The　helix　is　a　complex　geometrical　element.　During　the　process　of　a　dynamical　measurement　of　the　helical　deviations,　many　factors,　including　the　machine　and　the　environment,　lead　to　measurement　errors.　Although　ISO　as　well　as　national　standards　stipulate　the　tolerances　and　assessment　methods　for　helical　deviations,　these　standards　contribute　little　to　the　uncertainty　calculations　concerning　such　measurements.　According　to　the　Guide　to　the　Expression　of　Uncertainty　in　Measurement　(GUM),　all　measurement　results　must　have　a　stated　uncertainty　associated　to　them.　But　in　most　cases　of　helical　deviation　measurements,　no　uncertainty　value　is　given,　simply　because　no　measurement　uncertainty　calculation　procedure　exists.　For　the　case　of　helical　deviation　measurements　on　a　Computer　Numeric　Control　(CNC)　polar　coordinate　machine,　this　paper　analyses　in　detail　all　kinds　of　factors　contributing　to　the　measurement　uncertainty,　and　gives　the　calculation　procedure　of　the　measurement　uncertainty　of　helical　deviation.　As　an　example,　the　calculation　of　the　measurement　uncertainty　of　the　helical　deviations　of　a　worm　is　presented.