When someone mentions tapping a hole, we all envision cut tapping. Cut tapping has been around since the beginning of machining, and it is still the standard in which threads are made, but it is not the only way, and it may not be the best way.
The idea behind cut tapping is easily grasped; make a hole to the minimum size of a thread and feed a tap into it. The tap cuts out the thread we all imagine, and the hole becomes the minimum/minor diameter of the thread.
Form tapping is a little more difficult to imagine if you have never seen it before. You drill a hole which is larger than the minor diameter of the thread, and then feed in a form tap which pushes material around and creates the major and minor diameters.
What are the differences?
Cut tapping is commonplace and taps are readily available for every size and type of thread imaginable. The drills needed are just as common and the hole size and hole finish are forgiving and easily achieved. However, cut tapping creates chips which can adversely affect the thread making process. Also, the design of a cut tap has inherently weakened the tap and will break if not careful.
On the other hand, form tapping is not as commonplace and requires a higher skill level. The hole tolerance is tighter, and the surface finish of the hole is also important. Some thread sizes will align with drill sizes, but with an importance of the quality of the hole, reaming the hole size is now recommended. A form tap also requires a great deal of torque and is not possible to do outside of manufacturing. For the good side, a form tap has minimal material removed making it much stronger than a cut tap, and more difficult to break. Form taps also have no cutting edge to become dull and are capable of threading more holes before replacing. They also create no chips and are well suited for blind-hole tapping.
So which is better?
Cut tapping costs less upfront, readily available, easier to accomplish, and can be done in the field. Form tapping is chipless, less chance of breaking a tap, and produces stronger threads. Both of these tapping techniques have their benefits, and each have their flaws. It is up to you to decide if and when the benefits of form tapping outweigh the benefits of cut tapping; it all depends on your situation
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