Archive expert content: the article conveys a historical introduction to technical drawing and design. It is not a substitute for current SRPS, EN, ISO or other applicable standards, regulations, terms of reference, calculation, technical documentation or approval. The old JUS markings, displayed dimensions, tolerances, formats and projection rules must be checked according to the type of documentation, the contracted standard and the applicable requirements. For structural, mechanical, electrical, gas and other safety-critical works, an authorized designer of the appropriate profession is needed.
Today, Savo Kusić is focused on wooden windows, wooden-aluminum windows, custom windows, doors and requests for quotation. This article remains a historical archive and does not constitute a design service.
Design before construction
Whatever we make or repair, the first stage of work, not only in chronological order but also in importance, is designing. Often this stage of work is done only in the mind. Success with this kind of designing “only in the head” can be expected for simpler works. For more complex jobs, it is necessary to design, on a sketch or a drawing. Someone else can design for you, and your task is just to conform to the design.
The method of making technical drawings, applied markings, lines is determined by JUS standards, which you can find with a lot of other useful information, e.g. in the book Technical drawing by Todor Pantelić, Belgrade, or Technical drawing by Branko Kovač, Zagreb.
Our text, of course, cannot be measured against a single set of regulations. We only have space for the most important regulations from technical drawing (picture 1).
Figure 1 — historical overview of basic technical-drawing conventions.
Standards: citing JUS standards and old manuals preserves the original context, but does not confirm that those rules are valid today. Each project should explicitly state the applied standard, units, projection system, drawing revision and responsible person.
Scales and types of lines
First of all, we need to draw the drawings in a certain scale (Figure 1b), i.e. to show the object in the drawing reduced or enlarged compared to the original.
For example: the scale M=1:5 means that a bar whose length is 850 mm will be drawn five times smaller in the drawing, i.e. 170 mm (but we will apply 850 on the elevation). Very small objects, for example the shaft of a clock, will be enlarged in the drawing, for example, in the scale 10:1 or 2:1. Magnification is also shown by the “inverted” scale, e.g. M=2:1.
On one sheet, let’s draw the pieces only in a certain scale, and in the lower right corner, mark the scale. If the details have different scales, we indicate the corresponding scale next to each detail.
Recommended dimensions
Floor plan 1:100 - 1:50 (reduced in the drawing)
Building plan 1:20 - 1:10 (reduced in the drawing)
Interior 1:10 - 1:5 (reduced in the drawing)
Small machines 1:5 - 1:2 (reduced in the drawing)
Hand tools 1:2 - 1:1 (reduced in the drawing)
Fine Mechanics Details 1:1 - 1:2 (enlarged in drawing)
Watchmaking finesse 2:1 - 5:1 (enlarged in the drawing)
The scales 1:5, 1:50 or 5:1 should be avoided if possible, because they are difficult to recalculate by heart and do not give a round number. At the ratio 1:5, for example, a bar with a length of 763 mm should be drawn 152,6 mm and we should calculate that measure beforehand. In the case of the ratio 1:10 the drawn length will be 76,3 mm and will be obtained without difficulty.
The indicated measurements should always be the actual measurements of the objects regardless of their size on the drawing (due to scale).
Important: the list of recommended scales is transferred from a historical source. The item is made according to the dimensions and specification, not by measuring the display on paper or screen, which may be scaled when printing or displaying.
With thick lines we draw the visible edges, and with thin lines we draw elevations and auxiliary lines during dimensioning (so that the elevations do not touch the main edges of the object). The dotted line indicates the invisible edges (which when viewed from the front fall behind the object and are invisible to us).
A dashed line with a dot between each break indicates the axis of symmetry of rotating bodies along the length of the body (for example, the mid-length line of pipes, shafts, etc.) and the line of section, if the body were to be cut using an imaginary plane to highlight those internal details that are inaccessible to the eye, for example a drilled hole. We mark the surface of the imaginary section with oblique lines (hatched). The ends of the lines indicating the elevations end with arrows that lie on the auxiliary lines.
A dotted line with two dots between each break indicates the places to bend. For example, in sheet metal works, the sheets should be bent in the places marked like this. The screw thread is marked with one thin line parallel to the thick line so that the thicker line comes from the outside. On the nut, the inner line is thicker and indicates the diameter of the hole, while the thread is marked with a thinner line.
It happens that not all dimensions can fit on certain items. In such cases it is necessary to give object projections viz. multi-sided view, possibly a section or section and projection (image 2 upper part).
Projections and sections
Figure 2 — projections, sections and drawing orientation.
When drawing certain projections (“views”), the “opposite” arrangement is prescribed. This means that the selected central projection (main view) comes in the middle. An object seen on the right is drawn to the left of the central projection. An object seen from the left, or viewed from below is drawn above, etc. (Figure 1d).
We try to draw as few projections (sections) as possible, but let’s not forget that the lack of any necessary projection leads to the creation of a scrap. The same applies to listing. The rule is that every required dimension should be indicated on the drawing, but not more than one. Elevations duplicated for safety are usually contradictory and cause a dilemma (Figure 3).
Dimensioning and tolerances
Figure 3 — comparison of unclear and controlled dimensioning.
The main dimensions should always be given on the drawing. For the presence of other measures, it is important whether there is an opportunity to measure the details and whether they are necessary for the production of the object. If possible, measures should be indicated from the end, ie. edge. Chained together, these can easily lead to an item crafting error. It is important that the measurements can be checked with a scale not only on the drawing but also on the object (image 4)
Figure 4 — dimensions must be sufficient, measurable and unambiguous.
On machine drawings, measurements are given in millimeters, on construction and wood industry drawings in centimeters, and on land plans in meters. Figures, which indicate measurements, are written above the horizontal lines, which indicate elevations, and on the left side, when they are vertical, at an angle of 90° in relation to the previous ones. The numbers are written nicely, unambiguously and legibly and in the manner described above so that the vertical elevations can be read with the head tilted slightly to the left (Figure 1e).
Do-it-yourself jobs rarely use a label for tolerances - the allowed deviation of an object from ideal measurements. A sign placed after the measurement number and a small number above it warns that the dimensions of the manufactured item may be larger or smaller than indicated. (For example 204+0 indicates that the actual measure may be smaller, but not larger). If 204-1+0,5 is indicated, the object can be longer by a maximum of half a millimeter, or shorter by one millimeter.
Note on tolerances: the historical record of the tags in this paragraph may be typographically incomplete. Tolerances, fits, geometric tolerances, roughness and measurement base must be written unambiguously according to the applied standard; they should not be copied from this article into the production drawing.
It is also important to indicate the quality of the surface treatment. A collapsed sign S (~) placed on the contour line indicates that the surface is not machined, and the sign N indicates the side of the world (see the lower part of the figure 2).
The format of the drawing can be the size of a sheet of typewriter paper, half the size, double or several times larger (in the first case, the paper is folded into two parts, and in the second drawing, it is formed by placing two or more sheets next to each other, in order to obtain a certain format). The designations of the formats obtained in this way are: typewriter paper A4, half A5, double A3, etc.
The letter R or r in front of the number indicates the radius of rounding. The diameter of the circular section (e.g. in the case of rod tubes, etc.) is indicated by the sign Ø placed in front of the figure. For example, the measurement for a disc with an R16 radius can also be marked with Ø 32, depending on which measurement is easier to measure.
For drawing, we use a pointed semi-hard graphite or ballpoint pen with an elongated tip. The blade of the ruler should rest on the paper, and we draw a line with the tool that we hold in a vertical position right next to the ruler. If we want to make the drawing with ink, then the oblique edge of the ruler must not lie on the paper but must be on the upper side, so that the ink does not smear on the drawing. It is recommended to make a frame on the drawing sheet 1 cm from the edge of the sheet.
Graph paper or graph paper can be used for sketching, on which cross lines can easily be drawn freehand and drawn to scale. What we have learned so far about technical drawing is only the basis for good design. In designing, it is not enough just to know the rules, but the ability to reason and self-initiative comes to the fore. There are also a few golden rules. One of them is matching the application with the load capacity of the material, choosing the optimal weight and price of the material.
If we have already imagined in outline what and how we want to make, we also determine the necessary material. Manuals give us the necessary but not sufficient help in choosing materials, because they assume that there is an ideal supply possibility. We are, however, forced to manage ourselves by our own stock of materials, or in most cases by the poor selection of unsuitable materials, which the shops offer us.
We will get to know some important materials in detail in the chapters that describe the processing of wood, metals and plastics. Therefore, we will only dwell here on some important general properties such as:
-the strength of a material is determined by its weakest part; a knot in the middle of a batten, a softened part of a steel spring, a frozen brick of a column, make the whole weaker.
The direction of wood fibers, the shape of the cross-section, the method of clamping, the method of support greatly affect the strength of the elements.
The -strength of the material increases with increasing cross-section. Thicker material basically has more strength. However, with a suitable cross-section and a smaller cross-sectional area (and therefore less weight and cost) the material can have the same and even higher strength. For example, a U-profile steel beam has almost the same strength as a beam of the same dimensions with a full profile.
Engineering note: general claims of “nearly the same strength” of different profiles are not a basis for choosing a load-bearing element. Load capacity depends on material, load axis, stability, buckling, torsion, joints, fatigue, fire, corrosion and boundary conditions. The bearing element must be calculated for a specific case.
-individual materials, depending on the type and direction of the load, have different strength.
Strength also depends on: heat (under its influence, some plastics soften), water (mainly attacks construction material), cold (makes tin very rigid and brittle), aggressive chemicals (harmful effect on almost all construction materials), etc.
When designing, we must take all this into account, never forgetting the limited possibilities. If we have the material, the technology should be established depending on the manufacturing capability. We will choose one material when we want to weld, and another, if we want to make the connection by riveting. For the lid of the artistically decorated box, a suitable wood material is needed, and for the basement shelves, another material. We will cut the insert for the back of the armchair from one piece of sponge foam. However, sponge foam scraps can also be used to fill the cushions of a work chair.
An important rule: a detailed plan is made only after the acquisition of the necessary materials. Certain constructions cannot be realized, for example, simply because there are screws on the market with larger dimensions than intended.
The drawing is a hoax! Often the details can be arranged very nicely on it, and during assembly it is found that it is impossible to fit them; and for others, that they cannot be disassembled after assembly.
We must never forget to secure the elements against slipping, shifting and unscrewing. As an example, here are a few options for securing the “connection” with the screw against opening: washer, spring washer, crown washer, split, screw end tear, nail polish, chewing gum, paint, lock nut, plastic washer, protective cap, etc. The most suitable fuse should be selected in advance and indicated on the drawing.
Important: nail polish, chewing gum, paint and other improvised means are not approved protection of the safety-critical screw connection. The method of securing is chosen by calculation and according to the standard, material, vibrations, temperature, corrosion and manufacturer’s instructions, with a defined tightening torque and control.
We also think about adapting to needs. We’re going to be more serious about designing a go-kart steering shaft than we are about designing a birdhouse. A constructor reasons differently when designing a model airplane with radio control, than when designing a toy-wooden boat for an annual vacation.
It is difficult to learn all the wisdom of design in one text. But, success will not be absent if we draw and design thoughtfully, accurately, cleanly and controlled and if we do not spare the effort (and a little financial resources) to obtain the necessary official approvals, standards and regulations for the work.
Final note: function, assembly, maintenance, disassembly, accessibility, tolerances, material, joints and all relevant risks are checked before manufacture. Drawing revision and approval must be traceable; for every safety-important product or construction, appropriate professional calculation and verification is required.