Machining cast iron is not considered to be problematic. This premise is partly due to the material’s higher graphite content compared to steel. Graphite makes generated chips brittle and short, and possesses anti-friction properties that contribute to cutting edge lubrication. And with its ability to absorb vibrations, graphite also improves machining stability. While these advantages may reinforce the above-mentioned premise, the nuanced world of cast iron demands a more detailed study of this statement.
As can be construed from its name, cast iron is intended for castings. Machining cast iron parts involves removal of non-uniform and variable stock - for example sand inclusions, casting skin, blowholes, hot tears and other casting defects, which affects cutting tools. From a machining point of view, the higher graphite content also has a disadvantage: it accelerates abrasion wear. This means that the cutting tools must have good wear resistance to ensure high productivity. It is also worth noting that the term “cast iron" may in fact refer to different types of ferrous alloy, for which machinability can vary significantly. The latter is often neglected, which can lead to wrong choice of cutting tools and incorrect definition of cutting data.
There are several types of cast iron. Grey, nodular and malleable cast iron form the application group K (red identification color) in accordance with the ISO 513 Standard classification. Hardened and chilled cast iron relate to group H (grey identification color). These specifications offer clear guidelines for manufacturers regarding the application of cutting tools - correct tool material, cutting geometry and cutting data selection.
Usually, machining ISO K cast iron is not a problem for manufacturers. Ferritic grey cast iron, for example, is an easy-to-cut material. However, machining ISO H cast iron is more difficult. Although similar to conditions for machining hard steels, the material’s specific features demand appropriate solutions from cutting tool producers. In addition, some types of cast iron demonstrate a certain duality in their machinability, emphasizing the broadness and heterogeneity of the definition "cast iron".
For example, machinability of Ni-resist cast iron can be compared with grey cast iron; however, the required cutting geometry seems more suitable for austenitic stainless steel. Workpieces from austenitic ductile iron (ADI) are delivered in different material conditions and hardness level that impact on selecting right cutting tools. Machinability of ADI before hardening is sufficient and is similar to cutting high-alloy steel. Nevertheless, if this cast iron is machined in a high hardness condition, only the tools intended for the ISO H application group will meet the customer's expectations.
The situation with cast iron of high hardness in ISO H group is challenging. Machining cast iron with a hardness of HB 400…440 is usually less of a problem for manufacturers. However, the picture changes radically when dealing with hard abrasion-resistant high-chrome cast iron. The general hardness may be around HRC 52…54 but in the thin-wall areas of a machined part, the hardness can reach HRC 60 and even more. In combination with the high chrome content, it makes machining extremely difficult and significantly diminishes tool life.
Table 1 shows the averaged machinability rating for different cast iron types. Pearlitic grey cast iron, which is specified as 100% rating, provides a base for comparison.
Table 1
| Material |
Condition |
ISCAR
mat. group* |
Machinabilty
% |
| Grey |
Ferritic |
15 |
130 |
| cast iron (GCI) |
Pearlitic |
16 |
100 |
| Nodular |
Ferritic-pearlitic |
17 |
75 |
| cast iron (NCI) |
Pearlitic |
18 |
70 |
| Malleable |
Ferritic |
19 |
115 |
| cast iron (MCI) |
Pearlitic |
20 |
93 |
| Compacted graphite iron (CGI) |
|
~17 |
80 |
| Austempered |
Soft |
~10 |
80 |
| ductile iron (ADI) |
Hardened |
41 |
35 |
| Ni-resist austenitic CI |
|
|
90 |
| Chilled cast iron |
HB 400…440 |
40 |
50 |
| Hardened cast iron |
HB 550…600 |
41 |
25 |
* ISCAR material group in accordance with VDI 3323 standard
Selecting the most suitable cutting tool for machining cast iron should be based on a detailed study of a cast iron type and its hardness. Cutting tool application specialists, who are involved in selecting right tools, need to be fully accurate when specifying characteristics of the cast iron that is intended for machining. In turn, cutting tool manufacturers make every effort to find the most effective solutions for machining cast iron, taking into account the diversity of the cast iron world. Among the main consumers of cast iron are the automotive, die and mold, machine tool, and heavy industries – all demanding increasingly efficient products from their cutting tool partners.
The tools for machining cast iron form a large part of ISCAR's product range. ISCAR has brought to the market a variety of interesting designs and tool materials targeted precisely for cutting this popular material. Some of these designs are quite indicative of their creators’ logic, which was aimed at finding an appropriate answer to customer needs.
On a firm basis
The previously mentioned difficult-to-cut hard/high chrome cast iron produces serious barriers for machining productivity. A cutting tool experiences high mechanical and thermal loading. In milling this cast iron by cemented carbide tools, for example, a typical cutting speed is low: 40-50 m/min (130-160 sfm). Intensive heat generation often forces manufacturers to apply wet coolant. As a result, the tool operates under conditions of a heat shock effect, which considerably shortens the tool life.
ISCAR developed the DT7150 grade especially for this type of operation. DT7150 is a "DO-TEC" carbide grade that has a tough substrate and combines medium-temperature CVD and PVD coating processes. Due to its extremely high wear and chipping resistance, DT7150 provides customers with an effective tool material for cutting hard cast iron.
A fundamental significant change in productivity can be reached with the use of cubic boron nitride (CBN), which enables a considerable increase in cutting speeds. In machining hard cast iron, for instance, the speeds are 2-5 times higher when compared with cemented carbide. ISCAR's high-performance milling cutters carrying tangentially clamped inserts with CBN tips are extremely popular in the automotive industry. For hard turning applications, the company expanded the range of CBN tipped ISO-type inserts for both continuous and interrupted cut (Fig. 1).
In ISO K applications (machining grey, nodular and malleable cast iron) in medium-duty loading, ceramic tools have demonstrated good results. Peripherally ground tangentially clamped TANGMILL milling inserts, made from the silicon nitride (Si3N4) grade IS8, ensures increasing cutting speed up to 3 times and provide excellent surface finish. In turning, cutting speeds of up to five times faster can be achieved by using the CVD-coated silicon nitride inserts, even for roughing operations.
The role of geometry
Cutting geometry and edge preparation are crucial for tool performance. There are several types of cutting-edge preparations: rounded, chamfered and so on. Although selecting the required preparation might appear to be a simple task, it is not so easy. Which width or angle of chamfering will be the most effective? How to ensure the defined angle during tool production? These questions are particularly critical when using ceramic or CBN inserts. The answers require appropriate professional skills and experience. Today, tool engineers are armed with a powerful design instrument – computer modelling of chip formation - that seriously contributes to finding an optimal shape. This instrument considerably shortens developing cutting geometry and represents an important factor in successful tool design.
A good example of an optimized edge condition in combination with a carbide grade, which is dedicated for machining cast iron, is the TGMA grooving insert. This insert was introduced over the last years and enriched ISCAR's TOPGRIP family. The frontal and side areas of the insert cutting edge feature the chamfered condition forming so called T-land to increase the edge strength and extend tool life (Fig. 2). Computer modelling played a crucial role in optimizing the edge geometry. The insert is made from the CVD coated carbide grade IC5010 that was developed especially for grooving cast iron.
Extra fine finisher
ISCAR's recent launch of new products in its “LOGIQ” marketing campaign included the TANGFIN family of face milling cutters designed for extra fine surface finish (Fig. 3), which has been of great interest to producers of cast iron parts. The tangentially clamped inserts are positioned in a TANGFIN cutter with a gradual displacement in both radial and axial directions. The design causes each insert to cut only a small portion of the material. This concept, together with the high-rigidity tangential clamping principle and the long wiper edge of the insert, results in an impressive machined surface finish, with an Ra up to 0.1 μ (4 μin) roughness parameter.
Customized solutions
The automotive industry is one of the largest producers of cast iron parts. In efforts to reduce cost per part in the mass production of automotive components, cutting tool manufacturers have developed customized highly engineered tools that perform specific machining operations with maximum productivity and lead to reducing the non-cutting component of cycle time.
A customized solution for machining steering knuckles (Fig. 4) provides an excellent example of these combined tools. ISCAR proposed this solution as a part of a turnkey project for one of the biggest car producers. The combined assembled tool performs several operations - cutting inner thread in holes for dust shield by tapping; milling two grooves (for snap ring and grease seal) by helical interpolation; and milling the outer face. The tool carries different radially and tangentially clamped inserts and has a tapping attachment with a misalignment compensating mechanism. The strict tolerance limits for linear dimensions ensure successful utilization of the tool in multi-spindle machines.
It turns out that machining cast iron is not such a simple matter after all, as is sometimes believed. Understanding the colorful world of this material and following the cast-iron rules for correct cutting tool application will ensure maximum efficiency and demonstrate the tool’s "IQ" capabilities to achieve – and sustain - highly effective machining practices.
