Who Knows What Kind of Trouble Lurks in the Darkness?

A "blind" hole is one that is drilled, bored, or cast to a specific depth without breaking through to the other side of the workpiece. Unless you are Superman, daylight will not be visible when looking into a blind hole.

If you search the Web for solutions to problems experienced while tapping blind-holes, you will find more suggestions than you can shake a tap at. Perhaps it's simpler to start with the basics, and formulate options from there.

The first challenge when tapping blind holes is "chip evacuation". There is only one way for chips to exit the hole, and flute space has its limits. A natural force, Gravity, is not likely on your side, as it might be with a "through" hole in a vertical operation. The application of spiral-fluted tap encourages chip flow away from the chamfer and pulled up and out of the hole. Another option is the use of a coolant hole through the tap. A coolant-thru tap, with either straight or spiral flutes, will utilize the force of the coolant against the bottom of the hole to blow chips backwards, exiting the hole.

Adding to the issue of chip evacuation is thread depth. This has an effect on the volume of chips, and the torque applied to the tap as the depth increases. Increasing depth also challenges the ability to provide adequate lubrication, or coolant, to the cutting edges of the tap. Chips packed in the flutes can also cause damage to the threads just cut, especially as the taps is reversed.

As you can see, there are plenty of issues. Here are some suggestions.

Consider the physical design of the part being tapped. Has Engineering specified depth of hole, and minimum full-thread needed? How much room is left at the bottom of the hole, once depth has been achieved, for the chamfer of the tap? (Chamfer length affects chip-load, and chip-load affects ease of chip evacuation.) Try to take full advantage of available chamfer by matching tap point-size to the specified hole-size. Ordering a "special tap" allows any of these critical dimensions of chamfer to be addressed in manufacturing.

Have a plan for chip removal. Consider the depth of thread required, and the clearance available for tap chamfer when full thread depth is achieved. Use a tap with the longest chamfer possible to reduce chip-load, but remember that the threads left by the chamfer toward the bottom of the hole will be partial.

Deciding to use a "forming tap", if the material is ductile, will eliminate the chip evacuation issues. Forming taps don't create any chips! Hole size will need to be increased to accommodate material flow with any "form" tap application. If the part design will not allow that alteration, forming is no longer an option. Because forming taps should always run at higher speeds, depth control in a blind hole is critical. When any type of tap, cutting or forming, hits the bottom of the hole, something has to give.

Tapping "blind" holes should not be a "no-brainer". Carefully consider the details of the application before ordering the tool. Alterations to the design of the tool will likely be easier to approve than later alterations to the design of the part.

Tags: blind hole, blind threading, forming tap