Ultimate Guide to Loft Sheet Metal

Estimated reading time: 31 minutes

Sheet metal lofting is generally carried out on the lofting workbench. Before lofting, you should be familiar with the drawing and check whether the dimensions of each part of the drawing are correct. If there is no problem, you can prepare the marking tool for sheet metal loft operation.

Sheet metal lofting operation steps

• The reading of drawings is an important process to have a comprehensive understanding of the parts to be made. According to the principle of orthographic projection, read the content of the construction drawing (including the shape, size, technical requirements and content of the title bar of the parts), through the comprehensive analysis of the drawing, imagine the position, size and shape of various parts in the space.
• Preparation the measuring tools and tools used for lofting must perform specific operations during lofting, marking the center line, contour line, positioning line, etc. In addition to clear lines, the most important thing is to ensure the accuracy of the size. In the lofting operation, in order to ensure the accuracy of the various dimensions of the parts and improve the work efficiency, it is necessary to master the geometric drawing of various basic figures and use measuring tools and tools correctly. Generally, the measuring tools and tools required for lofting of workpieces include steel straightedge, tape measure, disk ruler and drawing needle, as well as compasses, ground rules, powder line, hammer, sample punch, etc. If necessary, brushes and paints should be prepared for color draw patterns.
• Selection of lofting datum. Lofting datum refers to the starting point and datum line when staking out. Since parts have dimensions in three directions: length, width, and height, each direction must have at least one datum. Regarding the determination of the datum, the symmetry plane, the bottom surface, the important end surface and the axis of the slewing body can generally be selected. Normally, the lofting reference can be selected according to the following 3 types.

1. Take the central axis of symmetry as the reference.
2. Take two mutually perpendicular planes (or lines) as the reference.
3. Take a plane and a center line as the reference.

The basic method of sheet metal lofting

The method of sheet metal lofting is generally: the direct lofting method using the scoring tool to directly scribe the lofting and the scoring lofting method by using the lofting template. The selection of the lofting method should be done after familiarizing with the drawing, understanding the structural characteristics of the workpiece, the production batch size and the assembly technical requirements and other conditions. Generally, the marking template should be made for the components with higher assembly connection requirements or mass production.

The marking template can be processed by CNC laser cutting, punch machine, wire cutting, etc. According to the production equipment of the enterprise, or it can be cut manually and then trimmed or milled.

For larger size samples, the template is prone to deformation, which affects the accuracy of lofting and is inconvenient to use. In order to ensure the convenience and accuracy of the template, the template can be flat and upward (not exceeding the shape) by riveting or screwing appropriate vertical ribs, such as small angle steel, and adding handles at appropriate positions. But it should be noted that the stiffening ribs and handles are not allowed to be connected by welding to prevent deformation of the template.

Since the plate lofting is only drawn on the plane, the lofting template used is mostly a flat structure, which is relatively simple, and the profile has a certain cross-sectional shape, so the lofting is more complicated and requires some special tools. Specific methods of operation.

• 90° angle ruler , a type of 90° angle ruler formed by a thick ruler and a thin ruler at a right angle. When setting out the profile, use this 90″ square ruler to draw a straight line perpendicular to the edge of the profile on the profile, as shown in the below figure

• Straight-line gauge , the straight-line gauge is composed of a gauge blade and a gauge seat. It is a special tool for profile lofting. Among them, the tool steel for the gauge blade is made by forging, quenching and sharpening. The straight line gauge is mainly used to draw the center line of the processed hole on the profile, as shown in the figure.

• Sample bar card , the sample bar card is made of thin steel plate. When the sample rod is long, hang the sample rod on the profile with a sample rod clip to prevent the bending of the sample rod from affecting the accuracy of the lofting, as shown in the below figure.

• Scribing template , when the workpiece has various end shapes, in order to make lofting accurate and fast, prepare the corresponding end shape scribing template before lofting, as shown in the figure.

The square holes and rolled flanging on the template are for convenient hand-holding when in use, which is conducive to the scribing operation.

Precautions for sheet metal lofting operation

• Choose the benchmark for lofting when setting out the drawing, a rule must be followed when drawing lines—start from the benchmark. On the design drawing, the datum used to determine the position of other points, lines and surfaces is called the design datum. When setting out, the design basis of the pattern is usually selected as the basis of setting out.
• Lofting of plane graphics can be done directly on the blank, but when lofting, it must be combined with the use and processing method of the parts, and after the lofting datum is selected, the lofting can be carried out. The following figure shows the lofting sequence of steel reinforced ribs. From the analysis of the part shape and actual application situation shown in the drawing, combined with the design basis for judging the drawing, the AOB section of the contour line is obviously the basis for lofting.

The sheet metal lofting steps are as follows :

1. Draw the lofting baseline AO⊥AB, see the figure;
2. Cut AO=450 mm on AO, and OB=300 mm on OB. Make the vertical line of AO through point A and cut AD=100 mm; make the vertical line of OB through point B and cut BC=100 mm, see the figure;
3. Connect the CD to complete the lofting of the part.

• Lofting of assembly datum , it is usually marked with a stone pen on the working platform. When the actual sample drawing is used for a long time or is repeatedly used, the sample can be printed on the datum point, the scribe line and the important contour line punch your eyes so that you can redraw when you are unclear.

The lofting of the assembly datum is also to determine the design datum of the drawing first, make the lofting datum, and then draw the line in the order of first outside then inside, first big and then small.

The figure below is a drawing of a component base, and its lofting sequence is shown in Figures (b) ~ (d). The base is composed of channel steel. From the dimensions and characteristics of the drawing, the outline edge and horizontal center line of the right frame are design basis and lofting basis of the drawing. Such components are often assembled using ground-like assembly methods. That is, the actual sample of the component is drawn on the platform, and then the channel steel parts are assembled according to the contour line and the joint position.

The sheet metal lofting steps are as follows :

1. Draw the outline edge of the right frame and the horizontal center line perpendicular to it as a reference for lofting, and draw the outer outline of the box based on this reference.
2. Using the outline edge of the right frame and the horizontal center line as the reference, draw the position of each channel in the frame, as shown in the figure.
3. Draw the direction of the channel steel, draw a clear handover relationship of all handover positions, that is, complete the lofting of the assembly datum, see the figure below.

• Ensure the accuracy of lofting The accuracy of lofting should be guaranteed, otherwise it will directly affect the product quality. This requires maintaining the accuracy of the measuring tools used, and regularly checking the accuracy of the measuring tools in accordance with the regulations. At the same time, the measuring tools of the corresponding accuracy level should be selected according to the different accuracy requirements of the products. Before the construction of important components with high quality requirements, the accuracy of the measuring tools must be inspected. For the generally required processing parts, the size tolerance of the general marking can be carried out as required by the table 1-1.

Table 1-1 Dimensional tolerance of marking

Section steel (the most used in production is mainly angle steel and channel steel) cut shape of the bent part directly affects the bending quality of the section steel. The cut is usually drawn according to the drawing requirements and the actual sample is drawn, and then the sample number is used or the ruler is directly applied to the workpiece. Numbering materials are performed on the upper part, and finally according to the processing requirements of the bent part, the cuts are processed by punching, cutting or milling.

Angle steel bending incision shape and material length

The cutting of angle steel bending incision is generally carried out on site, and it mainly has several forms such as internal bending and external bending. The calculation of the curved incision shape and material length mainly includes the following contents.

• Angle steel bends inwardly at any angle and external sharp corners. The figure shows the cut and material length when the angle steel is bent inwardly at an acute, right, and arbitrary angle. The left figure shows the shape and size of the workpiece after it is formed, and the right shows the cut shape and length of the material.

• Inside-bending 90°external fillet of angle steel. The following figure shows the inward-bending 90° external fillet of angle steel, in which: Figure (a) shows the incision on the 45° angular line; Figure (b) shows the incision on the 45° angular line Shape and material length; Figure (c) shows the incision on the right-angle sideline; Figure (d) shows the incision shape and material length on the right-angle sideline.

The calculation formula for the arc length c of the curved surface center in the figure is :

c——the arc length of the central layer of the curved surface, mm;
R——the radius of the inner arc, mm;
d—–The thickness of the angle steel, mm.

• Various frame shapes of angle steel inward bending . The figure shows various frame shapes of angle steel inward bending, among which, figure (a) is the cut shape and material length when the angle steel is inwardly bent rectangular frame; figure (b) is the angle steel inwardly bent and outer rounded corner The cut shape and material length of the rectangular frame; and Figure (c) is the cut shape and material length of the trapezoidal frame when the angle steel is bent inward.

The calculation formula for dimension c in the figure is :

Channel steel bending incision shape and material length

Like angle steel bending, channel steel mainly has several forms such as internal bending and external bending. The calculation of the curved incision shape and material length mainly includes the following contents.

• Channel steel flat bending arbitrary corner fillet cut shape .The figure shows the channel steel flat bending arbitrary corner fillet cut shape. The key to the number of materials is to calculate the length of the arc c.

The calculation formula is :

c—-The arc length of the center of the curved facade, mm;
h—-Channel steel width, mm;
t—-Thickness of wing plate, mm;
α—- Bending angle, (°);

• Channel steel side bend right-angle rectangular frame .The figure shows the channel steel cut side bends 4 right angles to form a rectangular frame. Figure (a) is the workpiece forming drawing, and Figure (b) is the material length and cut shape drawing (only in the figure) Show half length).

• Channel steel side bending rounded rectangular frame .The figure shows the material length and cut shape of the channel steel cut side bend rounded rectangular frame; Figure (b) shows the material length and cut shape of the channel steel flat bend with 90° round corners.

The calculation formula for dimension c in the figure is :

Basic principles of sheet metal lofting and making-off

Lofting should carry out the basic principle of reasonable use of materials and maximal utilization of raw materials. Under the premise of technology permission, the application of reasonable lofting method, carefully budget to arrange the position of the workpiece in the material, so that the material is fully utilized. Specific methods to improve the utilization rate of materials are as follows.

• The centralized nesting lofting method of the plate is to gather the workpieces of the same thickness but different shapes and sizes together for lofting and cutting. The specific method is: lay all the workpiece templates of the same specification and material on the steel plate, and discharge the materials according to the principle of from large to small, and from more to least. Make overall arrangements and collocation to make full use of raw materials.
• Plank layout nesting lofting method. When the number of lofting workpiece is more, in order to make the lofting reasonable, to carefully arrange the position of the workpiece graph. One shape of the workpiece can be used for layout, and several different shapes of the workpiece can be used for nesting.

If one of the workpieces is used for nesting alone, and the utilization rate of the material is not high, two types of workpieces should be considered for nesting to maximize the use of raw materials. However, it should be noted that while considering the improvement of material utilization, the improvement of processing technology and production efficiency after the layout of the plate should also be fully analyzed.

• The statistical calculation lofting method of profile. When the length of the shaped workpiece is inconsistent, quantity is not unified, and raw materials are the same, can be used to conclude all the same specification material artifacts together, from long to short and the number of artifacts to undertake collocation method for layout, every root materials can get reasonable use, the first statistical arrangement, lofting method again, is the profile of statistical calculation lofting.

The profile lofting generally adopts this statistical calculation method. When lofting, the profile is placed neatly, according to the pre-calculated order, from one end of the profile side by side, which can improve the work efficiency, but also can be very convenient to grasp the number. The order of marking is usually to draw long profiles first, then short profiles.

Key points of layout operation

For the production of large and complex components, it is often necessary to divide the product into sections and pieces. After assembling, the welding method can be used to form the product. In order to ensure the quality of the splicing, how to reasonably divide it is the key to layout.

The so-called layout refers to the reasonable arrangement of welds, the relative position of parts and the shape of each sheet, according to the structural shape, welding and other requirements of the expanded drawing of the processed component and the specifications and dimensions of the existing steel plate on the expanded drawing. Working process of size and numbering. The layout work is the key work to ensure product quality and rational use of materials. It is often not determined once, and needs to be adjusted and compared many times to obtain a more reasonable layout chart. The following takes the discharging of a large cylindrical member cylinder as an example to describe the main points of the discharging operation.

The principle of layout

1. According to the design requirements, the size of the existing material specifications and the capacity of processing equipment, to determine the size of the barrel.
2. Reasonable arrangement of welds to reduce welding stress and avoid large deformation after welding. The main aspects can be considered from the following aspects: in the case of meeting the design requirements, as far as possible to reduce the weld, especially longitudinal weld; No special requirements, welding seam arrangement should be uniform and symmetrical, to avoid weld concentration; The circumferential weld shall be perpendicular to the axis, and the longitudinal weld shall be parallel to the axis. Avoid cross welds.
3. Machining allowance, counterpoint clearance and welding shrinkage should be considered. There are the following requirements for discharging the cylinder of the tower and the pressure vessel: the shortest length of the cylinder. For the cylinder section made of carbon steel and low alloy steel, not less than 300mm; The cylinder section made of stainless steel is not less than 200m; The length of each steel plate shall not be less than 800mm; Avoid cross weld, such as the use of cross weld butt, the longitudinal seam on the adjacent cylinder section should be positive; When the T-shaped weld is used, the distance between the longitudinal weld on the adjacent joint or the end of the head weld and the longitudinal weld of the adjacent joint should be greater than 3 times the wall thickness, and not less than 100mm; The distance between the parts in the cylinder and the welding seam of the cylinder body should not be less than the thickness of the cylinder wall, and not less than 50mm. The welding seam of the container opening and its strengthening plate must be staggered with the welding seam of the shell for more than 50mm; The girth weld of the horizontal vessel should be located outside the bearing surface of the support. The longitudinal weld shall be located outside the range of 140° on the lower part of the shell; The distance between the welding edge of the support and the cylinder and the circumferential welding seam of the shell should be greater than 4 times the wall thickness, and not less than 10mm, as shown in the figure.

The steps of layout

1. According to the design drawing, draw the expansion drawing at appropriate scale.
2. On the expansion diagram, draw the center line of the four directions of 0°, 90°, 180° and 270° to determine the positions of all kinds of openings and parts, such as supporting ring and strengthening ring.
3. According to the existing sheet size plate layout. According to the principle of material arrangement (plate arrangement), the welding seam layout is reasonable, the welding stress is small and the reasonable material is used.

The following figure shows the layout diagram of the vertical container and its barrel. The wall thickness of the simplified body is 8mm.

The operation points of marking-off material

Check the grade, specification and surface quality of the material before marking-off.

Achieve reasonable use of materials, as far as possible to improve the utilization rate of materials. If there is no trimming, 30~50mm of trimming should be set aside. According to the thickness of the sheet material, the slot clearance of different sizes of material should be considered, as shown in the table below.

Cutting clearance of marking-off material (mm)

The figure below shows the cylinder section plate marking-off

• When marking material, the fiber direction of the material rolling should be considered, so that the rolling plate in the processing, the bending direction of the steel plate is consistent with the fiber direction of its rolling, to ensure its strength.
• For the blank of sheet material, the drawing number, work number, part number, material, specification, quantity and steel number of the product should be indicated, and the inspection line, center line, actual consumption line and planing edge line should be marked.

Method of secondary marking on the head

The head is often assembled with the cylinder section to form a cylinder. However in general, the mouth of the formed head is often different in length and wrinkles. In order to facilitate the assembly of the head and make it meet the requirements of the pattern, it is often necessary to perform secondary marking and cutting to remove the unqualified or The surplus part makes the head and mouth even. This secondary scribing and cutting work is called the net material after the head is formed. There are mainly the following methods for the commonly used clean material after the head is formed.

• Use the underlined plate number to clean the material, and the operation steps are as follows.

1. Lay the head on a flat surface and align the head with a pad, as shown in the picture.

Figure (a) shows alignment of head with manhole. During operation, the ruler is placed on the long and short axes of the elliptical manhole, and the distance between the ruler and the platform is equal with the cushion block, that is, alignment. Figure (b) shows alignment of manhole head. Put the 90° square on the four corresponding directions of the head, and make the straight edge of the head coincide with the 90° square by means of adding pad block, that is, alignment. The distance from the ruler to the platform is 1/2(a + b).

2. Determine the height of the needle and ensure the head height h, as shown in the figure.

3. With the platform as the benchmark, the marking plate is marked along the circumference of the head to obtain the net cutting line of the head.

• Use a hose level gauge to making-off the material. The following figure shows the schematic diagram of using the hose level to clean the material. The operation steps are as follows.

1. Put the head on the platform, align it and pad it firmly.
2. Determine a point on the outer wall of the head, the distance from the platform is the height of the head, and this point is used as the reference point.
3. One person holds one end of the hose level, and when the height of the glass tube level is consistent with the height of the reference point, another person holding the other end of the hose level can draw a series of points on the outer wall of the head that are consistent with the reference point.
4. Connect the points smoothly to obtain the net material cutting line of the head.

• Use water level making-off net material. The method of using water level to clean materials is shown in the figure below. The operation steps are as follows.

1. Put the head on the platform, align it and level it.
2. The inside of the head is filled with water, and the height of the horizontal plane H is determined.
3. Taking the horizontal plane as the reference, draw a series of points on the inner wall of the head with a height equal to the height h of the head, and mark these points on the outer wall correspondingly.
4. Connect the points on the outer wall with a steel ruler to obtain the cutting line of the net material of the head. If the inner diameter of the head is large enough, you can connect points directly on the inner wall of the head, and cut the line and implement the cutting.

• Use a rotating tire to cut at a fixed point. There is a hollow boss in the middle of the rotating tire, and the lower part is connected with the mechanical transmission part. The use of fixed-point cutting of the rotating tire can improve work efficiency and reduce labor intensity. The operation steps of fixed-point cutting for rotating tires are as follows.

1. Place the head on the rotating tire, and use the self-made big angle steel bending ruler to align the head, as shown in the figure.
2. Fix the cutting nozzle on the bracket, the height is equal to the net material height of the head. Before cutting, turn on the tire first to check whether the position of the cutting nozzle is appropriate to ensure the cutting quality.
3. Ignition cutting. When cutting, the position of the cutting nozzle can be adjusted in time to keep it at a proper distance from the head.

Drafting of the axial center line of the cylinder

The following cylinder body is taken as an example to illustrate the drawing of the axial center line. There are mainly the following commonly used.

• The double vertical method is shown in Figure (a). First adjust the bracket so that the level on the bracket is in a horizontal position. The two wire hammers are hung on both ends of the bracket, so that they are both 5mm away from the outer wall of the cylinder or tangential to the cylinder wall. Then on the line of the two hammer lines, at 1/2 distance of the bracket, use the universal square to lead the vertical line to the point on the cylinder wall. At the other end of the cylinder, use the same operation to make another point. Finally, connect the two points with a pink line pop-up, resulting in a center line. This method is suitable for the straight cylinder of 1000~2000mm.
• As shown in Fig (b), a line hammer is used to hang on the end of the cylinder so that arc abc = arc adc and the circumferential equinox points are obtained on the cylinder wall. On the other end, do the same thing. Then connect the two points and pop them out with a pink line to get the center line. This method is applicable to a certain rigidity of the cylinder, not limited by the diameter.
• The water level method is as shown in Figure (c). The water level method is to fix one end of the glass tube of the hose level on the left side of the cylinder, and move the other end of the glass tube to the right side of the cylinder to make the arc bad = arc bcd. As a point on the other end of the cylinder, the glass tube at point B is still not moving. The glass tube at point d should be moved to the left and right sides of the other end of the cylinder to obtain points b ‘and d’ (not shown in the figure), so that arc b ‘a’ d ‘= arc bcd. Finally, the point is popped out with a pink line to obtain the axial center line. This method is suitable for the installation and positioning of large diameter cylinder and various large components.

• Horizontal square method. As shown in the figure, a 90° square (level instrument) with bubbles is placed on the outer wall of the cylinder to make the bubbles level (located in the middle), and point A is obtained. On the other end of the barrel, if you do the same thing, you get another point A ‘(not drawn). Finally, the result pops out with a pink line to obtain the axial center line. This method is applicable to the scribing of small diameter cylinder and large diameter steel tube.

The method of marking and arranging holes

The hole on the component, generally play a role in the connection and assembly between components in the whole parts, usually the processing accuracy is relatively high, is generally completed in the component group welding, and after the completion of the component assembly, welding, correction. The methods and steps of marking holes in several complex components are described below.

• The method of marking holes in the cylinder body

1. Draw the axial center line. Delimit axial center line method can be according to the specific situation, targeted to choose the above method, as shown in the figure, with double vertical method to make the center line, and then to center line Ι as the benchmark, with equal arc length to draw the center line Ⅲ, and draw the center line Ⅱ, Ⅳ.

2. Draw the ring to the reference line. As shown in the figure, first determine the position of the hoop reference on the center line of the I axis at point A, use a scribing rule to take point A as the center, and use the appropriate length as the radius to draw the center line of stray intersection I at points B and C, and then use points B and C as the center of the circle and draw arcs with different radii to obtain a number of intersections D, E, F, and G. Connect each intersection point to obtain a circular reference line. Use the same method to make other sections of the hoop reference line, and finally draw a complete hoop reference line.

3. Row holes. As shown in the figure, the position of the socket hole is marked. When arranging holes, it is necessary to know the axial position size and the circumferential position size of the socket hole. Circumferential position size l≈0.01745Ra. When arranging holes, the benchmark should be determined first, and then the size should be measured.

• The method of scribing and arranging holes on the head. Under normal circumstances, the scoring and arranging holes of the head are carried out after the number of clean materials, and the scoring and arranging holes can be carried out according to the following steps and methods.

1. Making-off net material. Before making the material, the head should be aligned, as shown in the figure.
2. Draw the center line. Use the equal arc length method to draw 4 center lines, as shown in the top view in Figure (a).

3. Draw a cross reference line. As shown in the figure, taking the platform as the reference, place the 90° square on the center line of Ι and Ⅲ, and slowly move down on the 90° square with the pink line to print a line on the surface of the head. Similarly, put the square ruler on the center line of Ⅱ and Ⅳ and draw a line to obtain the cross reference line.

4. Row holes. As shown in the figure is a schematic diagram of the row of holes. On the platform, the upper and lower center lines are used as the basis to measure the distance to the left; the left and right center lines are used as the basis to measure the distance forward in m. Place the 90° square at the marked m and n points. Use the pink line to close the square and move it down to pop up the cross line. The intersection point is the center of the hole.

• The method of scribing and arranging holes for beams. Figure (a) shows a box beam that needs to be scribed and arranging holes. The scribing and arranging of holes can be carried out according to the following steps and methods.

1. Draw a longitudinal center line. As shown in Figure (b), use a 90° square on one end of the beam, with the upper (lower) cover as the reference, draw one side of the square against the inside of the web, and draw the remaining lines in the same way. Then take points a and b at 1/2 of the distance between the two lines. Using the same method, get the midpoints a’ and b’ at the other end of the beam (not shown in the figure). Finally, a straight line pops up between points a and a'(b and b’), which is the upper (lower) center line (longitudinal center line).
2. Draw a horizontal center line. Determine the horizontal reference point A on the upper center line, and cross point A to make one side of the 90° square coincide with the upper center line, draw point B (B′) along the other side, and then pass B (B′) point, and draw with a 90° square Point C, D (D’), connect BD (B’D’) and DD’ (lower cover), as shown in figure (c).
3. Row holes. As shown in Figure (c), the measurement of the vertical dimension should be based on the horizontal middle line; the measurement of the horizontal dimension should be based on the vertical middle line.