【正文】 d are free of chips, and use a tapping fluid specifically designed for the type of material you are cutting. TIP: Tap drill size is the size of the hole required for a specific tap. For 75% effective threads the formula that will determine the correct drill size is: D – 1/N, whereD = major diameter of the tap and N = number of threads per inch A tapped hole with 75% of thread depth has only 5% less strength than 100% thread and takes only 1/3 of the cutting force of a 100% thread. END MILL An end mill is shaped similar to a drill, but with a flat bottom. It is used primarily to cut with the side of the tool to contour the shape of a workpiece. TIP: Programming an end mill to cut contour or pocket tool paths using cutter pensation (G41 and G42) allows you much more flexibility in adjusting the size of machined features. Using cutter pensation allows you to adjust the amount of stock removal. As an end mill wears, minor offset adjustments allow you to make every part the same size. You may also use a different size end and have the machine cut the same part features as with the end mill originally programmed for that tool path. BULL END MILLA bull end mill is the same as a regular end mill except that there is a radius on the corner where the flutes meet the bottom of the end mill. This radius can be any size up to onehalf of the tool39。s diameter. This gives the tool a spherical shape at the tip. It can be used to cut with side of the tool like an end mill. TIP: The primary purpose of a ball end mill is to machine lofted surfaces. The spherical shape of the tool is able to move along any undulating surface and cut anywhere along the cutter39。ll have to make when choosing a cutting tool. Most all of the cutters described above are available in three basic materials: highspeed steel, solid carbide, and carbide insert style. Almost all of the basic cutting tool materials can be used to cut almost all materials. It really boils down to performance. Highspeed steel cutting tools have very high toughness but lack wear resistance. Carbide, on the other hand, has a very high wear resistance but chips and breaks easily. Carbide will always be able to cut materials at higher speeds and feeds, but is more expensive. Carbide insert cutting tools are very useful in highproduction situations because the inserts are designed with multiple cutting edges on each insert. When they bee worn out, you index the inserts to the next cutting edge, and when all cutting edges are used, you only replace the inserts and not the whole tool. TIP: If you are using a highspeed steel drill, always use a center drill to get the hole started. Then drill the hole. This will ensure that the drilled hole is in the correct location. If you are using a carbide drill, it is not necessary to center drill first because carbide drills are ground with a selfcentering tip. Using a carbide drill to drill a hole that is already center drilled will damage the drill. The outer cutting edges will contact the tapered walls before the tip of the drill begins to cut. This will shock the outer cutting edges and cause the drill to chip. Carbide drills must begi