The main malfunctions of machines. Carrying out repairs of CNC machines

Defects- deviations from the quality of the material provided for by the technical specifications in terms of chemical composition, structure, continuity, surface condition, mechanical and other properties.

Defects that occur during the operation of equipment can be divided into three groups:

1) wear, scratches, risks, nadir;

2) mechanical damage (cracks, tooth chipping, breakage, bending, twisting);

3) chemical and thermal damage (warping, shells, corrosion).

Most large and medium mechanical defects are detected during external examination. In some cases, the check is carried out with a hammer: a rattling sound when the part is tapped with a hammer indicates the presence of cracks in it. To detect small cracks, various methods of flaw detection can be used. The simplest are capillary methods that allow you to visually determine the presence of cracks. The method of magnetic flaw detection with longitudinal or rotational magnetization is more complicated. Defects located inside the material are determined by fluoroscopic or ultrasonic methods. Ultrasound can also be used to detect cracks.

Wear(wear) - a change in size, shape, mass or state of the surface due to the destruction of the surface layer of the product. The following types of wear are distinguished: permissible, critical, limiting, premature, natural, and many others, the name of which is determined by physical and chemical phenomena or the nature of the distribution over the surface of the part.

Of all the possible types of wear, the main ones in machine tools are mechanical, seizing and oxidative.

At mechanical wear there is abrasion (cutting) of the surface layer of the jointly working parts. It is often exacerbated by the presence of abrasive dust, solid particles, chips, wear products. In this case, the rubbing surfaces are additionally destroyed due to scratches. Mechanical wear occurs at zero and different relative speed of movement of the mating surfaces, in the presence of long-term loads, high specific loads and a number of other factors. Proper design and processing can significantly reduce this wear.

Seizure wear occurs as a result of the setting of one surface with another, deep pulling out of the material. This happens with insufficient lubrication and significant specific pressure, when molecular forces begin to act. Seizure also occurs at high sliding speeds and high pressures, when the temperature of the rubbing surfaces is high.

Oxidative wear manifests itself in machine parts that are directly affected by water, air, chemicals and directly temperature.

The wear of parts and assembly units can be judged by the nature of their work (for example, noise), surface quality, shape and size of the machined part.

To reduce the wear of the mating surfaces, liquid lubrication (including gas lubrication), rolling friction, a magnetic field and special anti-friction linings, gaskets and materials are used.

Monitoring the wear of critical interfaces of machine tools is necessary to determine the need for repairs, to assess the quality of machine operation, and to develop measures to improve the durability of the machine.

Wear can be measured during operation (especially during scheduled inspections), during periods of scheduled repairs or when testing machines.

There are various methods for measuring wear, which can be divided into the following groups:

1) integral methods, when it is possible to determine only the total wear on the friction surface, without setting the amount of wear at each point of the surface, these include weighing, the use of radioactive isotopes;

2) the method of micrometering, based on measuring the part with a micrometer, indicator or other devices before and after wear; micrometerage, especially measurement using indicator devices, is often used when machine parts are worn out in production conditions; the method does not always give an accurate idea of the shape of the worn surface;

3) the method of "artificial bases" used to assess the wear of the friction surfaces of the base parts of the machine; it consists in the fact that holes of a certain shape are pre-applied on the wear surfaces, which have practically no effect on the change in the friction mode, since their dimensions are small; according to the first method (imprint method), holes 2 are applied to the friction surface either by indentation of a diamond pyramid 1 (Fig. 8.4, A), or a rotating carbide roller 3 (Fig. 8.4, b). The second method, which is called the "wipe" method, more precisely because of the absence of expanded metal.

Rice. 8.4. Forms of prints

4) the surface activation method, like the “artificial bases” method, is used in automatic lines due to the large number of controlled equipment and limited access to rubbing surfaces; the essence of the method is that the working sections of guides, spindle assemblies, gear and worm gears, screw gears and other critical mechanisms are subjected to surface activation in cyclotrons by a beam of accelerated charged particles (protons, deuterons, alpha particles); the depth of the activated layer must correspond to the expected value of the linear wear of the part; for large parts, pre-activated special inserts are used. The amount of wear of the activated surfaces is judged by periodically measuring the radiation energy.

The choice of method depends on the purpose of the test and the required measurement accuracy. The allowable wear of the guide beds of screw-cutting and console-milling machines is normalized depending on the required machining accuracy and the dimensions of the part. If the wear of the guides exceeds 0.2 mm, the vibration resistance of the machine is significantly reduced, and although, according to the conditions for ensuring the specified accuracy of parts, it is permissible to continue the operation of the machine, it is necessary to stop it for major repairs due to deterioration in the quality of the machined surface (traces of vibration) or loss of productivity.

Permissible wear of the guides of longitudinal planers and longitudinal milling machines is determined by the formula

U max \u003d d (L o / L 1) 2,

where d is the machining error on the machine (tolerance for the part); L o and L 1 - the length of the guides of the bed and the workpiece, respectively.

For flat guides, wear is equal to the distance from some conditional straight line passing through points at the non-worn ends of the guides to the worn surface.

For machines with V-guides or triangular guideways with base angle α permissible wear

U max \u003d dcos α (L o / L 1) 2.

The wear of the frame guides, depending on the mode of operation of the machine and proper operation, is 0.04 ... 0.10 mm or more per year.

The wear of the bed guides of lathes and turrets operating in the conditions of individual and small-scale production is on average about 30% of the wear of the guides of machine tools employed in the conditions of large-scale and mass production.

The main consequence of the wear of the guides of heavy machine tools, such as, for example, longitudinal planing, longitudinal milling, boring, carousel, etc., as well as medium-sized machines with high speeds of movement along the guides, is contact seizing - jamming. Accompanying it in this category of machine tools is abrasive wear.

To check the guides, universal bridges are used. They are installed on machine guides of various shapes and sizes. With the help of two levels, the straightness and twisting (i.e., deviation from parallelism in the horizontal plane) of the guides are simultaneously checked, and the parallelism of the surfaces is determined with indicators.

The bridge is located approximately in the middle part (along the length) of the frame so that four supports are located on the prismatic part of the guides. Then, levels are fixed on the upper platform with a division value of 0.02 mm per 1000 mm of length and the position of the levels is adjusted using screws so that the bubbles of the main and auxiliary level ampoules are located in the middle between the scales. Next, the device is shifted along the guides and returned to its original place. In this case, the bubbles of the main ampoules should return to their original position. If this does not happen, it is necessary to check the fastening of the columns and thrust bearings.

The guides are checked when the bridge is stopped sequentially through sections equal in length to the distance between the bridge supports. According to the level set along the guides, the non-straightness is determined. The twisting of the surfaces is determined by the level located perpendicular to the guides.

The level readings in micrometers, measured in individual sections, are recorded in the protocol and then a graph of the shape of the guides is built.

On fig. 8.5, A an example of checking the guides of a triangular profile (often found on the beds of turret lathes) is given. Indicator 4 determines the parallelism of the left guide base plane; according to level 2, located across the guides, their twisting is established. The second side of the right guide can be checked by level by installing support 3 on this side, or, without moving the support, by the indicator (in the figure this is shown by a dashed line).

Rice. 8.5. Guide test patterns

On fig. 8.5, b shows the installation of a fixture on the bed of a lathe to check with indicator 4 the parallelism of the middle guides of the base surface, i.e. from the plane under the rack and check the helical twisting level 2.

To check the beds of grinding and some other machines with a similar combination of guides (Fig. 8.5, V) for straightness and twisting, four supports 1 are placed between the generatrices of the V-shaped profile guide, and one support 3 is placed on the opposite flat guide. Testing is carried out at level 2.

When the dimensions of the guides do not allow placing devices between them that form all the supports (Fig. 8.5, G), then only two supports 1 are installed.

On fig. 8.5, d supports 1 are moved apart in accordance with the size of the prismatic guide bed.

When checking the flat guides of the bed (Fig. 8.5, e) two of the supports 1 abut against the side surface, the other two and support 3 are placed on horizontal planes. This ensures stable level 2 readings.

Using a universal bridge, using various holders for mounting the indicator, you can control the parallelism of the axis of the lead screw and the guides of the lathe bed. The scheme for checking the parallelism of the axis of the screw of the jig boring machine with the guides of the bed is shown in fig. 8.6.

Rice. 8.6. Scheme for checking the parallelism of the axis of the screw of the jig boring machine with the guides of the frame

The design of the universal bridge is simple, so setting up the device takes no more than 5 minutes. It is handled by a medium-skilled locksmith.

Corner bridge. Angular bridges are used to check guides located in different planes (for example, the guide surfaces of the traverse of a coordinate boring machine model KR-450).

On fig. 8.7 shows a diagram of such a device for measuring with an angular bridge.

The short arm 3 is located perpendicular to the elongated 5. The roller 1 is fixed, and the roller 4 can be moved and installed depending on the size of the guide. When this rollers 1 and 4 are placed in V-shaped guides or cover the surface of the prismatic guide. The support 7 is reinstalled along the groove of the shoulder 5 and adjusted in height.

An adjustable block 2 is installed on the shoulder 3 along the guides. level and check their straightness. The twisting is checked when the level is perpendicular to the guides. Using indicators 6 determine the non-parallelism of the surfaces, as well as the non-parallelism of the axis of the screw to the guides.

It is convenient to check the parallelism of the guides of the dovetail form, as well as other forms, using special and universal devices equipped with indicators.

The guide can be checked for parallelism with indicator devices only after the preparation of the base ones. Shown in Fig. 8.8 the device is used to check the parallelism of male and female guides of various shapes and sizes with contact on the upper or lower surfaces.

Rice. 8.8. Schemes for checking the dovetail guides

The device consists of a beam 3 with a hinged lever 1 and an adjustable measuring rod 8 , stands 2 with indicator and interchangeable swivel support 5 with control roller 6 . The support 5 can be installed at various angles and on any part of the bar 3 along its groove. The position of the support 5 is fixed with a bolt 4 .

When checking the guides of the dovetail form with contacts along the lower plane, a replaceable support is selected with a roller diameter that provides contact approximately in the middle of the height of the inclined plane (Fig. 8.8, A And V). Support 9 is adjusted along its groove and also fixed with a bolt (not shown in the figure). On the cylindrical surface of the measuring rod there is a scale, which determines the value of the division of the indicator, depending on the difference in distances A And b(Fig. 8.8, A). In this case, the value of one division of the indicator scale is 0.005 ... 0.015 mm , which must be taken into account when measuring.

Various methods are used to restore parts (Table 8.1). When choosing a restoration method, it is necessary to assign a repair, repair free or repair regulated size.

Table 8.1

Detail recovery methods

Name recovery method	Characteristics
Treatment cutting	The repair dimensions method is used to restore the accuracy of machine tool guides, worn holes or necks of various parts, lead screw threads, etc. Of the two conjugated parts, a more expensive, labor-intensive and metal-intensive part is restored and repaired, and a cheaper one is replaced. Worn parts are transferred after appropriate processing to the next repair size. When restoring the joints of the guides, compensators are used
hardfacing	Welding fix parts with kinks, cracks, chips. Surfacing is a type of welding and consists in the fact that a filler material is deposited on the worn area that is more wear-resistant than the main material of the part. After surfacing, the service life of the part, which can be reused many times, is significantly increased, however, warping of the parts is possible in this process. For the repair of steel parts, arc welding with metal electrodes is more often used, using certain methods, depending on the chemical composition of the steel. Gas welding is used to restore cast iron and steel parts with a thickness of less than 3 mm. Welding of gray cast iron can be hot, semi-hot and cold
Welding - soldering	Cast iron recovery. Used brass wire and rods made of copper-zinc tin alloys
	Ductile iron is repaired using brass electrodes or monel electrodes (an alloy of nickel with copper, iron and manganese)
Metallization	Metallization consists in melting the metal and spraying it with a jet of compressed air into small particles that penetrate into the surface irregularities, adhering to them. Metallization is applied to parts made of various materials operating under a calm load. Gas or arc metallizers are used. The surface must be free of grease and rough
Chrome plating	Chrome plating is the process of restoring a worn surface by electroplating chromium. Chrome-plated surfaces have increased hardness and wear resistance, but do not tolerate dynamic loads. Chrome plating is less versatile compared to plating due to the small thickness, the difficulty of coating parts of complex configuration. It has undeniable advantages over other restoration methods: a partially worn chromium layer can be easily removed by galvanization (dechrome plating), parts can be repeatedly restored without changing sizes

Repair is the size to which the worn surface is processed when restoring the part. Free repair size - a size whose value is not set in advance, but is obtained directly during processing, when wear marks are removed and the shape of the part is restored. The corresponding size of the mating part is adjusted to the resulting size by the individual fit method. At the same time, it is impossible to pre-manufacture spare parts in a finished form. Regulated repair size - a predetermined size to which the worn surface is processed. At the same time, spare parts can be manufactured in advance, the repair is accelerated.

Methods for restoring parts during repairs are discussed in detail in the technical literature, some of them are shown in the diagrams in Fig. 8.9. The use of one or another repair method is dictated by the technical requirements for the part and is due to economic feasibility, depends on the specific conditions in the production, on the availability of the necessary equipment and the timing of the repair.

Methods using polymeric materials have become widespread for the restoration of parts. This requires injection molding equipment that is simple and materials such as polyamides that have sufficient adhesiveness to metal and good mechanical properties.

In a bored bushing (Fig. 8.9, A) make radial holes, then the sleeve is heated, placed on the press table, pressed against the nozzle (Fig. 8.9, b) and pressed. The restored sleeve is shown in fig. 8.9, V.

To restore a worn shaft journal (Fig. 8.9, G) it is pre-cut (Fig. 8.9, d), and then the process is repeated, as in the previous case (Fig. 8.9, e).

Rice. 8.9. Schemes for the restoration of machine parts

Restoration will be of high quality only if the casting conditions and process technology are observed.

Sliding screw drives can be restored using self-hardening acrylic plastics (styracryl, butacryl, ethacryl, etc.), consisting of two components - a powder and a monomer liquid. After mixing the powder with the liquid, the mixture hardens in 15–30 minutes.

Broken shaft (Fig. 8.9, and) can be restored by pressing in a new part 1 (Fig. 8.9, h) or welding method (Fig. 8.9, m) with subsequent turning of the weld.

Worn thread in the body part (Fig. 8.9, To) are reamed and reamed, a sleeve is pressed into the resulting hole, which, if necessary, is fixed with a locking screw 2 (Fig. 8.9, l). In a similar way, they proceed when repairing smooth holes.

An exact fit on the sides of a worn splined shaft can be restored if, after annealing the shaft, the splines are expanded with core blows, followed by hardening and grinding of the sides (Fig. 8.9, m).

The inner diameter of the bronze bushing can be reduced from d 1 to d 2 by upsetting, i.e. reduce its height with a constant outer diameter. The draft is carried out under pressure (Fig. 8.9, n).

The technology for restoring sliding screw gears may be as follows. Restore the constancy of the pitch of the sliding lead screw of the slotted thread. The thread in the lead nut is cut off and bored to a diameter of 2 ... 3 mm larger than the outer diameter of the lead screw. The surface to be bored is, if possible, ribbed. The repaired lead screw is heated to 90°C and immersed in molten paraffin. After cooling, a thin paraffin film remains on the surface of the screw. The paraffin-coated screw is mounted with a bored nut, simulating the working condition of the transmission. The ends of the nut are sealed with plasticine. Then, the newly prepared mixture is poured into the side, specially drilled hole of the nut with a syringe. After a few minutes the mixture hardens and the screw can be removed from the nut.

Ball screws are repaired if the wear of the screw thread is more than 0.04 mm. Recovery technology is as follows. Fix the center holes of the screw by grinding or lapping. If there are nicks and dents in the center holes, then they bore and install plugs with center holes on the glue. After restoring the centers, if necessary, the screw is straightened according to the indicator in the centers. Then the accuracy of the thread pitch is restored by machining. During processing, the thread groove is expanded along the entire length of the screw to the width at the most worn area. The outer and inner thread diameters remain unchanged. The axial clearance is chosen by adjusting the nuts. Nuts are most often not repaired, but if necessary, they are interchanged.

Correction of worn guide beds is carried out in the following ways: 1) manually; 2) on machines; 3) with the help of devices.

Correction manually by sawing and scraping is used for guides with a small surface area with a small amount of wear. Scraping guide beds can be done in two ways: 1) using a control tool; 2) according to a pre-scraped or ground mating part.

When the wear of the guide beds exceeds 0.5 mm, they are repaired by machining. For this, special grinding, longitudinal planing and longitudinal milling machines are used.

When the guide beds are worn 0.3 ... 0.5 mm at some plants, they are processed by the finishing planing method. The accuracy of processing by this method allows you to almost completely abandon scraping and limit yourself to only decorative scraping.

By grinding, the bed guides are repaired on special grinding machines or longitudinal planing or longitudinal milling machines with special stationary devices.

Large beds that cannot be machined must be machined with fixtures. Devices, when used correctly, provide a sufficiently high quality of the treated surfaces. Processing is carried out without dismantling the frame, which reduces the repair time and reduces its cost. Portable devices move, as a rule, along the frame that they process. A specially prepared plate or sometimes a part of a repaired machine is used as a base for a fixture (carriage).

The most widespread are planing and grinding devices.

Processing with fixtures does not require special equipment. The disadvantage of the method is lower productivity compared to processing on machines and the need for manual work on preparing the bases. The advantage of machining with fixtures is that it saves time for dismantling, transporting and re-assembling the bed, which is inevitable when machining on machines.

Of great importance for the restoration of guides is the selection of technological bases. According to the nature of the bases, the beds can be divided into four main groups.

1) Beds in which spindles are mounted (horizontal milling machines, vertical milling machines with an integral head, some types of gear shaping, etc.). When repairing the beds of this group, alignment is carried out from mandrels installed in the machine spindle, materializing the axis of rotation.

2) Beds with non-working surfaces, machined along with the workers (longitudinally milling, longitudinal planing, circular and intra-shaft grinding).

3) Beds with partially worn guides. As a base, working surfaces are taken that wear out little and not all the way during operation. In such frames, first, little-worn surfaces are restored, then, based on them, the rest of the worn-out working surfaces are restored. Typical for this group are the beds of lathes, turret machines with a detachable headstock, etc.

4) Beds with separate unworn sections of the guides. This group includes beds that do not have other processed surfaces, except for wear workers (gear and thread milling machines). Unworn or slightly worn sections of working surfaces to be corrected are taken as a base.

To restore the required properties of the guide beds, they are subjected to heat treatment. From the variety of methods, here are some of the most common.

Surface hardening with induction heating by high frequency currents ( HDTV ) . The quality of the cast iron layer hardened by HDTV depends on the frequency of the current, power density, heating time, design of the inductor, the gap between the inductor and the hardened surface, as well as on the cooling conditions. The initial state of the cast iron (its chemical composition and microstructure) also affects the final results of hardening.

When gray cast iron is heated for the purpose of subsequent hardening, part of the carbon dissolves in austenite, while the rest remains in a free state in the form of graphite inclusions. As a rule, cast iron must have a pearlitic structure before hardening. If the initial structure of cast iron is unsatisfactory for surface hardening, then the concentration of bound carbon should be increased (increase the content of pearlite in the structure) by preliminary heat treatment - normalization.

The maximum achievable hardness of cast iron, obtained after hardening of high-frequency current at a temperature of 830 ... 950 ° C (depending on the composition of cast iron), is HRC 48-53. A further increase in the hardening temperature leads to a decrease in hardness.

The cooling rate during quenching has little effect on hardness. When quenched in oil, the hardness of cast iron decreases only by 2-3 units. HRC compared to water hardening.

Surface hardening with heating of high-frequency modified cast iron makes it possible to obtain greater hardness and layer depth compared to hardening of conventional pearlitic cast iron. In terms of microstructure, hardened modified cast iron practically does not differ from pearlitic cast iron.

Before hardening lathe beds, the following must be done:

1) install the frame on the table of the planer and align it for parallelism with the base surfaces with an accuracy of 0.05 mm and then bend it by 0.3 ... 0.4 mm (value of deformation during hardening);

2) plan all guide beds until they are parallel to the table travel. After detaching the frame (from the table), due to elastic deformation, a bulge is formed corresponding to the deflection value;

3) install the frame (without alignment) on the hardening platform, edged with a cement shoulder to collect the used hardening water;

4) install a portable machine on the guides of the bed, fix two brackets on both sides of it; connect the roller chain with the machine drive sprocket;

5) adjust the gap between the inductor and the hardened bed using the vertical and horizontal support of the machine. Then supply water to the inductor;

6) turn on the current and harden. Since the surface of the frame to be hardened is located in a horizontal plane, cooling water floods the flat, not yet fully heated area, and thereby makes hardening difficult. As a rule, the depth of the hardened layer at the top of the prism is greater than in the flat area (3...4 mm at the prism, 1.5...2.5 mm in the flat area).

Example. The mode of hardening the guides of the bed of a screw-cutting lathe mod. 1K62.

Generator voltage, V ……….………………………………. 600-750

Current strength, A………………………..…………………………………. 95-120

Capacitor battery capacity, uF ….…………………….. 300-375

Used power, W ………………………………………. 55-70

Gap between the inductor and the hardened bed, mm ………..2.5-3.5

Inductor movement speed during heating, m/min….. 0-24

Bed surface heating temperature, °С …………………850-900

Hardening depth, mm …………………………………………………..3-4

HRC ……………………………………………………….…………. 45-53

Bed hardening time, min………………………………….……. 60-70

The leash of the bed after hardening (toward the concavity), mm ... 0.30-0.50

During hardening, the guide beds sag, while compensating for the bulge obtained during planing. Thus, a small metal removal is ensured during the subsequent grinding of the guides.

Flame case hardening

For surface hardening of guide beds by flame hardening, stationary and mobile installations are used in repair practice. The former are usually installed in special areas of mechanical repair shops. In this case, the beds must be delivered there for heat treatment and subsequent recovery. For frames that cannot be removed from the foundation for production reasons (lack of lifting equipment and transport, the need to preserve the foundation, etc.), mobile units are used.

Flame surface hardening of guide beds can be carried out with an oxy-acetylene or kerosene-oxygen flame. Heating with an acetylene-oxygen flame is more intense than with kerosene-oxygen, since with the help of the first one it is possible to heat up to 3150 ° C, and with the help of the second - only up to 2400 ° C. As a combustible mixture, propane-butane and oxygen or natural gas mixed with oxygen are also used.

The quenching medium is water. The flame hardening plant is simple in design and reliable in operation, it is operated by one worker.

Tempering with a snake . In some factories, instead of continuous hardening of the guide beds of lathes, so-called serpentine hardening is practiced, in which, by heating with a gas burner, crossed zigzag hardened strips are formed on the surface of the guides.

During the hardening process, a crossed zigzag line 6 ... 12 mm wide is applied to the guide surfaces of the frame With step 40 ... 100 mm (Fig. 8.10).

Rice. 8.10. Hardening snake pattern

The hardening pattern is done by hand and usually has an irregular shape. The distance from the edge of the bed to the hardening line must be at least 6 mm . Torch travel speed along guide rails approx. 0.5 m/min , which provides heating up to 750…800 °C.

The hardening pattern is recommended to be applied like this. First, you should apply a zigzag line in one pass on the first guide, and then move on to the second guide. During the application of a zigzag line on the second guide, the first one cools down to 50 ... 60 ° C, and a crossed hardening line is applied to it.

Therefore, it is necessary to carefully monitor the heating process and timely adjust the speed of the burner relative to the hardened surface of the guide frames, preventing metal melting.

Wear of cutters.

Due to sliding friction and the action of high temperature at the points of contact of the cutting wedge with chips and the cutting surface, wear occurs by removing microparticles from the working surfaces of the cutter.

The wear of the cutting tool occurs at constantly renewing rubbing surfaces, high pressures and temperatures. In this regard, there are three types of wear: abrasive, molecular and diffusion.

Abrasive wear occurs as a result of scratching - cutting off the smallest particles of the tool by solid inclusions of the material being processed. Such wear is mainly observed when cutting cast iron, high-carbon and alloyed tool steels, which have very hard carbide grains in the structure, as well as when processing castings with a hard and contaminated crust.

Molecular wear is accompanied by pulling out the smallest particles from the tool surfaces by chips and the cutting surface of the workpiece due to the action between them of significant forces of molecular adhesion (adhesion, welding) and relative slip. This type of wear mainly occurs during the processing of ductile metals, especially hard-to-cut steels (heat-resistant, stainless, etc.).

At high temperatures, diffusion occurs in the cutting zone - mutual dissolution of rubbing bodies - as a result of which the chemical composition and mechanical properties of the surface layers of the tool change, which accelerates its wear.

sewn on the front and back surfaces. On the front surface, the chip chooses a hole, and on the back surface, a platform ground to the cutting surface without a back angle is formed. In the initial period of the formation of the hole, the cutting process is facilitated due to the increase in the rake angle in this place. However, as the distance f decreases from the edge of the hole to the cutting edge, the latter is weakened and destroyed. From the very beginning of its appearance, the wear area along the rear surface of the short-circuit increases friction and the heating temperature of the cutting edge, and worsens the finish of processing.

Tool wear can be slowed down by reducing the work expended on the deformation of the cut layer and external friction, which is achieved by the correct choice of cutting mode, cutter geometry, its finishing and the use of lubricating and cooling fluids.

The nature of wear depends on the cutting conditions. When machining steels in the zone of medium speeds, wear mainly occurs along the front surface, at very low and high speeds - along the back. When cutting brittle metals (cast iron, hard bronze), it is mainly the rear surfaces of the tool that wear out.

The increase in wear over time can be divided into three periods. During the first period (segment OA), the friction surfaces are run-in when the roughness remaining after tool sharpening is smoothed out. The duration of this period can be shortened by fine-tuning the cutter. The second period (segment AB) is characterized by a normal (slow) wear rate. This period is the longest and accounts for about 90-95% of the cutter's operating time. The third period is a period of increased wear, upon reaching which the tool must be removed from the machine for regrinding. Otherwise, to restore it by sharpening, you will need to cut off a significant layer of metal, which will greatly reduce the total duration of the tool.

Signs of maximum allowable wear (blunting criteria), indicating the need for regrinding, depend on the nature of the work performed.

In roughing, when accuracy and cleanliness are not the ultimate goal, the allowable wear is practically determined by the following external signs: the appearance of a shiny strip on the cutting surface when machining steel or dark spots when machining cast iron; a sharp deterioration in the purity of the treated surface; changing the shape and color of the chips.

When finishing, tool wear is determined by the deterioration of the cleanliness and accuracy of processing below the allowable.

The regrinding time can also be set according to the allowable width of the platform L8 along the rear surface, the value of which is given in reference books. For example, for carbide cutters when roughing steel, Le = 1 -1.4 mm, when finishing - L3 = 0.4 - 0.6 mm,

In mass production, permissible wear is limited by forced regrinding of tools at certain intervals corresponding to their durability.

Review questions

MAIN FAULTS OF THE ELECTRICAL EQUIPMENT OF THE LATHE

The electrical equipment of the lathe is designed to be connected to a network with a voltage of 220 to 380 V and consists of:

asynchronous electric motor;

· magnetic starter;

a transformer.

This article is about rules andlathe control technique . Your safety depends on compliance with the rules for working on a lathe. confidentlathe control technology affects the quality of the product and the productivity of controlled work. If your goal is to learn more about turning business , follow the guide.

Step 1. Checking the lathe before starting

Before start lathe , tolerance control must be carried out, namely:

During shift work in production, the shifter who hands over the lathe to you is obliged to report the problems noticed in it (orally, in writing, by phone). The absence of comments implies that the lathe is in good condition.

In production by eliminating lathe malfunctions is in charge of the repair service. The machine operator should only inform them about the occurrence of a malfunction.

Before turning on the lathe make sure the power supply:

That there is no warning on the machine, such as ( do not include lathe in repair ) ;
Covers, doors, hatches that cover the main parts, and lathe mechanisms must be closed.
The control knobs for the spindle, feeds, uterine nut must be in the neutral position.
The cooling supply is off, the liquid supply nozzles are directed downwards.
RPMs and feed steps are set to what you want them to be when the spindle starts up.
The part you installed to be processed must be securely fastened.
The floor near the lathe should be clean, and there should be no unnecessary objects under your feet.
Turner's clothing should be neat (no hanging flaps).
Do not forget the key in the chuck (always take care to remove the key from the chuck).

Having completed the access control: turn on the main switch of the lathe, additional switches, if there are any. Next is carried out lathe lubrication .

Step 2. Spindle control.

Before starting the spindle or the main engine, be sure to make sure that the rotating elements on it, in particular the chuck, will not be obstructed by rotation from the stationary parts of the machine. Special danger when starting the spindle at high speeds are thin bar blanks protruding beyond its limits.

This also applies to parts of large diameters with a significant overhang from the cartridge and the center of the tailstock not pressed from the other end.

As already stated in the first lesson "The device of the lathe", spindle speed settings produced by installing switches and levers on its nodes in a certain position according to the table located on the machine.

Switching rules can be summarized as follows - “You cannot switch or bring to the end of the switch if they cause a characteristic sound of gear teeth not engaging. In this case, the necessary switching should be done at a complete stop.

On all lathes direct turns are included by feeding the power handle towards yourself, and reverse from yourself. At the handle with a vertical stroke (pull it up), and at the handle with horizontal movement (pull it to the right, respectively).

Forward revolutions on all lathes correspond to clockwise spindle rotation as viewed from the back of the spindle. Spindle braking at high speeds due to the reversal of the clutches or the reverse thrust of the main engine, this is unacceptable, as it leads to overload and overheating of the mechanism. Braking must be done by the brake. And if the effectiveness of the brake is not enough, then it should be restored by adjustment or repair.

For fastening parts in a three-jaw chuck, one “0” socket is usually used to insert a key into it, which requires that this socket be set to the upper clamping and wringing position. In machines with a mechanical clutch, this action (with some skills) can be performed with the clutch control handle.

When cutting it is impossible to stop the spindle when the feed is on and the cutter is not withdrawn from the part (this leads to breakage of the cutter).

Step 3. Lathe Feed Control

Manual feed control implies the supply of a tool for short lengths (during processing, settings, eyeliners).

Manual control filing allows you to quickly lead, interrupt and resume, as well as instantly change its speed (depending on changing conditions and processing situations). Manual feed in longitudinal direction driven by a handwheel with or without a horizontal handle. Rotating the flywheel counterclockwise moves the caliper to the left, and clockwise to the right.

Longitudinal movement of the caliper on a lathe is carried out by gear rack and pinion. Such gears have backlashes or gaps in the contacts of parts and its mechanisms.

Manual cross feed control (performed with a T-handle with a horizontal handle). Turning the handle clockwise moves the sled tool forward, that is, away from you, turning the handle counterclockwise moves the tool towards you. On our machine there is an accelerated inclusion of the movement of the sled. There are different flywheel rotation techniques one and two hands, which are applied depending on the work performed on the lathe.

On the top sled, turning the handle clockwise moves the sled forward, and turning it counterclockwise moves it backwards. Quick idle movement of such handles can be done using one of the handles. In this case, the sled must be adjusted for easy movement. We will consider in more detail about the adjustment of mechanisms, sleds, lathes in the following turning lesson.

Step 4. Managing mechanical feeds

Mechanical feeds work from the drive through the running shaft, and their control is done by the handle of the 4-position switch. The direction of movement of the switch handle corresponds to the direction of movement of the tool on the caliper.

Before turning on the mechanical feed in any direction, you need to visually make sure that there are no obstacles at all points of the caliper from other parts of the machine, especially rotating ones. A frequent oversight of beginner turners is an attempt to bring the caliper closer to the chuck with the sled shifted to the right, which leads to a collision. Therefore, you should check the free movement of the caliper in advance.

It is necessary to work out manual feed techniques so that the cutter does not stop or the stop is minimal.

Step number 5. Rapid feed lathe

On machines with rapid feed such requirements must be met.:

To prevent accidental pressing of the rapid feed button, the feed selector lever must be operated by applying a hand from the side, but not from above.
Before starting rapid feed, you need to make sure that there are no obstacles to advance at any points on the support, including the tool, in the direction where you want to feed.
It is forbidden apply rapid feed for short movements, especially when approaching rotating elements.
Heavy calipers of medium machines have inertia, which is enhanced by the accelerated feed of its drive mechanism.

There are combined feeds of lathes (by type of drive, by directions). Such lathes are used for processing irresponsible cones (irrelevant chamfers) and shaped surfaces.

Threaded feeds

For threading caliper feed is carried out by closing the uterine nut with the lead screw. Turning the mother nut on and off is done with a separate lever. Spindle and lead screw rotate synchronously regardless of the set thread pitch. Changing the direction of rotation of the spindle leads to a change in the direction of movement of the caliper. Also, changing the spindle speed leads to a change in the speed of movement of the caliper. Getting into a previously cut groove is ensured by the synchronization of the rotation of the spindle and the lead screw and, accordingly, the stroke of the caliper.

It is possible to cut both right and left threads using a switch on the headstock, which changes the direction of movement of the screw relative to the spindle. When cutting threads, it is not recommended to get carried away with high spindle speeds, since its rotation is directly related to the movement of the caliper.

Locking the tailstock of a lathe is carried out by a lever, as the working stroke of which increases the clamping force. When machining with heavy loads, requiring a better fixation of the tailstock, the impact on the lever should be vigorous. It is important not to confuse the resistance of the lever when clamping with its hard stop at the end of the stroke. When the tailstock is used with minimal loads, its maximum fixation with the bed is not needed. The tailstock clamp is rationally commensurate with the upcoming load.

Tailstock quill driven by manual feed by rotating the handwheel. Fixing the tool and fixtures in the quill cone is carried out in the following order:

Checking the cones of the quill and tool for contamination;
Inserting the outer cone into the cone of the quill and finding the position of the match of the lock connector in the quill with the foot on the tool cone (not required for tools that do not have a foot).

Toolholderis a fairly accurate mechanism that ensures the rigidity of the cutter in the specified positions. Correct holder handle position when clamped, it should correspond to the position of the hour hand at 3-4 hours. This position is ensured by the position of the spacer washer under the tool holder handle nut. The lever is clamped with an average elbow force. And you can’t press the handle with the pressure of your weight in order to avoid weight loss. The wringing of the handle is done by one or more short pushes with the base of the palm in a counterclockwise direction. Before turning the tool post, make sure that there are no obstacles for itself and the tool fixed in it. Obstacles from the rotating elements of the machine are a great danger.

In the process of work, any turner will sooner or later have to face unforeseen situations when working on a lathe.

Possible situations when working on a lathe :

Spontaneous stop of the lathe during operation, during a power outage or mechanical failure;
Collisions between rotating elements and caliper elements;
Turning a part in a chuck;
Pulling a part out of a lathe fixture;

Lathe malfunctions can be expressed in extraneous noise, the smell of burning electrical wiring, etc.

Leaving the lathe is prohibited (do not leave the lathe unattended).

For an emergency stop of processing the part, quickly move the cutter away from the part, turn off the feed, stop the spindle and turn off the main engine. When stopping the spindle, the main thing is not to turn on the reverse speed, but to turn on exactly the neutral position. Malfunctions of the lathe should be reported to management immediately.

		Remedy
Ovality of the part. Hole inner diameter not maintained	The runout of the workpiece in the chuck. Offset of the tailstock in the transverse direction (when drilling). Weak fastening of the tailstock. Incorrect drill sharpening	Adjust the workpiece in the chuck for runout. Expand fists. Adjust the tailstock to the spindle axis. Attach the tailstock. Sharpen the drill
Part hole axis offset	Insufficient centering depth. The axis of the tailstock quill does not match the axis of the spindle. Incorrect drill sharpening	Center the workpiece, observing all the rules. Adjust the tailstock to the spindle axis. Sharpen the drill
Taper of the machined part	Offset spindle and tailstock centers	Adjust the alignment of the centers of the headstock and tailstock
The presence of spiral (screw) risks on the part during the reverse stroke of the cutter. Unclean end of the part from the side of the cut	Incorrect cutter setting. Incorrect sharpening of the cutting edges of the cutting tool (the right auxiliary surface has a small auxiliary angle in the lead and a small clearance angle)	Set the cutter slightly above the center. Sharpen the cutter
When cutting the end, the size along the length of the part is not maintained	Weakly fixed workpiece. Incorrectly defined place for processing the end face of the workpiece	Firmly clamp the workpiece in the chuck. Comply with all trimming rules
Crushed surface treated	Gaps in caliper guides. Weak fastening of incisors. Weak fixing of the workpiece in the chuck (centers). The workpiece vibrates during processing. Great cutting edge. Blade not centered	Tighten caliper bars and wedges. Firmly fasten the cutter. Reduce cutter overhang. Set the cutter exactly along the axis of the centers. Hold the workpiece firmly

		Remedy
Broken thread on a shank or hole	Very soft and viscous workpiece material. The workpiece diameter does not meet the requirements. High cutting speed	Reduce the diameter of the threaded rod or increase the diameter of the threaded hole. If possible, replace the workpiece. Decrease spindle speed

Close the spindle with a casing and lower the protective screen;

Turn on the main drive motor by pressing the start button.

During the execution of work it is necessary:

Switch on the drive of the main movement with the control handle of the friction clutch of the main drive;

Bring the cutter to the workpiece until it touches, using manual longitudinal and transverse feeds, using the appropriate handwheels;

Set the cross and longitudinal feed dials to zero. Before setting the dials to zero, be sure to select the backlashes (air gaps) in the feed mechanisms;

Remove the cutter from the processing zone;

Set the required cutting depth on the dials;

Turn on the automatic longitudinal or transverse feed mechanism;

To process the workpiece at a given length (along the end). At the end of turning, turn off the automatic feed;

Take the cutter away from the workpiece and return it to the starting point at rapid feed or manually;

Continue processing in the same sequence until the workpiece has the appropriate dimensions;

When cutting, monitor the flow of chips and, if necessary, remove them, while the cutting process must be interrupted. Chips should be removed only with a special hook.

Upon completion of the work it is necessary:

Switch off the electric motor with the stop button;

Turn off the input circuit breaker;

Remove the machined part, processing tool and additional equipment from the machine;

Set the tailstock to the extreme right position;

Move the machine support to the right to the tailstock;

Clean the machine from chips and dirt using a hook, scraper, brush and rags;

Lubricate the machine at lubrication points and rubbing surfaces.

Broaching defects and ways to prevent them

Most lathes have a similar structure: the headstock, spindles and bed. On lathes, the processing of parts is carried out mainly in a horizontal plane. Screw-cutting machines differ from standard lathes in the presence of a front and rear working headstock, an elongated bed and caliper, as well as a feed box.

A feature of the repair of screw-cutting machines is the work with cutters, drills and other equipment for internal processing of parts.

The main causes of breakdowns of screw-cutting lathes

As practice shows, often the spindle speed control assembly fails first of all. It is the tapered roller bearings used in such machines that are subject to the highest degree of wear. Depending on the type of machine and the type of circular lubrication system, periodic adjustment or replacement of the bearings is required.
In addition, malfunctions often occur in the fixing holder of the caliper. As a result, the workpiece is unevenly moved in the longitudinal and transverse directions.

Various models of screw-cutting machines differ in the types of workpieces, dimensions and device. For example, there are automatic and semi-automatic machines. They differ in the presence of special sensors for the supply of working heads. In practice, it is automatic systems that work longer, since during manual feeding, the size of the part and the degree of its processing may be incorrectly calculated.

Repair of screw-cutting lathes

A machine of this type is classified exclusively as complex equipment. Especially if the equipment is additionally equipped with heads for milling or grinding. Therefore, it is recommended that the repair of machines be carried out by specialized companies.

Of course, its own staff of engineers can carry out current repairs, consisting of lubrication of working parts and a control inspection of the condition of working elements. But a complete overhaul is best done by a professional.

Specialists in the repair of screw-cutting lathes carry out their work in accordance with uniform norms and standards for work, taking into account the features of your specific model. In addition, such repairs will always be carried out with a guarantee for parts and the work itself.

Therefore, in order to save money and reduce downtime, which means additional losses, you should not resort to repairing screw-cutting lathes with your own hands. After all, imaginary savings can lead to unthinkable waste.