【正文】 r 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。s 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 centerdrilled 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 centerdrilled hole is not critical, drill as deep as you want this diameter to be. With a standard, 60degree center drill below diameter, the hole diameter produced will be close to the depth you drilled. With larger center drills – inch and above – the depthtodiameter ratio bees larger, so you could be off by as much as to 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 to 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 on diameterstock of 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 doesn39。s major diameter. Threaded connections will not increase in strength if the contact length is more than times the diameter of the fastener. If you need threads that are deeper, machine tap them first and handtap them to finished depth. If you tap deeper than x the hole diameter, your chances of breaking the tap increase dramatically. Chip control bees 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 tappe