Press Brake

What sheet metal shops wish you knew: Minimum flange, bend radius and tooling access

Here we are going to concentrate on certain guides about the designs of sheet metals, as proper comprehension of the designing process will result in preferable CAD solutions.

Fig. 1 a

One of the recent questions by our readers was about the guidelines on 3D CAD usage to design elements of sheet metals. First, let us sum up the way sheet metals are bent. Herein we are delivering certain tips on bend process possibly unfamiliar to those out of fabrication shops.

  • Certain parts of sheet metals are possible to shape through folding machines.
  • Certain ones are possibly to bend through press brakes.
  • Particular designs are shaped through stamp and coin procedures by the means of a special tool within stamp presses.

A fold machine, also referred to as a fold, box or pan brake realizes clamping of work pieces upon the static bed. Folding machines are vastly used in structural, decorative and duct trade.

No matter what type of equipment is applied, sheet metals have got bend specifications determining the appearance of accomplished products:

  • While bending, sheet metals get stretched. They might be cracked as well rather than stretch, which depends on certain shifts, such as used tools and the directions of the sheet micro-grains.
  • If the applied tooling is not special, the inner radii will differ within batches and shops. Most commonly it is regulated as a compensation for deviations in a flat work piece. Sheet metals have to be bent to gain the required non-restrained bending angles

Stretching Time: Flat Lay-out 

A flat lay-out is the forecast of accomplished parts (Fig. 1a). Fig. 1b depicts the part appearance prior to bending. Flat lay-out must be the subject of care for any designer. Meanwhile, it must be considered that adjusting flat lay-outs will provide such vital facts for bending as accessible tooling, materials and enginery. 

Arranging a flat lay-out by designers will result in more preferable designs of cut while notching the internal protrusions or flanges. The estimated cut breadth will probably determine if the work piece could be stamped, laser cut, or perforated / nibbled on sheet blanks.

Note this CAD tip: The default slot breadth equals the thicknesses of materials. The offered tip is especially crucial for perforated work pieces. A less large notch breadth, as little as that of the cutting hole, is convenient for pieces with laser or water-jet cutting.

Precise flat lay-out promotes the arrangement of materials and the estimation of costs. Material arrangement predicts the volume of economic order. Examples of flat pieces are depicted in Fig. 1c. 

For one thing, flat lay-out is commonly done by just one flick of a switch in 3D CAD. It implies simplicity. For another, precise lay-out suggests certain challenges for fabrication. The tooling, that the brake carries, impacts greatly the mode materials stretch in bending. Alterations in thicknesses and speeds influence the work piece reaction to the tooling.

Note this CAD guideline: In rare situations, when precise flat lay-out determines the designs and functions the CAD jokey has to be knowledgeable about particular manufacturing processes.

Fig. 1 b

In most cases it`s enough for CAD jockeys just to check the design capability of unfolding. Application of the same subtraction / bending tolerance as in a fab store is excellent experience, though not so compulsory.

Input the allowance in finished designs making your predictions on the whereabouts of the possible deviations as the work piece is exposed to varied fabrication steps.

Every single component of equipment used for cutting or bending changes the work piece. Plain parts emerging from laser or nibble typically correspond in the limits of ± 0.004 inches. Any accuracy brake usually repeats in limits of ± 0.004 inches.

It is generally offered that ± 0.005 inches work directly at the accuracy limit of plain sheet metals under the circumstances of a work shop. In case of bent sheets changing the thicknesses of raw materials makes the suggested accuracy to ± 0.010 inch for each bending. 

Method of Lay-out

Sheet metals stretch when exposed to bending. When bends are examined more closely, it becomes apparent that sheets stretch over the outside face and the inner face tends to get contracted. In case all dimensions of flange inner depth of get summarized, using bending tolerance for pre-stretching plain materials is crucial to make them compressed to their finished sizes. On the contrary, if the outer dimensions are summarized, then bending deduction becomes compulsory for the preliminary shrinkage of flat work pieces. So, this appears to be the most common method applied in fab-shops to calculate flat lay-outs.

It is crucial for CAD jockeys to take into consideration the calculations of fabrication workshops to prepare the lay-out for manufacture. It is quite simple to standardize base flanges in CAD, applying the pressure gauge chart. The metric data table may refer to the factory warehouse reductions in the K-factor for the three-dimensional CAD framework.

Design Guidance No.1: Depth of Flanges

The V-die breadth in press brakes puts limitation on flange sizes being bent. While bending, sheet metals should completely overlap the V-shaped stamp.(Fig.2) V-shaped die should be 5-8 times as thick as the work piece itself. Fragile sheets might demand a V-die ranging from 8 to 12 times more thickness.

In case V-dies are 5 times as thick as the materials, about 6 times more thickness is recommended to securely overlap dies. The top tool divides it into halves, this in its turn requires 3 times more thickness than that of the least flange sizes to air bend through press brakes.

A relatively small V-die brings about the formation of bending radii more suitably corresponding to the top tool radii. The reason of this is the rise of pressures in tool over the sheets. The wider the die, the gentler is the bend. Increase of pressures cause more injuries upon the work piece, as well. They look like polished and embossed lines running collateral to the flange lengths. The commonly suggested least inner depths of flange is 3 times as thick as the work piece, however, fold machines do not bear such a V-shaped die. Short flange depth gets shaped through folders in comparison with air-bend press brakes.

Note that consulting the fabrication shops will help to confirm the manufacturing capabilities for the required bending radii. Nkar Fig.1c

Fig. 1c

Design Guidance No.2: Check the inner radii.

The production workshop tends to adjust the tool setting to offset for deviations in flat work pieces. Any changes in the choice of tool, for instance the breadth of the V-shaped die or the radii of the top tool, change the method of compression / extension of flat work pieces.

In case three-dimensional designs begin with internal radii, the same as the thicknesses, in the factory workshop, there are not a few many options as regards to equipment and settings. This can bring on less waste and shorter production times. But it requires costs.

As a method for turning flat billets into precise complete products, the choice of a wide V-shaped matrix allows a smaller billet to turn into large common parts. To certain extent, large radii in the top tool also improve the final bending force.

When the bending radii become relatively small, the stress in work pieces rises. Exceeding the sheet wear points brings about crack formation. Fragile metals, for example aluminum, usually require considerably large bending radii compared to tough materials like cold rolled steels. For instance, bending 6061-T6 aluminum requires great efforts. The demanded inner bending radii are at least 6 times as thick as the material. Bend 5052-H32 aluminum requires nearly the same ways as those of mild steels. 3003-H14 material appears to be too tough, the same way as copper. Smaller bending radius and shorter flange depth are efficient in case of tough materials.

To sum up 1×thickness=inner radii rule is suitable in case of steels, stainless or other tough sheets. Consult with fabrication workshops about the usage of bending radii smaller than the sheet thickness in your design.

Design guidance No.3: Corner Processing

The intersection of two edges brings about the formation of an angle, at which flange length might be overlapped, under-lapped or butted. The key trouble of fabrication shops relates to the memory of sheets. Excessive bending of every flange is needed so that it can bounce back at the proper unlimited angles.

Design No.4: Work with U-channel

In case of 2 or more distant bend formation, manufacture designing becomes excessively decisive. There are various modes to model a U-channel as a component of based-flange drawing, pair-edged flange or angled flanges.

 In fabrication shops, the U-channel is formed using a succession of two bending procedures. The 1st bending nearly always suggests simplicity, yet, some design limitations emerge. The entire design is restricted by the utmost depths of the flanges to the radii of the cuff-for instance, 29 inches, and the lengths of flanges get restricted to the tool stand and the brake frame — for instance, 72 inches.

The 2nd bending in U-channels carries even more limitations. As soon as this bending is finished, the 1st flange rotates to the stance to accomplish The U-channel. Fig. 3a and 3bThe brake frame prevents the rotation from completion. To get a smaller U-channel, the tool itself might interfere.

DFM tip is as the following: it might require making U from L and I weld together.

While setting up in production shops, brake tool profile patterns are possible to apply for visual selection of the top punches. Through one-to-one complete U-channel design, shops will be able to locate the tool templates to ensure proper approach for the completion of the 2nd bending. Then the true tool is advisable to load in the brake confidently.

Thus, in fabrication shops brake tool designs are applied to estimate the manufacturing accessibility. The latter as well as tool force are major requirements.

Tons, necessary for bending, are no less significant. Relatively long flanges and great thickness demand higher tonnage. Increase bending radii through wide V-dies, which will contribute to the tonnage reduction necessary for bending completion. In case of uncertainty, consulting the fabrication workshop on capabilities is advisable.The top jaw of a folder restricts the size ranges of the U-channels when the 1st flange is rotated towards it. A narrow U-shaped channel is common for folders. A press brake experience more difficulties in the formation of tightly placed bends, as the designs require being pressed rather than folded. The force demanded to finish the bend just takes place. Goose-neck tools are generally applied to shape a comparatively narrow u-channel. Still, these tools are not as strong as direct punches. Consult your fabrication workshop as regards to the sheet materials, as well as the least size ranges of U-channels.

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