Putting iron machinability to the test.
* The AFS Iron Division's Control Committee (5-J) is developing a way to determine an iron casting's machinability. This standard test could help vindicate metalcasters in disputes over "bad" machinability.
* Detailed within is a description of how the test works and how metalcasters can use the test when producing iron castings.
The machining process, a final step in finishing a casting, is often deemed a reflection on the metalcaster because the time and cost of machining is a major component of manufacturing. When an iron casting proves to be problematic during machining, the metalcaster often is blamed, even though metallurgical release parameters, such as hardness, mechanical strength and microstructure, may not fully explain the differences in machinability of a component.
Poor machinability could be related to factors not detected by the metalcaster with standard tests nor controlled by the machinist in the machining process. These complaints also could be related to tooling or machining variables selected by the part-maker. In response to this dilemma, the AFS Iron Division's Quality Control Committee (5-J) is developing a way to determine an iron castings machinability. A standard machinability test provides cast iron producers with an additional measurement tool to detect material and processing concerns affecting machining.
"(Often) the metalcasting facility is blamed for a machinability problem, yet further testing revealed that the problem was related to factors other than the metallurgical quality of the material," said George Goodrich, president of Professional Metallurgical Services, Buchanan, Mich.
The standard machinability test initially will be used as a reference for comparing the same grade of iron to:
* material from different lots made in the same metalcasting facility;
* material from different metalcasters;
* a benchmark material.
One of the goals behind the test is to show that machinability in an iron casting could be attributed to the grade's own composition and processing. If it can be determined that specific grades of iron have a specific and different machinability "value," then machinability eventually could become a material specification, such as hardness or mechanical properties.
A 10-in. (25.4-cm) diameter disc-shaped casting was selected to validate the cutting tool wear test concept initially on ASTM A159 G3000 and G1800 gray cast iron grades.
A pattern with a hub on the disc for chucking in a turning center was designed. It was believed that the 1-in. (2.54-cm) thick disc is the most appropriate shape to capture the microstructure and mechanical properties of cast metal components.
For the test, it was decided to adapt the ISO 3685-E specification for face turning by selecting and verifying the most effective tooling and machining parameters, such as speed, feed and depth of cut. The observable part of the test to be measured is the maximum cutting speed for a 30-min. life of a cutting tool (the [V.sub.30] Value) based on a (cutting) tool life data base for each grade of cast iron.
The machinability test, which could be completed either in-house or at a testing lab, is run on a representative disc for 30 min. at the specified speed for that cast iron grade. After the test, the cutting tool (insert) is removed and measured for flank wear. If the flank wear falls at or below the end-of-test criterion, the heat or lot of cast iron passes the machinability test.
Life of a Cutting Tool
Several machining tests were completed on the two grades of cast iron while measuring flank wear of the cutting tool insert. The results of the tests were
used to calculate the Taylor line (best fit) on a (cutting) tool life plot over a range of different cutting speeds. Several types of cutting tool inserts and holders were selected, along with speeds and feeds, to provide a tooling life of 10-60 min. The investigators also worked t to find the turning parameters that resulted in an even wear on the cutting tool inserts as well as a narrow band of the Taylor line.
The observed tool life for each insert and grade combination test was plotted along with the best-fit line, and comparisons were made about the type and progression of wear on each cutting tool insert.
The speed for the [V.sub.30] Value was calculated from the Taylor line for the two grades from cutting tool life tests. Investigators then tested a cutting tool insert at the [V.sub.30] Value until it reached 0.015 in. (0.038 cm) of flank wear, which is the end-of-test criterion.
The time taken to reach the end point was plotted as a separate point for fit within the error bands of the Taylor lines (Figs. 1-2).
[FIGURES 1-2 OMITTED]
Several tests with different grades and types of cutting tool inserts and cutting tool holders were run before a successful combination was found that provided a good fit of the [V.sub.30] Value for the cast iron discs from both grades (Figs. 1-2).
The combination included an uncoated tungsten carbide insert with a chip breaker and holder providing a zero-degree lead angle. This tooling was used to develop the machinability values for the gray cast iron, and when cutting, the tool was flooded with a water-soluble cutting fluid. Machining parameters of 0.008 in. per revolution (0.2 mm/rev) feed and 0.050 in. (0.127 cm) depth of cut provided a uniform wear pattern on the flank of the insert without spinning the disc in the chuck of the turning center. The plan is to use the same tooling and machining guidelines, with modifications where necessary, for developing the [V.sub.30] Value for two grades of ductile iron castings. The researchers would then develop (cutting) tool life data bases and [V.sub.30] Values for all the standard grades of gray and ductile cast iron.
Saving Time and Money
Usually, an extensive amount of time and money is spent on chemical, tensile and microstructure rechecks of the castings when machining costs are higher than they should be. Now, the [V.sub.30] Value tool life test will be available to vindicate the metalcaster in disputes over "bad" machinability of cast iron.
Before a metalcaster begins casting a component, the machinability test can be used to determine the machinability value, which would enable him to modify the composition and/or processing if necessary. The metalcaster also may routinely determine and record the [V.sub.30] Value of each heat as meeting the machinability specification. Either way, the metalcaster can breathe easy knowing machinability problems won't be pinned on the casting process.
For More Information
"Machinability of Gray Cast Iron," R.D. Griffin, H.J. Li, E. Eleftherion, and C.E. Bates, Paper No. 02-159 Transactions of American Foundry Society, Schaumburg, Ill.
Michael Finn, TechSolve, Cincinnati