Moisture Content of Southern Pine as Related

Holes 3-1/2 inches deep were bored with a I-inch spur machine bit in southern pine having specific gravity of 0.53 (ovendry weight and volume at 10.4 percent moisture). The bit was rotated at 2,400 rpm and removed chips 0.020 inch thick. For wood moisture contents ranging from ovendry to saturation, thrust was lower when boring along the grain (average 98 pounds) than across the grain (average 138 pounds), while torque was higher when boring along the grain (average 42 inch-pounds) than across the grain (average 33 inchpounds). For both boring directions, torque and thrust increased with increasing moisture content to a maximum at about 5 to 10 percent, then decreased to a constant value at about the fiber-saturation point. For the type of bit tested, net power at the spindle required to cut 0.020inch-thick chips at speeds of 2,400 rpm or less should not exceed 2 hp., regardless of boring direction or moisture content; thrust should not exceed 200 pounds. Chip types resembled those obtained in orthogonal cutting. Direction of borioB;=tangential, radial, longitudinal. Nominal wood moisture content: 0, 3, 6, 10, 1', 30, '0, and 80 peIceDt. Held roobnt were: Drill type: spur mad1ine bit. Drill diameter: 1 ind1. Spindle speed: 2,400 rpm Olip thickness: 0.020 inch (plunge speed of 1.6 indICS per S«ODd). Wood specific gravity: 0.'3 (ovendry weight and volume at 10.4 percent moisture rontent). The dlip thickness and spindle speed are representative of commercial practice. The spur madtine bit was selected because it produces holes of good quality in southern pine; geometrical specifications are given in Figure 1. New bits were used for ead1 replication and ead1 drilling direction. Except d1at ~ minor imperfections were corrected by hand honing, bits were put into test as they ame from the ~ of acturer . Several hundred bO81'd feet of rough-sawn PJtberD pine 4 by 4'5 were kiln-dried to 12 percent moisture con. tent and arolratdy surfaced on four sides to 3-1/2 by 31/2 indIes. They were then crosscut o foml 3-1/2-ind1 cubes; only dear, defect-free wood was accepted. The cubes were placed on stickers (widt the end grain exposed) in a room maintained at 60 pettmt relative hu. midity and 73°F. Fans assured adequate air circulation dtroughout the stacks until the samples read1ed constant weight. WOOD MOISTURE CONTENT affects tool forces and thus the energy consumed in madtining. Moisture also interacts with other factors to influence the process of chip fo~tion and, hence, affects the shape and size of the separated particle. While these subjects have been explored for such operations as periphetal milling and sawing. few specific data are available for macltine boring (5, pp. 347-358). In the research tq)()rted here, an analysis was made of the rdationship of wood moisture content to the thrust force and torque required to madtine-bore southern pine (Pm.s spp.) in the tangential, radial, and loogihldinal directions. Limited consideration was also given to certain aspects of chip formation. The authors are Principal Wood Scientist and Ass0ciate Wood Scientist, Southern Forest Experiment Station, USDA Forest Service, Pineville, La. They acknowledge the assistance of Mr. Arthur Flack, H~t, Inc., MIlwaukee, Wis., in desi~ the boriDI machine. This paper was recevied for publication in January 1972. ProcedUR A factorial experiment with three replications was designed with variables as follows: FOREST PRODUCTS JOURNAL Vol. 2a. No.ll 55 FI",re 1. Geometrical .pedflcatlons of spur machine bit. A = m_n rake an91_20. P = mean sharpne.. angle--6O. p. = sharp_s angle of spur.-35. 'Y = clearance angle of 11,_10. . = angle of lead (.pur to lip, measured at drcumferencej-180. h. = height of brad-O.20 inch hs = height of spur-O.1 0 Inch L = le"gth of spur at root-O.53 InCh D = bit dlameter-1.00 Inch ft = bit radlu-o.50 inch ft = radius of brad at root-O.09 Inch