In this article we will discuss about the sheet metal manufacturing process and its operation types.
Introduction to Sheet Metal Work:
Most of the sheet metal work is done on presses where a die and punch or other formed tools are required. In press work large force is applied on thin sheet metals to give the required shape or to cut it in to the desired shape. Press work is highly economical method of manufacturing.
Various manufacturing methods on sheet metals can be classified as follows:
(a) Cutting and Shearing- Here the sheets are cut by using various tools. Blanking, piercing, perforation, notching etc. are various types of shearing operations.
(b) Bending Operation- The sheet metals are straining around a straight axis.
(c) Forming- Flanging and tube forming are forming operations to give shape to the sheets.
(d) Drawing Operations- Cupping, drawing, deep drawing are all drawing operations in which sheet metals are given cup or shell type shapes.
(e) Reducing Operations- Necking type of operations.
(f) Squeezing- For example coining operation is done in closed dies applying large force.
Press is a method of forming sheet metals and plates by cold working into required shapes by applying large force through press tools. The presses may be manually or power operated.
(a) Hand Operated Press:
It is small press which is operated by hand. As the arm of the press is rotated, the ram moves up or down and the stored energy is transferred in doing work on the sheet metal. Hand presses are used for small jobs.
(b) Power Press:
For large jobs and where high production is required hand operated presses are not suitable and power presses are used. There can be many types of power presses on the basis of power used—electric motor driven, pneumatic or hydraulic press etc. or on the type of jobs to be done like-shearing press, coining or punching press etc.
(c) Die and Punch:
The sheet metal work on the press would require die and punch depending on the type of job. Die and punch for making a circular disc. The die and punch are made of materials harder than that of the sheet metals.
(d) Hand Tools:
For sheet metal working following hand tools are used:
(i) Measuring tools—Like steel rule, folding rules, flexible, push- pull rule, vernier caliper, micrometer, thickness gauge etc. are used to measure the parts during manufacture.
(ii) Straight Edge—It is used to scribe long straight lines.
(iii) Scriber—It is a long thin rod with pointed end and handle at the other side. It is used to mark lines on the sheet metal.
(iv) Divider—Used to draw small circles.
(v) Trammel Compass—It is used to draw large circles on sheets.
(vi) Punch—To locate centre or to make mar.
(vii) Hammer—Used for forming.
(viii) Mallets—These are soft hammers with rubber, raw hide, soft metals etc.
(ix) Miscellaneous tools like chisels, scissors etc.
Typical Sheet Metal Working:
(a) It is a forming process by pushing a punch against a flat sheet and forcing it into a die to take the required shape. Or in other words drawing is the operation of producing thin walled, hollow shaped parts from sheet metal. Both the die and the punch have shape of the part to be manufactured . Classifications of drawing operations could be cupping, redrawing and deep drawing. Components like cups, shells, household utensils etc. can be made by drawing.
(b) In the drawing operation soft material are used since the material is required to undergo large permanent deformation to acquire the intended shapes in the presses. Drawing operations could be for cup making or deep drawing.
(c) For making cup shapes on the press first a circular blank of proper size is taken. The die is usually open from the bottom side and has an ejector system placed below a pad for the removal of the component after pressing it into shape. The pressure pad ensures smooth flow of the blank over the die/punch.
(d) The ratio of the diameter of the blank (D) and that of the cup (d) is called draw ratio (Did). If draw ratio is greater than about 1.8 then the cup cannot be drawn in one draw. Such items may require redrawing in several steps.
(e) In drawing operations lubricants are extensively used.
It is a forming operation performed on a press to bend a sheet metal or a strip through required angle.
Salient features of bending are:
(a) It requires a press and tooling to do the bending operation.
(b) The sheet metal strip is permanently deformed to the required shape.
(c) When the sheet metal is pressed for bending in the die the outer portion is stretched while inner sides (towards the punch) are compressed.
(d) The radius of the corner should not be small i.e., the angle at the bent should not be too sharp—otherwise material can crack at the bend.
(e) After removal of the bent component from the press there is usually spring back up to about 4 degrees depending on the material hardness. Hard material have greater tendency to spring back while softer materials have less tendency to recover shape.
(a) Blanking is an operation of cutting an object of given shape from sheet metal strip. It may or may not be necessary to perform further operations on the blank. Piece detached from the metal strip is called blank. Examples are discs, washers.
(b) Blanking operations are extensively used in sheet metal working to make components like washers, discs etc. If needed other operations may be combined with the blanking operation. Blanking is usually the first operation in sheet metal.
(c) A blanking operation to make disc from a steel strip would require-
(i) A press
(ii) A punch and die with clearance at the lower side so that the disc can fall down freely.
(iii) The material for blanking operation is usually hard.
It is a distinct process of making a hole in sheet metals.
It is characterized by:
(a) The punch is sharp and pointed tool which is able to penetrate or pierce through the sheet metal.
(b) There is no scrap from the hole.
(c) The hole has rough flanges around the hole.
Notching is removing small quantity of material from the edges of a sheet metal part. Notching may be done to avoid overlapping of material after bending at the seams. Proper notching would give the sheet metal joints a better fit.
(a) Modern industry has been developed on the basis of interchangeable manufacturing. By interchangeability it is meant that the parts manufactured anywhere can be assembled without rework. The production of any component requires interaction between man, machine and material. Defects or variations in any of these would result in variation in the dimensions of component.
(b) The variations may arise due to:
(i) Difference in the skills of the machine operator
(ii) Machine tool may have inherent inaccuracies.
(iii) The cutting tools may have variations and get worn out.
(iv) The materials may have defects.
(c) Cost of producing the components to exact dimensions would be very high and rather impossible. However it is possible to manufacture them with such variations which would not affect the working of the assembly.
A component is said to have interchangeability if with the recognized deviations the component is able to be assembled at random with the mating part and also has required tightness or looseness after assembly. If the parts are not interchangeable selective assembly would be needed which is costly and time consuming.
Advantages of interchangeability are as follows:
(i) It is possible to manufacture components on mass scale at various places.
(ii) It is also possible to replace worn out parts.
This leads us to the concept of limits; tolerance and fits. In this connection knowledge of following is essential.
(a) Basic Size:
It is the exact theoretical size of a component. Basic size is chosen on the basis of design considerations. Limits and variations are in relation to the basic size. Zero line coincides with the basic size. (Fig. 41.7).
(b) Nominal Size:
It is the size designation given in the drawing for the sake of convenience and identification. It is usually rounded off to whole number. Nominal size is given for identifying a component and it does not represent accurate measurement. The nominal size of a shaft and a hole are same.
(c) Actual Size:
It is the dimension of a part as it actually measures is actual size. Actual size should be within the specified limits.
(d) Zero Line:
The limits and fits are represented graphically in relation to the zero line which represents the basic size.
(e) Limit of Size:
It is the two extreme limits of sizes between which the actual size may lie.
(a) It is impossible to manufacture large number of pieces to an exact dimension and there will always be some difference in size. Then limits of acceptable dimensions are found so that the component would perform satisfactorily within these limits. The component can differ from the proper size by the small amount and still be able to be used.
(b) For example if a component has basic size of 25.00 mm and it is found that it would perform satisfactorily if the size is between 25.02 mm on the higher side and 24.97 mm on the lower side.
Then upper deviation is- The maximum size minus the basic size (in this case 25.02 – 25.00 = 0.002). Lower Deviation – Minimum deviation = Basic Size – Lowest size = (in this case 25.00 – 24.97 = 0.003)
(c) Maximum Limit or Upper Limit = Max. dimension which can be allowed = Basic size + Upper deviation = 25.02.
(d) Minimum Limit or Lower Limit Minimum dimension of component which can be allowed = Basic size – Lower deviation = 24.97.
(a) It is the total amount by which a dimension is allowed to vary. The Tolerance is the total amount by which the size of the component can differ from the Nominal Size. The difference between the maximum and the minimum limits of sizes is tolerance. It is equal to the algebraic difference between the upper and lower deviation.
(b) Types of Tolerances:
A Tolerance is said to be Bilateral if it is spread over both sides of the Nominal Size. If it is on side it is called unilateral tolerance.
(c) Example of Unilateral tolerance—
(i) If basic size is 20.00 mm and component can be accepted up to 20.01 min. It can be written as 20.00 + 0.01 where +.01 represents unilateral tolerance.
(ii) If the dimension is given as 16 –.02, it implies that the component has unilateral tolerance of –.02 and the component can vary between 16.00 – 02 = 15.98 to 16 mm.
(d) Examples of Bilateral Tolerances:
Here the tolerance are spread on both sides of the zero line. For example 20.01 would mean that component can lie between 19.99 to 20.01 mm.
(e) Tolerance Zone:
The zone between the two limits of sizes as represented graphically in Fig. 41.7.
(a) Allowance is basically the gap between components that work together. Allowance between parts that are assembled is very important. For example, the axle of a car has to be supported in a bearing otherwise it will fall to the ground. If there was no gap between the axle and the bearing then there would be a lot of friction and it would be difficult to get the car to move.
If there was too much of a gap then the axle would be jumping around in the bearing. It is important to get the Allowance between the axle and the bearing, correct so that the axle rotates smoothly and easily. Allowance is intentional difference between a shaft and hole dimensions for any type of fit.
(b) The allowance is the intended difference in the sizes of mating parts. This allowance may be: positive which means there is intended clearance between parts; negative for intentional interference: or “zero allowance” if the two parts are intended to be the “same size”. This last case is common to selective assembly.
Tolerance is applied to a single component. When two such components are assembled and are required to work together as a unit—the assembly characteristics is known as fit between mating parts. The relationship between parts when one is inserted in to another with degree of tightness or looseness is called fit.
Generally speaking following are most common fits:
(i) Clearance Fit:
When there is relative motion between the shaft and the hole it is called clearance fit. In clearance fit there is positive clearance between the maximum shaft size and the minimum whole size.
(ii) Interference Fit:
When there is no relative movement between the shaft and the hole it is called interference fit. Here the shaft size is bigger than the hole.
(iii) Transitional Fit:
When there is neither interference nor clearance fit-then it is called transitional fit.
Fits can be based on the basic hole size or basic shaft size.
Basic Hole System:
1. Design size of the hole is taken as the basic size for both the hole and the shaft.
2. It is most common system for limit dimensions. In this system the design size of the hole is taken to be equivalent to the basic size for the pair. This means that the lower limit of the hole dimension is equal to design size. The basic hole system is more frequently used since most hole generating devices are of fixed size like drills, reamers etc.
3. The position of the tolerance zone of the shaft may be above or below the zero line. Depending on the tolerance zone it can give various types of fits—interference, clearance
4. Capital letters H is used for the hole. Shaft have different letters to indicate the position of tolerance zone for desired fit.
1. The design size of the shaft is taken as basic size for both the hole and the shaft.
2. When designing using bought out items with fixed outer diameters like bearings, bushings, etc. a basic shaft System may be used. Taking the basic size of a common reference in both following relationships exist.
3. The position of the tolerance zone of the shaft may be above or within the tolerance zone of the hole.
4. Small letter h is used for the shaft and the holes have different letters.