The benefits of an option of coolant-through the tool should not be underestimated. In the case of Thread Rod , the tooling is rotating, and at recommended SFM's, the tool's RPM will be quite fast. The centrifugal forces of this high rotation, will attempt to negate the benefits of the applied coolant. aussiessay
One of the main guidelines in designing fastener joints is that the length of engagement should be long enough so that the bolt will break rather than strip the threads in the tapped hole if the joint is overloaded. It's generally easier and more cost effective to replace a broken bolt than to have to repair or replace the part with the tapped hole.
A limitation of this design though, is with through holes, where other than making a mess, the coolant does not get applied where we need it - at the insert's cutting edge. Another affected application will be with horizontal jobs where gravity will tend to pull the coolant towards the bottom of the hole, as a result varying the amount of coolant applied. This will have different effects as the tool travels around the circumference of the hole.
