Sheet Metal Design – What is a K-factor?

Introduction

One of the phenomena sheet metal fabricators must contend when bending parts is something called bending allowance. The reason for this is that when metal is bent, the material on the inside of the bend compresses, while the material on the outside stretches, and not by equal amounts (for reasons that get too technical even for this article). The result is that the workpiece actually “grows” in overall size if not compensated for. This compensation can be calculated using something called the K-factor, which considers variables such as the type of material, its thickness, the inside radius of the bend, and how it is bent.

What does this mean to the part designer? Even if your part is designed as a sheet metal part in a CAD Solids Modeler, when flattened, the flat development may or may not be correct, depending on the above-noted variables. It is for this reason that reputable metal fabricators may revise the model, and add their particular press brake bending parameters to create an updated flat development that will yield the part you require, and within your required tolerances.

Knowing your fabricator’s process and press brake tooling available (notably the radius of the male break dies), goes a long way to save time and potential surprises. The purpose of this article is to demonstrate to engineers and designers the technical aspects of how we (and you!) can determine the K-factor in sheet metal design.

Problem Statement

When files are provided by the customer, the 3D model has no bending parameters specified if it’s a STEP, IGES, or Parasolid file, and bending parameters are set to a default value in SolidWorks. When the flat pattern is developed for the part to be cut, it will often have incorrect dimensions to yield the customer’s desired bent part. In order to overcome this problem, your sheet metal fabricator should calculate the K-factor and revise the part’s flat development to yield your desired part within your specified tolerances.

Why is K-factor important in sheet metal manufacturing?

The K-factor in sheet metal design is important since it is used to correctly calculate flat patterns. This is because it is directly related to how much material is stretched during bending. K-factor is the ratio of the neutral axis to the material thickness. K-factor plays a key role in understanding the limits a material can handle during sheet metal bending.

General equation for K-factor:

BA= Bend Allowance

μ= Material Thickness

ρ = Inside Radius

β = Bend Angle (in degrees)

Calculating K-factor

1. Measurements taken from a bracket we manufacture:

β = 90°
ρ = 0.142 in (see Fig. 1)
μ = 0.135 in (Hot Rolled Steel)

FL1 = 1.378 in (see Fig. 2)
FL2 = 1.462 in (see Fig. 3)
Initial length = 2.583 in (see Fig. 5)

FIG. 1
Fig. 2
FIG. 3

2. Now that you have your measurements, we’ll determine the BA. To do this, first determine to bend outside set back (OSSB)

OSSB=Tan (β/2)(ρ+μ) Tan (90/2)(0.142+0.135)
OSSB = .277 in
FIG. 4
FIG. 5

Then determine the bend deduction (BD is not part of the formula but can be used instead of the K factor if desired).

BD = FL1 + FL2 – Initial length

3. Calculate bend allowance using the following equation

BA = 2*OSSB
= 2* 0.277
= 0.297 in

4. Plug the Bend allowance (BA), the Bend Angle (β), Inside Radius (ρ), and Material Thickness (μ) into the below equation to determine the K-factor (K).

Conclusion

A practical method to calculate the K-factor

1. PREPARE SAMPLES

- Begin by preparing 5 sample blanks which are of equal and known sizes .
- Record material type and thickness in test report
- The blanks should be at least a foot long to ensure an even bend, and a few inches deep to make sure you can sit them against the back stops.
- Measure Initial length of metal blank

2. BEND TESTING

- Set up the press brake with the desired tooling you’ll be using to fabricate this metal thickness. Make sure to use the same die and punch for all samples. If different punch or die is used it needs to be mentioned in the test report.
- Place a 90° bend in the center of the piece.

3. MEASURE SAMPLES

- Measure the flange length and inside radius of each piece, record length A,B and radius R as shown in the Fig. 6.
- The test report should have material type, material thickness, initial blank length, inside radius, flange length A and B at this step.

4. CALCULATION

- Calculate the K-factor by following the procedure mentioned earlier with the measurements obtained.
- This K-factor value is to be used in the trail-and-error approach in the following steps.

5. MODELLING

- Use the flange lengths measured to create a model on SolidWorks as shown in Fig. 7 and set the K-factor value obtained from the calculation (Step 4) in the SolidWorks model.

6. VERIFY

- Finally, develop a flat pattern from the model, and adjust the K-factor in SolidWorks until the flat pattern dimensions are equal to the measured initial length of the metal blank. The right K-factor of the metal that is being tested is when the overall flat pattern dimensions are equal to the measured initial length of the metal blank.

FIG. 6
FIG. 7

Die Maker

Job Summary

Reporting to the Director of the tool room, the person holding this position has a main tasks to manufacture and repair various tools such as punches, progressive and simple dies. 

Main Tasks

  • Studying the plans and specifications, the drawings, instructions for tooling, jigging, dies and punch projects; planning the workflow;
  • Calculating the dimensions and tolerances of the part to be manufactured or modified; executing sketches if necessary;
  • Operating various machines and tools such as milling machines, grinding machines etc;
  • Adjusting and assembling parts in a die or mechanism;
  • Verifying dimensions and alignments of machined parts using control and measuring instruments;
  • Testing dies and punches, tools, templates or completed assemblies to ensure their proper operation;
  • Removing and repairing worn or defective tools; repairing or modifying die elements;
  • Checking the results according to quality standards;
  • performing various machining operations to modify, repair or manufacture spare parts and installation structures;
  • Any other related tasks;

Required Skills

  • Strong skills in machining / mechanical engineering
  • Ability to provide support and solutions to production
  • Excellent ability with dies. (Evaluated according to experience)
  • Knowledge in heat treatments of steel would be an asset
  • Ability in mathematical calculation
  • Ability to read and interpret modification and manufacturing plans
  • Experiments in CNC milling and Wire edm would be an asset
  • Good knowledge of measurement and verification tools

Required Skills

  • Autonomy and versatility
  • Spirit of innovation and improvement
  • Adaptation to changes
  • Rigor and manual skill
  • Ability in communication and teamwork
  • Skilled in training and knowledge transfer
  • Sense of responsibility
  • Meticulous

 

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Machinist

Job Summary

Reporting to the Director of the Tool Room, the employee in charge of this position has the primary duty of adjusting and operating all conventional machine tools. 

Main Tasks

  • Studying the specifications of the machined parts, drawings or models and planning the various operations accordingly.
  • Calculating the dimensions and tolerances of the part to be manufactured or modified; preparing sketches of execution if necessary;
  • Regulating and operating conventional machine tools.
  • Verifying the results (inspection) according to the required quality standards.
  • Any other related tasks

Required Skills

  • Master in machining techniques.
  • Experience on conventional machines such as, lathes, milling machines, grinding machines, drills, saws.
  • Ability to occasionally provide support and solutions to production.
  • Ability in mathematical calculation.
  • Ability to read and interpret modification and manufacturing plans.
  •  Knowledge in heat treatments of steel would be an asset.
  • Good knowledge of measurement and verification tools.
  • Experience on CNC milling machine and / or Wire EDM would be an asset.
  • Master Cam programming experience for CNC and / or Wire EDM milling machines would be an asset.
  • Knowledge of punches and dies would be an asset.

Required Skills

  • Autonomy and versatility
  • Spirit of innovation and improvement
  • Adaptation to changes
  • Rigor and manual skill
  • Ability in communication and teamwork
  • Skilled in training and knowledge transfer
  • Sense of responsibility
  • Meticulous

 

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Machinist/CNC Programmer

Job Summary

Reporting to the Director of the Tool Room, the primarily responsibility of the employee will be adjusting, operating and programming CNC Milling Machines and Wire EDM machines. A secondary responsibility will be to operate conventional machines and Manufacturing, modifying or repairing single pieces for die or various machines.

Main Tasks

  • Programming for CNC milling machine and Wire EDM.
  • Regulating and operating CNC and conventional machine tools.
  • Studying the specifications of the machined parts, drawings or models and planning the various operations accordingly.
  • Calculating the dimensions and tolerances of the parts to be manufactured or modified; preparing sketches of execution if necessary;
  • Verifying the results (inspection) according to the required quality standards.
  • Any other related tasks

Required Skills

  • Master in machining techniques.
  • Knowledge of Master Cam programming software.
  • Experience on CNC milling machines.
  • Experience on Wire EDM machine.
  • Experience on conventional machines such as milling machines, grinding machines, drills, saws.
  • Ability to occasionally provide support and solutions to production.
  • Ability in mathematical calculation.
  • Ability to read and interpret modification and manufacturing plans.
  • Knowledge in heat treatments of steel would be an asset.
  • Good knowledge of measurement and verification tools.
  • Knowledge of punches and dies would be an asset.

Required Skills

  • Autonomy and versatility
  • Spirit of innovation and improvement
  • Adaptation to changes
  • Rigor and manual skill
  • Ability in communication and teamwork
  • Skilled in training and knowledge transfer
  • Sense of responsibility
  • Meticulous
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Matriceur, (Die Maker)

Sommaire du poste 

Relevant du Directeur de la salle d’outillage, la personne titulaire de ce poste a comme tâches principales de fabriquer et réparer différents outillages tels que des poinçons, matrices progressive et simple. Disponible pour faire du surtemps (overtime) et pour travailler sur le quart de soir au besoin.

Tâches principales

  • Étudie les plans et devis, les dessins, les instructions propres a des projets de fabrication d’outils, de gabarits, des matrices et de poinçons; planifie le déroulement des opérations;
  • Calcule les dimensions et tolérances de la pièce à fabriquer ou à modifier; exécute des croquis si nécessaire;
  • Règle et opère diverses machines et outils tel que tours, fraiseuses, rectifieuses, aléseuses, etc.;
  • Ajuste et assemble les pièces dans une matrice ou un mécanisme;
  • Vérifie les dimensions et les alignements  des pièces usinées en se servant d’instruments de contrôle et de mesure;
  • Essaie les matrices et poinçons, outils, gabarits ou montages terminés afin de s’assurer de leur bon fonctionnement;
  • Démonte et répare les outils usés ou défectueux; fait la réparation ou modifie des éléments des matrices ;
  • Vérifie le résultat selon les normes de qualité;
  • Peut effectuer différentes opérations d’usinage en vue de modifier, réparer ou fabriquer des pièces de rechange et de structure d’installation;
  • Toutes autres tâches connexes;

Compétences requises

  • Fortes compétences en technique d’usinage/mécanique
  • Capacité d’apporter du support et des solutions à la production
  • Excellente aptitude en matriçage. (A évalué selon expériences)
  • Connaissance en traitements thermiques des aciers serais un atout
  • Habilitées en calcul mathématique
  • Capacité à lire et interpréter les plans de modification et fabrication
  • Expériences en usinage CNC (milling) et Wire edm serais un atout
  • Habilité à souder au tig et mig serais un atout
  • Bonne connaissance des outils de mesure et de vérification

Aptitudes requises

  • Autonomie et polyvalence
  • Esprit d’innovation et d’amélioration
  • Adaptation aux changements
  • Rigueur et habilité manuelle
  • Aptitude dans la communication et le travail d’équipe
  • Habile à former et au transfert de connaissances
  • Sens des responsabilités
  • Méticuleux
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