Position-fixed or float with thread cutting screws?
Position-fixed or float with thread cutting screws?
(OP)
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
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





RE: Position-fixed or float with thread cutting screws?
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,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at http://eng-tips.com/market.cfm?
RE: Position-fixed or float with thread cutting screws?
Thread cutting screws are fixed. The undersized hole is equivalent to a tapped hole.
JHG
RE: Position-fixed or float with thread cutting screws?
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at http://eng-tips.com/market.cfm?
RE: Position-fixed or float with thread cutting screws?
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at http://eng-tips.com/market.cfm?
RE: Position-fixed or float with thread cutting screws?