Thursday, December 8, 2022

Introduction to Lathe Inserts

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Faced with the expansion of high-performance machinery for turning and with CNC control for about 40 years, the boom that inserts or interchangeable plates have gained was notorious, to the point of making them currently the tool A must for high speed turning, even despite its cost. AND

Today’s great variety of inserts Y insert holder in the market has determined its standardization under standards ISO (or ANSI in the United States, which use English measurements) in order to facilitate the right choice for each application.

In this article, which does not pretend to be exhaustive due to the immense diversity of inserts, we are going to know the two ISO standards that regulate the carbide inserts for turning, as well as the main types of insert clampings to insert holders, also regulated by ISO. In a later article, we will continue to deepen the topic to know more details about it.

Insert standardization

a) ISO 1832 standard

This standard groups carbide inserts into 10 categories different ones that contemplate diverse parameters. Each of these categories presents a multiplicity of inserts that are designated with symbols composed of capital letters and/or numbers, forming a insert identification sequence which follows a strict order.

The specification of the categories 1 to 7 that we will see next is mandatory in the identification sequence of all types of inserts, while that of categories 8, 9 and 10 is optional and depends on each manufacturer. Category 10 is generally used to offer special information about the insert, for example, the characteristics of the chipbreaker. If the symbol for category 10 appears in the insert identification sequence, it is separated from all other symbols by a hyphen.

Let’s take a closer look at each of the 10 categories that make up the ISO insert identification sequence, as well as the symbols that represent those categories.

  1. Insert Shape: is a letter that indicates the shape of the top face of the insert. The standard categorizes 16 shapes and the most common are: round, square, rhombic (of various angles), triangular and trigonal.
  2. Front relief angle or incidence angle: is a letter indicating the 90° difference measured in a plane normal to the cutting edge generated by the angle between the flank and the top surface of the insert. It allows the cutting edge to work freely and does not rub against the part to be machined.
  3. Dimension Tolerance: is a letter that defines the maximum and minimum tolerances of the size of the insert, designated by the largest circle that can be inscribed within the perimeter of the insert.
  4. Clamping and chipbreaker system: is a letter indicating design differences not specifically provided for in the other categories of the sequence. The most common differences are the existence of clamping holes, countersinks and special characteristics of the leading surfaces.
  5. Cutting edge length: is a two-digit number (with or without a leading zero) indicating the size of the inscribed circle (IC) for all inserts that have a true IC (shapes round, square, triangular, trigonal, rhombic, etc.). The symbol of this category is represented only with integers and decimal figures are not considered; if the diameter of the IC is less than 10 mm, a zero is prefixed. For rectangular and parallelogram shaped inserts, which do not have a true CI, the width and length dimensions are used.
  6. Thickness: is a number or letter + number that indicates the thickness of the insert in millimeters. The symbol of this category is represented only with integers and decimal figures are not considered.
  7. Radius of the nose (or tip): is a number or letter + number that indicates the radius of the tip and generally varies from 0.03 mm to 3.2 mm. The symbol of this category is represented only with integers and decimal figures are not considered.
  8. Cutting edge: is a letter (or two, depending on the manufacturer) that defines special conditions, such as edge treatment and surface finish.
  9. Cutting direction: is a letter that indicates the cutting direction that the insert must have during the process. It can be R (right), L (left) or N (neutral or both ways).
  10. Product customization: at the discretion of the manufacturer.

In its inserts catalog, each manufacturer provides the ISO code tables that help identify each of their products.

For a better understanding of how this identification process works, let’s look at an example.

Suppose that a certain manufacturer’s label on an insert gives us the following insert identification sequence:

C N M G 12 04 08 E N – MP

How do we interpret each of these symbols? The figure that follows reproduces part of those tables provided by the manufacturer, where we can identify with the red oval the indicated characteristics corresponding to the insert in question.

ISO 1832 tables for interchangeable insertsISO 1832 tables for interchangeable inserts

From the interpretation of these tables, it follows then that a insert with the code: CNMG 12 04 08 EN – MP has the following characteristics:

  1. “C”: 80º rhombic shape.
  2. “N”: angle of relief or incidence of 0º.
  3. “M”: the dimensional tolerances in the measurements of the insert are: height “m” of the rhombus from ±0.08 mm to ±0.18 mm, diameter of the inscribed circle D1 from ±0.05 mm to ± 0.13 mm and thickness S1 ±0.13mm
  4. “G”: Regarding the clamping and chipbreaker system, the code “G” indicates that it is an insert with a central hole (cylindrical) and with chipbreakers on both sides.
  5. “12”: with this number “12” for a rhombic shape “C” indicated in category 1, we see that the length of the cutting edge (represented by the diameter of the inscribed circle) is 12.70 mm.
  6. “04”: Indicates that the thickness of the insert is 4.76 mm.
  7. “08”: indicates that the radius of the nose is 0.8 mm.
  8. “AND”: rounded cutting edge.
  9. “N”: cutting direction in both directions.
  10. “PM”: characteristics of the chipbreaker (exclusive information of the manufacturer that it provides in its catalog of inserts).

b) ISO 513 Standard

Now, what are all these parameters that we have just seen for? It is evident that we must take them into account when choosing the right insert for the work we want to do. Features such as the type of material that makes up the part (hardness Y specific shear force), the type of cut (roughing, finishing, grooving, parting off, drilling, threading, etc.) and the winch capacity will determine the material, the shape, the depth of cut, the Advance, the nose radius, the size and the thickness of insert.

Regarding the material with which the insert, the rules ISO513 define 6 categories of hard metal represented by a letter and a color different. Within each category, various subcategories with numbers ranging from 1 to 50, which take into account the tendency of the insert to be tough or hard, where lower numbers represent lower toughness and higher numbers represent lower hardness.

In the following table we detail each of the six categories, as well as the corresponding subcategories, with their applications.

ISO 513 table for carbide inserts and applicationsISO 513 table for carbide inserts and applicationsISO 513 table for carbide inserts and applications

In terms of versatility, toughness, vibration and power requirements, there are insert shapes that will be better suited for the job at hand. For example, 80º rhombic inserts are ideal for roughing and finishing of inside and outside diameters, while the 55º or 35º rhombic inserts are used for copying.

The nose radius affects the surface finish of the piece. For finishing operations, the lower radius values ​​are used, capable of making finer cuts, while for roughing and heavy roughing, the higher values ​​are used, which offer a strong edge and resistant to significant feeds. Let us remember that the Advance is the speed of the tool towards the part; it is preferably measured in mm/rev or mm/min. The following table shows the relationship between lead and nose radius, with recommended values ​​for the former.

Relationship between lead and nose radius

Nose radius (mm)

Recommended feed rate (mm/rev)

0.40

0.12 – 0.25

0.80

0.25 – 0.50

1.20

0.36 – 0.70

1.60

0.50 – 1.00

2.40

0.70 – 1.60

In turn, progress is related to cutting speed, defined as the speed with which a point on the circumference of the part passes through the cutting tool in one minute and is expressed in meters per minute.

Relationship between feed and cutting speed

Material

Feed (mm/rev)

Cutting speed (m/min)

roughing

Finish

roughing

Finish

threaded

machine steel

0.250 – 0.500

0.070 – 0.250

27

30

eleven

tool steel

0.250 – 0.500

0.070 – 0.250

twenty-one

27

9

Molten iron

0.400 – 0.065

0.130 – 0.300

18

24

8

Bronze

0.400 – 0.650

0.070 – 0.250

27

30

8

Aluminum

0.400 – 0.750

0.130 – 0.250

61

93

18

Standardization of insert holders

The choice of tool holders for him insert It is performed according to different machining modes such as external, face and copy turning. To this end, the insert clamping system to the tool holder is also standardized by ISO and, although there are several, there are four categories main, symbolized by letters:

Q: the insert is fixed by means of a lever which pushes it over its seat on the tool.

C: the insert is held by a flange pressure, which keeps the insert pressed against the seat in the tool holder.

S: the insert hole is conical in shape and the insert is fixed by screw.

M: the insert is fixed by a cradle (or flange and screw) that simultaneously holds the top and side of it.

The table below exemplifies each of the main fixing types, as well as its characteristics and applications.

Main types of attachment of the insert to the insert holder

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