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Cnc Turning

What is Cnc Turning?

Cnc Turning refers to lathe machining, which is part of machining. Lathe machining mainly uses lathe tools to rotate workpieces. Lathes are primarily used for machining shafts, discs, sheaths and other workpieces with rotating surfaces. They are the most widely used type of machine tools in machine manufacturing and repair shops.

Technical briefs

It is on the lathe that the shape and size of the blank is changed by the rotational movement of the workpiece and the linear or curved movement of the tool and is machined to meet the requirements of the drawing.

A lathe is a method of cutting a workpiece by rotating the workpiece against a tool on the lathe. The cutting energy of turning is mainly provided by the workpiece, not the tool. Lathes are the most basic and common cutting method and occupy a very important position in production. Suitable for rotary machining. Many workpieces with a rotating surface can be processed by rotary machining. Tools such as inner / outer cylindrical surfaces, inner / outer conical surfaces, end surfaces, grooves, threads, and rotomized surfaces are mainly rotary tools. Lathes are the most widely used in all types of metal cutting machine tools, accounting for about 50% of the total number of machine tools. You can use a lathe to rotate a workpiece with a turning tool, but you can also use a drill, reamer, tap, or knurling tool to drill, ream, tap, or knurl. Lathes can be divided into horizontal lathes, floor lathes, vertical lathes, turret lathes, and copy lathes, depending on various characteristics of process, layout, and structural characteristics.

Technical problem

Turning is the most widely used type in the machine manufacturing industry. The number of lathes is large, the number of people is large, and the Machining range is wide. Due to the large number of tools and fixtures used, rotational safety and technical issues are of particular importance. The main tasks are: Drilling

1.Chip damage and protective measures.

The steel parts machined on the lathe are more flexible and the chips generated during rotation are plastic crimped and have sharp edges. High-speed cutting of steel parts creates red hotspots and long swuffs that are very vulnerable and are often wrapped around workpieces, rotary tools and tool holders. Therefore, the hooks need to be cleaned and pulled in a timely manner while working. If necessary, you need to stop and clear. However, do not remove it by hand. Chip breakage, swaf flow control, and various protective baffles are often employed to prevent chip breakage. Tip breakage countermeasures are to grind the steps of a tip breaker or turning tool. Use a suitable tip breaker to mechanically clamp the tool.\

2.Work mounting.

During turning, many accidents occur, including improper installation of the feature, damage or breakage of the tool, and damage to the machine tool due to the fall or popping out of the workpiece. Therefore, special care must be taken when loading workpieces to ensure safe production of lathes. For parts of different sizes and shapes, you must use the appropriate fixtures, regardless of the connection of the 3 jaws and 4 jaw chucks or the special fixtures and spindles. The workpiece must be securely clamped and clamped, and large workpieces can be clamped into sleeves.

When the work rotates at high speed and cutting force is applied, the work does not move, fall off, or fly. If necessary, it can be reinforced by an upper center and a central frame. Lock immediately after removing the spanner.

3.Safe operation.

Before working, the machine should be thoroughly inspected to make sure it is intended for use. Installing the workpiece and tools ensures the correct position, certainty and secure position. During the machining process, tools must be stopped when changing tools, loading and unloading workpieces, and measuring workpieces. Do not touch the work piece with your hands or wipe it with cotton when rotating it.

Overload machining is not allowed in order to properly select the cutting speed, feed rate and stress depth. Workpieces, work clamps, and other debris are not placed on the surface of the tool rest and bed. When using the crusher, move the rotating tool to a safe position with your right hand on the front and your left hand on the back to prevent the sleeves from getting caught. Machine tools require a person responsible for their use and maintenance and should not be used by anyone else.

Precautions

The CNC lathe Machining process is similar to a regular lathe. However, since the CNC lathe is installed once and all lathe work is completed automatically, the following points should be noted.

1.Reasonable choice of cutting amount:

The materials to be machined, the cutting tools, and the cutting conditions are the three main factors for high-efficiency metal cutting. These determine Machining time, tool life and Machining quality. An economical and effective machining method must be a rational choice of cutting conditions. The three elements of cutting conditions: cutting speed, feed rate and cutting depth directly cause tool damage. As the cutting speed increases, the temperature of the tooltip rises, resulting in mechanical, chemical and thermal wear. Cutting speed is increased by 20% and tool life is reduced by half.

Feed conditions and wear behind the tool are generated within a very small range. However, the feed rate is high, the cutting temperature rises, and the rear surface wear is large. It has less effect on the tool than the cutting speed. Effect of deep cutting on cutting tools There is no cutting speed and feed rate, but if the cutting depth is small, the work material has a hardened layer, which also affects the life of the cutting tool. The user selects the cutting speed to be used based on the material to be processed, hardness, cutting conditions, material type, feed amount, cutting depth, and the like. The selection of the optimum Machining conditions is selected based on these factors. Average and stable wear is an ideal condition for achieving life expectancy.

However, in actual operation, the choice of tool life is related to tool wear, machine size to be processed, surface quality, cutting noise, and Machining heat. When deciding the Machining conditions, it is necessary to conduct a survey that is in line with the actual situation. For difficult-to-machine materials such as stainless steel and heat-resistant alloys, coolants and rigid blades can be used.

2.Reasonable choice of tools:

  • (1) Select a tool with high strength and good durability to meet the requirements of a large cutting depth during rough cutting and a large feed amount during rough cutting.
  • (2) After turning is completed, it is necessary to select a tool with high accuracy and good durability in order to ensure machining accuracy.
  • (3) Clamps and blades should be used as much as possible to reduce tool change time and facilitate tool setting.

3. Reasonably select jig: Jig

  • (1) Use the universal fixture to clamp the workpiece so that no special fixture is used.
  • (2) The component positioning criteria are matched to reduce positioning error.

4. Determine the Machining route:

The Machining route is the trajectory and direction of the tool with respect to the machining part of the index controlled machine tool.

  • (1) It must be possible to ensure the requirements for Machining accuracy and surface roughness.
  • (2) It is necessary to shorten the machining path as much as possible in order to shorten the idle running time of the tool.

5. Machining route and Machining allowance:

In general, excess margins for blanks, especially margins for forged and cast hardcovers, should be machined on regular lathes, provided that CNC lathes have not reached universal use. If you need to use CNC lathe machining, you need to be careful about the flexible placement of the program.

6. Jig mounting point:

The connection between the hydraulic chuck and the hydraulic clamp cylinder is achieved by a pull rod. The main points of the hydraulic chuck clamp are as follows.

First, remove the nut of the hydraulic cylinder by hand, remove the pull tube, and pull the chuck out from the rear end of the spindle. Then remove the chuck fixing screw to remove the chuck.

Tool settings

1) The tool shank extends from the tool holder and should not be too long. The general length should not exceed 1.5 times the height of the toolbar (excluding holes, slots, etc.)

2) The centerline of the lathe tool holder must be perpendicular or parallel to the cutting direction.

3) Adjusting the height of the tip of the knife:

  • (1) When rotating a surface, rotating a conical surface, turning a screw, rotating a molded surface, and cutting a solid workpiece, the cutting edge should generally be at the same height as the axis of the workpiece.
  • (2) Roughing wheels, finishing holes, and tool tips should generally be slightly higher than the workpiece axis.
  • (3) When cutting a thin shaft, rough hole, or hollow work, the tool tip must be slightly lower than the work shaft.
  • 4) The bisector of the thread angle of the thread cutting tool must be perpendicular to the geographic axis.
  • 5) When inserting the turning tool, use a small and flat washer under the tool bar, and tighten the screw that pushes the turning tool.

Assembling the workpiece

  • 1) When clamping a workpiece for roughing or finishing using a 3-jaw automatic centering chuck, if the workpiece diameter is less than 30 mm, the overhang length must not exceed 5 times the diameter. .. If the diameter of the workpiece exceeds 30 mm, its overhang length must not exceed 3 times the diameter.
  • 2) When clamping irregularly weighted workpieces using 4 jaws, single action chucks, face plates, angle irons (bending plates), etc., additional weight is required.
  • 3) When machining a shaft type workpiece at the tip of the shaft, adjust the central axis of the tailstock so that it coincides with the axis of the lathe spindle before rotation.
  • 4) If you want to machine a thin shaft between two centers, you need to use a tool holder or a center frame. Care must be taken to adjust the best tightening during the process. Care must be taken to lubricate the top dead center and center frame.
  • 5) When using tailstock, the sleeve should be stretched as short as possible to reduce vibration.
  • 6) When clamping a vertical lathe, it is necessary to have a small support surface, a high workpiece height, a high claw height, and press with a pull rod or press plate in an appropriate place.
  • 7) When forging by turning the wheel, it is necessary to correct the unfinished surface in order to make the wall thickness after Machining uniform.

Turning

  • 1) When rotating the step shaft, it is generally necessary to rotate the large diameter part first and the small diameter part later in order to ensure rigidity during turning.
  • 2) If the shaft has cutting grooves in the workpiece, it should be performed before finishing to prevent deformation of the workpiece.
  • 3) If you turn the screw shaft finely,
    After threading, the non-threaded part rotates precisely.
  • 4) Before drilling, the end face of the work must be flattened. If necessary, first drill the center hole.
  • 5) When drilling a deep hole, the tip hole is generally drilled first.
  • 6) When rotating a (Φ10-Φ20) mm hole, the diameter of the tool shank must be 0.6-0.7 times the diameter of the machined hole. If the hole diameter is larger than 20 mm, you should use a knife holder with a clamp head.
  • 7) When turning the multi-head or multi-head screw Adjust the replacement gear before performing a test run.
  • 8) When using an automatic lathe, adjust the relative position of the tool and workpiece according to the adjustment card of the machine tool. After adjustment, it is necessary to perform a test run. The first piece can be modified and then processed. During the machining process, tool wear, workpiece size and surface roughness are always taken into account.
  • 9) When turning on the vertical lathe, do not move the beam freely when adjusting the knife holder. When the vertical lathe is turned on, the crossbeam cannot move freely when the tool rest is adjusted.
  • 10) If the associated surface of the workpiece has a position tolerance, the lathe is performed with a single clamp whenever possible.
  • 11) When turning a spur gear blank, the hole and reference end face must be machined with a single clamp. If necessary, the marking line should be drawn near the end of the gear indexing circle.

Error compensation

Modern machine manufacturing technology is moving towards high efficiency, high quality, high precision, high integration and high intelligence. Precision and ultra-precision machining technology has become the most important element and direction in modern machine manufacturing, and has become an important technology for improving international competitiveness.

Turning error has become a hot topic of research with a wide range of applications in precision machining. Since thermal and geometric errors make up the majority of various errors in machine tools, reducing these two errors, especially thermal errors, is a major goal. Electroconvulsive technology (ECT) has emerged with the continued development of science and technology. The loss due to thermal deformation of machine tools is considerable. Therefore, it is very important to develop a high precision, low cost thermal error compensation system that can meet the actual production requirements of the factory to compensate for the thermal error between the spindle (or workpiece) and the cutting tool. Needed for. Improve machine tool accuracy, reduce waste, increase production efficiency and economic benefits.

Basic definition

The basic definition of error compensation is to artificially create a new error to cancel or significantly reduce the original error of the current problem. By analyzing the original error, statistics, derivation and learning characteristics and laws, a mathematical model of the error is established, producing an artificial error equal to the original error and vice versa, thereby reducing machining error. And improve the dimensional accuracy of the parts.

Early error compensation was implemented in hardware. Hardware compensation is mechanical fixed compensation. To change the amount of correction when the mechanical error changes, you need to recreate the part, calibrate the rules, and readjust the correction mechanism. Hardware compensation also has the disadvantage of not resolving random errors and inflexibility.

[1] The development of software compensation is characterized by the comprehensive use of advanced technology and computer control technology in various fields to improve the accuracy of the machine tool without changing the machine tool itself. Software compensation overcomes many of the difficulties and shortcomings of hardware compensation and takes compensation technology to a new level.

Characteristic

Error compensation (technology) has two main characteristics: science and engineering.

The rapid development of scientific error compensation technology has greatly strengthened the theory of precision mechanical design, precision measurement, and precision engineering and has become an important branch of discipline. Technologies related to error compensation include detection technology, detection technology, signal Machining technology, optoelectronic technology, material technology, computer technology and control technology. As a branch of new technology, error compensation technology has its own independent content and features. Further research on error compensation techniques is theoretical, systematic, and of great scientific significance.

The engineering significance of engineering error compensation technology is very important and has three implications.

  • First, the use of error compensation technology can easily achieve the level of accuracy that “hard technology” requires.
  • The second is the use of error correction technology that can solve levels of accuracy that “hard technology” cannot normally achieve.
  • Third, if error compensation technology is adopted under certain accuracy requirements, equipment and equipment manufacturing costs can be significantly reduced, which is very economically significant.

Occurrence and classification of thermal error due to turning

As the machine tool accuracy requirements are further improved, the rate of thermal error continues to increase overall. Thermal deformation of machine tools is a major obstacle to improving machining accuracy. Machine tool thermal errors are primarily due to thermal deformation of machine tool components due to internal and external heat sources of the machine, such as motors, bearings, transmission components, hydraulic systems, ambient temperatures, and coolants. Machine tool geometry errors result from manufacturing defects in the machine tool, matching errors between machine tool components, dynamic and static displacement of the machine tool components, and so on.

Basic method of error compensation

In the abstract and related references, it can be seen that rotational error is usually caused by the following factors:

  • Machine tool thermal deformation error;
  • Geometric error of machine parts and structures
  • Error due to cutting force.
  • Tool wear error.
  • Other error causes such as machine axis servo error, CNC interpolation algorithm error, etc.
  • There are two basic methods for improving the accuracy of machine tools: error proofing and error correction.

Error proofing methods seek to eliminate or reduce the possibility of errors through design and manufacturing methods. The error prevention method can effectively reduce the temperature rise of the heat source, equalize the temperature field, and reduce the thermal deformation of the machine tool to some extent. However, it is impossible to completely eliminate thermal deformation, it is very expensive, and applying thermal error compensation methods opens up an effective and economical way to improve the accuracy of machine tools. ..

Related conclusions

Research on turning errors is an important part of the development of the modern machine manufacturing industry and has become an important technology for improving international competitiveness. Errors are generated in a variety of ways. Thermal error analysis and research can help improve turning accuracy and technical requirements.

Error correction technology can meet the high accuracy and low cost of the actual production requirements of the factory. Thermal error correction technology compensates for thermal drift error between the spindle (or workpiece) and the tool, improves machine tool machining accuracy, reduces waste, and improves production efficiency and economic benefits.

Frequently Asked Questions

When a conventional lathe rotates a thread with a large pitch, saddle vibration occurs, the machined surface undulates, and heavy work can become a crushing knife.
And when cutting, workers often carry knives and cutters.

There are many reasons for the above problem. This section mainly describes the phenomena and solutions by analyzing the stress conditions of the tool.

1. Causes of problems and their causes

  • We know it:
  • When the yarn is rotated at a small pitch, the usual knife cutting method (straight feed in the direction perpendicular to the work axis) is common.
  • When rotating a thread with a large pitch, the cutting force is often reduced by using the left and right knife cutting methods. (By moving a small slide, the thread cutting tool is cut with the left and right cutting edges, respectively).

When the thread is rotated, the movement of the saddle is carried out by the rotation of a long screw, which drives the movement of the nut to open and close. There is an axial gap in the long screw bearing and there is also an axial gap between the long screw and the split nut. When the main cutting edge on the right side of the right turn is strongly machined by the left and right borrowing cutting methods, the tool is applied to the force P (ignoring the friction between the tool and the insert) given by the workpiece. Then, the force P is decomposed into the axial component Px and the radial component, the axial component Px and the feed of the tool are in the same direction, and the tool transmits the axial component Px to the saddle-shaped portion. Therefore, the saddle moves back and forth quickly and quickly after being pushed to the side of the gap. As a result, the tool vibrates back and forth, wrinkling and even damaging the machined surface. However, this is not the case when cutting with the left main cutting edge. When cutting with the left main cutting edge, the axial component force Px of the tool is opposite to the feed direction, and the direction of the gap is removed. At this point, the saddle moves at a constant speed.

When cutting, the movement of the central sliding plate is achieved by the rotation of the central sliding platen screw that drives the nut. The screw shaft bearing has an axial clearance and there is an axial clearance between the screw shaft and the nut. When cutting a lathe, the rake face of the tool (with front corners) receives the force P (ignoring the friction between the insert and the rake face) given by the workpiece, breaking the force P into force Pz and radii. Directional force. The radial component is the same as the cutting direction of the cutting tool. It points to the work piece and pushes the tool towards the work piece. This moves the intermediate slide in the direction of the clearance, causing the knife or workpiece to bend (break).

When cut, the movement of the intermediate slide plate is achieved by the rotation of the intermediate sliding platen screw of the drive nut. The screw shaft bearing has an axial gap, and there is an axial gap between the screw shaft and the nut. When cutting a lathe, a force P exerted by the workpiece acts on the rake face (having a front angle) of the tool. (Ignoring the friction between the tip and the rake face), the force P is decomposed into the force Pz and the radial force.
The radial component is the same as the cutting direction of the cutting tool. It points to the work piece and pushes the tool towards the work piece. This will cause the intermediate slider to move in the direction of the gap, causing the tool or workpiece to bend (break).

Solution

If the lathe has a large pitch and you want to use left and right borrow-cut threads, you will need to adjust the clearance between the saddle and the bed rail as well as adjust the relevant parameters of the lathe to make it slightly tighter. .. The possibility of saddle turbulence is reduced to increase friction during movement, but this gap cannot be adjusted too tightly, so it is appropriate to rock the saddle steadily.

Adjust the clearance of the central skateboard to minimize the gap. Adjust the tightness of the small skateboard and lightly tighten it to prevent the rotating tool from rotating during rotation. Workpieces and toolbars should be as short as possible and the main blade on the left should be used as much as possible. When cutting with the main blade on the right side, you need to reduce the amount of back knives. Increase the rake angle of the main cutting edge on the right side. The edge of the cutting edge must be straight and sharp in order to reduce the axial component force Px of the tool. Theoretically, the larger the rake angle of the right main blade, the better.

 

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