High-Performance Composites

MAY 2014

High-Performance Composites is read by qualified composites industry professionals in the fields of continuous carbon fiber and other high-performance composites as well as the associated end-markets of aerospace, military, and automotive.

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M A Y 2 0 1 4 | 5 1 FEATURE/MACHINING UPDATE S I D E S T O R Y Hole quality defned edges/corners, an increase in feed rate through the hole, to 0.1 mm/rev (.004 inch/rev), reduces the total length of time the tool is in contact with the mate- rial, and, thus, further limits heat trans- fer throughout the tool. Although chang- ing the speed and feed rate might seem elementary, choosing a slower speed and a higher feed rate is the opposite of what is normally recommended for drilling of CFRP laminates. • The chips. At a reduced RPM and higher feed rate, the tool cuts more deeply per revolution. "Now a problem arose in that this type of tool gener- ates extra material, which has to go somewhere," says Mueller-Hummel. "In traditional metal drill tools, when that buildup tries to squeeze the tool, the tool pushes that material into the hole edges, which, in effect, workhardens the edge and improves its fatigue life." This does not apply in composites drilling, so excess material must be evacuated as soon as possible. A progressive flute design now helps to evacuate the chips. In addition, the chip size is reduced by means of a technique called micropecking: The drill motor is designed to alternately apply force and retract, many times per second. This action literally hammers the chips into smaller pieces (see "Learn More"). In manufacturing — and the prerequisite engineering — it is not possible to make components to an exact size every time. Thus, the amount the actual molded component's size may deviate from the target dimensions must be known and listed along with its nominal measurements. A component's maximum and minimum permissible size are known as its limits and their difference is termed the toler- ance. These defnitions also apply to the holes drilled to accommodate fasteners when joining multiple components to form an assembly. ISO 286: System of Limits and Fits is a coordinated system of hole and shaft tolerances that is used in both engineering and produc- tion. It is defned and maintained by the International Organization for Standardization (ISO, Geneva, Switzerland), an independent, nongov- ernmental organization that comprises members from the national standards bodies of 161 countries. The ISO 286 standard states, "hole basis fts [as opposed to shaft basis fts] have four preferred hole toler- ances (H11, H9, H8, and H7)." Hole basis fts are used when holes are made with standard machine tools (drills, reamers or end mills). The above hole tolerances are used to indicate the precision of drilled holes in aerostructures. Fig. 1 shows a typical hole size and tolerance designation where size is most commonly given in milimeters (mm). The smaller the IT number, the more precise the hole diameter. Cutting-tool manufac- turers often use H8, H7, etc. interchangeably with IT8, IT7, etc., to describe the precision of drilled holes. According to Dr. Peter Mueller-Hummel, senior manager for the aerospace and composites business unit of Mapal (Aalen, Germany), "the standard for drilled hole tolerance in aerospace metals is H7, meaning that the diameter does not vary more than .010 mm [0.0004 inch or 0.4 mil]." He notes that the hole tolerance in The Boeing Co.'s (Chicago, Ill.) 787 Dreamliner program was initially relaxed to H8, or ±0.018 mm (0.0007 inch or 0.7 mil), but because the holes being drilled still could not consistently meet that, the tolerance was dropped again to H9 at ±0.04 mm (0.002 inch or 2.0 mil). The other measure of hole drilling precision given is CPK, or C pk , which is defned as process capability index, a statistical tool that measures the ability of a process to meet specifcations. It compares the process data distribution — e.g., hole diameter — to the specifcation limits and predicts future performance. A CPK >1.3 is required for "reliable and safe" drilling operations in aerostructures, according to Mueller-Hummel, and 1.7 is necessary for OEM qualifcation (a higher number indicates better performance). Thus, the CPK of 2.4 that Mapal reports for its new tool in the dry drilling of a wingbox assembly's stacked materials is impressive. Fig. 1 — Description of ISO 286 hole tolerance. Source: (top) generic textbook example and (bottom) ISO 286. 40 H 8 Tolerance Zone symbol 40 f 7 Tolerance Zone symbol Internal Dimensions (Holes) Basic size Fundamental deviation (position letter) International tolerance grade (IT number) External Dimension (Shafts) Basic size Fundamental deviation (postion letter) International tolerance grade (IT number) Tolerance grade (or zone) IT grade Basic size Fundamental deviation Example: (B) Hole only Ø25 H8 0514HPC_MachiningUpdate-OK.indd 51 4/22/2014 3:21:43 PM

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