【转帖】position-fixed or float with thread cutting screws

yang686526

position-fixed or float with thread cutting screws?
greetings,
this is my first thread.  i have followed the site off and on for years.  i have my asme technologist level certification so i am fairly book smart but not always world smart.  
i understand the position callout, fixed fastener vs. floating.  i understand the bonus tolerance as you leave mmc and so forth.
my question- two injection molded plastic parts.  part "1" has multiple clearance through holes for a self cutting screw.  part "2" has undersized holes to give the treads an area to bite in.  the fixed and floating formulas are written as if both parts have clearance holes.
what is done to figure the position tolerance?  
for the sake of discussion, qty 4 #8 thread cutting screws. part 1 has dia .185 clearance hole. part 2 has a dia .128 hole.
thanks,  troy
per asme y14.5m-1994 appendix b the fixed formula is not "written as if both parts have clearance holes", it is written as if one part has threaded holes (or equivalent) and the other clearance.
do you have any idea of how 'off center' the screws can be from the center of the holes and still cut a good thread/work?
if so i'd maybe add this value to the 'position' value calculated for the 'threaded' holes, calculated using the fixed fastener equation & using the thread max major dia rather than whatever size pilot hole the part actually has.
my only concern is that the screws will probably tend to 'self center' and may cut into the 'clearance holes' of the mating part (or push it sideways) so more conservatively you may just want to do the standard fixed fastener calculation, using the thread max major dia rather than the pilot hole dia.
that's just my idea though, i can't reference a standard or anything.
kenat,
troy442,
   thread cutting screws are fixed.  the undersized hole is equivalent to a tapped hole.
                        jhg
yeah, i've thought about it some more and agree with drawoh, just do the fixed fastener calc, but use the max major thread dia not the pilot hole dia.
kenat,
i nearly forgot
drawoh and kenat have it right.  but, a word of caution - the typical thread-cutting screw for plastics has a lot of "slop" between the throat of the screw and the i.d. of the pilot hole, and the screw can wander from the centerline much more than a typical metal tap or metal self-tapping fastener.  so, do the calc., but add in some extra wiggle room for the above reason.