fraisa.com 337 i Technical notes regarding use of thread cutting tools Hardness increase during drilling For drilling, care has to be taken to ensure that a sharp, intact drill is used. If the wear is already too great, there is a risk that hardening of the periphery will occur during drilling. One result of this hardening may be the tap breaking. Age or precipitation hardening and upsetting the countersink When preparing for tapping, quality tools should be used which are adapted to the material. If the counterbore is too worn, the countersink will harden or be upset. This means the tap will encounter problems as soon as it starts to cut. This can lead to breakage right at the beginning. The countersink is important in guiding the tap. Deeper threads For deep threads, tools must be chosen with sufficiently long flutes. There is otherwise a risk of tool breakage due to chip build-up, because the chips cannot flow out of the hole. With Rigid Tapping, pecking in several steps can be programmed, in order to shorten the chips. Sufficient cooling is important. Functional performance of the compensation chuck A satisfactory compensation function of the chuck must be guaranteed (no jamming). Otherwise, despite the chuck, the thread may be miscut or pitch errors can arise. The torque level of the compensation chuck must be selected according to the thread diameter. Correctly adjusted coolant jet A specifically aimed coolant jet is important in tapping work. The direction should be from above, in the direction of the flutes, so that the cooling is also effective at depth. If the coolant jet is not aimed, the problem of heat development in the drillhole arises, with the risk of cold welding-together of the material, which in turn may lead to reduced service life and poor chip formation. Safety clearance When using Rigid Tapping, a sufficient approach distance must be programmed. If the clearance is too small, the problem arises of the machine spindle not synchronising with the feed before entering the drillhole. This results in pitch error of the thread, even with synchronous working. Suitability of the machine On a number of machines, there is a possibility that the recommended RPM cannot be applied. In this case, the RPM of the machine should be programmed accordingly. Inconstant RPM Programming too high RPM can lead to problems: if a RPM is selected which cannot be achieved (due to the inertia of the machine), the result may be premature breakage of the tool or miscutting of the thread. If observed closely, it may often be seen that a high RPM does not in fact produce the time saving which was intended.
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