Lack of knowledge..true position
Lack of knowledge..true position
(OP)
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.
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.





RE: Lack of knowledge..true position
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
ctopher's home (updated 10-27-06)
RE: Lack of knowledge..true position
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.
RE: Lack of knowledge..true position
Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca
RE: Lack of knowledge..true position
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.
RE: Lack of knowledge..true position
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
RE: Lack of knowledge..true position
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 ‘relax’ tols and take advantage of mmc/bonus tol but due to the need for backward compatibility we often can’t ‘relax’ the tolerances as much as we’d like.
RE: Lack of knowledge..true position
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.
RE: Lack of knowledge..true position
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
RE: Lack of knowledge..true position
Good luck!!!
Dave D.
www.qmsi.ca