Sheet metal Forming

What sheet metal shops wish you knew: Hems, jogs and forming tools

Fig. 1a

The acceptable CAD workflow is to form with the sizes and functions of final products as a model goal. The final stage of the workflow is the extraction of flat workpieces from production data about the finished products.

Otherwise, you might design plane workpieces later making additional bending to model the finished product. 

Work on Hems

Hemming is a one-point bending of 180 degrees. Hemming can be of closed type. Mostly its inner bending radii equal 0. In case hemming bears radii above 0, it gets a U shape at crossed sections. Not like the U shape channel hemming bears single bend rather than 2. 

In three-dimensional CAD hems are quite simple to shape. Fig. 1a depicts that the process implies picking a hem or hems, setting deepness and making a decision on the clearance. It is possible to adjust hemming angles above 180 degrees to shape an enclosed tear-drop or open hooks. For beading edges, just select the types of hems out of hem settings. Fig.1b depicts edge hemming.  

 As a model component, hemming forms a look of larger thickness with no additional heft. This shapes a rather smooth rib hiding the rough rib of the part. From time to time it is a way to increase the durability of painted parts. Hems can do good sheet metal panels by stiffening them. They can create flat edges hiding the rough edges of a work piece. This can make the painted elements even more durable. Hemming may toughen sheet metal panels as well. Hemming has its disadvantages, though. Due to metal coated steel elements, the inner parts of hems might cause difficulties to protect against erosion. This might be troublesome, since the hem inner parts get wet easily, but hardly get dried.

In a stunning workshop, hems are formed through a few bends or machine circles. Machine circles require time, and in the effect, any hem increases design costs. The complementary cost, the edge demands a special tool setting, separately from the setting for a bend of 90 degrees.

Fig. 2a depicts the plane piece position being bent for the 1st time. The 1st bending implies forming sharp angles over 120 degree to identify the central area of hem bending. As soon as the upper punches lower to touch the work piece, spring-load dies get pressed. (Fig. 2b)

Fig. 1b

The 1st sharp hem bending usually finishes not so quickly. The reason of this may be the flopping and whipping of the work piece while bending for the 1st time. Even the work piece of medium sizes sometimes has to be supported by more than one machine operator to prevent it from whipping. Additional efforts are another reason of hemming cost rise unlike plain bending of 90 degrees. 

When the machine finishes its 1st cycle, the upper punches raise, the load-spring returns to its initial state. This is when the work piece releases. Next, the work piece positions to get bent finally. (Fig.2c) This appears to be swift process carried out by plane tops within sets of dies. The upper punch lowers onto the V-shaped die compressing the spring in dies. The initial sharp bending turns into finished angles of 180 degrees in this process. (Fig. 2d). The radii upon open-hem bending is exposed to deformity in this process.

This time also, when the machine finishes its cycle, the upper punches raise, the spring returns to its initial state. At this point the work piece comes out.  (Fig. 2e) 

 A tool like this is time-saving compared to tuning with a multi-purpose tool, but the selection of hem dies is restricted to edge sizes.

A CAD suggestion: Escape extra tool costs. Consult the fabrication-shop about hemming tool sizes and properties existing in stock.

Jog Bending

Jogging is composed of both matching opposed bends close to each other to form a Z off-set, that equals the sheet thicknesses. This provides suitable covering, as ell as stiffens edges with no additional volume or thicknesses.

Just like in case of hemming, jog modeling is simpler than fabricating. Special tool setting and operation are required for jogging, which appear clearly distinct from those that create bending of 90 degrees.

A DFM suggestion: Jogging incurs its own one-time design expenses and customization demands for entire manufacturing expenses. This and hemming are alike in these points.   

Fig. 3a depicts the setting of three-dimensional CAD through designing lines for bending location, adjusting off-set distances, deciding on bending radii, selecting bend angles.

One more DFM suggestion: For matching the virtual jogging tool performance cancel the bending radii on jogging features and adjust them to smaller values, say 0.004 inches. Cancel bending angles as well and adjust them to no more than 90-degree ones. 80-degree inner angles appear quite acceptable start. A jog bending is depicted in Fig. 3b. 

Fig. 2a

In fabrication shops jogging gets formed through die selection pressing bends in the work piece concurrently. (Fig. 3c)Jogging requires more pressures compared to one-time bending.

Jogging characteristically finishes like bottoming. There is no coining here, yet, the full contact with work pieces is ensured. In case of bottoming adjust the inner jog angles close to 90-degree ones rather than the suggested 80-degree angles. (Fig. 3d)

Jogging leads to a little more diverse stretch of work pieces. Turn to the fabrication shops on the plane layout calculation at hemming and jogging areas. While matching virtual tools, radii adjusting of jogging and angles is advisable.

Miter Flanges

Miter flanges save time. Elementary sketches form a single maybe more bends. (Fig. 4b) upon 1 or more ribs while making a one-step design. If more than one rib is chosen to lay sketches for flanges, the result angle is smoothed at an angle of 45 degrees to form the necessary plane lay-out.

A DFM suggestion: To set up miter-flanges, note that bending is going to over-bent for the compensation of the back spring, as well as consider clearance of 0.015 inches at miter angles.

Sketch Bending

The difference between this and jogging lays in single bending for sketched bending. (Fig. 5b) Like jogging, one-lined sketching realizes bending location.

Fig. 5b depicts a sample of sketching.

Unfold/Fold vs. Bending

There may be more advantages in feature modeling for plane work pieces compared to the finished products. CAD tooling provides designing features based on parameters for unfolding a bend in models. Fig. 6a depicts the process of appending relief incisions at the plane angles. The work succession of CAD implies the following:

  1. Adding unfolding features to smoothed parts.
  2. Adding certain features like extruding.
  3. Addition of folding features to bring the parts to their stable state.

Like Unfold tooling, the Flattening icon (Fig. 6b) switches the mode between plane lay-out and defaults. This switching seems a suitable mode to shift from folding to flattening.

CAD jockeys might as well learn from Flat icon to form gradual folding. In case the bending gets repressed, it cannot flatten. Fig.6 shows the case. The initial action should be to free the bending procedure from suppress to gain smooth lay-out. The quickest means of doing it is clicking the Flat icon.

Then, bending process should get suppressed to gain the requires bending. For that, decide on bends, click the right-side mouse, select Suppress. The bending emerges. Fig. 6 shows bending in process or already in flattened condition.

Fig. 2b

Through added configuration it is possible to make rapid switches from flats and folds in a particular succession. While illustrating, this method of bending is quite convenient.

CAD Form Tooling VS. Hard Tools

Within fabrication shops a punch machine applies tools to stamp an extruded hole, formation louver, raising lance, hammering countersink.  CAD jockeys might apply Formation tooling, CAD simulation for the tool die selection.

Formation tooling models are free from the work piece design models, Fig. 7a shows a sample of bridge lances. The Formation tooling designs depicts the embossing inner side. In the creation of the Formation tool models, CAD jockeys define the stop faces. Specify incisions for the creation of an opening. The bridged-lance each side is opened.

 Figure 7b shows the Formation Tooling model is dragged out of the folder on the surface of the sheet parts. The Formation Tooling can be positioned and circled to get accurate position. Fig. 7c shows the effects of the bridged-lance application.

The model depicted in Fig. 8 has turned to smth. requiring a few settings. Its modeling delivers simplicity. So does its edition.

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