英文原文CUTTING TOOLS When selecting cutting tools for a job, the first thing to consider is what type of operation needs to be performed. Here is a quick description of the basic cutting tools most often used in milling operations. DRILL A drill is used to create a round, cylindrical hole in a workpiece. Drilled holes can be through holes or blind holes. A blind hole is not cut entirely through a workpiece. Quite often, an engineering blueprint will specify a drilled hole to be drilled to full diameter depth. This means that the hole diameter must be a specified depth without regard to the angled tip of the drill. When you measure your tool length offset, you are measuring the length of the drill and its tip. So how deep do you drill the hole so that the full diameter depth is correct? Well, you need to know how long the drill point is. TIP: The length of the drill point is determined by the tool point angle and the drill diameter. You can calculate the length of the drill point by multiplying the drill diameter by a constant; the value of the constant depends on the drill point angle (most standard high-speed steel drills have a tool point angle of 118 degrees). For a drill point angle of: 118 degrees 135 degrees 141 degrees Multiply the drill diameter by: 0.3 0.207 0.177 Using these constants allows you to calculate the drill point length within a few thousandths of an inch. CENTER DRILLA center drill is a small drill with a pilot point. It is used to create a small hole with tapered walls. When a holes location must be held to a close tolerance, use a center drill first and then use a twist drill to finish the hole. The tapered walls of the center-drilled hole will keep the twist drill straight when it begins to drill into the workpiece. TIP: Many machinists use this rule of thumb: If the tolerance of the diameter of a center-drilled hole is not critical, drill as deep as you want this diameter to be. With a standard, 60-degree center drill below 0.375-inch diameter, the hole diameter produced will be close to the depth you drilled. With larger center drills 0.375 inch and above the depth-to-diameter ratio becomes larger, so you could be off by as much as 0.080 to 0.100 inch. REAMER A reamer is designed to remove a small amount of material from a drilled hole. The reamer can hold very close tolerance on the diameter of a hole, and give a superior surface finish. The hole must be drilled first, leaving 0.005 to 0.015 inch of stock on the walls of the hole for the reamer to remove. TIP: The ideal situation for hole size accuracy and location when reaming is to process the hole with the following steps: the hole is first drilled, then bored, then reamed. TIP: Stock allowance for a reamed hole will depend on the size of the hole. A general rule is: for holes less than 1/2for holes greater than 1/2stock of less than 0.0150 on diameterstock of 0.030 on diameter The type of workpiece material and the method used to create the hole will affect the stock allowance. TIP: A reamer produces the best, most uniform surface finish when it is fed into and out of the hole using the G85 (bore in, bore out) canned cycle. Many people try to save time by using the G81 (drill) canned cycle, which will feed into a hole and rapid out. It is quicker than G85, but will usually leave a helical swirl mark on the cylindrical surface of the hole. Although this swirl mark is only a cosmetic flaw and doesnt affect the size of the hole, the appearance of the hole may be rejected by some customers. TAP A tap is used to create screw threads inside of a drilled hole. NOTE: Great care must be taken when using a milling machine to perform a tapping operation. TIP: If you are using a machine with rigid tapping, feedrate (in inches per minute) = thread pitch x revolutions per minute. Also, you should never tap more than 1.5 x the taps major diameter. Threaded connections will not increase in strength if the contact length is more than 1.5 times the diameter of the fastener. If you need threads that are deeper, machine tap them first and hand-tap them to finished depth. If you tap deeper than 1.5 x the hole diameter, your chances of breaking the tap increase dramatically. Chip control becomes a problem. When tapping blind holes, always drill as deep as possible to avoid packing chips below the tap. Using a spiral flute tap will bring the chips up, out of the hole. To further reduce tapping headaches, make sure all holes to be tapped 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 tapp