Recent studies in CGI machining have focused on continuous cutting conditions that occur during high-speed cylinder boring.1,2  Boring of engine cylinders is one of the more critical operations in engine production, requiring high-quality surfaces to be produced at relatively high cutting speeds. It is at these elevated cutting speeds (250-700 m/min) where the machinability differences between CGI and conventional gray cast irons are most pronounced.

Previous studies have reported insert wear rates to be 20-30 times greater in the continuous cutting of CGI relative to results obtained in machining of gray cast iron under equivalent conditions.1 The differences in machinability between gray cast iron and CGI have been attributed to differences in graphite structure and also to the presence of manganese sulfide inclusions in gray cast iron.

Manganese sulfide inclusions (not present in CGI) form a protective and lubricating film on the cutting tool surface during machining of gray cast iron, protecting the tool surface and decreasing rates of tool wear.3,4,5  It has been shown that the formation and activity of these lubricating films is highly dependent on cutting speed (likely cutting temperatures) with gray iron machinability and tool life improving as cutting speeds increase.6

Based on these factors and the current level of understanding of the microstructural and compositional features of CGI, recent work at Quaker Chemical has led to the development of new metalworking fluid technology — Quakercool 7020-CG — useful for increasing tool life in CGI machining.7

To assess further the utility of this new technology and especially its impact during CGI machining at high speeds under continuous cutting conditions, tests were conducted to assess the performance of this fluid in a turning operation using tungsten carbide cutting inserts at cutting speeds of 250 m/min, and also PCBN cutting inserts at cutting speeds of 700 m/min.

Fluid 37, a fluid commonly used during machining of conventional gray cast iron, was included in this testing and served as a baseline for this study. In addition to assessing fluid performance in the machining of both gray cast iron and CGI, dry machining of CGI was performed using both tool materials and conditions. This was done to assess the impact of wet versus dry machining in this operation.

Machining CGI v. Gray Cast Iron

Machining was conducted on a Victor Fortune TNS-2 turning center. Multiple turning passes were made on test cylinders of gray cast iron and Grade 450 CGI, both supplied by SinterCast. Two separate cutting insert materials were used. Coated carbide inserts were used at cutting speeds of 250 m/min. and PCBN inserts were used at 700 m/min. Details of the machining conditions used for these two tool materials are shown in the table.

 

Machining Conditions

 

Carbide

PCBN

Insert

Kennametal, Carbide Grade-KC 9230

Sumitomo BNX10

 

80°, 5° radial rake angle

80°, 5° radial rake angle

 

5° axial rake angle

5° axial rake angle

Speed

250 m/min. (820 SFM)

700 m/min. (2,296 SFM)

Feed

0.3 mm/rev (0.012 ipr)

0.3 mm/rev (0.012 ipr)

Radial depth of cut

0.20 mm (0.0079 in.)

0.15 mm (0.0059 in.)

Tool failure criterion

0.3 mm flank wear length

0.3 mm flank wear length

 

A. Carbide Cutting Inserts @ 250 m/min. — 
In turning of the CGI cylinders using the carbide inserts at 250 m/min, abrasive wear on the flank face of the cutting insert was seen to develop quickly and progressed rapidly with continued machining. The progression of flank wear that occurred can be seen in Figure 1. In the current study, machining was continued until 0.3 mm flank wear length was reached. At 0.3 mm flank wear, considered as the failure point for the tool, noticeable loss of the rake face geometry was observed also.

The insert wear measured for the two metalworking fluids as well as for dry machining, using the carbide inserts are shown in Figure 2. As seen, wet machining offers significant benefits in regard to tool wear over that obtained under dry machining conditions. Tool life improvements of between 79 to 124% were obtained using fluid lubrication. In assessing the relative performance of the two fluids tested, the enhanced performance offered by Quakercool 7020-CG over the conventional ferrous machining fluid (Fluid 37), and certainly over dry machining, is clearly seen. Quakercool 7020-CG resulted in a 32.6% increase in tool life over that for Fluid 37. A 124% increase in insert life was seen over that obtained for dry machining.

Looking at the insert wear measured in the machining of gray cast iron using the conventional cast iron machining fluid (Figure 8), the lower machinability of CGI relative to gray cast iron is clearly seen. In the turning of CGI, tool failure was reached after 10 Km of metal distance cut, whereas in the machining of the gray cast iron, minimal wear was observed even after 20 Km of cutting distance.

The dramatic difference in wear between the two metals can be seen also in microphotographs of the flank surfaces of the cutting inserts following 10 Km cutting distance (Figure 9.) Rapid abrasive wear and crater wear occurs very quickly with the machining of CGI whereas only slight abrasive wear occurs on the flank face of the tool during gray cast iron machining. This difference in machinability also can be seen in the condition of the rake face surface of the inserts following 10 Km cutting distance on both metals (Figure 10.) While abrasive wear can be seen on the insert rake face surface used with gray cast iron, the cutting edge is still retained. In contrast, the rake surface of the insert used with CGI shows significant abrasive wear with deformation and loss of the cutting edge.

B. PCBN Cutting Inserts @ 700 m/min.— Polycrystalline cubic boron nitride inserts are typically used at considerably higher cutting speeds than many other tool materials, and thus may present even greater challenges for effective machining of compacted graphite iron. In turning of the CGI cylinders using the PCBN inserts at 700 m/min. cutting speed, abrasive wear developed quickly and at a much greater rate relative to that which occurred with the carbide inserts used at lower cutting speeds. Failure of the PCBN inserts (0.3 mm flank wear length) occurred between 1.1 to 1.8 Km of cutting distance, a very short amount of machining.

The progression and type of wear observed can be seen in Figure 11. Abrasive wear occurs quickly and continues in severity as machining continues, likely due to the high hardness and wear resistance of this tool material. Along with abrasive wear, a noticeable amount of plastic deformation of the tool material is seen along the cutting edge.

The PCBN insert wear measured for the two metalworking fluids along with that for dry machining, are shown in Figure 12. As shown, while wear occurs very rapidly on this insert at the cutting conditions used, the benefit of wet versus dry machining still can be seen with a 55% improvement in insert life being obtained by using Quakercool 7020-CG relative to that obtained during dry machining conditions.

This difference is evident also in the condition of the cutting inserts used for dry machining and wet machining using Quakercool 7020-CG, (Figure 13.) Measurable differences in performance between the fluids is seen too, with Quakercool 7020-CG yielding an 18% increase in insert life over that obtained with use of the conventional ferrous machining fluid, Fluid 37.


In looking at the insert wear measured in the machining of gray cast iron relative to CGI, the significant differences and challenges that exist with high-speed boring of CGI using PCBN inserts is very apparent. The wear rates measured in the machining of gray iron along with CGI are shown in Figure 14.

A significant difference in machinability between the metals is clear. This difference is seen as well in the microphotographs of the flank surfaces of the cutting inserts following 2.85 Km cutting distance as well as at 25.7 Km cutting distance for gray iron (Figure 15.) Rapid abrasive wear occurs very quickly with the machining of CGI whereas only slight abrasive wear occurs on the flank face of the tool during gray iron machining. This difference in machinability was also seen in the condition of the rake face surface of the inserts where significant abrasive wear and loss of the cutting edge occurred after approximately 2.85 KM cutting distance in CGI machining, while in the machining of gray iron retention of the cutting edge was maintained through the entire 25.7 Km distance.