Maintenance 3.0 aims to prevent the systematic maintenance on machine tool equipment, which is decided by hours of operation or for periods of time. It is based on a very logical principle: if it works, do not touch. Do not touch it, but watch it, analyze it, and only if the equipment gives failure symptoms, then Act! and inmediately please.
Systematic strategy, organizing maintenance downtime, up-time keeping or maintenance hours defending the interests of machine tool equipment manufacturers, but not the interests of the owner of an installation. Maintenance by Condition or Maintenance 3.0 is also wrongly called predictive maintenance, since it does not predict anything.
Maintenance 3.0 bases are:
• Change the concept of Maintenance Plan for Inspection Plan
• Change the concept of Predictive Maintenance for Maintenance by Condition
• Eliminate systematic tasks and replace (when possible) for inspection tasks
• Update facilities as far as possible to make them maintenance-free
• Exploiting the maintenance status and diagnostic tasks, so that underlie the maintenance
• The maintenance tasks for status and diagnosis must be made whenever possible with the plant running.
• There are three basic types of maintenance tasks diagnostic condition with ground-up, diagnostic tasks stopping floor (to avoid), corrective maintenance scheduled arising from maintenance condition, and corrective unscheduled maintenance (to avoid).
• Create ‘windows’ of maintenance, which is carried out corrective maintenance Scheduled estimated from inspections. Maintenance windows are determined by a limiting factor, such as replacing a consumable, if not, would stop production or generate a fault powerful.
• Subjecting all maintenance activities
• operating personnel is responsible for a part of maintenance, namely the maintenance of class or conductive maintenance: readings of parameters, data collections, basic sensory inspections, testing equipment in reserve, rotation equipment
• Change in the management of parts, with the creation of kits of rotation and replacement kits that accelerate interventions
• Really useful indicators selection
•• Experienced fitters and inspectors carry out the acceptance test of a new machine tool in the manufacture’s works. These men know how to use measuring instruments and how to assemble the machine in such a manner that manufacturing tolerances of individual components have a compensating and a cumulative effect as far as the accuracy of the whole is concerned.
•• Machine tools, which satisfy the specified accuracies in the standards of machine tool acceptance tests, will produce components that will meet the requirements of modern production in accordance with standard limits and fits. However, if closer tolerances are needed, an expensive additional operations and tedious selective assembly have to be carried out. For the manufacture of components to medium or coarse tolerances, wide tolerances may be permissible.
•• When the machine tool is under load, deformations and vibrations are created in the frames and other parts of machine tool during machining operations. Tests that cover the vibration behavior of any machine tool are difficult.
•• With regard to vibrations, various investigations concerning the causing and elimination of chatter on different machine tools have carried out. Some of them serve for determining the conditions under which a machine tool can be used without the danger of chatter, others serve for the testing and improving of newly designed and prototype machines.
•• As far as acceptance tests for standard machines rather than prototypes are concerned, finishing cuts be still taken for determining the machine performance.
1.- GEOMETRICAL TEST: 1.1.- Measuring Equipment and Methods
Any type of equipment may be used as long as the specified measurement can be carried out with the required degree of accuracy.
1.1.(a) Dial Gauges
1. The graduation must be clear and normally they don’t need to be finer than 0.01 mm/m. Finer graduations that are required in special case graduation down to 1 µ/m may be used.
2. The initial plunger pressure should vary between 40 and 100 grams; for very fine measurements a pressure as low as 20 grams is desirable or even 100 mNw.
The dial gauge must be fixed to robust and stiff bases and bars in order to avoid displacements due to shock or vibration.
1.1.(b) Test Mandrels
The most widely used inspection tool during manufacture and acceptance tests of new machine tools, and the repair of old ones, is the test mandrel, the quality of which (especially as far as straightness and roundness are concerned) is of paramount importance for accurate results. Two types of test mandrel are used:
1) Mandrels with a cylindrical measuring surface and a taper shank, which can be inserted into the taper bore of the main spindle 2) Cylindrical mandrel that can be held between centers.
However, the “natural sag” as the deflection caused by the weight of the mandrel should be kept within permissible limits. Table below shows the permissible deflection of cylindrical mandrels with 1000 mm in length and held between centers. Outside diameter, (mm) 75 80 80 80 100 100 100 125 125 125
Bore diameter, (mm) solid solid 50 60 solid 60 80 solid 80 100
Deflection, (µm) 13.20 11.60 8.35 7.45 7.40 5.50 4.55 4.75 3.40 2.90
All mandrels must be hardened, stress-relieved and ground.
1.1.(c) Straight – Edges and Squares
Straight – Edges of cast iron or steel should be heavy, well ribbed and free of internal stresses. Their bearing surfaces should be as wide as possible. The error at the top of a standard square should be less than ± 0.01mm, and for a precision square less than ± 5µm.
1.1.(d) Spirit Levels
Spirit levels are used in the shape of a bubble tube, which is mounted on a cast-iron base. The two main types are the horizontal, and the frame spirit level. Spirit levels used for high-precision measurements (tolerances 0.02 to 0.04 mm/m) should have a sensitivity of about 0.03 to 0.05 mm/m for each division.
1. Spirit levels which are too sensitive are difficult to bring to rest in a workshop in which machines are running, while too low a sensitivity result in insufficient reading accuracy.
2. It is often advisable to use a bridge piece, the feet of which are about 300 millimeters apart. The spirit level can then be placed on the scraped surface of the bridge. This method avoids errors which could be caused be irregular scarping of the surface to be measured.
The sensitivity E of the spirit level is the movement of the bubble in millimeters, which corresponds to a change in slope of 1 mm/m. The scale value ‘S’ indicates the change in slope (mm/m) necessary for producing a bubble movement of one division. If the distance between two divisions is called ‘d’ then S = d/E. The sensitivity of the level depends, only on the radius of curvature of the bubble tube ‘R’, and not on the length of its bearing surface.
1.2 – GEOMETRICAL TEST: Magnitude & Direction of Tolerances
In the test charts, the tolerances are given in three different ways:
a) As plus or minus tolerances (example: ± 0.03 mm/m); With plus or minus tolerances, the permissible error is allowed to occur in either direction within the specified reference length. The total range of error is therefore double the specified tolerance.
b) As tolerances without signs (example: 0.03 mm/m); Tolerances without signs include the total range of error measured on the reference length, no matter in which direction the error appears.
c) As unilateral tolerances (example: 0 to 0.03 mm/m); With unilateral tolerances, the specified limits cover the total range of error across the total reference length, the direction of error being of great importance and always stated in the text of the respective test chart.
In detail, the tolerances are specified in the test charts and cover the following:
1.2.1- Straightness of Slideways and Flatness of Tables. These are tested by means of the spirit level. The tolerances are specified either: As plus or minus tolerances, or as unilateral tolerances.
Example: Lathe bed straight or flat in the longitudinal direction (convex only) with specified tolerance of 0 to 0.02 mm/m. The spirit level is allowed to deflect in one direction only, i.e. rising towards the center and within the limits of 0 to 0.02 as measured on a reference length of 1000 mm, with the result that the bed will be only convex.
The spirit level should be used on a bridge the feet of which are spaced about 300 mm. The measurement is started at the center of the bed with zero reading as accurately as possible. From this point the spirit level is moved to the right and to the left.
The slope has its largest permissible value if the spirit level indicates the maximum reading 0.02 per 1000 millimeters (convex) along the left, and the opposite reading along the right half of the bed. The largest permissible rise of a bed having a length of 1.5 meters, would be equal 0.75×0.02=0.015 mm.
1.2.2- Flatness of Slideways (Twist of Cross Rails and Arms)
These are tested by means of the spirit level. The tolerances are specified without signs. In these cases, the spirit level is moved along the surface to be tested. Then, the range of the largest readings taken in both directions indicates the error that must be within the specified tolerance.
1.2.3- Alignment of Slideway and Axes, or Center Line Parallel or perpendicular to each other
These are tested by means of the dial gauge or the spirit level. The tolerances are specified in the form of either: tolerances without signs, or unilateral tolerances. In each case the specified tolerance represents the total range within which the pointer is allowed to deflect.
Example: Lathe spindle parallel with the bed in the vertical plane (spindle rising towards the free end of test mandrel only) with specified tolerance of 0 to 0.02 millimeter per 300 mm. When the dial gauge is being moved along the test mandrel, the pointer is allowed to deviate in the stated direction only.
1.2.4- Alignment and True Running of Shafts
The tolerances for the true running of a shaft have to be taken as the admissible total deviation (range of deviation) of the dial-gauge pointer.
Example: Testing lathe spindle for true running, with specified tolerance of 0.01 millimeter. Then, during one revolution of the spindle, the dial pointer is allowed to deviate over range of 0.01 millimeter
1.2.5- Lead or Pitch Error of Lead-screws
The lead or pitch error is generally based on a reference length of 300 mm. Beginning from any given initial position, the test nut is moved over a number of threads corresponding to an accurate travel of 300 millimeters for metric or 12 inches for Whitworth thread screws. The actual travel of the nut may be either lager or smaller than 300 millimeters or 12 inches, respectively, by not more than the specified permissible lead error.
2- DETAILS FOR LATHES ACCEPTANCE TESTING CENTERS
2A) Tests the accuracy of the main spindle and of its alignment relative to other important parts of the machine.
2A-1) Installation and Leveling of the Lathe: Lathes are grouped in accordance with their uses, their sizes and the degree of accuracy required from them. Experience shows that lathe beds wear more rapidly in the center than at the ends. Moreover, the overhanging weights of the carriage and the cutting resistance force the front shears (apron side) down and lift the rear shears. Hence, the tolerance must be directed in opposition to this deformation. The front shears are, therefore permitted to be arched or humped upwards (convex) only, while the rear shears may be less convex or even slightly concave. To avoid the undesirable combination of a maximum convex tolerance for the front shears and a maximum concave tolerance for the rear shears, spirit level for twist in the transverse direction is also carried out.
Measurements are carried out with the spirit level the sensitivity of which has to be in accordance with the required accuracy.
2A-1-1) Leveling the lathe Bed
During the test of short machine the carriage must be in the middle of the bed.
In the case of long beds with more than two legs it must be between two legs.
1. A spirit level (scale value 0.04 mm/m) is best put first on the rear slideway (i.e. the slideway opposite the operators’ side). This slideway is usually plane whilst the front slideway may be intentionally convex. By checking positions ‘a’ and ‘b’ of the rear slideway and repeating the measurements for the front slideway straightness of the beds can be determined.
2. It is advisable to check the leveling in the transverse direction simultaneously with the previous step. This is done by means of a second spirit level alternatively placed in position ‘c’ and ‘d’. A ± twist tolerance is not permissible because the sliding surface of the carriage would not be properly supported by twisted slideway.
The above tests make it possible to ensure that the Four Corners of the bed lie in a horizontal plane, and this plane is the datum for all following measurements.
2A1-2) Testing the Quality of Slideways and Locating Surfaces
The slideway surfaces of the lathe bed are not only datum faces for leveling the machine but also working surfaces for guiding the carriage and the tailstock. The quality of these sliding surfaces, of whatever design, is of vital importance for the accuracy of workpieces produced on the lathe. Special care has therefore to taken in their manufacture. For this reason the acceptance test serves for making certain that the locating elements of the three main parts (headstock, tailstock and stay) are accurately aligned both vertically and horizontally with the carriage slideways over their full length. Moreover, the tailstock guides must be carefully scraped or ground. Putting the plunger of the dial gauge directly on their scraped or ground surface carries out the tests of the slideways.
If the surfaces are tested in this manner along three lines, this can be done in a few minutes and the alignment and the quality of these important sliding surfaces can be judged
Measurements concerning flatness, straightness and parallelisms of the principal machine ways should be introduced successfully.
The Lathe Bed must be straight longitudinally. In the case of beds up to three meters long, it is sufficient to place a spirit level on the slideways; if necessary an intermediate block, or a bridge piece, can be employed. The base of spirit level must always be parallel to the direction of the slideways. The straightness is checked by placing the spirit level at intervals of about 300 millimeters along the whole length of the bed. The difference between this test and the leveling of the four corners lies in the fact that for the straightness test the spirit level readings are taken in several positions along the bed.
For machines of more than three meters between centers, other methods are used for testing the straightness of the slideways, e.g. the taut wire.
The straightness and surface quality of the tailstock slideways as well as their parallelism with the slideways for the saddle is best tested with a dial gauge clamped to the saddle and measuring both surfaces of the Vee.
The full length of the slideways is tested, including the bridge piece, which covers the gap, if present. The plunger of the dial gauge indicates any small depression in the surface, which might be caused, for instance, by mottling. These surfaces are relatively small in relation to the load to which they may be subjected. They may be subjected to considerable wear, and their quality is of great importance
2A1-3) Testing the Accuracy of the Main Spindle and of its Alignment Relative to other Important Parts of the Lathe
The headstock should be so aligned that an arbor inserted in the spindle nose rises or inclines upwards only at its free end with respect to the bed ways, whilst inclining in the horizontal plane towards the tool post only. This will counteract the deformations resulting from the weight of the workpiece and the cutting force. Similarly, the tailstock spindle when fully advanced is only permitted to deflect in the corresponding directions.
These tests concern the following steps:
2A1-3.1 True running:
(a) The center;
(b) The internal taper;
(c) Cylindrical locating spigots, external tapes
Three source errors have to be covered and are often measured simultaneously these are:
1. Inclination of the spindle axis in relation to the axis of rotation (angle a)
2. Eccentricity (distance e) of the spindle axis with respect to the axis of rotation.
3. Lack of roundness of the surface tested as shown in the enlarged X-X cross-section.
For all these measurements, a precision dial gauge is used; the calibration of which should be checked every few months by means of slip gauges.
The test for true running of the center points is required. For external cylinders or tapers are used for locating a chuck on a lathe spindle. Resting the dial gauge plunger at right angle (radially) to the surface to be tested carries out these tests of true running. Readings of the dial gauge are taken while the spindle is slowly rotated.
Also, a test mandrel with a locating taper shank and a cylindrical portion of about 300 millimeters long is used.
Readings of the dial gauge are taken while the spindle is slowly rotated. Readings are taken with the dial gauge plunger both near the spindle nose (point a) and at the end of the test mandrel (point b).
The headstock should be so aligned that an arbor inserted in the spindle nose rises or inclines upwards only at its free end with respect to the bed ways, whilst inclining in the horizontal plane towards the tool post only. This will counteract the deformations resulting from the weight of the workpiece and the cutting force.
2A1-3.2 Axial Slip:
If the plunger of the dial gauge rests against a shoulder face, the total error indicated may be the result of three sources: errors in the thrust bearing, the face of the locating shoulder is not perpendicular to the axis of rotation, and the shoulder face is irregular.
All shoulder-face measurements for axial slip must be carried out at two diametrically opposite positions.
After the plunger is placed against the shoulder face of the spindle, readings are taken while the spindle (axially loaded against the thrust bearing) is slowly rotated. Measurements are repeated with the dial-gauge plunger resting against the shoulder face at a point diametrically opposite to that of the first measurement.
The influence of the errors of the shoulder face can also be eliminated by placing the dial gauge plunger as accurately as possible in line with the axis of rotation either against a face which is perpendicular to the axis of rotation, is often used when axial slip of the spindle is measured.
For testing the axial slip of lead-screws, a steel ball which rests in the center bore is employed.
2A1-3.3 Alignment between spindle axis and other axes:
This concerns errors of alignment between two axes or faces. It is often necessary to describe the plane in which the alignment error has to be measured. Alignment can be clarified by Parallelism and Perpendicularity measurements.
The alignment of the lathe spindle and the external diameter of the tailstock sleeve are not critical as along as the axes of the two internal tapers are in line.
2A1-3.4 Parallelism between spindle axis and slideways:
Parallelism between two axes, between two surfaces or between an axis and a surface is checked by measuring the distance ‘a’ and ‘b’ of two sets of points A1, A2, B1, B2
It is necessary to determine the angular deviation, which can also be measured by means of a spirit level or determined by relating the difference ‘T’ to the distance ‘L’. Parallelism is usually measured in two planes vertical and horizontal. Measurement in only one plane is sufficient when the position in the other plane is adjustable, (tailstocks, swiveling tables, etc.). In the case of the center lathe, parallelism has to be checked, as follows:
// Between the saddle slideway and The tailstock slideways, // between two surfaces
(b) The spindle axis, i.e. between a surface and axis The test mandrel is located in the spindle taper, the dial gauge is mounted on the saddle, the plunger touching the test mandrel. The saddle is moved along the mandrel by an amount equal to the reference length and the indication of the dial gauge noted. Measurements have to be repeated in the vertical plane ‘a’ and the horizontal plane ‘b’. The spindle must have been running for about half an hour before the measurement is taken so that the main bearing is at its working temperature.
(c) The outside diameter of the tailstock sleeve, i.e. between a surface and an axis
(d)The tailstock sleeve taper, i.e. between a surface and axis
The tailstock spindle when fully advanced is only permitted to rises or inclines upwards only at its free end with respect to the bed ways, whilst inclining in the horizontal plane towards the tool post only. This will counteract the deformations resulting from the weight of the workpiece and the cutting force.
The tailstock sleeve, which cannot rotate but can be axially moved, must be clamped during each measurement, as the effect of clamping may affect its working position.
(e) The lead screw axis, i.e. between a surface and axis
For measuring the alignment of the lead-screw, the saddle is moved to the middle of the bed and the nut closed. The located by the front Vee of the bed and freely supported by the rear slideway. The dial-gauge plunger touches the outside diameter of the lead-screw. The bridge piece is moved to the right ( I to III) and to the left ( I to II), and the procedure repeated both in the horizontal ‘a’ and the vertical ‘b’ plane. The lead-screw is also tested for true running (tolerance 0.1 millimeter).
The lead-screw is one of the most important parts of the lathe. It is of the same importance as the main spindle. Various errors may affect the accuracy of the screw thread produced on a machine
1. The pitch error of the lead-screw.
2. The axial slip of the head-screw due to faulty thrust bearings.
3. The alignment of the lead-screw axis in relation to the carriage slideways.
4. The axial slip of the main spindle.
5. Errors in the transmission of the feed drive from the main spindle to the lead-screw through a Norton gear box or other means.
The lead-screw is subjected to quite unusual loads. The end bearings locate it axially and radially. The nut in the apron, which transmits the drive from the lead-screw to the carriage, is usually split and has to be opened and closed repeatedly. It also supports the lead-screw at any point throughout its full length. The position of the split nut is determined by the slideways of the bed. It is therefore essential to test the alignment of the lead-screw relative to the bed slideways. The permissible errors (0.1 to 0.2 mm according to the size of the lathe) have been determined in relation to the influence, which the misalignment of the lead-screw exerts upon the pitch accuracy of the thread produced.
2- Between the spindle axis and the tool rest slideway
This requires a test similar to the one shown for step 1(b);
3- Between the spindle axis and the slideways
In a similar manner the height alignment of two axes is tested after each axis has already been checked for its parallelism with a common datum slideway. In the case of the lathe such tests concern the alignment of spindle axis and tailstock sleeve taper (clamped), and lead-screw nut (closed), and lead-screw bearings.
A hollow test mandrel, 300 to 500 millimeters long is held between the centers, the spindle bearing again at its working temperature. A dial gauge is mounted on the saddle is plunger touching the top of the mandrel. The saddle moved along the bed and the indication of the dial gauge noted. The tailstock center must be higher than the spindle center.
2A1-3.4 Perpendicularity between spindle axis and slideways: The tolerance for perpendicularity is specified in the same manner as that for parallelism. Perpendicularity between the cross movement and the spindle axis can be checked by a surfacing operation. For the surfacing test, a workpiece is fixed in the chuck or fastened to the face plate and a fine finishing cut taken starting from the inside diameter. A straight edge is then placed against two equal block gauges that rest on the outside diameter of the turned face, and the distance between straight edge and workpiece measured by means of block gauges. The turned surface must be concave. This test combines as alignment test and a cutting test. It would be wrong to check the machined surface by means of a dial gauge fastened to the cross-slide because the dial-gauge plunger would traverse exactly along the path of tool edge and would indicate a zero reading whatever the inaccuracy of alignment.
It is also possible to test the perpendicularity between cross-slide and spindle axis by means of a straight edge L fixed to the cross-slide, and a dial gauge mounted on the face plate. The straight-edge is set exactly perpendicular to the axis of rotation of the spindle (dial-gauge reading identical in two diametrically opposite positions a and a1). If the cross-slide with the straight edge is then moved along the dial gauge, alignment errors can be determined directly from the dial-gauge reading.
It may be repeated here that, in all tests concerning running conditions and alignment of the main spindle, the machine must be at its working temperature, as otherwise the spindle will not be in its normal position in the bearings.
The accuracy with which the machine has been manufactured. The accuracy of the workpieces produced on the machine: Testing the Accuracy of WorkPieces Produced during a Finishing Operation
It will often be left to the manufacture to choose workpieces and tools for testing, and to ensure that the machine is free from vibrations and other faults. An attempt has been made to establish specifications for performance tests of lathes, as shown in Table below
Test to be applied Dimensions of piece Tolerances
(a) Round turning, chucking Diameter =1/4 center height,
Length = center height 0.01 mm
(b) Parallel turning, chucking —————— 0.03 mm per 300 mm
(c) Parallel turning between centers Diameter = 1/8 length,
Length from ½ to 1 distance between centers 0.02 mm in any length
(d) Facing (concave only) Diameter = center height,
Length about center height 0.02 mm per 300 mm in diameter
(e) Screwing Diameter = 25 mm,
Length of thread, 50 mm
Length of thread, 300 mm ± 0.02 mm total pitch error
± 0.05 mm
3- Power Requirements, Speeds and Feeds
Every machine tool must be so designed that its parts will not be deformed beyond permissible limits when subjected to the maximum working load. For instance, many lathes are designed for taking the largest possible cuts when the tool is acting on the maximum diameter.
The performance test of a machine tool should also check the speeds and feeds with the suitable instrumentation.
An additional field of application is the current inspection of machine tools during their use in production, and after maintenance and repair work has been carried out.
4- Checklist Chart for Finishing Turning Lathes up to 400 mm height
Test to be applied, Permissible Error, mm
Bed straight in long. direction; apron side (convex only), (a) 0 to 0.02 mm/m
Bed straight in long. direction; opposite side (concave only),(b) 0.02 mm/m
Bed level in transverse direction; (c) ± 0.02 mm/m
Straightness of slide ways; 0.02 mm/m
Tailstock guideways parallel with movement of carriage, 0.02 mm/m
Center point for true running, 0.01 mm
Centering sleeve for true running. 0.01 mm
Work spindle for axial slip, measured at two points, displaced by 180° 0.01 mm
Taper of work spindle runs true:
1) Nearest spindle nose 0.01 mm
2) At a distance of 300 mm 0.03 mm
Work spindle parallel with bed in vertical plane (rising towards the free end of mandrel only 0 to 0.02 per 300 mm
Work spindle parallel with bed in horizontal plane (free end of mandrel inclined towards the direction of tool pressure) 0 to 0.02 per 300 mm
Movement of upper slide parallel with work spindle in vertical plane (hand feed)
When automatic feed is provided: in vertical plane 0.03 per 300 mm
in horizontal plane 0.02 per 150 mm or 0.04 per 300 mm
Tailstock sleeve parallel with bed in vertical plane (front end rising), 0 to 0.02 per 100 mm
Tailstock sleeve parallel with bed in horizontal plane (front end inclined towards the direction of tool pressure) 0 to 0.01 per 100 mm
Cone of sleeve parallel with bed in vertical plane (free end of mandrel rising), 0 to 0.03 per 300 mm
Cone of sleeve parallel with bed in horizontal plane (free end of mandrel inclined towards the direction of tool pressure 0 to 0.02 per 300 mm
Axis of centers, mandrel between centers parallel with bed in vertical plane and mandrel rising towards tailstock end 0 to 0.02
Lead screw for axial slip 0.01
Lead screw bearing aligned with each other in vertical plane (positions II and III) 0.1
Lead screw bearing aligned with each other in hor. plane (positions II and III) 0.1
Lead screw aligned with half nut in vertical plane, measurements taken with closed half nut, carriage in middle position, position I serving as starting point 0.15
Lead screw aligned with half nut in horizontal plane, measurements taken with closed half nut, carriage in middle position, position I serving as starting point 0.15
Lathe turns round within 0.01
Lathe turn cylindrically:
a) work between centers within 0.02 per 300 mm
b) work held in chuck within 0.02 per 300 mm
Lathes faces hollow or concave only within 0 to 0.02 per 300 mm in dia.
Thread cut on 50 mm length ± 0.02 per 50 mm
Accuracy in pitch of lead screw is assured within 0.03 per 300 mm
5- MACHINE TOOL MAINTENANCE The maintenance of the machine tool includes:
1. Checking the accuracy of the finished workpiece.
2. Preparation of materials and component parts necessary for repair work, including bought-out parts. Any repair work must be carefully planned beforehand.
3. Instructions to machine shop foremen and operators in order to ensure correct use of the machines.
4. Overhaul or rebuilding of the machine tool.
5. Emergency repairs.
6. Estimates of maintenance and repair cost.
The measuring and testing equipment used in the maintenance shop is practically identical with that used for acceptance tests. In addition, scrapers, surface plates, straight-edges, etc. must be available as well as special equipment necessary to suit the requirements of particular machines, which have to be repaired.
The general procedure is laid down along the following lines:
1. The machine is installed and carefully leveled by the proper use of an accurate spirit level.
2. Where necessary the straightness, flatness, parallelism and quality of the guiding and bearing surfaces of beds, uprights and base plates are tested.
3. The main spindle, one of the principal elements of the machine, is tested for true running, axial slip, and location and position of its axis relative to other axes and surfaces.
4. The movements of other main parts of the machine are checked.
5. Working tests are carried out in order to determine whether the machine as a whole produces workpieces within the specified limits of accuracy.
6- DYNAMIC PERFORMANCE TESTS FOR LATHES
The overall accuracy of a machine tool depends not only on its geometrical accuracy but also upon its stiffness, i.e. its resistance to a steady force between the tool and workpiece, and its liability to both temperature changes and wear.
Surface finish and metal removal rate depend on the dynamic stiffness, (i.e. the resistance to dynamic forces), the workpiece material and the type and state of the cutting tool involved. In practice two kind of cyclic force during cutting are involved:
1) Forces imposed inside or outside the machine, such as forces transmitted through the floor, out of balance forces in the machine, etc. These give rise to forced vibration between the workpiece and the cutting edge, and its amplitude depends on the magnitude of the exciting forces.
2) Self-excited forces arise in the machine/workpiece/tool system and the cutting process becomes unstable. This is called ‘chatter’.
The major limitation on machine tool performance is, therefore, one of the dynamic stiffness, and this affects surface finish, noises generated and tool wear with the result that a limitation on metal removal rate may be necessary to over come these undesirable effects.
It should be clear that the dynamic tests of a machine tool performance mean tests of metal removal capability with free of chatter workpieces. Also, the described cutting test procedure is for use when it is desired to compare the chatter-free machining performance of two or more lathes. This procedure enables the limiting width of cut (blim) at which chatter commences on each lathe to be compared.
Mainly in order to reduce the number of variables involved in the dynamic performance tests, the following sections are confined to machine for turning with single-point tools.
The factors likely to affect the limiting width are machine tool condition, workpiece material, speed, feed, warm-up time, ambient temperature, operator, tool overhang, tool center height, tool wear, tool clamping torque, and workpiece clamping torque.
The majority of these variables, except for tool wear and the warm-up time, can be controlled. The tool wear could be controlled by ‘running-in” the cutting edge and then restricting the number of tests before using a new tip. For the warm-up time, if controlled accurately, would have required the machine to be allowed to cool down to ambient temperature before beginning the next test. Also, it is recommended to take the surface temperature of the housing of the front headstock bearing as an alternative measure of warm-up time.