I used TTS ER20 holders for endmills previously. It lacked rigidity when milling steel. this is because TTS Er holders are designed quite inefficiently. there's unnecessary stickout right below the spindle nose registering area I decided, in order to maximize rigidity, I should get rid of all unnecessary part that lengthens the tool. therefore, the most efficient design is endmill holder type, which holds specific endmill holder shank size. also, the registering shouldn't register the spindle nose. instead the tool should register the flat part of TTS collet inside the spindle. i was able to reduce futher by doing so. TTS ER20 + 8.5mm endmill = 95mm from spindle nose. homemade endmill holder design = 45mm from spindle nose. the result is roughly 2X the rigidity. not only the tool is shorter, but the spindle is also lower. which lowers Z column flexing. for industrial type machines, such as bridgeport, TTS might still have enough rigidity. however, for most hobbiests and mid sized machine, rigidity cannot be sacrificed, especially when machining steel. hack, even bridgeport needs more rigidity, when machining demanding materials. this holder design achieves both benefits of: simple, fast, and easy to use TTS system maximum rigidity achievable by your milling machine (shortest tool stick out possible) hope this might help others achieve better rigidity for their machining projects! below is the pictures of the endmill holders and a 50 minute long video, that includes almost every process and details for making this endmill holder.