What is the significance of pitch controlled tapping in machining?

[krishnryn]

what is pitch controlled tapping? anyone can enlighten me.

 

[berkeman]

Welcome to PF.

A Google search turned up lots of hits -- maybe try the same search?

 

[Baluncore]

what is pitch controlled tapping? anyone can enlighten me.

A single point cutter, in a lathe, will cut any pitch selected by the lead screw gearing, but it will take many passes to cut the full profile. That is because material removal must be done as tight turnings that have a smaller section than the tooth profile or pitch.

A tap or die, with four flutes, tapered over five threads, will perform the equivalent of twenty cuts in one pass, with each tooth removing one twentieth of the profile. The turnings will be removed as five coils, in each of the four flutes.

Thread cutting in soft materials, requires that the tap or die, bite into the material and then feeds at the correct rate. Without pitch control, the tool may turn without advancing at the required rate, creating a conical section that lacks thread near the start of the thread. Getting the tap to advance is the challenge.

With pitch controlled tapping, the tap is forced to advance at the correct pitch rate, so thread cutting cannot stall, and will be advanced correctly from the very start of the thread.

Pitch controlled tapping is really only economic and required, in the high volume production environment.

When hand tapping, the tool must be pushed hard at the start, to ensure that it begins to cut and advance at the pitch rate, yet the operator will often taper the entrance to the hole, in order to get the tap started with the correct alignment. That taper reduces the depth of the thread profile at the start of the thread.

But all is not lost, there is a skill to hand tapping the entry to a hole. By holding the tap gently in the chuck, of the machine that was used to drill the pilot hole, the tap can be introduced into the work quickly, stalling the tap after about one turn, by allowing the tap shank to spin in the chuck, once the tap has bitten. The tap is then aligned and has the pitch determined, so the chuck can be backed off, and the thread can be finished by sensitive hands.