【转帖】best way to tolerence machined hole to center of boss

yang686526

best way to tolerence machined hole to center of boss?
i have a product that is getting seven new holes added. it's more important that the holes be drilled in the center of their bosses (so the self tappers wont bust out the side) then the holes be positioned to [-a-], [-b-], [-c-].
should i call each boss a datum tolerenced to a/b/c and then toerence the drilled hole to it's datum (boss)?
or can by tolerencing each boss to a/b/c and then say their holes must be concentric to them?
thanks
i would add a datum to one of the bosses and callout out the 7x holes using position to locate that feature to the boss datum.  then underneath the feature control frame callout 7x individually as well as calling out  7x individually next to the datum applied to the boss.
i think i see what you're saying.  the word individually means to make each boss an unnamed datum and check it like the one that is described.  
am i understanding ?
thanks again
joe
7x means individually. there really isn't a need for the word individually.
seems to me your 3rd option would work "or can by tolerencing each boss to a/b/c and then say their holes must be concentric to them?"
hard for me to make a suggestion without a picture.
chris
systems analyst, i.s.
solidworks 06 4.1/pdmworks 06
autocad 06
the feature control frame for concentric to should be ?
self tap hole diameter +/- tol
small circle in big circle (for concentric)1.0 to its boss?
thanks
if you have access check out asme y14.5m-1994.  i would stay away from using concentricity as this is very difficult to measure and usually used on things where balance is required like high speed shafts.  position is the control you want.  i still beleive that one callout for your holes will work.  it will not be long before the gd&t experts chime in.
i thought so too. but i thought there would be a better way then naming every boss a datum that would be seven new datums.  i think we're to h now so i have enough letters to make them all datums.  if i have to i will.
appreciate all the help.
any way you can show a pic?
chris
systems analyst, i.s.
solidworks 06 4.1/pdmworks 06
autocad 06
no i can't confidentiality agreement was signed with customer.  i wish i could.
sorry
let me explain again.  i have a print change which added seven bosses.  we will take metal out of the die and cast the bosses in.  then we machine one hole in each boss.  those holes need to be in the center of the bosses or the screw might break through as it self taps. the position to a/b/c is not as important. upholstery will be fastened to it in the assembly process at my customer.
my customer was upset about the gage cost to measure each to a/b/c. i have a target fixture that would need to be added to.  i told them we could check the bosses on the cmm at each die set then use a small mmc template to check each hole during regular production, to save the money.  
now they want me to dimension the print for them.
the example shown in y14.5 for this situation (fig.5-39) suggests that you would use "7x individually" next to both the new datum callout and the feature control frame.  so the first response to your question would be the most correct.