How to Detect Straightness Errors in the Vertical Plane of a Cylindrical Grinding Machine Table Movement?
A: To check for straightness errors in the vertical plane of a cylindrical grinder table movement, follow these steps:
- Place a bridge (either supplied with the machine or a special homemade bridge for inclined table surfaces) in the center of the table. Note: A bridge is not required for a platform surface.
- Position a gradienter in the middle of the bridge and parallel to the direction of the table movement.
- Move the table and record the readings every 250mm. For short beds, inspect the two ends and three positions in the middle of the maximum grinding length. Arrange the readings of the gradienter in sequence and draw the motion curve of the table.
- The maximum coordinate value between the movement curve and the connecting line of the two endpoints on a length of 1000mm is the straightness error on that length.
- Draw a parallel line to cover the movement curve, and the coordinate value between the covering line is the straightness error value on the entire length of the guide rail.
How to Detect Straightness Errors in the Horizontal Plane of a Lathe Slide Movement?
A: To detect straightness errors in the horizontal plane of a lathe slide movement, follow these steps:
- If the travel of the lathe slide carriage is less than 3000mm, use a long cylindrical test rod.
- Tighten the test rod in the front and rear center and fix a dial gauge on the slide carriage, positioning it on the side generatrix of the gaging mandrel.
- Adjust the tailstock so that the micrometer readings on both ends of the gaging mandrel are equal.
- Move the slide carriage to check the full range of the gaging mandrel.
- The maximum difference between the readings on the dial gauge over a travel of 1000mm is the straightness error.
- If the travel of the gaging mandrel is more than 3000mm, use a taut wire parallel to the bed rail and a microscope (or an optical straightness meter) to detect the error.
How to Detect Straightness Errors in the Horizontal Plane of a Long Lathe Bed Working Table or Slide Carriage Movement?
A: To detect straightness errors in the horizontal plane of a long lathe bed working table or slide carriage movement, follow these steps:
- If the travel of the slide carriage on double housing planers, milling planers, and horizontal boring and milling machines is more than 2000mm, or the horizontal lathe, screw lathe, or other slip plate stroke is greater than 3000mm, use tools such as a wire and microscope for detection. This method is similar to the detection method for straightness errors in guide rails.
- If possible, an optical flatness gauge can be used for detection.
- Place the optical flatness gauge at one end of the machine tool and the reflector on the slide carriage or working table.
- At the two extreme positions of the stroke, adjust the flattening gauge and reflector so that the bright cross image of the collimator coincides with the alignment of the movable reticle.
- Take readings every 500mm of worktable movement, arrange the readings to draw the movement curve of the worktable, and then calculate the error values on a length of 1000mm and the total length of the stroke.
How to Detect Tilt Error When the Slide Carriage Moves?
A: To detect tilt error when the slide carriage moves, follow these steps:
- Place a gradienter on the dovetail guide of the slide carriage near the knife holder, perpendicular to the lathe bed rail (i.e. the direction of slide carriage travel).
- Move the slide carriage, record a reading every 250mm (or 500mm or less), and check the full travel of the slide carriage.
- The maximum algebraic difference between the readings on a 1000mm stroke and the full stroke of the gradienter is the tilt error.
- The tilt error is indicated by the slope or angle value.
How to Detect Tilt Error When the Table Moves?
A: To detect tilt error when the table moves on cylindrical grinders, thread grinders, broaching grinding machines, double housing planers, and milling planers, follow these steps:
- Place a gradienter in the center of the table, perpendicular to the direction of worktable movement. If the grinder has an inclined table, place the gradienter on a special pad.
- When the worktable moves, record readings every 250mm (or 500mm or less) and take at least three readings on the short lathe bed for the entire travel of the worktable.
- The largest algebraic difference between the readings on a 1000mm travel and the full travel of the gradienter is the tilt error when the worktable moves.
How to Detect Tilt Error When the Beam Moves?
A: To detect tilt error when the beam moves, follow these steps:
- Place a gradienter in the center of the rail of the beam, parallel to the beam.
- When moving the beam, record a reading every 500mm (or less) and take at least three readings for the full travel of the beam.
- Move the two vertical tool holders (or milling heads) to a symmetrical position on the beam when checking over the full travel of the beam.
- The beam should only be moved from bottom to top, not back and forth.
- This method is commonly used for detecting tilt error when the beam moves on double housing planers, milling planers, and vertical lathes.
Which machine tools require higher positioning accuracy? How should it be tested in general?
A: Jig borers require relatively high positioning accuracy, so it is necessary to detect any positioning errors of the table or spindle box after moving to the desired coordinates.
Jig borers have two types of positioning systems: optical and mechanical (screw and calibration ruler). Although the systems may be different, the method of measuring positioning error is the same.
To test the positioning accuracy, a precise scribing ruler is placed along the longitudinal direction of the machine tool table. The scribing accuracy of the ruler should be verified using an error verification table, with an accuracy of within 0.0005mm. The scribing ruler should be placed in the center of the worktable and positioned at a height of l/3 to l/2 of the maximum distance from the worktable to the end of the vertical spindle.
A reading microscope (with a reading accuracy of 0.001 to 0.002mm) is fixed on the spindle sleeve, allowing for clear observation of the scribe lines on the scribing ruler. The movement of the table is then detected, generally by taking one reading for every 10mm movement. The table should be clamped during the readings.
The positioning error is calculated as the most algebraic difference between the actual values of the readings taken at any two positioning times.
How to Detect the Dividing Error of an Indexing Head?
A: To detect the dividing error of an indexing head, follow these steps:
- Fix the standard indexing disc to the spindle of the indexing head to eliminate any dividing error.
- Mount the reading microscope on the holder of the inspection plate to ensure a center overlap.
- Align the reading microscope and the index plate to the zero position by determining the starting position of the handle.
- Make a reading every 90° turn of the indexing head and record it.
- For comparative testing using an optical indexing head, insert an inspection rod with a Morse Taper at both ends. One end should be closely fit into the spindle taper hole of the optical indexing head, while the other end should be closely inserted into the spindle taper hole of the indexing head to connect the two.
- If the center of the main shaft of the inspected indexing head and the optical indexing head are not equal in height, flatten them.
- Disengage the indexing head and turn the handle to rotate the indexing head being inspected, and simultaneously drive the optical indexing head to rotate using the inspection rod.
- When the indexing head is in the starting position, record the rotation of the optical indexing head.
- Also, record the reading value of the optical indexing head’s starting position when the indexing head is in the starting position.
When the errors of the indexing disc and indexing worm are not considered, the dividing error can be calculated as follows:
The transmission relationship of the indexing system is that when the handle of the indexing head rotates n times, the main shaft rotates once. The formula for this relationship is:
n = z2 (number of teeth of the indexing worm) ÷ z1 (number of heads of the indexing worm)
Typically, the number of teeth of the indexing worm (z2) is 40, and the number of heads of the indexing worm (z1) is 1.
Therefore, when the handle rotates one cycle, the theoretical angle of the indexing head spindle rotation is 360°/40 = 9°.
After determining the position, record the reading of the standard dividing disk (or optical indexing head) once. The difference between the theoretical angle of the spindle’s rotation and the actual reading from the optical instrument is the dividing error of the indexing worm during each cycle of handle rotation.
When the spindle rotates one cycle, 40 indexing error values can be obtained. The largest algebraic difference of these error values is the maximum indexing error when the spindle rotates one cycle.
To detect this error, the spindle should be rotated clockwise for one cycle and then counterclockwise for one cycle.
When considering the errors of the dividing disc and indexing worm, the dividing error can be calculated as follows: The indexing worm handle does not rotate one cycle but rotates an angle α. This angle is equivalent to rotating 1/z cycles, where z is an integer between 8 and 12. This value is determined by the indexing transmission ratio and the number of holes of the original dividing disc of the indexing head being inspected.
During detection, when the handle rotates an angle α (i.e., 1/z cycles), record the reading of the standard dividing disk (or optical indexing head). The algebraic difference between the maximum and minimum values, when considering the theoretical rotation angle and actual reading of the indexing head spindle, is the maximum indexing error of the worm during one cycle.
To detect this error, the indexing handle should be rotated clockwise and counterclockwise each cycle.
The comprehensive error value of the indexing system is the sum of the maximum indexing error when the spindle rotates one cycle and the maximum indexing error when the worm rotates one turn.
How to Detect the Dividing Error of a Rotary Table Using an Octagonal Gauge?
A: The rotary table is a milling machine accessory with low indexing precision, which can be tested using an octagonal gauge.
- The external diameter of the octagonal gauge is 250mm.
- To ensure that the center of the rotary table coincides with the center of the circumcircle of the octagonal gauge, insert the positioning spindle of the octagonal gauge closely into the worktable’s tapered hole.
- Fix the inspected worktable on the test plate and place the dial gauge’s flange on the base against the guide groove in the test plate. Use a flat ruler to ensure the dial gauge’s measuring head is on top of one side of the octagonal gauge.
- Move the micrometer along the guide slot until the readings are the same on both sides (adjust by turning the worm handle).
- Turn the table 45° (360°/8) and check the micrometer over the full length of the other side. Repeat this process for each side.
- The maximum difference between the meter readings on either side of the table is the dividing error.
Note: This method is used when the indexing precision of the measurement is not high (the error is within 4′ to 8′).
How to Detect the Dividing Error of a Rotary Worktable by Comparison with a Precise Horizontal Turntable?
A: Detecting the dividing error of a rotary worktable by comparison with a precise horizontal turntable is a convenient method that has simple operation and high efficiency. The dividing error of the precise turntable may not be as accurate as using an odolite, but it is still useful for detecting dividing errors.
The method:
- Place the precise turntable on the plate or machine tool worktable and use a gradienter to level the turntable.
- Stack the inspected rotary table on the precise turntable and use a dial indicator to check the parallelism of the two turntables. Adjust the turntables so that the rotation center lines of the positioning holes coincide and fix the two turntables together with screws.
- Fix a locating piece on the surface of the inspected turntable and fix the dial gauge on the plate (or on the differential table on the machine spindle when tested on the machine table).
- Adjust the reticle and dial gauge pointers of the two turntables to the zero position.
- Turn the handwheel of the inspected rotary table to rotate it clockwise by a certain angle (an integer value), then turn the handwheel of the precise turntable to rotate the inspected table counterclockwise with the precise turntable.
- When the worktable touches the dial gauge probe and the dial gauge pointer returns to the zero position (initial position), stop rotating the precise turntable.
- Record the readings of the two turntables at this time and the difference between the two readings is the dividing error of the inspected turntable.
Note: Repeat this process at intervals of 10°, 5°, and 1°.
How to Detect Repetitive Positioning Errors in the Fast Feed Mechanism of a Grinding Carriage on a Cylindrical Grinder?
A: To detect the repetitive positioning error of the fast feed mechanism of a grinding carriage on a cylindrical grinder, follow these steps:
- Fix a micrometer on the machine table with the grinding carriage in the feed position.
- Place the micrometer’s measuring head on the axis of the grinding carriage near the grinding wheel.
- Use the fast feed mechanism of the grinding carriage to continuously feed it 10 times.
- The maximum difference between the micrometer readings is the repetitive positioning error of the fast feed mechanism of the grinding carriage.
How to Detect the Repetitive Positioning Error of the Working Stroke for a Relieving Tool Carrier?
A: To detect the repetitive positioning error of the working stroke for a relieving tool carrier, follow these steps:
- Fix a dial gauge on the back of the tool carrier on the slide carriage, with the measuring head on top of the side of the tool carrier.
- Rotate the drive shaft by hand to make the tool post reciprocate.
- Reciprocate the tool carrier 10 times.
- The largest difference between the readings of the dial gauge is the repetitive positioning error of the working stroke for the relieving tool carrier.
What are the Main Factors Affecting the Machining Accuracy of Workpieces in Machine Tools?
A: The machining accuracy of workpieces in machine tools can be affected by several factors, including:
- Tool geometrical form error and misalignment on the machine.
- Misalignment of the workpiece on the machine.
- Errors in the machine tool’s machining principle.
- Vibration, elastic deformation, thermal deformation, and tool wear during the cutting process.
- The geometrical form error of the machine tool.
- The transmission error of the machine tool’s transmission chain.
How to Improve the Accuracy of the Machine Transmission Chain?
A: There are several ways to improve the accuracy of the machine transmission chain:
- Minimize the number of transmission elements in the chain to reduce error sources.
- Arrange the transmission chain so that the speed decreases from the first end to the end of the chain, and maximize the transmission ratio at the end of the transmission pair (increase the number of worm gear teeth, reduce the number of worm heads, reduce the number of threads, and reduce the pitch of the screw).
- Avoid using helical gears, bevel gears, or clutches near the final transmission pair.
- Place the exchange gear as close to the final transmission pair as possible.
- Use a 1:1 gear transmission to compensate for transmission error.
- Improve the precision of transmission components.
- Use error compensation methods during assembly.
- Install an error correction device.
Improving the accuracy of an existing machine tool is challenging and requires precision installation of transmission components, use of error compensation methods, and use of error correction devices. While reducing the number of transmission components or rearranging the transmission chain can help improve accuracy in new machine designs, these changes are more difficult to implement in existing machines. Improving the accuracy of an existing machine typically involves restoring its original precision or improving it by one level.
What Types of Machine Tools Use Transmission Chain Drive to Detect Errors, and What are the Methods for Detection?
A: Machine tools that process gears and threads using the synthetic motion of the transmission chain must have a certain level of accuracy in order to achieve the correct dentoform, pitch, and tooth graduation accuracy. There are two main methods for detecting transmission errors in these machine tools: indirect and direct methods.
- Indirect Method
This method involves measuring the quality of the workpiece after it has been processed according to the machine tool manual’s requirements. By measuring the workpiece, it is possible to determine if the transmission error of the machine tool’s chain has resulted in the desired accuracy level for the workpiece. This method takes into account the overall error of the production process, including errors from the transmission chain and other factors.
- Direct Method
The direct method can be further divided into two sub-methods: static measurement and dynamic measurement.
How to Detect Static Indexing Transmission Errors in Gear Hobbing Machines?
The following is a method for detecting indexing transmission errors in a vertical hobbing machine:
- Configure the machine with an indexing change gear and adjust the indexing chain so that the number of indexing teeth is equal to the number of teeth on the indexing worm.
- Mount the hob spindle with a standard dividing disk and install a reading microscope on the column to determine the rotation angle of the hob spindle.
- Place a theodolite on the worktable, or install a sight vane alidade on the machine’s outside support, or suspend a calibration hair thread to determine the rotation angle of the worktable.
- When the hob spindle rotates for a cycle, the worktable’s indexing worm wheel should rotate 360°/z.
- After the hob spindle rotates for each cycle, the theodolite will return to its original position and the actual rotation angle of the worktable will be determined by the readings of the theodolite.
- The forward and reverse rotation of the worktable should be detected once each.
- The difference between the actual rotation angle of the worktable and the theoretical rotation angle is the indexing transmission error of the machine’s indexing drive chain.
How do hobbing machines dynamically measure transmission errors?
A: Dynamic measurement of transmission errors in gear hobbing machines can be performed using the rolling principle. The inspection gear shaper is used for this purpose.
To begin, adjust the hob shaft of the hob holder so that it is parallel to the axis of the worktable. Next, install a friction disc on the hob shaft and a fixed disc with a mandrel concentrically on the table. Place a friction disc over the mandrel. The diameter ratio of the two friction discs should be equal to the transmission ratio between the hob shaft and the worktable.
As the friction disk rotates, it drives the rotation of the other friction disk through friction, creating a standard comparative mechanism. When there is no transmission error between the hob shaft and the table transmission chain, the two disks will rotate synchronously. However, when there is a transmission error, the fixed disk and the friction disk will not rotate strictly synchronously.
An inductive probe is installed between the fixed disk and the friction disk to convert the asynchronous rotation into electrical signals. These signals are then outputted, amplified, rectified, and recorded. Based on the recorded curve, the transmission error can be analyzed both qualitatively and quantitatively to determine its size and source.
How to achieve dynamic measurement of transmission errors on horizontal lathes and precision lead screw lathes?
A: Dynamic measurement of transmission errors on horizontal lathes and precision lead screw lathes can be achieved through the use of precision nuts, dial gauges, and standard screws in accordance with professional precision standards for machine tools.
A standard screw with a precision nut is tightly placed at the front and rear center. The nut should be precisely aligned with the standard screw, or with a device to adjust the clearance. The standard screw rotates, while the nut only moves axially without rotating.
A dial gauge is fixed on the slide and positioned so that its probe is on the end face of the nut. The ratio of the standard screw pitch and the machine screw pitch is taken as the transmission ratio from the spindle to the transmission screw.
To perform the test, the machine’s slit nuts are closed and the machine is started at a slow speed. The dial gauge readings are taken at lengths of 25mm, 100mm, and 300mm. The maximum difference between the readings is the transmission error.
How to detect repeat positioning error of turret lathe turret?
A: To detect the repeat positioning error of a turret lathe turret, a gaging mandrel is tightly inserted into the tooling hole on the turret. A dial gauge is fixed on the machine tool with its probe placed on the surface of the gaging mandrel from the center of the turret L (L is specified by the inspection standard), and then the turret is rotated 360° to be returned to its original position.
After clamping, the measurement is taken again and repeated 5 times. The maximum difference between the 5 continuous measurement readings is the repeat positioning error.
This process should be repeated for each tool hole in the turret.
For different shapes of guide rails, which plane straightness error should be controlled for each surface separately?
A: The two common shapes of machine tool guideways are rectangular guideways and V-shaped guideways.
For the rectangular guide rail, the straightness error in the vertical plane is controlled by the horizontal surface of the guide rail, while the straightness error in the horizontal plane is controlled by the two sides of the rectangular guide rail.
For the V-shaped guide rail, the two inclined surfaces control both the straightness error in the vertical plane and the straightness error in the horizontal plane. This is because the guide rail is composed of two inclined surfaces.
What are the common methods for detecting straightness error of guide rails?
A: The common methods for detecting straightness error of guide rails include: the lapping spots method, the horizontal ruler pull table comparison method, the mat plug method, the pulling wire detection method, the gradienter detection method, and the optical flatness gauge (autocollimator) detection method.
What is the lapping spots method?
A: The lapping spots method is a method used to test straightness error using a ruler. A thin layer of red lead oil is evenly applied to the surface of the guide rail being inspected. The ruler is then placed on the surface of the guide rail and moved short distances with appropriate pressure, creating lapping spots.
After removing the flat ruler, the distribution and sparsity of the lapping spots on the surface of the guide rail are observed. If the lapping spots are evenly distributed along the entire length of the guide rail, it indicates that the straightness error of the guide rail has met the corresponding accuracy requirements for a straight edge.
The flat ruler used for the lapping spots method should be a standard flat ruler with an accuracy level that meets the precision requirements of the guide rail being inspected. The accuracy level should not be lower than 6. The length of the ruler should be no shorter than the length of the guide rail being inspected (or 1/4 shorter in the case of lower precision requirements).
Which types of guide rail straightness error detection are suitable for the lapping spots method?
A: The lapping spots method is commonly used when fixing the straightness error of guide rails through scraping. It is often used for testing shorter guide rails because longer rulers (over 2000mm) are more prone to deformation and can affect measurement accuracy.
For shorter guide rails, the straightness error is usually guaranteed by the accuracy of the straight edge, and the density of research points in a unit area also has certain requirements. Depending on the accuracy requirements of the machine tool and the importance of the guide rail’s position in the machine tool, the number of points per scratch can be specified as not less than 10-20 points per 25mm x 25mm.
When using the lapping spots method to detect guide rail straightness errors, it cannot measure the error value of the guide rail’s straightness. Therefore, it is generally not used for the final test when a gradienter is available.
However, it should be noted that in the absence of measuring instruments (such as gradienters or optical flatness gauges), the inspection straight edge produced by the mutual research method of three straight edges can effectively meet the detection requirements for the straightness error of short guide rails in general machine tools.
Which types of guide rail straightness error detection are suitable for the horizontal ruler pull table comparison method?
A: The horizontal ruler pull table comparison method is suitable for detecting straightness error in both the vertical plane and the horizontal plane of short guide rails.
To improve measurement stability, the length of the pad moving on the guide rail should not be longer than 200mm, and the contact surface of the pad and guide rail should be scraped to ensure good contact. Otherwise, it can affect the accuracy of the measurement.
(1) Detection of straightness error in the vertical plane:
The working surface of the straight edge is placed level next to the guide rail being inspected, as close as possible to reduce the influence of guide rail distortion on accuracy. A supporting scraping pad iron is placed on the guide rail and a micrometer stand is fixed on the pad iron, with the micrometer probe positioned on both ends of the ruler surface. The straight edge is adjusted so that the readings of both ends of the ruler are equal.
The pad iron is then moved, and the micrometer value is read every 200mm. The maximum difference between the readings of the dial indicator is the straightness error within the entire length of the guide rail. During measurement, to avoid the impact of scratching points on accuracy, it is best to place a measurement block under the micrometer probe.
(2) Detection of straightness error in the horizontal plane:
The working surface of the flat ruler is placed next to the guide rail being inspected, and the straight edge is adjusted so that the readings of the micrometer on both ends of the ruler are equal. The measurement method and calculation of error are the same as described above.
Which types of guide rail straightness error detection are suitable for the mat plug method?
A: The mat plug method is suitable for plane guide rails that have been ground to low surface roughness.
A standard straight edge is placed on the guide rail being inspected, and two equal height pads are supported under the straight edge at a distance of 2/9 of the length (L) from both ends of the straight edge. The clearance between the working surface of the straight edge and the measured surface of the guide rail is checked using shims or a feeler gauge.
For example, if the straightness tolerance of a horizontal lathe guide rail is (1000:0.02) mm, it means that any place on the guide rail within a length of 1000mm cannot be inserted with a shim or feeler gauge with a thickness equal to the height of the equal height pad plus 0.02mm.
When measuring precision machine tool guide rails, it is recommended to use shims with higher accuracy to correctly measure the straightness error value of the guide rail.
A dial gauge can also be used instead of a feeler gauge, but the height of the equal height pads should be increased to allow the dial gauge to enter the measurement mode.
What are the key features and considerations for using the pulling wire method to detect straightness errors in guide rails?
A: The pulling wire method uses a tight steel wire as a reference line to directly measure the straightness error of each component on the guide rail. This method is similar to the horizontal ruler pull table comparison method.
It should be noted that this method only detects straightness errors in the guide rail’s horizontal plane. To perform the measurement, a 500mm pad iron is placed in the machine tool’s slideway, and a graduated reading microscope is installed vertically on the pad iron, aimed at the wire.
At both ends of the guide rail, a small sheave is fixed, and a wire with a diameter less than 0.3mm is attached to it. One end of the wire is fixed to the small pulley, while the other end is suspended by a heavy hammer, with the weight of the hammer being 30-80% of the wire’s pulling force.
The wire is adjusted until it aligns with the engraved line on the microscope lens at both ends of the guide rail. The reading on the microscope’s movable splitter handwheel is recorded. The pad iron is then moved and the microscope is observed every 500mm to ensure the wire continues to align with the scribe. If there is no overlap, the handwheel on the reading microscope is adjusted to make it overlap, and the reading is recorded.
Measurements are taken along the entire length of the guide rail and recorded in order. The readings are then plotted on a coordinate paper and a graph of the pad iron’s movement is drawn. The maximum coordinate difference between the movement curve and the line connecting the two endpoints at each 1000mm length represents the straightness error over 1000mm.
If the formed curve is convex or concave, the straightness error over the full length of the guide rail is the coordinate difference between the most convex or concave point and the two ends of the connection. If the curve is a folded line, with points on both sides of the abscissa axis, the straightness error over the full length can be determined using the inclusive line method, which involves taking the coordinate difference between the two parallel lines with the smallest distance.
What should be taken into account when plotting the straightness error curve of a guide rail?
A: When plotting the straightness error curve of a guide rail, the size of the chart should be appropriate. If it’s too large, it can be difficult to draw and modify, and if it’s too small, the straightness error calculation of the guide rail may be inaccurate.
It’s recommended that when measuring medium to small machine tools, the length of the x-axis should be 200mm, for longer lathes it should be 200-400mm, and for extra-long machine tools, it shouldn’t exceed 500mm.
Once the length of the x-axis is set, the scale of the x-axis can also be determined.
In reality, the 0 to x-axis represents the spacing of the gradienter pad iron, which can be drawn at a ratio of 1:5 to 1:10, meaning that a measurement distance of 200mm is represented by 40 or 20mm on the chart.
The z-axis and y-axis represent the precision error of the guide rail, which can be scaled from 1000:1 to 2500:1, i.e. the accuracy error of 1μm is represented by coordinates of 1 to 2.5mm.
For special precision machine tool guide rails (generally shorter), a scale of 5000:1 can be used, meaning that an error of 1μm is represented by 5mm on the chart.
For example, if the rail of a machine tool is 1000mm long and the specification of the gradienter is 0.02/1000, with a gradienter pad iron length of 200mm, the reading will be zero when the gradienter is placed on the 0-200mm section of the guide rail surface.
When the gradienter moves forward to the 200-400mm section of the guide rail surface, the bubble in the gradienter will move forward one frame with a positive value, indicating that this section of the guide rail surface is inclined upward by 0.004mm.
When the gradienter is moved to the 400-600mm section of the guide rail surface, if the bubble of the gradienter moves backward to the zero position, it means that this section of the guide rail surface is parallel to the 0-200mm section.
Since the 200-400mm section of the guide rail surface is inclined upward, the 0-200mm and 400-600mm sections are parallel in different planes.
When the gradienter moves to the 600-800mm section of the guide rail surface, the bubble in the gradienter will move backward one frame with a negative value, indicating that the plane of this section of the guide rail is inclined downward.
The last section of 800-1000mm will return to the zero position, so if the measurement continues until the entire guide rail is measured, the curve formed by each line segment is called the guide rail straightness error curve (or measuring tool movement curve).
What are the features of straightness error measurement of guide rails using optical instruments?
A: The method of measuring straightness error of guide rails using collimators and automatic collimators (optical flatness gauges) is based on the fact that the beam motion is a straight line.
The advantages of using optical instruments for measurement are:
(1) The instrument’s accuracy is less affected by external conditions such as temperature and vibration during the measurement process, resulting in higher measurement accuracy.
(2) It can measure not only the straightness error in the vertical plane (uneven level) like a gradienter, but also replace wire and microscopes in measuring straightness error in the horizontal plane.
Therefore, it has become a commonly used tool in the manufacture and repair of machine tools.
However, for measuring guide rails longer than 10 meters, the image becomes unclear due to the larger light energy loss over the longer distance of the beam, so it cannot be measured directly and must be measured in sections.
How to identify surface distortion error in a single guide rail?
A: In addition to having straightness requirements in both the horizontal and vertical planes, the surface shape of each guide rail must also be controlled for distortion error, especially for large guide rails, to ensure proper cooperation between the guide rail and moving parts and to improve contact rate.
During scraping, to measure parallelism error between guide rails, it’s important to prevent significant distortion in the guide rail used for reference measurement.
The method for identifying surface distortion error in a single guide rail: For a V-shaped guide rail, a V-shaped gradienter pad block is used, and for a flat guide rail, a flat pad block is used.
Starting from either end of the guide rail, the gradienter pad block is moved and the value is read every 200-500mm. The largest algebraic difference of the gradienter readings represents the twisting error of the guide rail.
This error is specified in machine tool accuracy standards, mainly in the scraping and grinding process.
What are the requirements for measuring flatness errors of a machine worktable?
A: In the past, the method for detecting flatness errors of a machine worktable was the “two-point method,” which involved using tools such as a parallel motion protractor, dial gauge, contour block, and frame-type level to measure straight parts along the worktable and determine the maximum straightness error of any cross-section.
However, this method is inconsistent with the definition of flatness in the national standard for geometric tolerances. The current JB2670-82 “General Rules for Inspection of Metal Cutting Machine Tools” states that the measurement for checking flatness error is the distance between two parallel planes, which is the smallest distance that accommodates the actual surface.
This definition highlights the importance of including the plane as an evaluation benchmark and determining its location based on minimum conditions.
Therefore, it’s necessary to thoroughly understand the error situation of the measured surface first and then determine the evaluation benchmark based on certain criteria.
This means that the measurement must be divided into two steps: first, measure the height of several points on the actual surface relative to an ideal plane (measurement reference), and then use methods such as datum conversion to obtain the error value in accordance with the definition.
How to perform and evaluate flatness error when measuring with an indicator?
A: When measuring flatness error with an indicator, the part to be measured is placed on a plate, with the working surface of the plate serving as the measuring datum.
During measurement, the height of the three farthest points on the actual measured surface is usually adjusted to be the same as the plate (leveling), and the algebraic difference between the maximum and minimum readings measured by the indicator represents the flatness error evaluated using the three-point method.
Alternatively, the height of the two ends of one diagonal line on the actual surface can be adjusted to be equal to the plate, and then the height of the two ends of the other diagonal line can be adjusted to be equal to the plate. The algebraic difference between the maximum and minimum readings measured by the indicator represents the error evaluated using the diagonal method.
However, this leveling is more difficult, and the measured surface can be measured according to a certain wiring pattern and the readings recorded at the same time.
Typically, the algebraic difference between the maximum and minimum readings can be used as the error value. If necessary, the readings from each measurement point can be processed based on minimum conditions to determine the error value.
What should be considered when measuring flatness errors with a gradienter?
A: The flatness error measured by a level meter is based on the natural level as the measurement reference. Before measurement, the actual surface to be measured should be adjusted to approximate level.
The gradienter is placed on the slab bridge, and then the slab bridge is placed on the surface to be inspected.
The surface to be inspected is measured point by point following a certain routing pattern, and the readings of each measurement point (grid number) are recorded. The grid number is then converted to a line value.
Based on the measured readings (line value), the flatness error can be obtained through data processing.
This method can be used to measure the flatness of large surfaces.
What should be considered when measuring flatness error with an autocollimator?
A: When measuring flatness error with an autocollimator, the autocollimator is positioned outside the part to be measured on a base, and a reflector is installed on the slab bridge, which is then placed on the surface to be measured.
Before measurement, the autocollimator should be adjusted to be roughly parallel to the surface to be measured.
First, the readings of the measurement points on one diagonal are taken using the method for measuring straightness error based on the wiring pattern of the meter type. Then, readings of the points on another diagonal and the rest of the cross-section points are taken, and these readings are converted to line values.
Based on the measured readings (line value), the ideal plane is determined using the intersection of the two diagonals according to the diagonal law, and the flatness error value is obtained based on this ideal plane.
If necessary, the error value can be further determined based on minimum conditions.
Please note that this method cannot be used to measure the flatness of large surfaces.
How to evaluate flatness errors when measuring with an optical flat?
A: When using an optical flat to measure a small surface, the optical flat is placed on the surface to be measured during measurement. If the surface to be measured is concave or convex, a circular interference band will appear.
The flatness error is calculated based on the number of ring fringes and the half-wavelength of the light wave.
If the interference fringes are not closed, the optical flat and the surface to be measured can be slightly inclined at an angle to form an air wedge between them.
The error value is calculated as the ratio between the curvature of the fringe and two adjacent fringes multiplied by the half-wavelength.
However, it’s important to note that the error assessed using this method is actually a straightness error, not a flatness error.
In the past, optical flats could only be used to measure small surfaces. However, in recent years, the development of flat interferometers allows for larger surfaces to be measured using flat interferometry.
How to detect the perpendicularity error of the horizontal milling head spindle axis due to the movement of the milling planer worktable?
A: The method for detecting the perpendicularity error of the horizontal milling head spindle axis caused by the movement of the worktable on the milling planer involves the following steps:
- The horizontal milling head is fixed in a position close to the worktable surface, the spindle sleeve is clamped, and the rotatable angle of the milling head is adjusted to zero position.
- The worktable is moved to the middle of the guide rail, which is located at a distance of L/2 from the spindle axis. A special slider is placed in the T-slot of the worktable, so that the slider flange is close to the side of the central T-slot.
- An angle bar is fixed on the spindle, and the probe of a dial gauge is placed on the side of the slider. Readings are taken and recorded.
- The slider does not move as the worktable is moved a distance of L (which is specified in the accuracy inspection standards for machine tools based on the machine tool’s specifications). The worktable is then rotated 180 degrees, and the dial gauge is used to take another reading.
- The maximum difference between the two readings taken by the dial gauge represents the perpendicularity error.
How to detect spindle cone hole oblique circular runout error?
A: The spindle cone hole oblique circular runout error can be detected by fixing a dial test indicator on the machine tool and placing the micrometer probe on the inner surface of the spindle cone hole. The micrometer probe should be perpendicular to the inner surface of the cone hole. The spindle is then rotated, and the maximum difference between the readings taken by the micrometer is the oblique circular runout error.
This method is commonly used to inspect the main spindle cone hole of an internal grinding head.
How to detect the radial and oblique runout errors of spindle centering journal?
A: The spindle of a machine has various centering methods to ensure that the workpiece or tool remains stable during rotation, so it requires that the surface of the centering journal and the spindle axis of rotation are coaxial. To detect this coaxiality error, the radial and oblique circular runout errors are measured.
A dial gauge (or a micrometer for high accuracy measurements) is fixed on the machine and the dial gauge probe is held against the surface of the spindle centering journal (if it is a conical surface, the probe should be perpendicular to it). The spindle is then rotated and the maximum difference between the readings taken by the dial gauge or micrometer is the radial and oblique circular runout error of the centering journal. The measurement is performed in the direction of the surface being measured.
How should the straightness error be detected when the lathe slide carriage moves in the vertical plane?
A: Straightness error detection of lathe slide carriage moves in the vertical plane: shake the handle, move the frame to the central line, a gradienter is placed on the slide carriage close to the knife rest on an equal footing with the lathe guide rail.
Begin by positioning the slide carriage near the spindle box and recording the gradienter reading.
As the slide carriage moves in the direction of the tailstock, record the gradienter reading at least four times along the entire travel of the slide carriage, every time the slide carriage moves 500mm or less.
Organize the gradienter readings in sequence to create a movement curve for the slide carriage.
The straightness error along a 1000mm travel of the slide carriage is the maximum coordinate value between the movement curve and the line connecting the two endpoints.
The straightness error for the entire stroke is the maximum coordinate value between the movement curve and the line connecting the two ends of the motion curve.
Note: Prior to testing the machine’s motion accuracy, ensure that the machine is properly level.
Place the lathe test slab bridge on the lathe guide rail, and place two gradienters on the slab bridge, one parallel to the guide rail and the other perpendicular to it.
Check the level of the machine tool at both ends of the guide rail, and ensure that the readings of both levels do not exceed the specified value.
For high precision machines, the specified value is (1000:0.02)mm. For common precision machines, the specified value is (1000:0.04)mm.
How to detect straightness errors in the vertical plane when the gantry planer table moves?
To detect straightness errors of the gantry planer worktable moving in the vertical plane, place a gradienter parallel to the direction of the worktable’s movement at the central position of the worktable surface.
Move the worktable from one extreme position to the other, recording the gradienter’s reading every 500mm (or less) after each movement.
Record the gradienter’s readings in each position throughout the worktable’s entire travel.
Sequence the gradienter’s readings to draw the worktable’s motion curve.
The maximum coordinates of the movement curve at the two endpoints of the 1000mm travel length indicate the straightness error on the 1000mm length.
Create mutually parallel straight lines to encompass the movement curve, and the coordinates of the two parallel lines with the smallest distance represent the guide rail’s straightness error on the entire travel length.
How to detect the perpendicularity error of a horizontal milling head spindle axis when the worktable moves on the milling planer?
A: To detect the perpendicularity error of the horizontal milling head spindle axis, follow these steps:
- Fix the horizontal milling head close to the working table surface, clamp the spindle sleeve, and adjust the milling head to the zero position.
- Move the worktable to the middle of the guide rail, which is L/2 away from the spindle axis. Put a special slider in the T-slot of the table with its flange close to the side of the central T-slot.
- Fix an angular table rod on the spindle and place the dial gauge measuring head on the side of the slider. Record the meter reading.
- Without moving the slider, move the table a distance of L (determined by the machine tool accuracy testing standards for different machine tool specifications). Then, place the spindle 180° and inspect the dial gauge by touching it to the side of the slider again.
- The maximum difference between the two readings of the dial gauge is the perpendicularity error.
