Transcription
Regarding the values of angular dimensions, they can be arranged in various ways. Let's look at this case here. We see the dimensions indicating the angles, placed above the dimension line. The dimensions must be readable only from the base of the drawing. In this case as well, we see the dimension of 70 mm, which is above the dimension line. In some cases, dimensions can be inserted inside the dimension line. The angular values of the dimensions must be inserted according to the scheme shown below. The same applies to angular dimensions. Therefore, let's now see with what criterion dimensions can be arranged. A first dimensioning system is called chain dimensioning. In it, each dimension is determined with respect to the previous one. This system can be adopted when only the accumulation of construction errors cannot compromise the functionality of the object. In fact, since no reference element is established, all partial errors add up. Let's see a classic practical example. Indeed, here the dimensioning of various parts of this cylinder, plus diameters. Each dimension concerns a single part. However, we can imagine that clearly these measurements, by those who make them in the workshop, cannot be realized perfectly with zero error margin. And therefore, inevitably, in one of these, there is a measurement that can be in excess or in deficit, even by a few hundredths of a millimeter. All these excess or deficit values add up, and therefore the total dimension suffers. That is, in essence, there is a sum in absolute value of the errors. Therefore, this system, as mentioned, is adopted in cases where the total dimension is not important. Here there are various types of dimensioning, precisely in series. In fact, you see here that 168 is placed in parentheses as it is not an important measurement, as are 127 and 85, or 136. Let's move on to another dimensioning system: dimensioning systems with a common origin. There is parallel dimensioning. This system avoids the possibility of accumulating construction errors. It allows, in fact, to establish tolerances on each of the dimensions independently of the others. It is particularly indicated in cases where the tracing, execution, and control of the parts are carried out with coordinate machines. In this case, we see that the reality of this object has two elements, two origins. In fact, the dimensions on the right take this original reference element. The reference element is precisely the beginning of the object. The 16, then the 33, the 52 all have the same origin for the dimensions on the right. The same applies to the 10 and 28, obviously. The drawing should be completed with the position of the horizontal axis and the diameter of the holes. In this case, we have a parallel reference system that allows for the exact position of each hole with respect to a common origin. This is the dimensioning system used for so-called numerically controlled machines, which are not operated by the operator but by a program inserted into the machine. And this machine is zeroed, as it is technically called, on the origin of the part, and then advancement impulses equal to the dimensions indicated are given. In this drawing, we see a dimensioning system that is practically analogous to the previous one. In fact, it is always a dimensioning system that has a common origin, but it is a so-called progressive system. That is, the difference compared to the previous system is that, as you can see, there is a dot at the origin indicating zero, and then there is always only one arrow in the increasing direction of the dimensions. That is, each measurement, moreover, to differentiate it from the parallel dimensioning system, is not written on the dimension line but on the reference line. These are indeed two very important signals because, in essence, this value of 20, for example, could not be written here; it could be mistakenly taken as the distance between the first and second hole. Instead, written in this way, it is understood as the distance from the second hole to the origin. So, in essence, what is this? It is a system of progressive or superimposed dimensions. It is a simplified parallel dimensioning system because there is less drawing to do, fewer lines to trace. A single measurement line is used, which is this one, and the origin element assumes the dimension zero. Here there are other dimensioning systems. In some cases, it may be convenient to use the superimposed dimensioning system in two positions, in this one and in this one, horizontally and vertically. Here we are in the presence of a hypothetical plate with holes distributed randomly. In industrial reality, a drawing and an object are not chosen to be represented solely by the chain dimensioning system, by the parallel one, or by the combined or progressive one, but in practical reality, all the dimensioning systems that are needed will be applied. In this case, for example, we see that there is a mixed parallel and chain dimensioning system. In fact, 16 and 27 are in parallel, while this 10 and 16 are in chain. Let's move on to the last system of dimensioning by coordinates, in which the dimensions are grouped in appropriate tables. This can be convenient for both the execution and the control of the part. Let's see here, for example, a plate with holes. Instead of dimensioning each hole diameter and giving the x and y coordinates for each hole, i.e., the horizontal dimension and the vertical dimension, using, for example, this top-left vertex as the origin, it is chosen to put all the measurements in a table, which is ordered according to the number of holes. In fact, you see that each hole is accompanied by a number, and for each of these holes, the x-coordinate, the y-coordinate, and the diameter are given. This dimensioning system is highly recommended in cases where, in addition to having one plate to drill, there are several similar but not identical ones. Then there is the polar coordinate dimensioning system. This dimensioning is used especially when it is necessary to define geometrically complex profiles, as in the figure, for which we see the realization of a polycentric curve, for which there are indeed several centers with different radii, and therefore for each piece of arc of the figure, the center and the radius must always be defined. This is also another realization that concerns polar coordinates. Let's now see some realizations for dimensioning some drawing details. For example, how to dimension an angle. Here you see an example, or an arc, understood as its length. The same applies to chords, understood as the straight-line distance between one point and another of a curve. How to dimension circles and cylinders. A circle is always dimensioned by its diameter and not its radius. This is also because in practical realization, in the case of a cylindrical hole, you can think of this to help remember the concept: when a circular hole is to be made in a workshop, one must look for the drill bit, the helical bit, and this hole will be made with a drill and a bit. Therefore, the choice of the bit, which is a cylindrical bit, is entrusted to reading the caliper, and its diameter is read. Therefore, the dimension of the diameter must be preceded by the symbol whenever it is not evident from the drawing that it is a diameter. So let's see in this case: practical reality says that these are circles, and therefore you see that the symbol does not show the diameter symbol. The measurements 20, 25, 38, and 45 are simply the diametral distances of these circumferences. Whereas, in this case, we are still in the presence of cylindrical elements, but the drawing does not clearly highlight that they are coaxial cylinders, i.e., all having the same axis. To indicate the diameter, the measurement must be preceded by the diameter symbol, the Greek letter Phi. Dimensioning of diameter on cylindrical surfaces in representations parallel to the axis. For reasons of space, the dimensioning can also be placed inside the object, because here you see that there would not be space on the left to place the dimension 20, and here for the dimension 34. How to dimension radii. Radius dimensions must be preceded by the symbol R. The radius is solely used to indicate a fillet between two surfaces. Therefore, remember that the fillet is always an arc, as in these cases, of a circumference, and not the complete extended circumference, because the complete circumference is indicated by the diameter symbol. Therefore, the radius fillet dimension must be exclusively reserved for arcs of circumference. The dimension lines must always have a radial direction, and the arrow must be placed inside, i.e., on the side of the center of curvature. In this case, it is possible to place the arrow outside, but the line must be extended beyond the arrow. That is, this small arc is assumed to have its center approximately at this point, and therefore the dimension line at 45 degrees indicates almost the opening radius. In this case, the radius would be almost close to the axis, but the dimension can be placed outside, provided that the measurement line is extended. This is a case that occurs quite frequently in drawings representing objects to scale when the center of the arc is located outside the limits of the representation. The measurement lines for radii can be broken or interrupted, as in this case. In this case, the broken line indicates that this center is not actually here but would fall outside the drawing. Therefore, to give meaning to the representation of the dimension, this small stratagem is used. Spherical parts are dimensioned by diameter or radius, preceded by the respective symbols, which appear here. Here is a realization, an example. This is a spherical end that could be confused with a cylindrical end seen frontally, whereas the letter S indicates that we are in the presence of a sphere or a spherical cavity. We are still defining the particularities of drawings. How to dimension a chamfer. A chamfer is defined as a conical section of length.