【转帖】lack of knowledge..true position

yang686526

lack of knowledge..true position
i have found that some of our customers are moving over to gd&t however they seem to not understand it completely. they are actually making the location of the holes >2.5 times "tigher". for example on some holes they used to call out +/- .005 in cartesian coordinates, they now use true position, and it reads .005 with respect the the appropriate datum features.
shouldn't the true position be .014?! i think they are missing the the concept that it is a diameter.
have you run across this often or am i completely wrong with my anaylsis.
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this is very common. i also see .005 used on most tol.
i don't know your design, so i don't know if .014 would be correct.
it is also very common for depts outside engineering to loosen the tol to make the part "cheaper".
very few companies get appropriate training in gd&t for various reasons..
chris
solidworks 06 5.1/pdmworks 06
autocad 06
i see the same thing as a checker and all you can do is show them the square and circle conversion chart.
i usually just calculate it using 1.414*a
if your tolerance use to be (+/-.005) .010 total it would be 1.414*.010=.014 dia positional tolerance.
cheers
i don't know anything but the people that do.
it's very common for engineering departments that are just starting to convert to gd&t.  it often comes from managers telling them that it's the same as before, without knowing enough to tell them there's a little math involved (try going from inch to mm & apply gd&t!!!).  most engineers may be rusty at the math, but showing them graphically the comparison of the square to circular/cylindrical tolerance zone and the underlying math will put it indelibly in their memories.
jim sykes, p.eng, gdtp-s
profile services
cad-documentation-gd&t-product development
then throw into the mix the feature modifiers it could create some confusion.  i would have to suggest they get some formal training before they put themselves out of business with increased part costs.
best regards,
heckler
sr. mechanical engineer
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never argue with an idiot. they'll bring you down to their level and beat you with experience every time.
precise-
you are getting good advise from these guys.
with us management at the wheel, however, i think the method will never be understood by those that need to understand it.  why?  because the method requires training, practice, discipline, committment, practice, and more training.  the method costs money to incorporate and pointy-haired mbas take too long to gain an understanding of the method much less quantify the long-run benefits.
tunalover
bear in mind to make real benefit of gd&t (positional) you often need to actually go back to first principles and do the calcs to determine what the required tolerance to guarantee interchangeability is (or reference a pre-prepared hole chart).
we do a lot of redrawing/designing old parts that had +- tols on holes etc.  we usually convert to positional.  converting +-.005 to dia .014 is often a simplification.  
if we do the analysis based on hole size and what's going through it we often end up tightening tolerance slightly as before there was frequently a chance of parts not matching up.  when possible we make use of the calcs and real requirement for interchangeability to 鈥榬elax鈥?tols and take advantage of mmc/bonus tol but due to the need for backward compatibility we often can鈥檛 鈥榬elax鈥?the tolerances as much as we鈥檇 like.
just to add to kenat's comment;    when doing a tolerance study as a part of your form fit & function requirement, use the line to line or net fit. i see a lot of engineers add .001 to the tolerance zone to assure clearance.
that is not required and has been studied to death by everyone from design to producibility engineers.
i may be straying from the original thread so i will get back to ebay where i know what i am doing.
cheers
i don't know anything but the people that do.
yeah, i knew a guy who did the math.  he kept the hole location tolerance at +/-.005/.014dia, and increased the clearance hole size.  he wound up with .281 clearance holes for the (14) x #10 screws that retained a mounting plate.
of course, when all the holes were anywhere near the basic location, the screw heads went right through the clearance holes, so we had to add washers to prevent that.
we also had to buy a fixture to locate the plate, because the holes no longer located it, and fixtures for each of our field service guys.
what a disaster.
it wasn't my project, so i couldn't just fix it.  
i couldn't pull rank.  the perp was a manager, i was not.  
he had attended classes, and had a certificate to prove it.  he was very proud of it.  i'm sure he had to study very hard to deal with the advanced mathematics and all.
worst part; i saw it coming, and couldn't stop it.
educating stupid people can get expensive.
mike halloran
pembroke pines, fl, usa
you are correct in the thinking that the diametric tolerance zone changes to .014 but designers new to gd & t sure do get messed up on this.
good luck!!!
dave d.