What tap should be used for deep holes with extra thread length? Why do some applications cause a “No Quote” or “No-Guarantee on Performance” response?
From the perspective of a tap manufacturer, the general rule is that any thread length more than 1-1/2 times diameter is considered “excessive”. It’s a good rule in terms of tap performance. Acceptable strength of thread as determined by length of fastener engagement might best be illustrated by the thickness of an Industry-Standard nut as assigned to its mating bolt. That functionality between nut and bolt seems to have had success! But, the debate rages.
The biggest challenge for a tap is chip evacuation. The size of the chip created (as determined by hole-size and length of available chamfer), and the geometry of the tap’s flutes (number of flutes, flute volume, and flute influence on direction of chip flow) set some limitations on tap performance. If the flutes of the tap fill with chips, the stress on the tool increases. If more chips are created than the tap can evacuate, something has to give! Usually, it’s the tap! Neither the tap manufacturer nor the machinist, want that to happen.
Obviously, chip creation and evacuation from the hole is not an issue for forming taps. But torque and tool stress is! As a forming tap “pushes” material away to assume the shape of the thread on the tap, the material “pushes” back. All materials have a “memory” in their molecular structure, some more than others. Even as they assume a new shape after tapping, they expand a little trying to return to their original shape. As the tap is run deeper, this “memory” puts more pressure on the tool. This creates friction. Friction increases torque. Too much torque causes bad things to happen. Tools break. Machines stall. Parts may be scrapped. Bad things!
With either type of tap, depth of thread needed to be produced is an issue. The material being tapped is certainly important. Engineers and designers have their own reasons for specifying thread length required in a part. Our job, and yours, is to find a way to make that happen. On occasion, we are allowed to request a revision to the original design. When that option is not available, we do the best we can. Taps are designed to allow additional reach into the hole. Chamfers are lengthened to reduce the size of the chips, making them more manageable and reducing stress on the tap. Tool geometry is engineered for the material being tapped, and flutes designed for more efficient chip flow and evacuation. Coatings are added to the tool to reduce friction. Coolants and lubricants may be experimented with. Tapping speed adjustments can be made. Common sense should be applied. Still, as with anything, there are limits.
Our limits are based in our experience and our own limitations in manufacturing capabilities. Our recommendations are focused on success. Ours and yours!