Zheng Lin

Abstract： "Mechanical drawing" is a basic course of engineering， and the quality of its teaching directly affects students' ability to read maps in production practice. How to improve the teaching quality of this course is of great significance to the teaching of engineering majors. This paper discusses how to improve the teaching quality of "mechanical drawing" in vocational schools from four aspects.

Key words： mechanical drawing; projection law; space imagination; orthographic views

"Mechanical drawing" is a technical language in the field of mechanical engineering. Scientific researchers and technicians express their design intentions according to engineering drawings， such as the shape， size， size of parts and related technical and assembly requirements， while technicians process and assemble according to engineering drawings， which are the silent technical language between scientific researchers and technicians. Engineering drawings are drawn according to the relevant standards and requirements of "mechanical drawing"， so it is particularly important for students in technical vocational schools to learn the course "mechanical drawing" well [1]. The source of students in vocational schools is complicated， and students' educational level is uneven. Some students are afraid of difficulties in this course. In the following article， the author will discuss the methods of improving the teaching quality of "mechanical drawing" based on the experience of teaching this course for many years.

I. Grasping the opportunity to improve students' interest in learning is the premise of improving teaching quality

As someone once said， "everything we need to know must be taught through the things themselves." that is to say， we need to find ways to make students truly feel what they want to learn， instead of just talking about things to them. There are rich and colorful contents in cartographic teaching， which require us to adopt different teaching methods for different chapters and different contents. Students often have curiosity about new things， at this time it is necessary to grasp this kind of psychology in students， compare the overall model in the teaching， use touch， look， contrast， thinking and other methods to cultivate students' interest. In addition， in the actual teaching， the author found that some particularly naughty but "potential" students， whose performance in other basic theory courses is poor， but their spatial imagination is very good， at this time， dont look at people through colored glasses. Instead， encourage and praise the advantages of such students to help them improve their confidence in themselves and their study， take them as the core of the class， promote work in all areas by drawing upon the experience gained on key points， drive the whole class to improve the class discipline and enhance the students' interest in the course of "mechanical drawing"， so as to improve the teaching quality of the course comprehensively.

II. Helping students understand the projection law is the basis of improving teaching quality

The projection law is the basis of projection mapping， and the projection mapping law can be simplified into three characters as： straight， uniform， even， which seems easy to remember， but for the students for the course of "mechanical drawing"， how to correctly understand and learn the law of projection of three characters is a difficulty. In the actual teaching， in order to help students correctly understand the law of projection， the author will firstly let students look and find the three projection system （in the corner in front of the student's right） after introducing the concept of the three projection system， so as to strengthen students' understanding of the system of three projection planes; Then students will be shown a system of three projection planes made with three boards. By using three boards perpendicular to each other to represent the abstract projection plane （front， side and horizontal plane）， so students can quickly and deeply understand the formation principle of three views. Ask a student to come to the lectern and place the geometry between the student and the front of the self-made three projection planes system. It should be noted that the student's eye line is equivalent to a set of parallel lines， so that the student looks at the geometry from front to back and perpendicular to the front， that is， looking straight ahead， orthographic projection to the front of the three-projection system from front to back， to get the main view of the geometry; then， let the geometry change its position and make orthographic projection to the side and horizontal plane of the three-projection system from left to right and from top to bottom， so as to obtain the left view and the top view of the geometry; then， the top view of the geometry obtained from the horizontal plane in the system of three projectors is rotated 90 degrees downward around the intersection line between the horizontal plane and the front， that is， the X-axis. Then， the left view obtained from the lateral projection of the geometry is rotated 90 degrees to the right around the intersection line between the side and the front， that is， the Z-axis. At this time， the side， horizontal plane and the front are in the same plane， that is， the main view， top view and left view of the geometry are in the same plane. Students can see the positions of the three views directly. In order to deepen students' further understanding of the upper and lower， left and right， and forward and backward projection directions， ask students to mark the corresponding six directions on the object and three views respectively， it should be notice that the distance between the top and bottom of the geometry is the height of the object， the distance between left and right is the length of the geometry， and the distance between front and back is the width of the geometry [2]. Finally， ask the students to discuss and communicate with each other to find out the relationship between different views of the object. Through this heuristic instruction， students can clearly find the following laws in the projection of three views： the length of the main view of the geometry is corresponding to the front （straight）， the height of top view of the geometry is at the same level （uniform） and the width of left view of the geometry is equal （even）. After this intuitive teaching operation， the process is replayed with the aid of the animation of the multimedia courseware， which breaks through the key and difficult points of the course "mechanical drawing" and strengthens the training of students' transformation from three-dimensional plane to three-dimensional thinking， in this way， with the difficult breakthrough， the learning effect will be naturally improved.

III. Cultivating students' spatial imagination is the key to improve teaching quality

Spatial imagination is a process of reconstructing objects in human brain according to language or pattern description. In the new century with the rapid development of science and technology and the constant updating of knowledge， imagination is even more important. Einstein once said： "imagination is the source of knowledge evolution"， training spatial imagination is the main task of "mechanical drawing"， teachers must cultivate students spatial imagination throughout teaching. In the course， drawing is the transformation of geometry from three-dimensional space to two-dimensional plane， which is， drawing plane view based on stereogram， and the view is to transform the geometry from two-dimensional plane to three-dimensional space， that is， to figure out the spatial shape of the object based on the plane view [3]. No matter in the process of view or drawing， students are required to present objects in three-dimensional form in their minds for necessary analysis， so it is necessary to have strong spatial imagination ability. This means that students have to build up their spatial imagination while studying "mechanical drawing".

For students who enter the school recently， their knowledge level still needs to be improved， so the space in their mind is only vague or even completely unclear， there is no doubt that this is normal， still， they can read some simple three-dimensional figure. Therefore， at the beginning of the course of "mechanical drawing"， students' spatial concept can be cultivated from the basic geometric form by utilizing the existing foundation of students; prepare some common basic geometry such as cylinder， sphere， cuboid， cube， prism， pyramid and so on. Let students see， touch， think， and then draw. First， draw a sketch that is close to the axonometric diagram， and then draw a three-sided view of the basic geometry. Then， look at the three views drawn， and reconstruct the three-dimensional stereogram of the basic geometry in mind， through repeated training and memory of basic geometry from "stereo-plane" to "plane-stereo"， students can establish perceptual knowledge of space in their brains. Guide students to accumulate the most basic spatial perception through the participation of vision， touch and other senses， so as to form the three-dimensional geometry of spatial geometry， laying a good foundation for the formation and development of spatial perception， and achieving the purpose of initially cultivating students' spatial imagination， at the same time it can greatly stimulate students' interest in learning， avoid explaining boring and difficult theoretical knowledge， and eliminate their fear of difficulties in the course of "mechanical drawing". Then gradually deepen on this basis， consolidate and strengthen students' concept of space from the combination， guide students to draw some combination that simply superimposed or cut from basic geometry. Prepare the dough and a knife， let the students think， cut， spell， and then draw. First， figure out what kind of basic geometry the assembly is made of， then figure out how the assembly is connected and what kind of surface the assembly is made of， that is， to analyze the shape and line of the assembly， it is best to come up with a three-sided view of the assembly and draw it directly according to the law of projection， if there is any difficulty， students can take out play-dough and cut it， break through the cognitive barrier， accurately draw the three views and reconstruct the three-dimensional stereogram of the combination in mind. To further strengthen the students' perceptual knowledge space can increase the students' space geometry information storage， the more storage space geometry information students have， the more space geometry image can be extracted when using， the stronger the space thinking ability， as result， space imagination of students nature can be raised， studying "mechanical drawing" will become no longer difficult.

IV. Mastering the correct learning method is the guarantee of improving teaching quality

The teaching of "mechanical drawing" is to start with sensibility， compare geometric drawing and projection drawing with real objects or models， and let students establish the three-dimensional concept and projection concept， and then abstract the geometric elements， describe the projection law of points， lines， surfaces and their relative positions in various situations， and return to the projection of geometry to deepen understanding， guide students to comprehensively use the projection knowledge to solve the problem of drawing and map recognition， therefore， it is necessary to repeat the basic theory and basic knowledge of cartography， so as to deepen the understanding and grasp knowledge point firmly. Therefore， the author summarized the learning method of this course as "listen"， "think"， "see"， "practice"， "remember" and "do". "Listen" is to ask the students to concentrate on the class， emphasizing that students must be prepared before class， listen to the class with purpose， and take lecture notes， timely review after class. "Think" is to emphasize that students should think about things by pictures， by things to think about pictures， and to think more， think repeatedly， associate the opposite relation of dimensional form and graph， so as to improve dimensional imagination and analytic ability ceaselessly. "See" is to emphasize that students should see the picture， see the real object， see the model comparatively， widely and repeatedly， so as to truly understand the picture， and constantly improve the ability to recognize the map. "Practice" is to guide students to practice drawing. Firstly， beginners should repeatedly carry out the practice of painting the basic body， observe the basic body model at first， so that to store a variety of basic three-dimensional shapes in mind， and draw orthographic views from the physical model， then draw orthographic views according to the memory of the three-dimensional image， so as to obtain the preliminary concept of space. Secondly， it is necessary to draw more axonal maps， paying attention to the easy before the difficult， so as to strengthen the formation of three-dimensional shapes and concepts， and build a "space shelf" and enrich the "inventory" of forms. With the amount of "inventory" getting large， the spatial sense will get strong. Thirdly， the arrangement of exercises should be combined with the content of classroom teaching and students' learning situation as far as possible. The topic should be targeted and arranged in different layers according to the degree of students， so as to help students summarize and master the main points of the textbook. Thirdly， the arrangement of exercises should combine with the content of classroom teaching and students' learning situation as far as possible. The topic should be targeted and arranged in different layers according to the degree of students， so as to help students summarize and master the main points of the textbook. Ask students to do more exercises on cutting cylinders and cuboids. Repeated exercises can activate thinking and enrich the inventory of three-dimensional graphics. "Remember" is to keep the basic theory and concept of drawing as well as the relevant provisions of the national standard of mechanical drawing in mind， and can use flexibly and skillfully when drawing and recognizing drawings. "Do" is to achieve the correct projection when do exercises and homework， with appropriate view selection and configuration， complete size， neat text specification， clean picture surface， and in line with the national standards of mechanical drawing. Develop the habit of using drawing method and drawing tool correctly， cultivate preciseness， meticulous style and serious learning attitude. If the learning method is right， students' subjective initiative in learning can be mobilized， students' potential will be brought into play， and the learning effect will be improved naturally， thus ensuring the quality of teaching.

Of course， how to promote students to learn "mechanical drawing" is a big topic， as long as we well prepare for teaching links of the "mechanical drawing" and study hard and constantly explore， we will be able to find more， better teaching methods and ways.

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