SHEET METAL

SHEET METAL DRAWINGS AND FABRICATION

This section establishes drafting and dimensioning practices pertinent to the preparation of drawings for parts to be fabricated from sheet metal. Additional information is presented to assist the draftsman in establishing proper dimensions for certain features of formed sheet metal parts.

DEFINITIONS.  (Alphabetically Listed)

Bend Allowance.  The length of material around a bend from bend line to bend line. 

Bend Angle.  The angle through which sheet metal is bent. It is measured from the flat through the bend to the finished angle after bending and is not to be confused with the included angle between the flange and adjacent leg. 

Bend Line.  The line of tangency where a bend changes to a flat surface. There are two bend lines for each bend. 

Bend Radius.  The minimum radius required to bend the material to prevent cracking or requiring additional work when forming the flanges. 

Bend Relief.  For optimum forming, interfering material at flange extremities is removed to a point behind the bend line to prevent cracking of the material when forming the flanges. See FIGURE 21-6 thru

Bevel, Closed.  The condition of a flange bent more than 90o from its flat condition.

Bevel, Open.  The condition of a flange bent less than 90⁰  from  its flat condition.

Blank.  A flat sheet metal shape of approximately the correct size to make a finished part. The part is usually trimmed to size after forming.

Center Line of Bend.  A radial line from the center of the bend radius which bisects the included angle between bend lines. 

Developed Length.  The length of a flat part which can be bent to make a part depicted on a drawing. This length is always shorter than the sum of mold line dimensions on the part.

Dimpling.  Stretching a relatively small shallow indentation into sheet metal. Stretching metal into a conical flange for use of a countersunk rivet head or screw. Dimpling is stronger than countersinking and should be performed on thin panels.

Flat Pattern.  A flat layout of a formed sheet metal part which can be bent to make the finished part without trimming after forming.

Form Block Line.  The inside mold line of a part.

Joggle.  An offset in the face of a part which has an adjacent flange. 

Mold Line.  The line of intersection of two flat surfaces of a formed sheet metal part. The term alone usually refers to the outside mold line. See form block line also.

Median Line.  A neutral axis through a bend where there is no stretching or compressing, located approximately 44% of the material thickness from the inside surface of the bend.

Set Back.  The amount of deduction in length when a flat pattern is developed across a bend. It represents the saving in material by going around a bend radius rather than around a square corner.

Tool Holes or Pin Holes.  Holes without a functional purpose in the end product. Used for aligning apart in the proper position on a die or form block, or for other fabrication or tooling purposes.

Fabrication of Sheet Metal Parts.  The fabrication of sheet metal parts involves methods which are quite different from those used for fabricating machined parts, and it is necessary that the draftsman understand these differences in order to properly prepare drawings. Sheet metal parts are frequently cut out of flat stock using developed patterns (templates) and then formed to finished parts. Other parts may be made from flat blanks and deep drawn or formed into complex contours by dies in a punch press or hydro press, or by drop hammer dies. These parts are usually trimmed after forming to produce the finished part. These are only some of the methods which force the establishment of certain special procedures for sheet metal drawings.

Characteristics of sheet metal bends

Location of Median Line for Bend Allowance Calculations.  When metal is bent, there is usually a local thinning or thickening of the material. This happens because the material is compressed on the inside of the bend and stretched on the outside, causing some displacement and plastic deformation of the material. It has been determined that there is a line through the bend where no stretching or compression takes place. This line, called the median line, is located approximately 44% of the material thickness from the inside surface of the bend. The location of the median line forms the basis for bend allowance calculations.

Bend Formulas.  The following paragraphs should aid in the preparation of flat pattern development for under defined drawings and provide information for calculating dimensions for developing flat blanks. Flat development of sheet metal parts is not permitted on engineering drawings except under defined drawings.

Bend Allowance.  The following empirical formula has been developed to determine the distance around a bend for all bends one inch or less: (.0078T + .0174R) times the number of degrees of bend where T =material thickness and R = inside bend radius.

Distance from Bend Line to Mold Line.  This distance can be calculated by trigonometry using the following formulas:

Set Back (Applicable To Closed Angles Only).  This is the deduction made in the length of a flat pattern development corresponding to the amount of material saved by bending around a radius instead of a sharp corner. It is equal to twice the distance from the bend line to the mold line minus the bend allowance. The formula is written as follows:

K = 2D – BA

WHERE:

K =SET BACK

D = DISTANCE FROM BEND LINE TO MOLD LINE

BA =BEND

ALLOWANCE EXAMPLE: Using the preceding examples, the “D” distance was calculated to be 1.400 and the bend allowance was .799;

Therefore:

K = 2D – BA

K = 2(1.400) --.799

K = 2.800 -- .799

K = 2.001

Dimensioning the Part.  Using the bends in the previous examples; the part is dimensioned as shown:

BEND RELIEF

Bend Relief Cutouts.  Whenever sheet metal bends intersect one another, it is necessary to remove material from the intersection area to prevent interference and buckling. Material must be removed at least .03behind the intersection of bend lines. There are many possible configurations for bend reliefs, two of which are illustrated in the usual practice is to radius the inside of a relief cutout, but this is not essential for the function of a relief cutout.  A sharp corner will do as well if it removes material .03beyond the intersection of bend lines.

Relief Cutout Dimensioning. 

The distance from the mold line to the bend line can be calculated by using “Distance From Bend Line To Mold Line” formula in distance can be used in various ways to establish dimensions for the relief cutout, depending on the shape of the cutout. Next two examples are shown in illustrate the possibilities.

Joggle Dimensioning.  Joggles are shown and dimensioned as shown:

Joggle Tolerances.  Dimensions "F" and "W" should have a tolerance of not less than ±.03. Dimension "D" should have a tolerance of not less than ± .010. It is preferable to allow the flange to be displaced as well as the face of the part, but if this is not permissible, it may be shown:

Angles Other Than 90 in Joggles.  When the angle in the face of the part is other than 90o to the mold line, the angle must be specified.

Flange Displacement in Joggles.  It is preferable to allow the flange to be displaced as well as the face of the part, but if this is not permissible, it may be shown as in

Double Joggles.  When both the face of a part and the flange are joggled in the same area, it is preferred to make both joggles coincide and use a washout length as determined by the deepest joggle.

Joggle Washout.  The "W" dimension varies with different materials.  It is greater for harder materials and is dependent on the depth of joggle. Joggle washout may be determined by using this table:

And for aluminum alloys use this table:

BEADS

Bead Dimensioning.  Beads are raised or depressed areas in sheet metal parts, usually for the purpose of providing rigidity. It should be recognized that beads necessitate stretching the material and normally are formed into parts by using dies. The plan view shows an outline of the bead at the mold line in phantom lines. It is necessary to show sections or breakouts to dimension the bead depth and the bead radii. When there is a straight section between two bend radii, the angle must be dimensioned.