In　order　to　improve　the　machining　precision　of　a　planar　enveloping　hourglass　worm　tooth　surface,　the　influence　of　the　machine　setting　parameter,　center　distance　error　on　the　helix　error　is　investigated.　Based　on　the　gear　meshing　theory,　both　theoretical　and　error　equations　for　the　tooth　surface　are　established,　by　which　an　expression　to　determine　the　normal　error　of　points　on　the　tooth　surface　can　be　derived.　The　helix　error　curve　of　the　reference　circle　is　studied　and　the　concept　of　influence　coefficient　is　put　forward.　The　impact　of　the　surface　error　caused　by　center　distance　error　can　be　obtained　using　the　influence　coefficient.　The　relationship　between　the　influence　coefficient　and　worm　parameters　is　also　studied,　and　meanwhile,　the　value　range　of　the　influence　coefficient　is　obtained.　The　influence　rule　is　verified　based　on　the　tooth　surface　precision　detected　by　an　hourglass　worm　and　hob　measuring　machine.　The　results　and　methods　of　this　work　have　a　great　significance　on　error　tracing　and　effect　analysis　of　other　machine　setting　parameter　errors.　©　2017　Taylor　&　Francis　Group,　London.