My holders do not slip on the post. What slipped was the post and base on the compound. I cured this by making a small of steel about 1/2" long by 1/8" thick by 5/8" wide which is the width of the compound slot on my craftsman 12x36 lathe. I welded this to the bottom of the base. This prevents the post and base from slipping slipping on the compound.
Chinggis if I understand your description of the pin I call that a roll pin. For a roll pin to work it needs to either be the width of the compound slot or fit into a hole in the T nut in the compound slot. Is yours set up like Martik777's?
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