Sheet Metal CAD Design in India: K-Factor, Bend Allowance, and Production-Ready Drawings, explained simply.
Sheet metal design is unforgiving — get the K-factor wrong and the part comes off the press brake the wrong size every time. This is the practical workflow for sheet metal CAD that ships to Indian fabrication vendors and comes back right the first time.
Core idea
What this blog covers
The single most common reason sheet metal parts come back wrong from Indian fabrication vendors: the CAD file used the wrong K-factor for the actual material and tooling combination at the vendor's press brake. The flat pattern is right in CAD and wrong on the floor. This is preventable with the right design workflow.
Main discussion
What K-factor is and why it matters
When sheet metal bends, the inner surface compresses and the outer surface stretches. The neutral axis — the line that doesn't change length — sits somewhere between. K-factor is the ratio of where that neutral axis sits, expressed as a fraction of material thickness. K=0.33 means the neutral axis is 33% of the way from the inner surface. Get this number wrong by 0.1 and a part with five bends is dimensionally wrong by millimeters. The K-factor depends on material (steel vs aluminium vs stainless), thickness, bend radius, and the specific press brake tooling.
Validating K-factor with the vendor
Before committing to final flat patterns, send a sample bend test piece to the vendor — a strip with a single 90° bend at the chosen radius. Measure it after bending; back-calculate the actual K-factor. Use that K-factor in your CAD. This 30-minute exercise prevents days of rework on production parts. Most capable Indian sheet metal vendors will run a test piece for free as part of the quote.
Bend radius selection — match the tooling, not your preference
Press brakes use V-dies; the inside bend radius is roughly equal to the V-die opening. Your CAD should specify a bend radius that matches a V-die the vendor actually has. Common Indian vendor V-dies: 6 mm, 10 mm, 16 mm, 22 mm, 32 mm. Specifying a 7.5 mm radius means the vendor either picks the closest die (with rounding error) or refuses the job. Specify in standard increments and confirm with the vendor before locking the design.
Material grade and gauge selection
Indian sheet metal commonly comes in mild steel (CRCA), galvanized (GI), stainless (SS304, SS316), and aluminium (AA6061, AA5052). Gauge tables differ slightly between vendors — a 'gauge 18' part might be 1.2 mm at one vendor and 1.25 mm at another. Always specify thickness in mm, not gauge number, in the production drawing. Specify the grade explicitly (SS304, not just 'stainless').
Production drawing essentials
A complete sheet metal production drawing has: (1) the 3D part view, (2) the flat-pattern view with bend lines marked, (3) overall dimensions on both views, (4) bend radius and direction (up or down) at every bend, (5) material grade and thickness, (6) finish (mill finish, powder coat, electroplated), (7) tolerances on critical dimensions, (8) GD&T where applicable, (9) part number and revision. Skip any of these and the vendor will fill in the gaps with assumptions — usually wrong ones.
Working with Yantrix on sheet metal CAD
We design sheet metal parts for Indian vendors with K-factor validation as part of the workflow. Deliverables include 3D parametric SolidWorks files, flat-pattern DXF, production drawings with full GD&T, and material / gauge specification. We coordinate K-factor test pieces with your chosen vendor before locking the design. Send us your part requirement and we'll come back with a fixed-scope quote within a business day.
Key takeaways
What readers should remember
- K-factor depends on material grade, thickness, and the press brake tooling — not a universal constant.
- Always validate K-factor with the vendor before generating final flat patterns.
- Provide flat-pattern DXF in addition to 3D STEP and 2D drawings — saves the vendor a translation step.
- Specify bend radius explicitly — don't assume vendors will pick the right one.
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