A "false flat pattern" in sheet metal fabrication is a part that has been cut to the wrong dimensions. The sheet metal has already been bent, so it’s too late. The flat pattern calculation relies entirely on how SOLIDWORKS estimates the material elongation during the bend. And by default, this calculation does not match your press brake.
- Incorrect flattening results from the calculation of elongation in the fold, which is set incorrectly by default.
- The K-factor Set the neutral fiber; SOLIDWORKS uses 0.5 by default, which is often too high for thin steel (0.4 is better).
- Three methods: K-factor (quick), margin/loss per fold (accurate but time-consuming), tables (the most reliable).
- Gauge table = standardizes thicknesses and radii; bending table = refines the developed pattern. The two work together.
- Best practice: Measure your actual values (3 test specimens per thickness) and create your tables. A half-day workshop, with accurate calculations right from the start.
What Happens Inside a Fold
When you bend a sheet of metal, the outer surface stretches, and the inner surface compresses. Between the two, one layer remains stationary: the neutral fiber. The developed length is the length measured along this neutral fiber. The challenge lies in determining where it is located within the thickness.
This is the role of the K-factor: a coefficient between 0 and 1 that positions the fiber neutrally. K = 0.5 places it in the middle of the thickness. The default value in SOLIDWORKS is around 0.5, and that’s the problem. The actual position depends on the material, thickness, bend radius, and tooling. For thin steel with a small bend radius, it is closer to 0.4. Blindly using 0.5 results in a flat pattern that is too long.
The Three Ways to Manage the Calculation
SOLIDWORKS offers three methods, ranging from the simplest to the most accurate. The K-factor, a single coefficient for the entire part, is fast and sufficient for prototyping, but imprecise for production.
Bend allowance or bend deduction: You enter the value to be added or subtracted for a given bend directly. This is more accurate, but time-consuming to enter bend by bend.
The tables. That's where things get serious.
Folding Tables vs. Gauge Tables
Two different concepts that are often confused. A bend table contains values for margin, loss, or K-factor based on radius, thickness, and angle. SOLIDWORKS interpolates between the lines. This is what produces an accurate flat pattern across a range of bends.
A gauge table standardizes your sheet metal thicknesses: for each gauge, it specifies the thickness, the default bend radius, and the K-factor. You select 12-gauge steel, and all the correct parameters are set at once. This prevents thickness errors and ensures that the entire team works with the same values.
The two work together: the gauge chart specifies the permitted materials and thicknesses, while the bending chart refines the calculation of the developed area.
Where They Live, and Why You Should Build Them Yourself
The files are Excel files located in the installation folder, under SOLIDWORKSlangfrenchSheet Metal Gauge Tables and Sheet Metal Bend Tables. The provided templates are generic. They do not take into account your press, your tools, or your material.
My position is clear: whether you bend in-house or at a regular subcontractor’s facility, measure your actual values and build your tables based on those. Three test specimens per thickness, one bend at a known angle, measure the actual developed area, then calculate the K-factor. It takes half a day in the workshop. After that, your flat patterns will be spot-on the first time, and you’ll stop having to trim the sheet metal during assembly.
As long as you stick with the default K-factor of 0.5, you'll have to compensate manually, piece by piece. It works, until the day someone forgets to compensate.
Are you designing a sheet metal production line using SOLIDWORKS? Talk to our CAD engineers.
Frequently asked Questions
Why is my SOLIDWORKS flat pattern incorrect?
Because the flat pattern is calculated using the default K-factor (around 0.5), which does not match your press brake or your material. The result is a flat pattern that is too long or too short.
What is the K-factor?
A coefficient between 0 and 1 that indicates the position of the neutral axis within the thickness of the sheet metal. At 0.5, it is in the center; in thin-gauge steel with a small radius, it tends to be closer to 0.4. This position determines the length of the developed pattern.
What is the difference between a folding table and a gauge table?
The bending table provides values for the allowance, loss, or K-factor based on the radius, thickness, and angle, and SOLIDWORKS interpolates between the rows. The gauge table standardizes your sheet metal thicknesses and sets the thickness, default radius, and K-factor all at once. The two tables work together.
Where are the sheet metal tables located in SOLIDWORKS?
These are Excel files located in the installation folder, under “Sheet Metal Gauge Tables” and “Sheet Metal Bend Tables.” The provided templates are generic: they do not take into account your press, your tools, or your material.
How can you get accurate layouts right the first time?
Measure your actual values: three test specimens per thickness, one bend at a known angle; you measure the actual developed area and work backward to the K-factor. You build your tables based on that rather than compensating manually, piece by piece.
In the same series: Symmetries in SOLIDWORKS and Master 3D Sketching.
