Turning uses a lathe to remove material from the outside of a rotating workpiece, while boring removes material from the inside of a rotating workpiece. #base
Turning is the process of removing material from the outside diameter of a rotating workpiece using a lathe. Single point cutters cut metal from the workpiece into (ideally) short, sharp chips that are easy to remove.
A CNC lathe with constant cutting speed control allows the operator to select the cutting speed, and then the machine automatically adjusts the RPM as the cutting tool passes different diameters along the outer contour of the workpiece. Modern lathes are also available in single turret and double turret configurations: single turrets have a horizontal and vertical axis, and double turrets have a pair of horizontal and vertical axes per turret.
Early turning tools were solid rectangular pieces made of high speed steel with rake and clearance corners at one end. When a tool becomes dull, the locksmith sharpens it on a grinder for repeated use. HSS tools are still common on older lathes, but carbide tools have become more popular, especially in brazed single point form. Carbide has better wear resistance and hardness, which increases productivity and tool life, but it is more expensive and requires experience to regrind.
Turning is a combination of linear (tool) and rotary (workpiece) motion. Therefore, cutting speed is defined as a distance of rotation (written as sfm – surface foot per minute – or smm – square meter per minute – the movement of a point on the surface of the part in one minute). The feedrate (expressed in inches or millimeters per revolution) is the linear distance that the tool travels along or across the surface of the workpiece. Feed is also sometimes expressed as the linear distance (in/min or mm/min) that a tool travels in one minute.
Feed rate requirements vary depending on the purpose of the operation. For example, in roughing, high feeds are often better for maximizing metal removal rates, but high part rigidity and machine power are required. At the same time, finishing turning can slow down the feed rate to achieve the surface roughness specified in the part drawing.
The effectiveness of a cutting tool depends largely on the angle of the tool relative to the workpiece. The terms defined in this section apply to cutting and clearance inserts and also apply to brazed single point tools.
Top rake angle (also known as back rake angle) is the angle formed between the insert angle and a line perpendicular to the workpiece when viewed from the side, front and back of the tool. The top rake angle is positive when the top rake angle is sloped down from the cutting point into the shank; neutral when the line at the top of the insert is parallel to the top of the shank; and neutral when it is tilted up from the cutting point. it is higher than the tool holder, the upper rake angle is negative. . Blades and handles are also divided into positive and negative angles. Positively inclined inserts have chamfered sides and fit holders with positive and side rake angles. Negative inserts are square in relation to the top of the blade and fit handles with negative top and side rake angles. The top rake angle is unique in that it depends on the geometry of the insert: positively ground or formed chipbreakers can change the effective top rake angle from negative to positive. Top rake angles also tend to be larger for softer, more ductile workpiece materials that require large positive shear angles, while harder, stiffer materials are best cut with neutral or negative geometry.
The lateral rake angle formed between the end face of the blade and a line perpendicular to the workpiece, as seen from the end face. These angles are positive when they are angled away from the cutting edge, neutral when they are perpendicular to the cutting edge, and negative when they are angled upwards. The possible thickness of the tool depends on the side rake angle, smaller angles allow the use of thicker tools that increase strength but require higher cutting forces. Larger angles produce thinner chips and lower cutting force requirements, but beyond the maximum recommended angle, the cutting edge weakens and heat transfer is reduced.
The end cutting bevel is formed between the cutting edge of the blade at the end of the tool and a line perpendicular to the back of the handle. This angle defines the gap between the cutting tool and the finished surface of the workpiece.
The end relief is located below the end cutting edge and is formed between the end face of the insert and a line perpendicular to the base of the shank. Tip overhang allows you to make the relief angle (formed by the shank end and the line perpendicular to the shank root) larger than the relief angle.
The side clearance angle describes the angle under the side cutting edge. It is formed by the sides of the blade and a line perpendicular to the base of the handle. As with the end boss, the overhang allows the side relief (formed by the side of the handle and the line perpendicular to the base of the handle) to be larger than the relief.
The lead angle (also known as side cutting edge angle or lead angle) is formed between the side cutting edge of the insert and the side of the holder. This angle guides the tool into the workpiece, and as it increases, a wider, thinner chip is produced. The geometry and material condition of the workpiece are major factors in selecting the lead angle of the cutting tool. For example, tools with an accentuated helix angle can provide significant performance when cutting sintered, discontinuous, or hardened surfaces without severely impacting the cutting tool’s edge. Operators must balance this benefit with increased part deflection and vibration, as large lift angles create large radial forces. Zero pitch turning tools provide a chip width equal to the depth of cut in turning operations, while cutting tools with an angle of engagement allow the effective depth of cut and the corresponding chip width to exceed the actual depth of cut on the workpiece. Most turning operations can be effectively performed with an approach angle range of 10 to 30 degrees (the metric system reverses the angle from 90 degrees to the opposite, making the ideal approach angle range of 80 to 60 degrees).
Both the tip and the sides must have sufficient relief and relief to enable the tool to enter the cut. If there is no gap, no chips will form, but if there is not enough gap, the tool will rub and generate heat. Single point turning tools also require face and side relief to enter the cut.
When turning, the workpiece is subjected to tangential, radial and axial cutting forces. The greatest influence on energy consumption is exerted by tangential forces; axial forces (feeds) press the part in the longitudinal direction; and radial (depth of cut) forces tend to push the workpiece and tool holder apart. “Cutting force” is the sum of these three forces. For zero angle of elevation, they are in a ratio of 4:2:1 (tangential:axial:radial). As the lead angle increases, the axial force decreases and the radial cutting force increases.
The type of shank, corner radius, and insert shape also have a large impact on the potential maximum effective cutting edge length of a turning insert. Certain combinations of insert radius and holder may require dimensional compensation in order to take full advantage of the cutting edge.
Surface quality in turning operations depends on the rigidity of the tool, machine and workpiece. Once stiffness has been established, the relationship between machine feed (in/rev or mm/rev) and insert or tool nose profile can be used to determine the surface quality of the workpiece. The nose profile is expressed in terms of a radius: to a certain extent, a larger radius means a better surface finish, but a too large radius can cause vibration. For machining operations requiring less than optimum radius, the feed rate may need to be reduced to achieve the desired result.
Once the required power level is reached, productivity increases with depth of cut, feed and speed.
Depth of cut is the easiest to increase, but improvements are only possible with sufficient material and forces. Doubling the depth of cut increases productivity without increasing cutting temperature, tensile strength, or cutting force per cubic inch or centimeter (also known as specific cutting force). This doubles the required power, but tool life is not reduced if the tool meets the requirements for tangential cutting force.
Changing the feed rate is also relatively easy. Doubling the feed rate doubles the chip thickness and increases (but does not double) the tangential cutting forces, cutting temperature, and power required. This change reduces tool life, but not by half. Specific cutting force (cutting force related to the amount of material removed) also decreases with increasing feed rate. As the feed rate increases, the extra force acting on the cutting edge can cause dimples to form on the top rake surface of the insert due to the increased heat and friction generated during cutting. Operators must carefully monitor this variable to avoid a catastrophic failure where the chips become stronger than the blade.
It is unwise to increase the cutting speed compared to changing the depth of cut and feed rate. The increase in speed led to a significant increase in cutting temperature and a decrease in shear and specific cutting forces. Doubling the cutting speed requires extra power and cuts tool life by more than half. The actual load on the top rake can be reduced, but higher cutting temperatures still cause craters.
Insert wear is a common indicator of the success or failure of any turning operation. Other common indicators include unacceptable chips and problems with the workpiece or machine. As a general rule, the operator should index the insert to 0.030 in. (0.77 mm) flank wear. For finishing operations, the operator must index at distances of 0.015 in. (0.38 mm) or less.
Mechanically clamped indexable insert holders comply with nine ISO and ANSI recognition system standards.
The first letter in the system indicates the method of attaching the canvas. Four common types predominate, but each type contains several variations.
Type C inserts use a top clamp for inserts that do not have a center hole. The system relies entirely on friction and is best suited for use with positive inserts in medium to light duty turning and boring applications.
Inserts M hold the protective pad of the insert cavity with a cam lock that presses the insert against the wall of the cavity. The top clamp holds the back of the insert and prevents it from lifting when the cutting load is applied to the tip of the insert. M inserts are especially suitable for center hole negative inserts in medium to heavy duty turning.
S-type inserts use plain Torx or Allen screws but require countersinking or countersinking. Screws can seize at high temperatures, so this system is best suited for light to moderate turning and boring operations.
P inserts comply with the ISO standard for turning knives. The insert is pressed against the wall of the pocket by a rotating lever, which tilts when the adjusting screw is set. These inserts are best suited for negative rake inserts and holes in medium to heavy turning applications, but they do not interfere with insert lift during cutting.
The second part uses letters to indicate the shape of the blade. The third part uses letters to indicate combinations of straight or offset shanks and helix angles.
The fourth letter indicates the front angle of the handle or the back angle of the blade. For a rake angle, P is a positive rake angle when the sum of the end clearance angle and the wedge angle is less than 90 degrees; N is a negative rake angle when the sum of these angles is greater than 90 degrees; O is the neutral rake angle, the sum of which is exactly 90 degrees. The exact clearance angle is indicated by one of several letters.
The fifth is the letter denoting the hand with the tool. R indicates that it is a right-handed tool that cuts from right to left, while L corresponds to a left-handed tool that cuts from left to right. N tools are neutral and can cut in any direction.
Parts 6 and 7 describe the differences between the imperial and metric systems of measurement. In the imperial system, these sections correspond to two-digit numbers denoting the section of the bracket. For square shanks, the number is the sum of one sixteenth of the width and the height (5/8 inch is the transition from “0x” to “xx”), while for rectangular shanks, the first number is used to represent eight of the width. quarter, the second digit represents a quarter of the height. There are a few exceptions to this system, such as the 1¼” x 1½” handle, which uses the designation 91. The metric system uses two numbers for height and width. (what order.) Thus, a rectangular blade 15 mm high and 5 mm wide would have the number 1505.
Sections VIII and IX also differ between imperial and metric units. In the imperial system, section 8 deals with insert dimensions, and section 9 deals with face and tool length. Blade size is determined by the size of the inscribed circle, in increments of one-eighth of an inch. End and tool lengths are indicated by letters: AG for acceptable posterior and end tool sizes, and MU (without O or Q) for acceptable front and end tool sizes. In the metric system, part 8 refers to the length of the tool, and part 9 refers to the size of the blade. Tool length is indicated by letters, while for rectangular and parallelogram insert sizes, numbers are used to indicate the length of the longest cutting edge in millimeters, ignoring decimals and single digits preceded by zeros. Other forms use side lengths in millimeters (the diameter of a round blade) and also ignore decimals and prefix single digits with zeros.
The metric system uses the tenth and final section, which includes positions for qualified brackets with tolerances of ±0.08mm for rear and end (Q), front and rear (F), and rear, front and end (B).
Single point instruments are available in a variety of styles, sizes and materials. Solid single point cutters can be made from high speed steel, carbon steel, cobalt alloy or carbide. However, as the industry shifted to brazed-tipped turning tools, the cost of these tools made them almost irrelevant.
Brazed-tipped tools use a body of inexpensive material and a tip or blank of more expensive cutting material brazed to the cutting point. Tip materials include high speed steel, carbide and cubic boron nitride. These tools are available in sizes A to G, and the A, B, E, F, and G offset styles can be used as right hand or left hand cutting tools. For square shanks, the number following the letter indicates the height or width of the knife in sixteenths of an inch. For square shank knives, the first number is the sum of the width of the shank in one eighth of an inch, and the second number is the sum of the height of the shank in one quarter of an inch.
The tip radius of brazed tipped tools depends on the shank size and the operator must ensure that the tool size is suitable for finishing requirements.
Boring is mainly used for finishing large hollow holes in castings or punching holes in forgings. Most tools are similar to traditional external turning tools, but the angle of cut is particularly important due to chip evacuation issues.
Rigidity is also critical to boring performance. The bore diameter and the need for additional clearance directly affect the maximum size of the boring bar. The actual overhang of the steel boring bar is four times the shank diameter. Exceeding this limit may affect the metal removal rate due to loss of stiffness and increased chance of vibration.
Diameter, modulus of elasticity of the material, length, and load on the beam affect stiffness and deflection, with diameter having the greatest influence, followed by length. Increasing the rod diameter or shortening the length will greatly increase the stiffness.
The modulus of elasticity depends on the material used and does not change as a result of heat treatment. Steel is least stable at 30,000,000 psi, heavy metals are stable at 45,000,000 psi, and carbides are stable at 90,000,000 psi.
However, these figures are high in terms of stability, and steel shank boring bars provide satisfactory performance for most applications up to 4:1 L/D ratio. Boring bars with tungsten carbide shank perform well at a 6:1 L/D ratio.
Radial and axial cutting forces during boring depend on the angle of inclination. Increasing the thrust force at a small lift angle is especially helpful in reducing vibration. As the lead angle increases, the radial force increases, and the force perpendicular to the cutting direction also increases, resulting in vibration.
The recommended lift angle for hole vibration control is 0° to 15° (Imperial. Metric lift angle is 90° to 75°). When the lead angle is 15 degrees, the radial cutting force is almost twice as great as when the lead angle is 0 degrees.
For most boring operations, positively inclined cutting tools are preferred because they reduce cutting forces. However, positive tools have a smaller clearance angle, so the operator must be aware of the possibility of contact between the tool and the workpiece. Ensuring sufficient clearance is especially important when boring small diameter holes.
The radial and tangential forces in boring increase as the nose radius increases, but these forces are also affected by the lead angle. Depth of cut when boring can change this relationship: if the depth of cut is greater than or equal to the corner radius, the lead angle determines the radial force. If the depth of cut is less than the corner radius, the depth of cut itself increases the radial force. This problem makes it all the more important for operators to use a nose radius smaller than the depth of cut.
Horn USA has developed a quick tool change system that significantly reduces setup and tool change times on Swiss style lathes, including those with internal coolant.
UNCC researchers introduce modulation into tool paths. The goal was chip breaking, but the higher metal removal rate was an interesting side effect.
The optional rotary milling axes on these machines allow many types of complex parts to be machined in a single setup, but these machines are notoriously difficult to program. However, modern CAM software greatly simplifies the task of programming.
Post time: Sep-04-2023