The shrink fit tool holder is a high quality shank construction.The tool holder opening is slightly larger than the tools by using the thermal characteristics of the heat shrink machine, when the tool is placed in the tool holder, the tool holder bore size shrink to original and creating a huge compressiveforce. Since there is tittle expansion in the bore of the holder, tight tolerances are required on the holder to ensure installation every time. Please make sure end mill shank diameters with h6 tolerances, so that the tool will always work properly with shrink fit tool holders.