What is the Basic Operation Technique of Fitter

Estimated reading time: 35 minutes

Marking

The lines are divided into two types: plane and three-dimensional. A plane line is a line drawn on a plane, and a three-dimensional line draws on the surface. Although the shapes of the parts are varied and sometimes even complex, any complex figure is made up of basic lines such as lines, curves, circles, and arcs. To improve the line drawing quality, one must first master the basic geometric figure painting skillfully. For example, when drawing straight lines, if the line is shorter (less than 1000mm), use a ruler to push one side through the selected two points, and press the ruler firmly on the workpiece. Then draw a line with a needle or a stone pen along the bottom of the vertical ruler. The end of the stone pen should be smoothed so that the lines drawn are fine and accurate. The thickness of lines should not exceed 0.5 mm. In the case of long straight lines (greater than 1000mm), it is better to use a powder line to eject at a time, as this is difficult to do with a ruler.

In addition, the method of drawing lines should be mastered.

Underline datum

No matter how complicated the drawing size line is, it is necessary to select a certain point, line, or surface on the workpiece as a basis to determine the size, geometric shape, and relative position of the other parts on the workpiece. This process is called determining the scribing datum. The underlined datum refers to the base surface and datums that play a decisive role on the zero or parts. The basis for determining the location of any other point, line, or surface on a zero or part of a design drawing is called the design baseline. In principle, the underlined baseline selected should be consistent with the design baseline, which is referred to as the principle for determining the line baseline.

After determining the line marking baseline according to the design baseline, you can select the line drawing baseline based on the specific situation. It shows three ways to select an underlined baseline.

1. Take two planes (or lines) that are perpendicular to each other as a datum. As shown in Figure (a), the parts have dimensions that are perpendicular in both directions. It can be seen from the graph that many dimensions in each direction are determined by the outer plane of the part, which is the underlined datum for each orientation separately.
2. Take two central lines as the datum. As shown in figure (b), in this part, the dimensions in both directions are symmetrical to the centerline, and the other dimensions are marked from the centerline. These two central lines are the line in these two directions respectively.
3. Base on a plane and a central line. As shown in figures (c), the dimensions in the direction of the altitude are based on the bottom surface, which is the line gauge for the height direction; The dimensions of the width direction are symmetrical to the centerline, which is the line marking reference for width.

Find center line and borrow material

Looking for positive and borrowed materials is a commonly used processing means in line, the main purpose is to fully ensure the quality of line work, and to ensure quality premise, make full use of raw materials, reasonable use, to reduce costs to a certain extent, improve productivity. Forwarding refers to the operation of using a scribing tool to position the surface of the workpiece in a proper position.

When the error defects in the shape, size, and position of the blank parts can not be remedied by the method of finding the straight line, we should solve them by borrowing materials. Borrowing refers to several tests and adjustments, so that each processing surface of the reasonable distribution of processing allowance, borrowing each other, thus ensuring that there are sufficient processing allowances, and errors and defects can be eliminated after processing.

It should be pointed out that: the line when the positive and borrowing work is closely combined. Therefore, looking for positive and borrowed materials must be balanced, so that all aspects meet the requirements if only consider one aspect, ignoring the other aspects, can not do a good job of drawing.

Find a straight line

Different shapes and sizes of the components processed, the method for finding positive components is different, but there are mainly two kinds of common methods.

• Straightening based on unmachined surfaces. In the assembling of structural parts, when some parts have unmachined surfaces, find the positive surface, and then underline to determine the assembly location of other components, so that the dimensions of the structural parts and other parts can be uniform. The figure is a drawing of a part found to be correct according to the unmachined surface.

The outer circular surface of Part 1 (disk) is not machined in this process, and the inner circle surface is to be processed, the outer circle is the datum to find the inside circle. Then the line is drawn to determine the assembly position of part 2 (round steel).

• Find the correct surface (side) according to the larger or more important dimension. When the size of the structural parts is large and rigid, the method of fixing the holes (or assembling other parts, etc.) on the components is usually adopted. When looking for timing, we should generally choose the face (side) with larger length, more important or higher appearance quality as the main basis, and take into account other secondary sides (sides), so that the size of each hole (or other parts, etc.) is even from the edge of the component. The figure shows a sample of the part found to be positive based on the larger side.

The component is a frame assembled by 4 pieces of angle steel 100mm * 100mmm x 8mm. The dimension and width of the diagonal line and the length of the frame after assembly correction have some deviation from the ideal size. The position of each hole shall be determined by underlining based on two long edges of 4000mm, taking into account the other sides so that the size of the holes is even from the edge of the frame.

Only when both the inner and outer holes are considered, the center of the circle is properly scribed at an appropriate position between the forging inner hole and the outer round hole to ensure sufficient machining allowance for both, as shown in the figure.

Steps and basic criteria for marking

Line drawing is the most basic operation technique of the fitter. The quality of line drawing directly affects whether the shape and dimension of machined parts are correct or not. This can generally be done in the following steps and is subject to the basic rules set out below.

Steps for marking

In addition to clear lines, the most important thing is to make sure that the size of the lines is accurate. Striking can be done in the following steps.

• Observe the title bar of the drawings and understand the names, proportions, materials, etc. of crossed parts, as well as the technical requirements, noting in particular that some requirements relating to the lines can not be marked on the drawing but are written into technical specifications or annotated classes.

• Understand the individual views and analyze their correspondence to identify the dimensions of the connections, defining the focus of each view, and visualizing the spatial shape of parts to form an overall concept.

• Carefully analyze the dimensional chain to find the dimension benchmarks in three directions: length, width, and height, and the positioning dimensions and deviations of the parts.

• Establishment of marking benchmarks. Benchmarking should be done by determining the number of benchmarks according to the type of marking and minimizing the numbers of baselines while ensuring the smooth functioning of the marking process; As far as possible, the selected datum is in line with the design datums when drawing, thus reducing the error caused by non-coincidence. When drawing, as far as possible, the marked surface should be chosen as the marking reference, and there is no machined datum surface at all before drawing some parts. At this time, according to the actual situation analysis, the assembling base or installation base of the workpiece should determine the crossing reference. When determining the crossing point, we should not only take into consideration the above points but also consider the smooth and working efficiency under the premise of ensuring the quality of the line.

• A great deal of computation is often involved in the drawing process, especially the location of the holes in a plane or complex surface, often converted into coordinate dimensions and then underlined. This requires the fitter to be familiar with trigonometric functions.

• The binder datum for the workpiece scribing shall, to the extent possible, be consistent with that of the design, taking into account the characteristics of complex parts, which are often to be corrected or supported using fixtures or aids.

• Reasonable selection of support points to prevent the shift of the center of gravity and ensure safety during line drawing.

The basic rule of drawing

It should be emphasized that the drawing is only the basis and important basis of machining parts, and the measurement is the means to ensure the machining accuracy. To improve the quality of the drawing, the following provisions must be strictly adhered to.

• A vertical line must be used as a drawing line and can not be drawn with a protractor or 90 degrees, still less by visual means.

• When marking the steel plate in a circle, arc, or component dimension with a marking scheme, to prevent the marking toe from slipping, it must be punched out and then scribed.

• Check that steel plates and specifications meet the requirements of the drawings before the line-drawing operation, that the steel used for important products shall be subject to satisfactory tests and that its chemical composition and mechanical properties conform to those specified in the drawing.

• The surface of the steel before the line shall be level and, where the surface is wavy or uneven, will affect the accuracy of line marking and shall therefore be corrected and flattened in advance. Besides, the surfaces of steel should be clean and free from defects such as cladding, pocks, cracks, etc.

• Measurement tools used for marking (e.g., steel tape, tape, and angle) shall be regularly inspected and corrected. Use efficient tools, clamps, and gauges as much as possible to improve efficiency.

The practice of basic graphics

To accurately draw the contour of the part on the drawing, it is necessary to understand the drawing knowledge of various lines and the rules of their connection. For the sheet metal worker, this is another operation such as lofting and scribing (numbering). The basics. Generally speaking, there are mainly the following methods for basic geometric figures. Name

• The drawing methods of straight lines and angles. The drawing methods of various straight lines and angles are shown in Table 1-5.

• How to draw a circle, equal division. The equal division of a circle is the basis of a regular polygon, and it is also a common method used in sheet metal processing to determine the position of the unfolded material or the position of the dotted line of the drilled hole. The drawing method is shown in Table 1-6.

The equal division of a circle can also be calculated by the calculation method. The calculation formula is:

s=2Rsin180°/n   

Which means:    s——The chord length of the halving circle;

                 R——Radius of the circle;

n——Number of equal parts of a circle.

When using the calculation method to divide the circle, you only need to use the above formula to calculate the chord length s value of the divided circle, and then use the divider to directly intercept the points on the circle, and then directly connect the points. For example, when the calculation method is used to divide the circle into six, the chord length s of the divided circle can be calculated first s(s=2Rsin180°/n =2Rsin180°/6 =R) and then Using compasses, first take any point on the circle as the center, take s=R length as the radius, and draw arcs in sequence, then the circle can be divided into 6 equal parts, and each point can be connected in sequence to form a regular hexagon, as shown in Figure 1-21 Show.

• The drawing method of arcs and ellipses. Arcs are the basis of various graphics. The drawing methods of arcs are shown in Table 1-7.

The ellipse is also a common figure in sheet metal parts, and there are many drawing methods. The commonly used drawing methods of ellipses are shown in Table 1-8.

• The drawing method of arc connection. The arc connection is to smoothly connect two known straight lines, two arcs or straight lines, and one arc with an arc with a known radius. Various arc connections are the basis for forming some more complex shapes of connections. The key to arc connection is to find the center and connection point (tangent point) of the connecting arc. The drawing methods of various arc connections are shown in Table 1-9.

Chiseling, sawing, filing

Chiseling, sawing, and filing are the more important basic operating skills in a fitter’s work. They are mainly used in occasions where machining is inconvenient, or when there is too much margin to remove enough margin.

Chiseling

Chiseling is the operation of hammering a chisel with a hand hammer to cut metal, also known as chiseling. In sheet metal processing, it is mainly used for cutting the plane [see Figure 1-22 (a)] and cutting the sheet [ See Figure 1-22 (b)], the segmentation of the sheet [see Figure 1-22 (c)] and the cleaning of burrs on castings and forgings, etc., as shown in Figure 1-22.

Classification of chiseling. The chiseling operation is done by hammering the chisel with a hand hammer [see Figure 1-23(a)]. The chisel is usually forged with carbon tools T7 or T8, and the blade is quenched and tempered. It is made into different shapes according to the needs of the work, generally, the total length is 170～200mm. There are two types of chisel commonly used in sheet metal processing.

1. Flat chisel. A flat chisel, also known as a flat chisel, is the most commonly used chisel for fitters. Its cutting edge is flat and the width of the cutting edge is generally 10-20mm, as shown in Figure 1-23(b). It is mainly used to remove flanges, burrs, clipping planes, cutting off sheets, etc. on the plane. 
2. Sharp chisel. A sharp chisel is also called a narrow chisel. Its cutting edge is relatively narrow, about 5mm. There are inverted cones on both sides of the cutting edge to prevent the chisel from being jammed when deep grooves are made. See Figure 1-23(c). Mainly used for cutting grooves and dividing curved sheets.                                             

Operation method of chiseling

• How to hold the chisel. The chisel is mainly held by the middle finger and ring finger of the left hand, the little finger is naturally closed, the index finger and the thumb are in natural contact, and the head of the chisel extends about 20mm. The chisel should be held easily and firmly, and should not be held too tightly, so as to avoid excessive vibration in the palm of the hand when it is struck, or injury to the hand once the hammer is missed. When chiseling, the hand holding the chisel should be level with the forearm, and the elbow should not be drooped or raised. Figure 1-24 (a) shows the front grip of the chisel, and Figure 1-24 (b) shows the reverse grip.

• How to hold the hammer. The hammer generally uses the 5 fingers of the right hand to fully grasp the method, the thumb is lightly pressed on the index finger, the tiger’s mouth is aligned with the hammerhead, do not tilt to one side, the end of the wooden handle is exposed 15~30mm. Figure 1-25(a) is the hammer grip method, and Figure 1-25(b) is the loose grip method.

• Chiseling posture. During the chiseling, in order to give full play to the greater percussive force, the operator must maintain the correct standing position, as shown in Figure 1-26, the left foot is half a step forward, the two legs stand naturally, and the human’s center of gravity is slightly biased to the right foot. The line of sight should fall on the cutting position of the workpiece.

• Precautions when chiseling. When chipping, pay attention: first, keep the chisel sharp. The dull chisel is not only laborious, but the surface of the chisel is uneven, and it is easy to slip or hurt your hands. If the hammered part of the chisel has obvious burrs, it should be sharpened in time. To prevent iron chips from breaking and flying out and hurting people, the operator must wear protective glasses; when the wooden handle of the hammer is loose or damaged, it should be replaced in time to prevent the hammer from flying out; the part of the hammer, the head and the handle of the hammer The parts should not be stained with oil to prevent slipping; the workpiece must be firmly clamped, and the height of the protruding jaw should be 10~15mm, and a block should be added under the workpiece.

Chiseling method. In the processing occasions of different forms of chiseling, the method of chiseling is different.

• Chiseling the plane. The chiseling plane is generally carried out with a flat chisel, and the allowance of each time is 0.5-2mm. When chiseling a narrow plane (the width of the workpiece is less than the width of the flat chisel edge), the cutting edge of the chisel should preferably be inclined to a certain angle with the forward direction of the chisel to increase the contact surface and make the chisel grasp stable; while the chisel is large When flat, you should first use a sharp chisel to make grooves, and then use a flat chisel to round off the raised parts between the grooves. The number of grooves should be such that the width of each remaining part is slightly smaller than the width of the flat chisel.

When chiseling a plane, you should master the three stages of trimming, chiseling, and chiseling out, as shown in Figure 1-27

1. Start chiseling. When picking up the chisel, you should start from the sharp corner of the edge of the workpiece (except for the whole groove). After the cutting edge is close to the chiseled part, the chisel is held flat and perpendicular to the end of the workpiece. Tap the chisel to facilitate cutting.
2. Chiseling. When chiseling, keep the correct position and direction of the chisel, and control the size of the relief angle (generally, the relief angle should be kept at 5°~8°) and the hammering force is uniform. After hammering several times, withdraw the chisel, observe the processing, and also dissipate heat from the edge of the chisel.
3. Chiseling out. When the chiseling is almost to the end (approximately 10mm from the end), you should turn your head around and remove the remaining part to prevent the edge of the workpiece from cracking, especially when chiseling brittle materials such as cast iron and bronze.

• Chipped sheet. When chipping thin sheets with a thickness of not more than 2mm, use a bench vise to chip. Use a flat chisel to cut from right to left along the jaws and diagonally to the board (about 45°), and make the chisel tangent line parallel to the jaws.

When cutting thick sheets, you can cut on iron drills (or flat plates). A soft-iron material should be placed under the sheet to prevent damage to the cutting edge of the chisel. First, cut the dent according to the scribe line, and then use the hammer to break it. For a sheet with a larger size or a more complex shape, a row of dense small holes is generally drilled around the contour of the workpiece, and then a chisel is used for cutting.

• Heat treatment and sharpening of the chisel. The chisel is usually forged with carbon tools T7 or T8. The blade is quenched and tempered. The overall length is 170~200mm, which can be purchased directly from the market. Sometimes, according to work needs, it is necessary to conduct self-made heat treatment. During heat treatment, the cutting part of the chisel should be rough ground, and then the cutting part of the chisel should be inserted into the furnace (generally using a forging furnace) to a depth of about 25mm. 750~780℃, after dark fuchsia, take it out and quickly immerse in cold water to cool (the immersion depth is 5~6mm), and move slowly along the water surface, so that the boundary between the quenched part and the non-quenched part is not very obvious, reducing the There is a tendency to crack at this junction. When the part of the chisel that is not in the water turns black, take it out of the water, quickly rub the blade face on the masonry or emery cloth a few times to remove the surface oxide layer or dirt, and use the residual heat from the upper part for tempering. At this time, pay attention to the color change of the blade surface as the temperature rises: after taking it out of the water, it changes from grayish-white to yellow, and then from yellow to red, purple, and blue; when it appears yellow, immerse all the chisel. Cool in water. This tempering temperature is called “yellow fire”. When it is blue, immerse all the chisel in water to cool down. This tempering temperature is called “blue fire”. The hardness of “yellow fire” is higher than that of “blue fire”. Wear-resistant, but brittle and easy to break; “blue fire” is more suitable, so it is more used.

The new chisel and the blunt one should be sharpened on the grinding wheel. The sharpening of the chisel is usually done on a bench grinder. The grinder is not only used for fitters to grind tools, such as chisel, scraper, etc., but also for grinding various cutting tools, such as drills. The bench grinder consists of a grinding wheel, a motor, a grinding wheelbase, a bracket, and a protective cover, etc. Its structure is shown in Figure 1-28.

Because the texture of the grinding wheel is relatively brittle, the rotation speed is high during work, and improper use of force will cause the grinding wheel to break and personal accidents. Therefore, before grinding with the grinder, check whether the grinding wheel is cracked or broken and whether the protective safety cover is intact. When installing the grinding wheel, the grinding wheel must be in dynamic balance so that the grinding wheel does not vibrate when it is rotating. The use of grinders must strictly abide by the safety operating regulations.

In the grinding process, it is not allowed to grind the workpiece on the side corners of the grinding wheel. The operator must wear protective glasses, stand on the side of the grinding wheel, and do not stand in the direction of rotation of the grinding wheel, so as to prevent the grinding wheel from flying out and hurting people. When grinding, the force should not be too strong, and the grinding should be steadily moved up and down, left and right. Strictly grind non-ferrous metals (such as copper, aluminum, etc.).

The distance between the bracket of the grinder and the grinding wheel should generally be kept within 3mm, otherwise it is easy to cause the grinding piece to be rolled, or even cause the grinding wheel to break and fly out. After the work is completed, cut off the gas source and power supply in time, and clean up the surrounding area of the worksite.

When sharpening a chisel on a grinding wheel, first sharpen the chisel in the correct shape and make the cutting edge sharp. For this reason, it is required that the angles between the two cutting edges of the chiseled face of the central plane are equal; the width of the two cutting edges are equal and flat and smooth; the cutting edge should be straight.

When sharpening the chisel, hold the chisel with one hand up and the other hand down, so that the cutting edge is tilted upward and placed on the edge of the rotating grinding wheel, and moves smoothly back and forth along the axis of the grinding wheel. When sharpening, it is pressed against the chisel. The force on the nail should not be too large, and the direction and position of the grip must be controlled to ensure that the desired wedge angle is ground. In order to maintain the hardness of the cutting edge, it is often necessary to cool it with water during sharpening to prevent high-temperature annealing of the cutting edge.

When cutting steel, the wedge angle is generally 50°~60°, hard steel is 60°~70°, and non-ferrous metals such as copper and aluminum are used 30°~50°.

Sawing

Sawing is a method of cutting metal materials through the cutting motion of saw teeth. Sawing can not only cut metal materials but also cut, cut grooves, etc.

• Sawing tools. The hand saw is composed of a saw bow and a saw blade, as shown in Figure 1-29 (a). The saw bow is used to tighten the saw blade. There are many saw teeth on the saw blade, which are used to cut materials or workpieces to complete the cutting, cutting, or grooving processing. The saw blade can only cut when it is pushed forward. Therefore, when installing the saw blade, the direction of the tooth tip should be forward, as shown in Figure 1-29 (b). If it is installed backward, normal sawing cannot be performed. When installing the saw blade, the tension should be appropriate, too tight and too loose will affect the sawing, and it is also easy to break the saw blade.

Saw blades are generally made of cold-rolled carburized steel, but also made of carbon tool steel or alloy steel, which is hardened by heat treatment. The commonly used saw blade is 300mm long (the length between the two mounting holes), 12mm wide, and 0.8mm thick. Sawteeth can be divided into coarse teeth (t=1.6mm), medium teeth (t=1.2mm), and fine teeth (t=0.8mm) according to the size of the tooth pitch t. It can also be expressed by the number of teeth per 25mm length of the saw blade: coarse teeth are 14-18 teeth, the middle teeth are 22-24 teeth, and the fine teeth are 32 teeth.

The selection of the saw blade should be determined according to the hardness and thickness of the processed material. Generally, the number of teeth working on the saw blade at the same time is 2 to 4 teeth. Coarse teeth are used for sawing soft materials such as low carbon steel, copper, aluminum, plastics, and materials with thick sections; fine teeth are used for sawing hard materials, sheets and thin-walled pipes, etc.; processing ordinary steel, cast iron, and medium-thickness Material, multi-purpose saw blade with the middle tooth.

The basic operation of sawing. When sawing, you should be proficient in grasping the hand saw, starting sawing, and sawing operations.

• The grip of the handsaw. Hold the saw handle with your right hand, and lightly hold the front end of the saw bow with your left hand, as shown in Figure 1-30 (a).
• The method of starting sawing. Starting sawing is the beginning of sawing work, and the quality of starting sawing will directly affect the quality of sawing. There are two types of starting saw [see Figure 1-30(b)] and near-start saw [see Figure 1-30(c)].

When starting the saw, hold the left thumb against the saw blade, so that the saw blade can be sawed accurately at the required position. The stroke should be short, the pressure should be small, the speed should be slow, and the sawing angle is about 15°. If the sawing angle is too large, the sawing is not easy to be stable, especially when the sawing is close, the saw teeth will be jammed by the edge of the workpiece and cause chipping. However, the sawing angle is not easy to be too small. Otherwise, because the sawtooth has a large number of teeth in contact with the workpiece at the same time, it is not easy to cut into the material.

Under normal circumstances, it is better to use remote sawing, because the saw teeth gradually cut the material when the remote sawing, the saw teeth are not easy to be jammed, and the sawing is more convenient.

• Sawing operation. The sawing operation after starting sawing should make all the effective teeth of the saw blade participate in the cutting in each stroke as much as possible. During operation, push the hand saw with uniform application force, proper pressure and speed, and no impact, otherwise it will affect the quality of sawing, and it will also easily cause chipping and fracture of the saw blade; the saw blade should be raised slightly during the return stroke, and the speed should also be increased. In order to reduce the wear and return time of the saw blade.

Sawing method. When sawing different materials and different forms of metal components, different methods should be used in a targeted manner.

• Sawing of bars. If the sawing section is required to be flat, it should be sawed along the cutting line from the beginning of the sawing to the end. If the sawing section is not demanding, it can be sawed in several directions, so that the sawing surface becomes smaller and it is easier to saw in, which can improve work efficiency.
• Sawing of thin sheets. When sawing thin sheets, you should saw it down from the wide surface as much as possible. When sawing only from the narrow surface of the sheet, it can be clamped by two wooden boards and sawn together with the wooden block to avoid the teeth from being hooked. At the same time, the rigidity of the sheet is improved, so that no vibration occurs during sawing, as shown in Figure 1-31 (a). It is also possible to clamp the thin sheet directly on a bench vise, and use a hand saw to push the saw horizontally to increase the number of teeth contacting the sheet with the sawtooth and avoid the saw tooth chipping, as shown in Figure 1-31 (b).

• Sawing of deep seams. When sawing the groove, if the saw seam is not too deep, it can be cut out directly with a hand saw, as shown in Figure 1-32 (a). When the depth of the saw kerf exceeds the height of the saw bow, the saw blade should be rotated through 90° and re-clamped, so that the saw bow can be turned to the side of the workpiece for sawing, as shown in Figure 1-32 (b). Or turn the saw blade through 180°, and place the saw bow on the bottom of the workpiece to continue sawing, as shown in Figure 1-32(c).

Filing

Filing is the operation of cutting the workpiece with a file to achieve the required size, shape, and surface roughness. Filing is a relatively fine fitter’s manual operation, its processing accuracy can reach about 0.01mm, and the surface roughness can reach Ra2.2~1.6um. Filing can process the inner and outer planes, inner and outer curved surfaces, grooves, and various complex-shaped surfaces of the workpiece, especially those parts that are difficult or impossible to be processed by machining, and individual parts are trimmed in the assembly and repair process, etc.

• File. The file is made of carbon tool steel T12 or T13. After heat treatment, the hardness can reach 62~72HRC. The file is composed of a file body and a filehandle. The file body includes the file face, the file edge, the bottom teeth, and the face teeth. The structure of the file is shown in Figure 1-33.

According to different purposes, files can be divided into three types: ordinary fitter files, special-shaped files, and plastic files.

Ordinary fitter files can be divided into flat files (plate files), semicircular files, square files, triangular files, and round files according to their different cross-sectional shapes, as shown in Figure 1-34 (a).

Special-shaped files include knife-edge files, diamond files, flat triangle files, oval files, round belly files, etc., as shown in Figure 1-34 (b). Special-shaped files are mainly used for filing special surfaces on workpieces.

Shaping files are also called assorted files, which are mainly used to trim small surfaces on workpieces.

Different files have different purposes. When filing, a file must be selected reasonably to give full play to its efficiency and extend its service life. Generally, the choice of the cross-sectional shape and length of the file depends on the size and surface shape of the workpiece. The choice of file tooth pattern thickness grade is determined by the size of the workpiece machining allowance, the nature of the workpiece material, the level of machining accuracy, and the requirements of surface roughness. Coarse-tooth files are suitable for filing workpieces with large machining allowance, machining accuracy, and surface roughness requirements; while fine-tooth files are suitable for filing workpieces with small machining allowance, machining accuracy, and surface roughness requirements. It should be selected according to the shape, size, and processing conditions of the surface of the workpiece. Figure 1-35 shows an example of selecting the corresponding section file according to the processing shape.

The basic operation of filing. When filing operations, you should be proficient in the grip of the file and the changes in the force of both hands during filing.

• How to hold the file. When filing, you usually hold the filehandle with the palm of your right hand against the end of the wooden handle of the file, place your thumb on the wooden handle, and press the file with your left hand, as shown in Figure 1-36.

• The force applied when filing. When the file is advancing, it should keep moving in the horizontal plane. It is mainly controlled by the right hand, and the pressure is controlled by both hands. When the file is at any position on the workpiece, the torque on the front and back ends of the file should be equal to make the file. Straight and horizontal movement. The change in the force of both hands is shown in Figure 1-37.

At the beginning of the file, the pressure on the left hand is high and the pressure on the right hand is small. As the file advances, the pressure on the left hand should gradually decrease, and the pressure on the right hand should gradually increase. When the file is in the middle, the pressure on the two hands should be equal; when pushing forward again, the pressure on the left hand is again Gradually reduced, the pressure of the right hand gradually increases; when the file returns, no pressure is applied to both hands to reduce the wear of the tooth surface. If the force of both hands remains the same, the handle will deviate at the beginning, and when the filing is finished, the front end will sag. As a result, the filing will have a drum-shaped surface with low ends and convex middle.

• Clamping of the workpiece. Whether the workpiece is clamped correctly or not will directly affect the quality and efficiency of filing. Generally, the workpiece to be clamped should be clamped in the middle of the jaws of the vise as much as possible, and the jaws should not be too high, and the clamping should be firm, but the workpiece should not be deformed; when clamping processed surfaces, precision workpieces and workpieces with irregular shapes, Appropriate gaskets should be added to the jaws to avoid clamping the surface of the workpiece.
• Precautions when filing. When filing, pay attention: the new file should be used on one side first, and then the other side should be used bluntly. In use, it is used to file soft metals first, and then file hard metals after a period of use to extend the service life of the file; do not get oil or water on the file to prevent slipping or tooth corrosion during filing; unusable Files are used to file castings with sand or forgings with hardened surfaces, as well as hardened surfaces. Fine files cannot be used to file soft metals; files cannot be used as tools for assembly, disassembly, hammering, or prying; The iron filings on the surface should be brushed off with burrs along with the teeth. Do not blow with your mouth or remove it with your hands to prevent the iron filings from flying into your eyes or hurting your hands.

File method

• Plane filing. Commonly used plane filing methods include forward filing, cross filing and push filing, as shown in Figure 1-38 (a) ~ (c) respectively.

A forward file is a file that is always filed along its length. It is generally used for filing or filing, and it can get straight file marks.

Cross file is to file one layer in one direction first, then turn 90° to file a second time, and then repeat. In this way, the unevenness of the filed surface can be found from the file marks, and the flat surface can be easily filed. This method has a large contact surface between the file and the workpiece, and the file is easy to grasp and stable and is suitable for occasions with large machining allowance and leveling.

Pushing the file means that the movement of the file is perpendicular to its length. It is generally used for filing narrow and long surfaces or the surface of the workpiece has been filed flat and the machining allowance is very small. It is used for smoothing the surface or correcting the size. After the workpiece is filed, it is necessary to check the size and shape accuracy. Generally use steel ruler or knife-edge ruler to check flatness by light transmission method; use a square ruler to check verticality; use an external caliper to check parallelism and size.

• Surface filing. There are three types of curved surfaces: outer arc surface, inner arc surface, and spherical arc surface. Generally use a flat-file for the outer arc surface, and a round file or a semicircular file for the inner arc surface.

When filing the outer arc surface, generally file along the arc surface, as shown in Figure 1-39 (a). When filing, while the file moves forward, it swings around the center of the arc of the workpiece. When swinging, the right-hand presses down and the left hand lift the front end of the file so that the surface of the field arc can be smooth and edgeless. This method is not easy to exert strength, so the efficiency is not high, and the filing position is not easy to grasp, so it is only suitable for small margin or the outer arc surface of fine filing; when the margin is large, use horizontal arc surface filing, As shown in Figure 1-39(b). This method is easy to use and has high efficiency. It is often used for rough machining of arc surfaces.

When filing the inner arc surface, the rolling file method is generally used. The file must complete 3 actions at the same time, that is, forward and retract actions, left or right movement (about half or a file diameter) action, and rotation around the centerline of the file (clockwise or counterclockwise rotation about 90°). Only when three actions are coordinated at the same time can a good inner arc surface be filed.

• Filing of flat and curved surfaces. Under normal circumstances, the plane should be processed first, and then the curved surface, so that the connection between the curved surface and the plane is smooth and smooth. If the curved surface is processed first and then the plane is processed, when processing the plane, the side of the file can easily move left and right, which will damage the processed curved surface. At the same time, the joints are not easy to file smoothly, or the arc cannot be tangent to the plane.
• File with each other. Through filing, the filing process that makes the mating surfaces of two matching parts meet the requirements specified on the drawing is called filing. Its basic method is the first file the mating surface of one of the matching parts, and then file the mating surface of a matching part according to the filed one. Because the outer surface is generally easier to process than the inner surface, it is best to file one piece on the outer surface first, and then file one piece on the inner surface.