preheating and heating temperatures to minimize thermal gradients. The knives were then sharpened for turning tests on MDF in accordance with past methods. The turning test combinations were replicated nine times for each HSS with the depth of cut at 0.005-inch and 550 rpm. The tool force components parallel (Fp) and normal (Fn) to the direction of tool travel relative to the workpiece were recorded with an oscillographic recorder attached to a lathe dynamometer. The length of cut was approximately 7,600 inches for each replication.
Previous results have shown that tool forces are linearly related to edge recession. Thus, tool forces after a predetermined length of cut ( 7,600 inches) were selected as a tool wear index for these tests and applied as a method for ranking the HSSes for wear resistance in wood machining. The workpiece sample disks were MDF cut from a single source with a density of 46.5 pcf maintained at approximately 8 percent moisture content. The round ¾ -inch-thick MDF test specimens were randomly selected for the turning tests that were replicated nine times for each HSS and heat treatment combination. The terminal tool force components (Fp and Fn) were compared by an analysis of variance (least significant differences). For the purpose of this paper the results are summarized as 1/X where X is the normal force Fn (see Figure 1).
hardness. For example, the analysis of variance for the normal tool force at the high hardness heat treatment shows M- 2 had a lower force than T- 15, but T- 15 was slightly harder than M- 2. Further, M- 2 is the same hardness as Vasco Wear at the low hardness heat treatment but their mean normal tool forces are not the same (although they are not statistically different). Hence, chemical attack or hot corrosion could be responsible for some
of the wear resistance observed.
An HSS within each high- or low-hardness heat treatment may not necessarily wear less, as indicated by an inspection of the mean tool forces alone (Figure 1). For example, Alloy Z has the lowest mean tool forces in the low hardness heat treatment, but Alloy Z is also slightly harder than the other HSSes. If the tool steels had the same hard-
continued
Overall, HSS heat treated to a high hardness wore less than HSS heat treated to a low hardness (Figure 1). However, some HSS grades showed better wear resistance over other grades that had increased hardness. For example, M- 2 (Rc 65.5) had a higher wear index (1/X) and a lower normal force than T- 15 (Rc 66.3) as shown in Figure 1. Other examples in Figure 1 show that HSSes of the same hardness had different wear indexes. Consequently, although hardness is a general indicator of wear resistance, other factors influence wear.
Generally, harder tool materials are more resistant to chemical attack at elevated temperatures (refractory) and/ or are better heat conductors. The data in this study suggest that this factor may explain the variation in wear resistance for different HSS grades with the same
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