Sheet Metal K-Factor Reference Worksheet
A K-factor is only useful when it is tied to the exact material, thickness, radius and tooling that produced it. This worksheet is a clean place to log verified values and see the bend allowance that each one implies.
| Job / part | Material | Thickness (mm) | Inside radius (mm) | Bend angle (deg) | Verified K-factor | R / T ratio | Bend allowance (mm) | Verified on | Tooling / V die | |
|---|---|---|---|---|---|---|---|---|---|---|
1 | 4.3982 | |||||||||
1 | 3.3458 | |||||||||
1 | 6.5031 | |||||||||
1 | 1.2723 | |||||||||
1.5 | 12.2522 |
Log a K-factor only after measuring a formed test piece. Keep one row per material, thickness, radius, angle and tooling combination, and export to Excel to share with the programming and quoting teams.
Link verified bend data to ERP routings
Optional: talk through storing K-factors and bend tables against routing operations in your ERP.
Optional. The tool and every export work without this. No spam.
How to use the worksheet
Add a row each time you validate a bend on a first article. Enter the material, measured thickness, inside radius, bend angle and the K-factor you settled on, plus the tooling and the date. The sheet calculates the radius-to-thickness ratio and the bend allowance for that row so you can sanity check it against your CAD flat pattern. Export to Excel or CSV and share the file with programmers, estimators and the press brake team so everyone works from the same values.
- One row per unique combination of material, thickness, radius, angle and tooling.
- Record the date so old values can be retired when material suppliers change.
- Note the V die opening, because it controls the formed radius in air bending.
What drives K-factor
K-factor locates the neutral axis as a fraction of thickness, measured from the inside surface. With tight bends the neutral axis moves toward the inside, so K is smaller. As the radius grows relative to thickness, K moves toward 0.5. Material strength, springback, grain direction and bending method all shift it. That is why a single published value is a starting point, and why a verified log by material and tooling is far more reliable than a generic chart.
- Low R/T ratio usually means a lower K-factor.
- Harder, higher-strength materials spring back more and may need a different effective K.
- Bottoming and coining behave differently from air bending.
Back-solving K from a test bend
To find K from a measured part, form the bend, measure the two outside leg dimensions and the cut blank length, then compute the actual bend deduction as legs minus blank. Add the outside setback twice to get 2 x OSSB - BD, which equals the bend allowance, and solve K = (BA / (angle in radians) - R) / T. Enter the result in the worksheet. Check it with a second piece from a different sheet to catch lot-to-lot variation before you publish the value to the shop.
- Use calipers on the formed legs and a measured flat length.
- Repeat on at least two pieces.
Common mistakes
Do not copy a K-factor from a different material or a different press. Do not store values to more precision than the process can hold, since two decimals is usually plenty. Avoid logging the nominal gauge thickness instead of the measured one. Finally, keep the worksheet under document control, so a changed value gets a date and an owner. In an ERP environment, the same record can be linked to the routing operation so the CAD, quote and shop floor stay aligned.
- Do not mix inch and mm rows in one sheet.
- Retire values when tooling is replaced.
Frequently Asked Questions
Is there one correct K-factor for steel?
No. Published values for steel commonly range from about 0.33 to 0.50 depending on radius, thickness and method. The only reliable value is one you measured on your own press with your own tooling and material, which is why this worksheet logs it per combination.
How do I convert K-factor to bend deduction?
Compute bend allowance as angle in radians x (R + K x T), compute outside setback as (R + T) x tan(angle / 2), then bend deduction is 2 x setback minus bend allowance. The bend calculator on this site does it in one step.
Does the angle mean the included angle?
In this worksheet the angle is the bend deviation from flat, so a standard right-angle flange is 90 degrees. If your drawing gives the included angle, subtract it from 180 first to get the value to enter.
Can I import this into CAD?
Export to Excel or CSV and map the columns into the bend table format of your CAD package. Most systems accept K-factor or bend allowance by material and thickness, but the file layout differs, so a quick reformat is usually needed.
If you want these verified values to live next to your item and routing master data instead of a loose spreadsheet, Netray can help design the ERP link. We can work on-prem if your data cannot leave the plant.
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