, a 3D model. The PDF is the approval document, while the 3D file helps check the form. The file name should include the part number and revision.

DXF or another flat-blank file can be useful, but should not be submitted as the sole source of data for the formed part. A flat pattern describes the flat state; the customer needs the result after bending. If the flat-pattern file is already prepared, link it in the request to the drawing of the finished part and state whether the contractor may propose corrections after a manufacturability review. This avoids a situation in which the parties understand a reference dimension differently.

Do not send several files with the same name and different changes without explanation. It is better to prepare an archive with the part number, revision, model and a short parameter table.

Practical request-preparation algorithm

1. Select the current drawing of the formed part and check its revision number. 2. Enter material, thickness, coating, quantity and batch size in a table. 3. Mark all angles, radii, critical flanges, holes and cut-outs near bends. 4. Separate general tolerances from dimensions requiring special control. 5. Add a 3D model or additional view if a flat drawing is insufficient. 6. State who supplies the blank and which operations are included in the order. 7. Add a photo of a reference part only as supporting material, not in place of a drawing. 8. Ask the quotation to show assumptions, what is included in the price, and which data must still be confirmed.

A sound quotation does not hide uncertainty. If material grade or tolerance is not yet approved, that must be visible in the offer.

Typical mistakes in bending requests

Providing only a photo of the part. A photo shows the general appearance but not exact dimensions, angles, radii and tolerances.

Stating only thickness. Two parts of the same thickness may have different materials, geometry, bend count and surface requirements.

Writing “bend to sample”. A sample may be deformed, have unknown material or fail to show critical dimensions. It can be added as a reference, but not as a replacement for a drawing.

Not stating the inside radius and angle. Without these, the contractor does not know which profile must be obtained.

Mixing several revisions. Even a small change to a hole or flange may alter the route and price.

Demanding “perfect accuracy”. Convert this wording into specific dimensions, tolerances and acceptance criteria.

Not mentioning the visible surface. Tool marks may be acceptable on an internal part but unacceptable on a face panel.

Not stating the batch. The same quantity in different production schedules creates different preparation workloads.

Treating DXF as a full description. A blank file does not always explain the finished three-dimensional part and its after-forming requirements.

What cannot be determined without additional data

From one general description it is not possible to responsibly name the exact price, guaranteed angle, required press force, particular die or final number of operations. At minimum, a drawing, material, thickness, geometry and quantity are required. In complex cases, a technical review, test part or agreement on critical dimensions is needed.

A result for one material must not be carried over to another without checking. The actual result depends on the machine, tooling, material, geometry and inspection method.

This article does not establish universal minimum flange lengths, radii, forces or tolerances. They are not one constant number for every press brake and every material. For a responsible series, these parameters must be confirmed for the particular part and equipment.

Checklist before sending the drawing

• The drawing is of the finished bent part itself.
• The part number and current revision are stated.
• Measurement units are specified.
• Material, grade or acceptable material group is defined.
• Nominal thickness is stated.
• Angles, radii and flange lengths are shown.
• Holes, slots and cut-outs near bends are identified.
• Critical dimensions and tolerances are highlighted.
• The face or visible surface is identified where important.
• Part quantity and batch size are stated.
• It is clear who supplies the blank.
• The operations included in the order are described.
• A 3D model is added if the form is complex.
• The DXF flat pattern is linked to the finished-part drawing.
• Assumptions still requiring confirmation are stated.

Conclusion

A good request is an agreed package that makes the finished geometry, material, thickness, quantity, accuracy requirements and order boundaries visible. The drawing answers “what must result”, and the table answers “from what, in what quantity and under which rules it is made”.

If data is not yet available, it is better to mark it as undefined. This separates a preliminary estimate from a final quotation.

To start a quotation, it is enough to provide the current drawing, material, thickness, quantity and a list of critical requirements. The bending route, tooling, inspection and quotation format can then be clarified.

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