I am starting to get this LinkedIn thing. Articles get hundreds of views and posts get thousands, even tens of thousands. So, I am going to condense and repost some things from earlier articles.
Most toolholders other than side-locks use friction to hold the endmill shanks. So you need help to get repeatable tool changes. We found these wrench-free aluminum shaft collars from @Ruland. The aluminum won't damage the cutting edges and holds up to the heat of shrinking.
Starting with a balanced and preset tool assembly. Slip the collar over the endmill until it it against the end of the toolholder. Use the slot in the collar to orient the tip of one tooth, like a gunsight. Look through the slot and rotate the collar until the tip lines up with the slot. If you are using variable pitch endmills, identify tooth #1 and set to that each time. Close the collar lever. Put a witness mark on the end of the toolholder (an automatic center punch works well) and use that as your target when orienting the new tool with the collar's slot. You can use a caliper depth rod or depth micrometer to record the collar location from the tool point. Remove the collar and place on the next sharp endmill at that measured depth and sight in a tooth. Balance and offset are retained with each tool change!
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