Wednesday, November 30, 2022

Lathe Cutting Tools

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Basically, machining using a turnstile generates cylindrical shapes with a cutting tool or knife which, in most cases, is stationary, while the workpiece is rotating.

A cutting tool typical for use on a lathe (also known as burin) consists mainly of a Body, handle or stem, and of a head where is the cutting part. In turn, the head is made up of various parts, as we see in the figure below.

PARTS OF A TYPICAL LATHE TOOL

It is an essential requirement that the cutting tool Present high hardness, even at high temperatures, high wear resistance and great ductility. These characteristics depend on the materials from which the tool is made, which are divided into several groups:

Carbon Steel: of little application at present, the tools Made of carbon steel or unalloyed steel, they have a thermal resistance to redness of 250-300 ºC and, therefore, are used only for low cutting speeds or in the turning of wood and plastics. They are low cost tools and easy to heat treat, but above 300°C they lose their sharpness and hardness. Taps, taps, hand files, and other similar tools are made from carbon steel.

Fast steel: they are alloy steel tools with ferrous elements such as tungsten, chrome, vanadium, molybdenum and others. These steels acquire high hardness, high resistance to wear and a thermal resistance to redness up to temperatures of 650 ºC. Although on an industrial scale and in high-speed machining their application has decreased markedly in recent years, high-speed steel tools are still preferred for low-production or soft metal work because they are relatively inexpensive and are the only ones that can be resharpen on grinders or grinders fitted with an aluminum oxide grinding wheel, commonly used in most workshops.

The materials that follow are those with which the today so widespread inserts or inserts are built.

Cemented carbide or hard metal: These tools are made from carbide powder, which together with a portion of cobalt, used as a binder, give it a resistance of up to 815°C. The most common carbides are: tungsten carbide (WC or widia), titanium carbide (TiC), tantalum carbide (TaC), and niobium carbide (NbC). Due to their hardness and good resistance to wear, they are the most suitable tools for machining cast iron, non-ferrous metals and some non-metallic abrasive materials. Another category of hard alloy metals comprises coated cemented carbide, where the cemented carbide base is coated with titanium carbide, titanium nitride (TiN), aluminum oxide, titanium carbon nitride (TiCN), and titanium aluminum nitride (TiAlN).

cermets (material combination ceramic and metto the): although the name is applicable even to cemented carbide tools, in this case the base particles are TiC, TiCN and TiN instead of tungsten carbide. The binder is nickel-cobalt. These tools exhibit good wear resistance, high chemical stability and hot hardness. Its most suitable application is in materials that produce a ductile chip, steels and ductile irons.

Ceramics: There are two basic types of ceramics, those based on aluminum oxide and those based on silicon nitride. They are hard, with high hot hardness and do not react chemically with the materials of the piece, but they are very brittle. They are used in mass production, such as the automotive sector and auto parts, where given their good performance, they have managed to significantly increase the number of parts manufactured.

Cubic boron nitride (CBN): It is the hardest material after diamond. It exhibits extreme hot hardness, excellent wear resistance and generally good chemical stability during machining. It is brittle, but tougher than ceramic.

polycrystalline diamond (PCD): It is synthetic and almost as hard as natural diamond. It features incredible wear resistance and low thermal conductivity, resulting in up to 100 times longer tool life than cemented carbide. However, it is also very brittle, cutting temperatures should not exceed 600 ºC, it cannot be used to cut ferrous materials because there is affinity, and it is not suitable for cutting tough materials.

Standardization of cutting tools

Now that we have seen the main materials that make up a cutting tool for lathe, let’s see other important classifications that characterize each tool and that respond to the ISO and/or DIN international standards that we will detail below. The lathe tools can be classified:

1) According to the direction of advance of the tool:

  • Right Cut (R): they are tools that advance from right to left.
  • Left Cut (L): they are tools that advance from left to right.

2) Depending on the shape of the tool shank:

  • Straight stem: when a straight axis is observed from the end of the tool.
  • Angled stem: when from the end of the tool it is observed that its axis bends to the right or left, near the cutting part.

3) Depending on the purpose or application of the tool:

  • Cylinder capacity: the part is lowered longitudinally to generate cylindrical shapes.
  • Facing: the end of the piece is lowered so that it is at 90º with respect to the axis of symmetry.
  • Taper turning: the axial and radial movement of the tool is combined to create conical and spherical shapes.
  • Threaded: the part is lowered in a helical way to create a thread that can be used to place a nut or join parts together.
  • Boring: the inside of a hole is lowered to achieve very precise measurements.
  • Shape turning: the tool travels radially from the outside to the inside of the part. A constant depth cut leaves the shape slotted or ribbed, while a deep cut completely cuts the cylinder (bucking).
  • Boring: a drill is used to make holes in the piece and the tools used in drilling on the lathe are the same as those used in drilling machines. Basically two types of drills are used to make deep holes: twist drills with holes for forced lubrication and gun drills.
  • Reaming: pFor reaming on the lathe, in addition to single edge tools, tooth reamers, also called machine reamers, are also used. Reamers are made up of a number of straight or helical teeth that varies from 4 to 16, arranged symmetrically around the axis of the tool.

4) According to the tool manufacturing method:

  • Integral or whole tools: they are forged to the required shape in a single piece of the same material. They are manufactured in the form of a round, square or rectangular bar of forged tool steel, which has a cutting edge at one end.
  • Composite Tools: They are of different types that we can classify into three subgroups:
    • Tools made of different materials: usually the shank is made of constructional steel and the cutting part is made of high speed steel and butt welded.
    • Tools with welded plate: steel stem and cutting part of high speed steel or widia in the form of a small pill or welded plate. Welding each tool requires time and skill. Depending on the application, shank shape and feed direction, these tools are classified according to ISO and DIN standards (see table below). The welded plate can be resharpened when necessary and until the end of its useful life.
    • Tool holders with interchangeable plate: they consist of a handle or tool holder capable of being reused innumerable times, in which small pads or interchangeable plates called inserts, made of ceramic compounds, triangular, square, rhombic, round or other in shape, can be alternately assembled and disassembled. Inserts are designed to be interchanged or rotated as each cutting edge wears and are discarded at the end of their useful life, so no sharpening is required. The inserts are classified under strict ISO standards that we will see in detail in a future article.

ISO/DIN classification of carbide welded plate tools

In the following figure we see the main applications of the lathe tools, with the ISO/DIN classification specific to those with widia soldered plate, detailed in the corresponding table.

Lathe Cutting Tools

Lathe Cutting Tools

TABLE 1 - ISO/DIN CLASSIFICATION OF TOOLS WITH HARD METAL WELDED PLATETABLE 1 - ISO/DIN CLASSIFICATION OF TOOLS WITH HARD METAL WELDED PLATE

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