Turning is a machining process performed on a lathe, which is a crucial aspect of mechanical manufacturing. In this process, a rotating workpiece is shaped using a cutting tool called a turning tool. Lathes are primarily employed to work on components such as shafts, discs, and sleeves, which feature rotating surfaces. They are the most commonly used type of machine tool in both manufacturing and repair workshops.
There is no end to the skills of lathe operators. The most common lathe operators do not need too high skills. They can be divided into five categories, which are the most common in society.
1. Ordinary mechanical lathe operators, simple and easy to learn, CNC lathe processing factory.
2. Mold lathe operators, especially those specializing in plastic mold precision lathes, must adhere to strict requirements regarding tools and precise dimensions. It is important to understand which types of steel provide a good polishing effect, achieving a mirror finish. You should also know whether the product from this set of molds is made from ABS or another type of plastic, as well as the elasticity of the plastic parts measured in millimeters. Additionally, there are many other common knowledge aspects to consider. The finished parts should have a high-quality finish, be easy to polish, and attain a mirror-like effect. A solid foundation in plastic mold principles is essential.
3. Tool lathe operators work with various tools such as reamers, drills, alloy cutters, and tool stems. This type of lathe operation is often the simplest yet can be quite tiring. Typically, these tools are mass-produced, with double center, taper, and flow modulus being among the most common types.To achieve the fastest and most efficient results, it is essential to minimize tool wear. The hardness of the materials processed by this type of lathe operator is nearly as high as that of a quality white steel knife. The quality of alloy knife grinding is directly linked to the quality of the finished products.
4. Large equipment lathe operator: this type of lathe operator must have senior skills, and more teaching is required when using a vertical lathe.
Example: To turn a crankshaft, you must first read the drawing repeatedly many times, which one to process first and which one to process later, whether to lose grinding or directly process to size, whether the thread is positive or negative…
5. CNC lathe operator: this type of lathe operator is the simplest and the most difficult. First of all, you must be able to read drawings, program, convert formulas, and tool applications.
Turning to process
The lathe is a machine tool used to rotate a workpiece, while allowing for either linear or curved motion of the cutting tool. This process changes the shape and size of the workpiece to meet specific design requirements.
Turning is a cutting method performed on a lathe, where the workpiece rotates in relation to the tool. In this process, the cutting energy is primarily supplied by the rotating workpiece rather than the cutting tool itself. Turning is one of the most basic and widely used machining methods and holds a crucial role in manufacturing. It is particularly suitable for creating rotating surfaces. Most workpieces with such surfaces can be processed using turning techniques, including internal and external cylindrical surfaces, internal and external conical surfaces, end faces, grooves, and threads, as well as other rotating forms. The primary tools used in this process are turning tools.
Among various types of metal cutting machine tools, lathes are the most commonly used, making up about 50% of the total number of machine tools. Lathes can not only utilize turning tools for shaping workpieces, but also employ drills, reamers, taps, and knurling tools to perform operations such as drilling, reaming, tapping, and knurling. Depending on their process characteristics, layout forms, and structural features, lathes can be categorized into several types, including horizontal lathes, floor lathes, vertical lathes, turret lathes, and copy lathes, with horizontal lathes being the most prevalent.
Safety and technical issues
Turning is the most commonly used type of machining in the manufacturing industry. It involves various lathes, a significant number of personnel, a wide range of machining capabilities, and numerous tools and fixtures. As a result, safety and technical issues related to turning are particularly important to address. The key tasks are as follows:
1. Damage caused by chips and protective measures.
Various steel parts that are processed on lathes exhibit good toughness. During the turning process, the chips produced are typically plastic and curled, with relatively sharp edges. When cutting steel parts at high speeds, red-hot and elongated chips can form, posing a risk of injury. These chips can easily become entangled with the workpiece, cutting tool, and tool holder.
To ensure safety, iron hooks should be used frequently to clean or break the chips as needed. If necessary, the machine should be stopped to clear the chips, and they must never be removed or broken by hand. To prevent chip damage and breakage, chip flow control measures and protective baffles are commonly implemented. Effective chip-breaking strategies include grinding chip breaker grooves or steps on the turning tool, using appropriate chip breakers, and mechanically clamping the tool.
2. Clamping of workpieces.
During the turning process, various accidents can occur that may damage the machine tool, break or strike the tool, and cause the workpiece to fall or fly out, potentially injuring personnel due to improper clamping. To ensure safe production during turning operations, it is crucial to pay special attention to workpiece clamping.
For parts of different sizes and shapes, appropriate clamps must be selected. Whether using a three-jaw chuck, a four-jaw chuck, or a specialized clamp, the connection to the spindle must be stable and reliable. The workpiece should be clamped securely and tightly to prevent any movement.
For larger workpieces, a sleeve can be utilized to ensure that the workpiece does not shift, fall off, or become dislodged while rotating at high speeds and under cutting forces. If necessary, clamping can be further reinforced by using a center or center frame. Additionally, it is important to remove the wrench immediately after clamping to avoid any accidents.
3. Safe operation.
Before using the machine tool, it must undergo a complete inspection to ensure it is in good condition. The workpiece and tool must be clamped securely in the correct position to guarantee stability and reliability. The machine must be stopped when changing tools, loading and unloading the workpiece, or measuring the workpiece.
It is important not to touch or wipe the rotating workpiece by hand or with cotton thread. The cutting speed, feed rate, and working depth should be chosen appropriately, and overload processing is not permitted. Additionally, workpieces, fixtures, and other items should not be placed on the machine head, tool holder, or bed surface.
When using a file, move the turning tool to a safe position. Keep your right hand in front and your left hand behind to prevent your sleeves from getting caught. A designated individual should be responsible for the use and maintenance of the machine tool; unauthorized personnel are not allowed to operate it.
Notes
The processing technology of a CNC lathe is similar to that of an ordinary lathe, but since the CNC lathe is clamped once and completes all turning processes continuously and automatically, the following aspects should be noted.
1. Reasonable selection of cutting parameters
For efficient metal cutting, three major factors must be considered: the material being processed, the cutting tools, and the cutting conditions. These factors influence processing time, tool life, and processing quality. An economical and effective processing method requires careful selection of cutting conditions.
The three key elements of cutting conditions—cutting speed, feed rate, and cutting depth—directly affect tool damage. As cutting speed increases, the temperature of the tool tip rises, leading to mechanical, chemical, and thermal wear. For instance, a 20% increase in cutting speed can result in a tool life reduction of half.
The relationship between feed conditions and tool back wear occurs over a very small range. However, if the feed rate is too high, the cutting temperature increases, which can lead to significant back wear. While cutting depth has less impact on the tool than cutting speed and feed rate, cutting at a shallow depth can cause the material to form a hardened layer, which can negatively affect tool life.
When choosing a cutting speed, users should consider the material being processed, its hardness, the current cutting condition, the type of material, feed rate, and cutting depth. The most suitable processing conditions depend on these factors, with the ideal scenario being regular and stable wear that maximizes tool life.
However, in practice, tool life is also influenced by factors such as tool wear, changes in workpiece size, surface quality, cutting noise, and processing heat. Therefore, processing conditions should be analyzed based on the actual situation. For challenging materials like stainless steel and heat-resistant alloys, coolants may be utilized, or tools with enhanced rigidity may be selected.
General Code
General process code for turning (JB/T9168.2-1998)
Clamping of turning tools
1) The tool bar should not extend too far from the tool holder. Generally, the length should not exceed 1.5 times the height of the tool bar (except for turning holes, grooves, etc.)
2) The center line of the tool bar should be perpendicular or parallel to the direction of cutting.
3) Adjustment of tooltip height:When turning end faces, conical surfaces, threads, forming surfaces, and cutting solid workpieces, the tooltip should generally be at the same height as the workpiece axis.When rough-turning outer circles and finishing holes, the tool tip should generally be higher than the workpiece axis.When turning slender shafts, rough turning holes, and cutting hollow workpieces, the tooltip should generally be slightly lower than the workpiece axis.
4) The bisector of the tooltip angle of the thread-turning tool should be perpendicular to the workpiece axis.
5) When clamping the turning tool, the gasket under the toolbar should be few and flat, and the screws that hold the turning tool down should be tightened.
Workpiece clamping
– When clamping a workpiece with a three-jaw self-centering chuck for either rough or fine turning, if the diameter of the workpiece is less than 30 mm, the overhang length should not exceed five times the diameter. If the workpiece diameter is greater than 30 mm, the overhang length should not exceed three times the diameter.
– When clamping irregular and heavy workpieces using a four-jaw single-action chuck, faceplate, angle iron, or bent plate, it is essential to add a counterweight.
– When machining CNC shaft machining workpieces between centers, ensure that the axis of the tailstock center is aligned with the axis of the lathe spindle before beginning the turning process.
– For slender shafts being machined between two centers, a follower rest or center rest should be utilized. During the machining process, pay attention to adjusting the top tightening force of the center and ensure proper lubrication of both the dead center and the center rest.
– When using the tailstock, extend the sleeve as little as possible to minimize vibration.
– When clamping a workpiece with a small support surface and a tall height on a vertical lathe, use a heightened clamping jaw. Additionally, a tie rod or pressure plate should be added at an appropriate position to secure the workpiece.
– When turning wheel-type and sleeve-type castings and forgings, align the unprocessed surface correctly to ensure uniform wall thickness of the workpiece after processing.
Turning
– When turning a stepped shaft, it is generally recommended to turn the part with the larger diameter first to ensure rigidity, followed by the part with the smaller diameter.
– To prevent deformation of the workpiece, cutting grooves should be done before fine turning the shaft.
– When fine turning a shaft that has threads, the non-threaded portion should typically be fine-turned after processing the threads.
– Before drilling, the end face of the workpiece must be turned flat. If needed, the center hole should be drilled first.
– When drilling deep holes, it is advisable to drill a guide hole first.
– For a hole with a diameter between 10 mm and 20 mm, the diameter of the cutting tool should be 0.6 to 0.7 times the diameter of the hole being processed. When processing a hole with a diameter greater than 20 mm, a cutting tool with a clamping head should generally be used.
– When turning multiple threads or worms, a trial cut should be performed after adjusting the exchange gear.
– When using an automatic lathe, adjust the relative position of the tool and workpiece according to the machine tool adjustment card. After making the adjustments, carry out a test turn. Production can only proceed once the first piece is confirmed to be qualified. Continuously monitor the tool wear and the size and surface roughness of the workpiece during processing.
– When turning on a vertical lathe, once the tool holder is adjusted, avoid moving the crossbeam without proper cause.
– If there are positional tolerance requirements for relevant surfaces of the workpiece, aim to complete the turning process in a single clamping.
– When turning a cylindrical gear blank, both the hole and the reference end face must be processed in a single clamping. If necessary, a marking line should be turned near the gear pitch circle on the end face.
Common Problems
When ordinary lathes are used to cut large-pitch threads under significant force, the bed saddle may sometimes vibrate. This vibration can lead to ripples on the machined surface or even cause tool breakage. Students often face issues such as tool piercing or breaking while cutting. There are several factors that contribute to these problems. This paper primarily discusses this phenomenon and proposes solutions by analyzing the forces acting on the tool.
The cause and cause of the problem
When machining threads with smaller pitches, the straight-feed cutting method is typically employed. This method involves cutting in a direction that is perpendicular to the axis of the workpiece. On the other hand, for threads with larger pitches, the left-right borrowing cutting method is often used to reduce cutting force. This technique allows the small slide to move, enabling the thread turning tool to cut with both the left and right cutting edges alternately.
When turning threads, the movement of the bed saddle is controlled by the rotation of a long lead screw, which drives the motion of the split nut. There is an axial gap at the bearing of the long lead screw, as well as an axial gap between the long lead screw and the split nut. When the right-handed worm is rotated using the left and right cutting method, the tool experiences a force (P) exerted by the workpiece (ignoring the friction between the chip and the front cutting edge, as illustrated in Figure 1). This force (P) can be decomposed into an axial force (Px) and a radial force. The axial force (Px) aligns with the feed direction of the tool. The tool transmits this axial force (Px) to the saddle, causing it to move rapidly and forcefully toward the side with the gap. As a result, the tool oscillates back and forth, leading to a corrugated machined surface and, in some cases, tool breakage.
When using the left main cutting edge, there is no observed phenomenon affecting the cut. Instead, the axial force ( P_x ) experienced by the tool is directed opposite to the feed direction, acting to eliminate any gap present. During this CNC machining process, the saddle maintains a uniform speed.
The movement of the middle slide occurs through the rotation of the screw, which drives the movement of the nut. However, there is an axial gap at the screw bearing, as well as an axial gap between the screw and the nut.
While cutting on a lathe, the front cutting edge of the tool (which has a front angle) experiences a force ( P ) exerted by the workpiece. For the sake of simplicity, we will ignore any friction between the chip and the front cutting edge, as illustrated in Figure 2.
The force ( P ) can be decomposed into two components: ( P_z ) (the axial component) and ( Q ) (the radial component). The radial component force ( Q ) aligns with the cutting tool’s feed direction, pushing the tool into the workpiece. This action causes the middle slide to move toward the gap, which can lead to the cutting tool suddenly piercing the workpiece. Consequently, this piercing may result in the tool breaking or the workpiece bending.
Solution
When turning threads with large pitches and using the left and right borrowing tool cutting method, it’s important to adjust not only the relevant parameters of the lathe but also the matching clearance between the saddle and the bed rail. This clearance should be set slightly tighter to increase friction during movement and reduce the chance of saddle movement. However, it is crucial that this clearance is adjusted appropriately so the saddle can still move smoothly. Additionally, minimize the clearance of the middle slide.
For the small slide, adjust its tightness to be slightly tighter as well, which will help prevent the tool from shifting during turning. To enhance stability, shorten the length of the workpiece and the toolbar as much as possible. When cutting, use the left main blade whenever feasible. If cutting with the right main blade, reduce the back-cutting amount and increase the rake angle of the right main blade. Ensure the blade edge is straight and sharp to minimize the axial component force (Px) experienced by the tool. A larger rake angle for the right main blade results in better performance.
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