Form taps, or forming taps, are also called roll taps because they roll material into the thread form rather than cutting excess material away. They are also sometime referred to as fluteless taps because they create no chips and therefore need no flutes to evacuate them. However, form taps do need to have lubricant grooves to allow lubricant along the longitudinal axis of the tap. These “lube” groves also prevent causing a hydraulic seal and “lock” which can deform or destroy the part as the pressure builds up inside the part creating a pressure vessel.
When tapping soft, non-ferrous metals, and moderate steels, form taps enjoy long lifespans because their fluteless design gives them the rigidity to resist breakage and because of the absence of cutting edge, there is nothing to wear and get dull. If drilling a material creates chips that do not break, but rather form ribbons, the workpiece probably is conducive to thread forming.
One of the most important operational difference with forming taps is the need to drill a larger hole. Since there is no excess material being removed in the form of the cut chips, this difference is compensated by removing the excess material by first using a larger pre-tap hole size. Because material is being forced into the thread form, it may require experimenting with a specific hole size to reach the proper amount of material that will form into the required thread limit size for gaging purposes.
Second, form taps are typically run at speeds 1-1/2 to 2 times the speeds of a cut thread tap. Form tapping needs to be accomplished in a timely manner before the material can work harden from the forming action and create premature tool failure. The forming action coupled with the natural tendency for most materials to work harden will create a much stronger thread when compared to a cut thread tap. However, with the greater wear life and faster speeds, greater productivity can be attained using a form tap versus a cut thread tap.
Moving material into the thread form require more horsepower than cutting it progressively into the required thread form. The form tap will exert greater pressure on the hole walls than a cut thread taps and is typically unsuited for thin wall applications such as tubing or tapping near the edge of a part. Use of a good-quality cutting oil or the appropriate concentration of a water-based lubricant to limit friction and keep things at a reasonable temperature.
In addition to stronger and more durable thread structure, form taps carry several advantages:
No chips that must be fished out of blind holes
High-speed capability for greater efficiency