Profile control vs. Size and Location + possible refinement
Profile control vs. Size and Location + possible refinement
(OP)
I have a part with 3 holes arranged to provide various amounts of offset to position pins and maintain parallelism with respect to one another. Up until now the responsible engineer has relied on linear dimensions to control location without giving much thought to controlling orientation (parallelism) as shown on the first page of the attached.
A new print was presented to me for review with an attempt made to use a profile tolerance. I had a good notion of what he wanted to accomplish so I dialed in the drawing to make it more presentable. It's the 2nd page of the attached. I've 90% convinced my self that I need a secondary datum and am leaning toward the central plane based on the .400" width or maybe one of the other holes. Any thoughts?
Actually my bigger question is whether profile control is the best strategy? I'd be more inclined to rely first on the tolerance of size and location (projected or not) as my controls and then refine with orientation (parallelism) as needed as shown on the 3rd page.
Either method in my mind would communicate the same funcional requirements, but is there anything else to consider? Is one easier to inspect than the other using a CMM for example?
A new print was presented to me for review with an attempt made to use a profile tolerance. I had a good notion of what he wanted to accomplish so I dialed in the drawing to make it more presentable. It's the 2nd page of the attached. I've 90% convinced my self that I need a secondary datum and am leaning toward the central plane based on the .400" width or maybe one of the other holes. Any thoughts?
Actually my bigger question is whether profile control is the best strategy? I'd be more inclined to rely first on the tolerance of size and location (projected or not) as my controls and then refine with orientation (parallelism) as needed as shown on the 3rd page.
Either method in my mind would communicate the same funcional requirements, but is there anything else to consider? Is one easier to inspect than the other using a CMM for example?





RE: Profile control vs. Size and Location + possible refinement
Chris
SolidWorks 10 SP4.0
ctopher's home
SolidWorks Legion
RE: Profile control vs. Size and Location + possible refinement
RE: Profile control vs. Size and Location + possible refinement
Chris
SolidWorks 10 SP4.0
ctopher's home
SolidWorks Legion
RE: Profile control vs. Size and Location + possible refinement
Frank
RE: Profile control vs. Size and Location + possible refinement
The larger the hole (whether controlled by a size or profile tolerance), the more un-parallel the two pins might be which is why I specified LMC in the positional tolerance. In other words, as the hole shrinks the better the parallelism (in theory). With that said, I would say that MMC works against me. Does this make sense?
RE: Profile control vs. Size and Location + possible refinement
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: Profile control vs. Size and Location + possible refinement
Also keep in mind some of the rules about profile: Profile tolerances must be applied to a true profile, so you have to have a basic dimension for the diameter of the three holes; you can't have them be .098 +/- .003. And profile can't use the MMC modifier, since by defintion a surface has no such thing as a maximum material condition.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Profile control vs. Size and Location + possible refinement
A full analysis on what tolerances are needed for the desired function has not been done by the responsible engineer, so consider the values as placeholders for now. He originally specified a profile of .003 but I assumed he really meant twice that which is why I increased it to .006". The impetus for making a change in the first place was to loosen tolerances. Prior to this exercise the distance between holes was +/-.001" and the supplier couldn't hold that nor was that offset accuracy needed. Obviously the enginner was inadvertantly proposing a tighter tolerance with the .003" profile Since the print was being revised anyway, It was decided to explore adding orientational control.
RE: Profile control vs. Size and Location + possible refinement
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: Profile control vs. Size and Location + possible refinement
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: Profile control vs. Size and Location + possible refinement
J-P: The last page where all 3 holes are assigned a +/- tolerance was from what my colleague proposed. His dimensioning scheme also suggested that these should be controlled a a pattern and not individually. I was going to propose my method 2b once I convinced myself that a profile tolerance is the way to go (which is now losing) and once he determined what he really wants from the design.