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 a v When turning, the tool is made of
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. Wear area by rear surface kz from the very beginning of its appearance increases friction and heating temperature of the cutting edge, worsens the cleanliness 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 the right choice cutting conditions, cutter geometry, its finishing and the use of lubricating and cooling liquids.
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.
When roughing, when accuracy and cleanliness are not ultimate goal, allowable wear is practically determined by the following outward signs: the appearance of a shiny strip on the cutting surface when machining steel or dark spots when processing 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
electrical equipment lathe designed for connection to a network with a voltage of 220 to 380 V and consists of:
asynchronous electric motor;
· magnetic starter;
a transformer.
High demands on the accuracy of the dimensions of the part, on deviations from the geometric shape and on the roughness of the surface to be machined are feasible only if the finishing machines maintain their original accuracy. The errors of individual mechanisms, the errors of their mutual movements are regulated by the relevant standards. Knowledge of the relationship between malfunctions of finishing machines and machining errors allows you to quickly determine the cause of deviations in the process and restore the necessary machining accuracy.
Malfunctions of grinding machines. An analysis of the schemes of finishing (precision) external and internal grinding allows us to conclude that the surface being machined can be strictly cylindrical both in longitudinal and in cross sections only when certain conditions: a) the part and the grinding wheel must have a constant axis of rotation; b) the axes of rotation of the part and the circle must be parallel in the horizontal and vertical planes; c) the axes of the part and the circle during the cutting process must remain parallel to the direction of the longitudinal feed.
The accuracy standards for grinding machines for precision external and internal grinding are very high and allow for a long time to obtain parts with the maximum deviations that are indicated in the machine's passport. In this regard, the appearance of a processing error should be considered as a violation technological process in any of his constituent parts The determining role in matters of processing accuracy, of course, belongs to the state of the machine.
When the axis of the tailstock quill is displaced in the horizontal plane, the deviation from cylindricity arises from a change in the location of the rear center due to fluctuations in the lengths of the parts.
For internal grinding, the machining error can be calculated using similar formulas, depending on what kind of machine, tooling or grinding wheel malfunctions occur during hole machining. If, during internal grinding, the axis of rotation of the part in height does not coincide with the axis of rotation of the grinding wheel, then the deviation from cylindricity can be calculated by the formula.
Achieving high precision when grinding holes is the most difficult task of all finishing operations. Considering the scheme of the technological process of internal finishing grinding, it is easy to notice additional technical difficulties that adversely affect the accuracy of processing.
These features are determined by the fact that the grinding wheel must be smaller than the diameter of the hole being machined. If the hole has a significant length (two or three diameters), the tool is mounted on a mandrel of a relatively small diameter with a considerable length. Even slight cutting forces cause elastic compression of the mandrel with the abrasive wheel, and the axis of rotation of the wheel deviates from the direction of the longitudinal movement of the grinding spindle. In this regard, the increase in the rigidity of the grinding spindles (including the mandrel) is of exceptional importance. The rigidity of any mechanism or machine should be understood as the ability to resist the movement of a part that is under the action of a force. The rigidity of the grinding spindle of cylindrical grinding machines is 20-30 kN / mm, the mandrel of the grinding spindle of internal grinding machines has a rigidity 100-200 times less.
When grinding holes of small diameters and large lengths, no technical methods can significantly increase the rigidity of the mandrel. In such cases, to improve the accuracy of processing (to restore the parallelism of the working surface of the circle to its longitudinal movement), they resort to turning the grinding spindle in a horizontal plane at an angle equal to the angle of the mandrel during cutting.
Second major technical complexity Achieving high precision internal grinding is the low cutting speed due to the small diameters of the abrasive wheels. To achieve a cutting speed of 40–50 m/s, and in some cases even 30 m/s, a wheel speed of 100–200 thousand rpm is required. This is achieved by using electrospindles.
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Turning Failures and How to Eliminate Them
Accuracy with finishing types of turning can reach 7...8th grade, and the roughness of the machined surface - 1.6...3.2 microns. In table. 10.1 shows the main causes of failures in turning cylindrical surfaces, ends and threading with a cutter.
Consider the methods of performing some turning operations that are not reflected in Table. 10.1.
Table 10.1 Failures of lathes and ways to eliminate them
| Lathe failures and their causes | Solutions |
Turning cylindrical surfaces |
|
| Taper of machined surface: misalignment of the tailstock quill with the spindle axis large overhang of the workpiece from the jaws of the chuck, in connection with which it is squeezed out the cutter in the tool holder increased wear of the cutter | Align the axis of the tailstock quill and the spindle Fasten the cutter securely Replace the cutter, reduce the cutting data |
| Ovality of the machined surface: uneven wear of the spindle front bearing | Repair machine |
| Barrel shape of the machined surface: deflection of a non-rigid shaft | Reduce depth of cut and feed; apply a through-thrust cutter, an additional support-lunette; set the cutter above the center line |
| Saddle shape of the machined surface: back center push | Reduce the overhang of the quill and secure it firmly |
| increased wear of the guides at the tailstock, the rear center is located below the front | Repair machine |
| Part surface misalignment: radial runout of the working surfaces of the jaws of the chuck or front center | Boring chuck jaws; apply a split sleeve bored to the diameter of the fixed surface of the workpiece; replace the front center or grind its working cone in place |
| Blackness on the treated surface: small allowance | Check blanks and, if necessary, change their size |
| increased curvature of the workpiece | Discard bad parts |
| displacement of the workpiece in the chuck | Reinstall workpiece |
| displacement of center holes relative to the axis of the workpiece | Eliminate misalignment of center holes |
| Unsatisfactory roughness of the machined surface: big feed; low cutting speed | Set the correct cutting data |
| small radius of curvature of the cutter | Install a cutter with a large cutter tip radius |
| worn cutter | Sharpen the cutter |
| non-rigid fastening of the cutter from the workpiece | Reduce the tool overhang from the tool holder; apply a more rigid clamping of the workpiece on the machine |
| increased gaps in the caliper guides | Adjust clearances in caliper guides |
End processing |
|
| End flatness deviation: large depth of cut and feed | Finish the face with a shallow depth of cut and feed. With positive results, it is necessary to change the modes |
| non-rigid cutter attachment | Reduce tool overhang from tool holder |
| side shift of the caliper during cutting | Fix the carriage on the bed |
| The end face is not perpendicular to the axis of the part: skew workpiece in the chuck | Reinstall the workpiece in the chuck |
| Part of the butt remained unfinished: small allowance | Check blanks |
| misalignment of the end face of the workpiece or its incorrect installation in the chuck | Select suitable blanks. Place the workpiece correctly in the chuck |
deflection of a part under the action of cutting forces | Reduce cross feed |
| Unsatisfactory roughness of the treated surface: large or uneven feed | Reduce feed |
| work without coolant | Apply Coolant |
| high cutter wear | Apply coolant Replace cutter |
Threading with a cutter |
|
| Incomplete profile height or thread profile shear: wrong choice of workpiece for threading | Select the diameter of the workpiece according to the recommendations of GOST 19257-73 and GOST 19258-73, specify it when cutting trial workpieces |
| Thread profile misalignment: incorrect cutter setting | Set the cutter relative to the axis of the part according to the template |
| Deviation from thread profile angle: wrong corner sharpening | Replace cutter |
| The sides of the thread profile are not straight in the axial direction: setting the cutter not at the level of the line of centers of the machine | Set the cutter to the height of the machine center line |
| Thread Pitch Deviation: incorrect setting of the machine per step | Check the pitch setting according to the feed box table, and when setting up a machine with replaceable gear wheels, check the calculation and selection of wheels |
| The first thread is thickened, the through gauge cannot be screwed on: cutter deflection at the moment of plunging | Change the cutting allowance of the first coil |
| Unsatisfactory roughness of the machined thread surface: high cutter wear | Replace cutter |
| wrong choice of coolant | Choose coolant |
Cutting workpieces on lathes are made with cutting cutters, which, according to their design, can be straight and reverse. Straight cutters have a long and narrow head to cut through the workpiece to the center and ensure minimal material wastage into chips. However, they have insufficient strength and rigidity, so the cut point should be as close as possible to the chuck jaws, at a distance of no more than one diameter of the workpiece. The cutting cutter is set strictly at the level of the line of the centers of the machine and perpendicular to the axis of the workpiece.
When cutting workpieces of large diameters, the cutter may break at the end of the pass due to the fact that the thin bridge bends under the action of gravity and cutting forces and the cutting cutter is pinched in the slot. In this case, it is necessary to remove the cutter from the slot, not reaching the center by about 1.5 ... 2.0 mm, turn off the rotation of the spindle and saw off the part to be cut off manually. During the cutting process, it is forbidden to support the part of the workpiece to be cut off with your hands. The exit of chips from a narrow and deep slot is very difficult, so cutting should be done by gradually widening the slot.
Before drilling, countersinking or reaming the lathe should be carefully aligned for center alignment.
Important conditions for the drilling operation are: strong fixing of the workpiece; perpendicularity of its end to the axis of rotation; absence of bulges at the end; setting the initial direction of the drill. To do this, the workpiece in the machine fixture is installed with the smallest possible overhang, and the end face is smoothly cut before drilling. To set the initial direction of the drill in the center of the end, a recess is made with a center drill or a short hard drill; the drilling depth should be approximately equal to the diameter of the hole to be made.
drilling large diameter holes with manual feed is difficult due to the need for great effort on the part of the turner, therefore, holes with a diameter of more than 20 mm should be processed sequentially with two drills. The diameter of the first drill is chosen approximately equal to half the diameter of the resulting hole. Due to this, the jumper of the second drill is not involved in cutting and, accordingly, the feed force is significantly reduced.
filing used for cleaning surfaces, removing burrs, removing small chamfers, etc. It is performed with files of various shapes, with different notches, and only with a whole and tightly fitted handle.
Since filing is done manually, to prevent injury, the turner must stand at an angle of approximately 45 ° to the axis of the centers of the machine, with a turn to the right. The handle of the file should be clamped in the left hand, and its opposite end should be held with the fingers of the right.
Polishing used to reduce the roughness of machined surfaces. It is carried out with grinding skins of various grain sizes. During polishing, the skin is held with the fingers of either the right hand or both hands (the front edge of the skin must be held with the left hand, and the opposite edge with the right).
It is impossible to hold the skin on the part by wrapping it with your hand, as it can wrap around the part and pinch your fingers.
Usually, several incisors are fixed in the lathe support at the same time, therefore, when filing and polishing, one should beware of cuts to the hands with the sharp edges of the incisors, as well as when turning the incisor head and taking measurements.
Any equipment fails sooner or later, so it is simply necessary to repair a lathe, since the purchase of a new machine can result in a round sum, and, frankly, there is no point in it.
To begin with, we will consider what is turning, the device of the machine, and also talk about the overhaul of the tailstock of the lathe.
The technological process of turning is to reduce the diameter of the workpiece using a cutter, which is equipped with a special cutting edge.
Due to the rotation of the workpiece, the cutting process occurs, the feed and transverse movement are carried out by the cutter.
Thanks to these three components: rotation, feed, movement, it is possible to influence the amount of material removal, the quality of the machined surface, the shape of the chip, etc. also depend on this.
The main elements of the lathe:
- Bed with guides for the tailstock and caliper;
- In front of the bed is the headstock, as well as the spindle and chuck;
- A gearbox is attached to the front of the bed;
- Support with slide for transverse cutting;
- The cutter holder is located on the cross slide.
These elements are the main ones, depending on the modifications, you can get a center, turret-turning, multi-cutting and other machines that must undergo mandatory maintenance.
Preparation for repair
The most common problems can be considered wear of bearings, guides, gear forks, etc.
Major repairs can be done only after the preparation of the equipment.
Before stopping the machine, it is necessary to check how it is idling in order to determine the increased level of vibration, noise.
To determine the rolling state of the spindle bearings, the sample must be machined. The axial and radial runout of the spindle is also checked.
These actions will correctly identify the problems that have arisen, since they are not always obvious.
In general, the enterprise should carry out maintenance of machines according to the calendar schedule.
Thus, it is possible to eliminate breakdowns and shortcomings in a timely manner in order to avoid overhaul.
If you send the machine for overhaul, then it must first be washed from dirt and dust.
It is also required to drain oils and emulsions, check that all parts are in place.
See the video for cleaning and lubricating the rails.
Guide repair
Overhaul of the bed guides can be done in several ways: scraping, grinding, planing.
To determine the amount of wear on the guides with your own hands, you need to clean the surface, remove the nicks.
After that, the gap between the guides is measured with a ruler along the entire length of the machine. Measurements are taken in increments of 30-50 cm.
The straightness of the guides can be checked using very thin paper (no more than 0.02 mm), cigarette paper is suitable for such purposes.
The sheets must be laid out on guides and pressed down with a light object.
With normal straightness, it will not work to pull out the whole leaves from under the object, only in scraps.
Scraping is carried out after the equipment is installed on a coating that is distinguished by its rigidity (special stand).
It is also possible to determine deviations from the level that moves along the guides or along the bridge of the tailstock.
The guides on the bottom of the tailstock are usually chosen as a reference surface as they are subject to less wear.
Before grinding, it is necessary to clean the surfaces to remove all nicks.
To perform these works, it is necessary to use the table of the longitudinal planer, on which the bed will be installed.
After that, it must be checked for parallelism.
The bed must be securely fixed on the table, the final result will depend on this. The twisting of the guides is measured again (the indicators before and after installation should not differ) and proceed to grinding.
Repair of guides by planing begins with cleaning the surface, installing a planer on the table, after which it is checked for parallelism and fixed.
Surface treatment with a cutter for the best result is carried out 3-4 times.
After completing the work, it is necessary to check the straightness, parallelism and twisting of the guides and unfasten the equipment.
Features of processing guides
It should be borne in mind that the scraping process involves the use of a certain order of work, which may differ for different machines.
Consider below the technology for performing scraping of a screw-cutting lathe:
- First, the guides are processed, which are located at the bottom of the tailstock;
- Further - located under the clamping bars and from the bottom of the carriage, while parallelism deviations should not be allowed to exceed 15 microns;
- Then - scraping the transverse caliper. Small errors are allowed;
- The next step in the repair of a screw-cutting lathe is the scraping of the carriage (reciprocal guides). A trihedral ruler is used to determine the error. The difference between the screw axis and guides should not be more than 35 microns;
- With severe wear of the longitudinal guides of the screw-cutting lathe, it is necessary to use an anti-friction compound. Here, the important point is to achieve compliance along the axes of the running shaft with the landing zone, the travel rack must have good engagement with the gear in the longitudinal direction, in the transverse direction, the spindle axis must be perpendicular to the movement of the caliper;
- Next, the tailstock guides are repaired using an anti-friction compound.
See video about rough scraping.
It is better to trust the maintenance of machine tools to specialists, since the work must be carried out clearly, professionally, without deviations.
It will be difficult to achieve such results with your own hands.
Repairing the caliper carriage
Restoring the accuracy of the lower rails that mate with the base rails, regardless of wear, is where you need to start repairing the caliper carriage.
Also, when repairing the carriage, it is necessary to restore the perpendicularity of its plane under the apron to the base plane (under the gearbox).
The location of these planes is measured by the level. What thickness the probe will be placed under the carriage, this will be the level of deviation (value).
The parallelism of the longitudinal guides and their parallelism to the axis of the transverse feed are also subject to restoration.
Longitudinal and transverse guides must be exactly aligned with each other.
It should be noted that the repair of the caliper carriage is a very time-consuming process, it is very difficult to do it yourself, so the company should plan to maintain the device according to the schedule.
You can restore the carriage guides using compensation pads or with acrylic plastic.
The cross slide of a screw-cutting lathe can be repaired by grinding. Swivel sleds begin with scraping surfaces, after which they proceed to grinding.
If necessary, also repair the upper sled.
To do this, the surface is scraped, aligned, polished, after which the accuracy of the mating of the surfaces with the guides of the rotary slide is checked.
See the video for scraping the cross carriage.
Lead screw and drive shaft
During an overhaul, it may be necessary to align the axes of the lead screw and shaft, feed box and apron.
The feed box is installed and fixed on the base.
The carriage must be moved to the feed box until the ends of the frame touch. Next, you need to measure the clearance using a probe with a ruler.
Using the pads, scraping the guides, you can restore the alignment of the holes of the lead screw and the shaft.
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