Sheet Metal & Fabrication Drawings

K-factor, bend allowance and why your parts come out wrong

When metal is bent, the outside stretches and the inside compresses. Somewhere in between is the neutral axis, which keeps its length. The K-factor describes where that axis sits, as a fraction of the thickness from the inside face. Typical values fall roughly between 0.3 and 0.5, depending on material, thickness, inside radius and tooling.

The bend allowance, the length of material used in each bend, follows from it: bend allowance = bend angle in radians × (inside radius + K-factor × thickness). Get the K-factor wrong and each bend adds a small error, which on a box with four bends adds up to a part that does not fit.

The most reliable K-factor comes from your own press brake: bend a test strip of known length, measure the flanges and work back. If your fabricator has a bend table or preferred values, we use those; if not, we start from sensible defaults for the material and note them on the drawing so they can be corrected.

Design rules that keep fabrication simple

• Inside bend radius at least the material thickness for mild steel and stainless, more for harder aluminium grades, matched to your available tooling.
• Holes and slots kept clear of bends, commonly at least 2.5 times the thickness plus the bend radius from the bend line, so they do not distort.
• Bend reliefs where a bend meets an edge, so corners do not tear.
• Flange lengths long enough to sit on the die; your press brake operator can tell you the minimum for each thickness.
• One inside radius throughout where possible, so the whole part bends with a single tool set.
• Grain direction noted for materials and finishes where it matters, such as brushed stainless.

Flat patterns for laser, plasma and waterjet

The flat pattern is what actually gets cut, so it has to be right. Ours are exported as DXF at 1:1 in millimetres, with every profile closed, no duplicate lines, bend lines on their own layer (or removed, if your cutter prefers), and engraved part numbers on a separate layer if you want parts marked. Kerf allowance is left to the cutting company unless asked for, because it depends on their machine. Read more in 2D CAD drafting.

We also supply a bend drawing showing each bend's direction (up or down), angle, inside radius and suggested sequence, so the press brake operator does not have to work it out from the 3D model.

Fabrication and weld drawings

Fabricated frames, guards, brackets and enclosures need drawings that show how pieces are cut, positioned and joined. Our fabrication drawings include a cut list of sections (box, angle, flat bar, tube) with lengths and end mitres, dimensions from clear datums, and weld symbols to ISO 2553 showing weld type, size, length and which side. Notes cover material grade, finish (galvanised, powder coated, painted) and any post-weld machining. Drawings follow the general rules in technical drawings, including projection, tolerances and revision control.

Weld procedure, welder qualification and structural calculations remain the fabricator's or engineer's responsibility. We draw what is specified; we do not sign off structural adequacy.

Materials and finishes on the drawing

A fabrication drawing should name the exact material, not just 'steel'. That means the grade and thickness, for example S275 mild steel, 304 or 316 stainless, or a named aluminium alloy and temper, because bendability, weldability and corrosion resistance all differ. Finish matters too: powder coating and galvanising add thickness, so close-fitting parts, threads and holes may need masking notes or extra clearance. We put material, thickness, finish and any masking on every drawing, so the cutter, folder, welder and finisher all work from the same information.

Price

Sheet metal and fabrication CAD is £45 an hour + VAT. A single folded bracket with a flat pattern is a short job; an enclosure with doors, hinges and a drawing pack takes longer and is usually fixed-priced. Fabricators sending regular work can agree a standing process and file format. See prices or design for manufacturers.

What you get

• Sheet metal 3D models and STEP exports
• Flat pattern DXFs for cutting, with bend lines on a separate layer
• Bend and fold drawings as PDF
• Fabrication and weld drawings with a cut list
• Assembly drawings with a bill of materials

How it works and what it costs

Logos from £150, brochures from £295, photography from £295 a half day and CAD at £45 an hour, all + VAT. You get a fixed price before we start.

1. Tell us the brief

   What it is for, who will see it, where it will be used and any examples you like. Rough is fine.

2. Fixed price

   We confirm the price and what you get, in writing, before any work starts.

3. First designs

   We design, shoot or model it and share the first version for your comments.

4. Refine

   We make the changes, check every detail, and prepare the files properly for print, web or manufacture.

5. Final files

   You get the finished files in the formats you need, ready for your printer, website, social channels or manufacturer.

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