Tap wrench
From Biocrawler, the free encyclopedia.
A tap wrench is a hand tool used to operate any small tool that has a square driving head as part of its body. These are usually cutting tools of which the most common are taps, however there are other small hand tools such as hand reamers that they may be employed with.
The image shows two Bar style tap wrenches, these two examples share a similar method of tightening or loosening the cutting tool. One of the handles is fixed while the other is threaded, turning the handle in the required direction will exert pressure directly on the square head of the cutting tool or, in the case of the lower wrench, able to move a shaped jaw which in turn applies the required clamping pressure. There are numerous sizes of Bar tap wrenches required to cover the range but generally speaking the smallest size that will accommodate the square head is chosen, the risk of breaking the cutting tool by the use of excessive force is therefore reduced.
The second style of wrench shown is a "T" tap wrench, the body of this model uses a collet design with the threaded nosecap used to close the four collet fingers, these fingers exert the clamping pressure on the cutting tool's drive faces. This style is used where space is at a premium and extra reach is required. Sizes of this model vary in two ways, length of the main body (extra reach) or diameter of body (size of cutting tool used). The hole through the body, that forms the collet fingers, may be stepped (usually by two different diameters),this increases the range of cutting tools that may be held by the one size of wrench.
These wrenches all have two handles, this ensures that with correct usage a balanced twisting action is applied to the cutting tool reducing the chance of snapping the hardened cutting tool.
Proper use
Taps and reamers are hardened by heat treating when they are manufactured. While hardness is desirable to ensure long tool life, it induces a brittleness and so the tool must be used with more care than other hand tools. Unlike drills, taps cannot automatically remove the chips created by the operation. A tap cannot be driven with a single rotation to cut the threads as this will cause the creation of long chips that will not clear and will thus jam the tap, perhaps causing breakage of the tap. When cutting threads, normal usage is to cut the threads between 1/2 and 2/3 of a complete turn (180 to 240 degrees of angle). The tap is then backed (turned opposite to the thread cutting direction) for about 60 degrees - until the chips created by the cutting action are broken by the back edge of the cutters. Proper lubrication is also important during thread cutting, with the lubricant specific to the material being threaded, generally light machine oil for most metals and kerosene for cast iron. (High speed machine tools require specialized water-oil emulsions.) It may be necessary to periodically remove the tap completely to clear the chips from the tool and from the hole, especially when a "blind" hole (closed bottom) is being threaded.
Reamers are never reversed in use. To clear any chips the whole tool is withdrawn and the chips carefully removed from both the hole and from between the reamer's flutes. The timing of the withdrawal is gauged to prevent any heavy packing of the chips, the likelihood of a breakage is reduced considerably if there is free space for the chips to move into. Reamers may be manufactured with either straight or spiral flutes to assist the cutting action as well as the movement of chips.
Even though this is a slow speed tool, the use of safety glasses is highly recommended owing to the possibility of a metal shard being released at high speed should the tool break. Also, when threading blind holes, compressed air may be used to clear chips from the workpiece, an operation absolutely requiring the use of safety glasses.fr:Tourne-à-gauche
