【转帖】proper way to define an oversize thread

yang686526

proper way to define an oversize thread
question folks and i hope this is the right forum to do just that...what is the proper way to call out an oversize thread? i want to tap an #8-32 unc-2b hole with an oversize of .020 (h5 limit). i have not consistently found what is the standard or best way to produce a note on a drawing that states the above. any thoughts or suggestions would be great.
ed gebo
mechanical designer/drafter
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i always call out the thread designation and the pitch diameter if it's not standard.
best regards,
heckler
sr. mechanical engineer
sw2006 sp 5.0 & pro/e 2001
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never argue with an idiot. they'll bring you down to their level and beat you with experience every time.
if i'm understanding correctly, basically you want to increase the pre-drill size?  i've done this by spec'ing the pre-drill size (and typically depth too) then the tap size (#8-32 unc).
jim sykes, p.eng, gdtp-s
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ed,
  i'm not sure what you're asking about because drilling the tap hole .020 over would pretty much eliminate any material for cutting threads into, save for a few thousandths. i also can't imagine .020 over size on the pitch diameter as that would cause that size of a screw to basically flop around in the hole, so i'll give you what i know.
here's how the thread designation breaks down:
8-32 unc-2b
8= standard screw size (about .164 od)
32= threads per inch
unc- unified national coarse as opposed to unf for fine threads
2= the fit class. this is what you're looking for.
b= internal threads. a = external threads.
  as you see the "2" is the fit class so if you are looking for a looser fit, you should specify 8-32 unc-1b or 8-32 unc-3b for a tighter fit. the classes go up to 5 and get tighter and tighter. class 5 is usually called a unjf and is called a jam fit because that's what you pretty much have to do. a class 2 fit is a standard assembly fit that most threads are specified as. i'm curious to know what are you making that a class 2 fit on a #8 screw would not be sufficient. at this size a class 1 fit will be as loose as you could go and still be able to hold something together.  i hope this helps and i hope i wasn't completely off in my interpretation of what you were asking about.
powerhound
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i think you'll find the op wants to know how to specify the use of an oversize tap. the usual purpose for this is to create a thread which allows for plating or anodizing.
dear corblimeylimey...
"yes"
sorry for the bad napolean dynomite reference but it seemed to have a purpose here. nonetheless, that is exactly what i am doing. creating on oversize tapped hole for powder coating. i don't want to plug the holes but i do need to fasten items together. i don't want to chase the holes afterwards neither. your suggestion was the one i was leaning towards but was unsure if it would be understood. i do agree with adding a note to specify the type of oversize tap. thanks for your help and if anyone else wants to offer their experience/knowledge please share with the kid.
edson gebo
mechanical designer/drafter
sw2007 sp2.0
cadra 2006 (yup 2d still exists)
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sometimes success begins at failure
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powder coating into a #8 thread ????? .... no way!
sorry edsongbofff"> but that's asking for trouble. the thread depth is only about .014", so the resultant thread engagement would be almost entirely paint.
have you considered using a thread-cutting or thread-forming screw instead of tapping the hole.
in all honesty it's not me doing the task and that's exactly what i thought too. however, i was told by the person with the task they were able to screw an 8-32 bolt (could be a threadforming screw) that will hold as required with 8-32 oversize (h5) tap after powdercoating. i was unable to see the end result...i was merely just getting info to call out the hole correctly. this was one of those tasks that you do as asked and move it along. as a yute...i made a living at doing just the opposite and getting whacked for it. as i get older i want to say i'm getting wiser...time will tell?!?
edson gebo
mechanical designer/drafter
sw2007 sp2.0
cadra 2006 (yup 2d still exists)
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sometimes success begins at failure
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paint isn't designed to hold loading either. what happens when the powdercoat compresses and losens the screw?
but sometimes you have to step into a situation and correct the illinformed just because it's the right thing to do.  my company pays me to make sound engineering judgements and sometimes i get whacked.
best regards,
heckler
sr. mechanical engineer
sw2007 sp 2.0 & pro/e 2001
dell precision 370
p4 3.6 ghz, 1gb ram
xp pro sp2.0
nvidia quadro fx 1400
      o
  _`\(,_
(_)/ (_)
never argue with an idiot. they'll bring you down to their level and beat you with experience every time.
when a part is powdercoated, the screw threads are always masked. i agree with the others, it's asking for trouble.
i have done the same as heckler, "i always call out the thread designation and the pitch diameter if it's not standard".
chris
solidworks 06 5.1/pdmworks 06
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