Bend drawings are 2D drawings, typically of sheet metal parts, that include bend lines which result in 3D shapes. Adding bends to sheets creates highly functional parts at low cost. 


Basics

To create parts made by bending sheet metal:

  1. Draw the flat shape (prior to bending).
  2. Select the outer shape lines and set Z to the sheet thickness.
  3. Select the holes and set Z to Air Inside.
  4. Draw straight lines where you want bends and choose Line | Machine | Bend.
  5. Choose View | 3D to check the 3D view.

 

 

sheet_metal_box_2d

Box Wizard

The Box Wizard is a feature that allows for the automatic design of sheet metal boxes. The wizard considers a number of subtle issues including material thickness, part fit, clearance, overhang, optimal design shape, screw holes, etc.

For simple two part enclosures it is recommended to use the wizard instead of designing from scratch as the box wizard considers many details. 

To use the box wizard:

  1. Draw the flat shape (prior to bending).
  2. Select the outer shape lines and set Z to the sheet thickness.
  3. Select the holes and set Z to Air Inside.
  4. Draw straight lines where you want bends and choose Line | Machine | Bend.
  5. Choose View | 3D to check the 3D view.

 

 

Box Wizard

Bend Modeling

When designing parts with bends where the parts must fit together, it is important to verify, on screen, that parts fit properly, that holes align, etc. The Job | Model Bends command helps you do so by creating a side view drawing of the bends. You can use the side view to:

  • Check fit by sliding parts into place on screen
  • Estimate distances between or heights of bend legs
  • Add tolerance lines to show a desired bend dimension
  • Adjust bend lines positions to achieve a desired bend model

case_model.gif (2765 bytes)

For mating parts, slide the side views on screen (and rotate if needed) to see if parts mate properly. For new or changed complex designs it is recommended to do a short run initially.

Also see Bend Specifications

 

    Note that:

  • Actual parts will vary from the model due to: limitations of the model, tolerance of parts before bending, tolerance of bend position, tooling variation, etc. 
  • You should incorporate allowance for these variations in your design (it’s better for pieces to be a bit loose than to not fit at all).
  • You must apply the command again if you change your design or Job | Specifications.
  • The general linear tolerance does not apply on exterior or interior dimensions of the part after bending. If needed you can specify critical dimensions and tolerances on the bend model using Comments to Machinist. For example, you might specify that the distance between legs in a U bend is 1.234″ +/- .01. 
  • For metal enclosures consider using File | New | Box which takes into account several details automatically. 
  • The model considers the sheet thickness, bend radius, and the compression and expansion of material that occurs at a bend.

 

Limitations

Some odd or complex shapes can confound the bend model computation.  If the bend model is clearly off, try copying your design to a separate file, simplifying the shape to a simple rectangle and generating the bend model on the simplified shape.

Adjusting Bends

You may need to make a part with bends such that a certain dimension is met after the bend occurs. You can do this easily using the technique shown below.

For example, suppose we design two parallel bends on a rectangular metal sheet to create a U shape and we want the outside distance across the U to be 1″. The example below shows how to do this with each step corresponding to the respective numbered drawing.

  1. Place the bend lines 1″ apart, knowing full well that it’s not quite that simple.
  2. Choose Job | Model Bends and move the computed bend model tangent to the leftmost bend line.
  3. Create the small “adjustment” rectangle to measure the error by placing it between the rightmost bend line and the rightmost edge of the U shaped bend model. Note that the horizontal size of the adjustment rectangle represents how much the outside of the U shape exceeds the desired 1″.
  4. Remove the bend model and flip the adjustment rectangle to the other side of the bend line.
  5. Move the right bend line to the other side of the adjustment rectangle. Then rerun the Job | Model Bends command and check the numeric bar to verify that the horizontal size of the bend model is exactly 1″. (Notice that one leg of the U is now longer than the other. This is easily corrected by drawing a horizontal line from the other leg and applying the intersect command. Also note that the same technique, with minor variation, can be used to target a specific length of each leg.)

bend_model

Example Design

The following example of a calculator-style enclosure illustrates some of the finer points of designing mating sheet metal parts.

The objective is to design a calculator-style enclosure of two parts that appear as shown to the right.

case_3d.gif (8220 bytes)

1. Draw top and bottom parts of the enclosure.

case_2d.gif (2363 bytes)

2. Choose Job | Model Bends.

case_model.gif (2765 bytes)

3. Rotate and slide on screen to test fit.

case_fit1.gif (458 bytes)

case_fit2.gif (1503 bytes)

4. Notice that neither drawing fits properly. Calculate the error by measuring with a temporary line.

5. Revise the design and try again. Delete the original bend model and choose Job | Model Bends again and test fit again.

case_rev_fit1.gif (498 bytes)

The above fits properly.

case_rev_fit2.gif (1290 bytes)

The above fits with desired overhang.

6. Check alignment of screw holes by drawing temporary guide lines as shown below. Then rotate and/or move the guide lines to the opposing piece.

case_rev_fit3.gif (3323 bytes)

 

 

7. Check the fit of the flat view of the bottom to the side view of the top.

case_rev_fit5.gif (1420 bytes)

8. Zoom in to check corners.

case_rev_fit_6.gif (538 bytes)

Bending Tips

Below are a few tips to consider in designing parts with bends:

1. Provide for allowance – it is better for pieces to be a bit loose than to not fit at all.

2. For two-part enclosures it is generally advisable to add at least some overhang.

3. To specify the general radius of bends choose Job | Settings | Specifications: Bend.

4. You can specify helical twists.

5. You can approximate a model of a curved sheet or a large radius bend by making several parallel bends, each of a small angle. 

Limitations

Not all shapes are practical in conventional bending. Here are some tips: 

1. Avoid overly intricate bends.

2. Avoid deep U shapes.

3. Avoid several parallel bends that add up to more than 180 degrees unless the shape is very open.

4. Avoid very complex bends with several bends at several angles. 

5. Consider splitting a complex design into two more parts that are bolted together.

Five Side Boxes

For enclosures where you want sides to meet with minimal gap (such as for welding), example A will have an unwanted gap between the flanges after bending. Example B will make edges meet symmetrically at their edge corners with minimal gap.

Weld_corner1.jpg (1212 bytes)                     B weld_corner2.jpg (1327 bytes)

To modify drawing A to B:

  1. Ungroup the lines.
  2. Extend the bend lines to meet.
  3. Draw a circle at the bend intersection of diameter approx. material thickness.
  4. Move the flange edge lines inline with the center of the circle.
  5. Select all lines.
  6. Intersect
  7. Delete the appropriate segments. 
  8. Look closely at the 3D view and adjust accordingly or, for more precision:
  9. Use Job | Model Bends.
  10. Adjust so that each flange fits the inside edges of the bend model.

You can arrange edges to meet so that the edge of one flange if flush against the surface of the other flange:

  1. Adjust the flange lengths so that the inside of the bend model of the horizontal flange equals the length of the vertical flange and:
  2. The outside of the bend model of the vertical flange equals the length of the horizontal flange.

Bend Specifications

The following specs apply to bends:

Linear bend position – limits variation of the position of the center of a bend – that is where the male punch contacts the flat sheet – only for bends directly adjacent to an edge.

Angle tolerance – limits variation of angle after the part is bent.

Radius of bend tool – actual radius of the male punch tool. The inside radius on the bent part will usually be a little larger – how much depends on the thickness, material, etc.

Die width – this does not influence the radius or angle of the bend – generally use the smallest value that does not give an error.

Minimize bend marks – reduces visibility of marks caused by the bending process.

Generally you should use the Model Bends command to estimate the results of a bend.

IMPORTANT: Note that dimensions between features on parts after bending can vary substantially due to an accumulation of tolerances, especially on complex bends. You must use Comments To Machinist to specify any needed tolerances such as the distance between two legs, the distance between holes after a bend, etc.