Form tapping are the process of creating thread in metal, but form tapping is different than cutting tapping. Cutting tapping remove some of the metal to form the tapped hole; form tapping push the ductile metal outward to form the threads. Form tapping requires that a hole of a larger diameter than the cutting tapping hole are drilled into the metal component.
If the hole is drilled too small for form tapping, the tap will get stuck and break. The size of the drill bits that is used for form tapping will depend upon the size of the threads that is to be created. For instance, a 1/4-20 UNC thread will require a #7 drill bit of 0.201 inches for cutting taps, but the process will require a #3 drill bit of 0.213 inches for form taps.
Form Tapping: How It Works and Simple Tips
The extra 0.012 inches of diameter allow for the metal to take the form of the tap without breaking the metal component. Additionally, the larger diameter of the hole result in stronger threads; formed threads are 30% more stronger than cut threads, and formed threads also have a smoother finish to the threads which is an advantage in high vibration environment. The materials from which the component is created also have an impact on the tapping process.
Metals like aluminum, copper and mild steel are ductile; they will move to take the shape of the cutting tap. It isnt recommended, however, to use form tapping on brittle metal like cast iron and hardened steel. Brittle metals will crack when form tapped instead of form the required threads.
Lubrication are necessary for form tapping to reduce friction and prevent heat buildup in the tool. For aluminum components, it is recommended to use light oil for form tapping. For steel components, sulfurized lubricant is used to handle the high pressure of the steel component.
If lubrication is not used during the form tapping process, the heat that build up in the tool will cause the tool to gall. Faster speed can be utilized during form tapping than cutting tapping operations. The RPM can be set to 50% to 100% of the cutting tapping speed.
For softer metals, this can result in the tapping operations being completed up to 75% more faster. Additionally, form tapping are performed for blind holes. Form taps do not create metal chip; therefore, the chips do not have the potential to become pack into the blind hole.
Mistakes should of been avoided during the form tapping process. For example, it is important to ensure that the hole that was drilled is not too small, the lubrication is not too little, and the machine is not set to turn at too slow speed. These mistake will result in the breaking of the form tapping tool.
A go/no-go gauge should be used after form tapping is completed to ensure that the threads are of the appropriate size for acceptance into the component. To be succesful in form tapping, the component should be prepared using an oversized drill bit, the taps should be properly lubricate, and the machine should be set to a high speed.
