Sandbag bumping is one of the most difficult methods of  hand forming sheet metal because there is no exact forming  block to guide the operation.  [Figure 4-162]  In this method,  a depression is made into the sandbag to take the shape of  the hammered portion of the metal. The depression or pit has  a tendency to shift from the hammering, which necessitates  periodic readjustment during the bumping process. The  degree of shifting depends largely on the contour or shape  of the piece being formed, and whether glancing blows must  be struck to stretch, draw, or shrink the metal. When forming  by this method, prepare a contour template or some sort of  a pattern to serve as a working guide and to ensure accuracy  of the finished part. Make the pattern from ordinary kraft or  similar paper, folding it over the part to be duplicated. Cut the  paper cover at the points where it would have to be stretched  to fit, and attach additional pieces of paper with masking tape  to cover the exposed portions. After completely covering the  part, trim the pattern to exact size. Open the pattern and spread it out on the metal from which  the part is to be formed. Although the pattern does not lie flat,  it gives a fairly accurate idea of the approximate shape of the  metal to be cut, and the pieced-in sections indicate where the  metal is to be stretched. When the pattern has been placed on  the material, outline the part and the portions to be stretched  using a felt-tipped pen. Add at least 1-inch of excess metal  when cutting the material to size. Trim off the excess metal  after bumping the part into shape. If the part to be formed is radially symmetrical, it is fairly  easy to shape since a simple contour template can be used as a  working guide. The procedure for bumping sheet metal parts  on a sandbag follows certain basic steps that can be applied  to any part, regardless of its contour or shape. A joggle, often found at the intersection of stringers and  formers, is the offset formed on a part to allow clearance for  a sheet or another mating part. Use of the joggle maintains  the smooth surface of a joint or splice. The amount of  offset is usually small; therefore, the depth of the joggle is  generally specified in thousandths of an inch. The thickness  of the material to be cleared governs the depth of the joggle.  In determining the necessary length of the joggle, allow an  extra  1 ⁄ 16-inch to give enough added clearance to assure a fit  between the joggled, overlapped part. The distance between  the two bends of a joggle is called the allowance. This  dimension is normally called out on the drawing. However,  a general rule of thumb for figuring allowance is four times  the thickness of the displacement of flat sheets. For 90°  angles, it must be slightly more due to the stress built up  at the radius while joggling. For extrusions, the allowance  can be as much as 12 times the material thickness, so, it is  important to follow the drawing. 4-81 Joggling Sandbag Bumping  2.  Determine the amount of metal needed, lay it out, and  cut it to size, allowing at least  1 ⁄ 2-inch in excess. 3.  Place a sandbag on a solid foundation capable of  supporting heavy blows and make a pit in the bag with  a smooth-faced mallet. Analyze the part to determine  the correct radius the pit should have for the forming  operation. The pit changes shape with the hammering  it receives and must be readjusted accordingly.  4.  Select a soft round-faced or bell-shaped mallet with  a contour slightly smaller than the contour desired on  the sheet metal part. Hold one edge of the metal in the  left hand and place the portion to be bumped near the  edge of the pit on the sandbag. Strike the metal with  light glancing blows.  5.  Continue bumping toward the center, revolving the  metal, and working gradually inward until the desired  shape is obtained. Shape the entire part as a unit. 6.  Check the part often for accuracy of shape during the  bumping process by applying the template. If wrinkles  form, work them out before they become too large. 7.  Remove small dents and hammer marks with a suitable  stake and planishing hammer or with a hand dolly and  planishing hammer. 8.  Finally, after bumping is completed, use a pair of  dividers to mark around the outside of the object. Trim  the edge and file it smooth. Clean and polish the part. 1.  Lay out and cut the contour template to serve as a  working guide and to ensure accuracy of the finished  part. (This can be made of sheet metal, medium to  heavy cardboard, kraft paper, or thin plywood.) Figure 4-162.  Sandbag bumping. 