Position vs True Position

[@shankar554]

I may not comment here often but do know that I truly appreciate your videos on GD&T. You are doing god's work for us mechanical engineers 😘

[@Gdandtbasics]

Thank you for the kind words, Shankar! We try!

[@trexinvert]

True position is basically a "datum axis" where the "cylindrical volume tolerance" is anchored. That's why you have [basic x, y] dims to locate that. Now, you just calc positions away from that "datum axis" coord point. The diameter of the "cylindrical tolerance" is dependent on the "measured hole/feature diameter".

[@davidfowler5546]

One thing I may have missed in this video is how big the blue cylinder should be. In the example shown, the value is .005. Does that mean the diameter of the blue cylinder is .005, or does it mean that the position of the whole must be within .005 of the true position, so the blue cylinder would have a diameter of .010?

[@Gdandtbasics]

Thanks for the question! The diameter of the blue tolerance zone cylinder is the value in the feature control frame. So if the tolerance states .005 in the control frame, then our tolerance zone diameter is .005

[@marshalldtech3908]

ตกลงค่า true position คือตำแหน่งที่สมบูรณ์แบบ ส่วน position คือตำแหน่งที่ไม่สมบูรณ์แบบ คือค่าที่มี tolerance ผมเข้าใจถูกไหม

[@MrAkosFintor]

Yessssir. great video! Thank you! Can you make one how to check true positions on cylindrical parts (round parts) and how it's different from run out ? thank you!

[@dillibabu6402]

Please do the video about the selection of instruments

[@HectorHernandez-by9dc]

Could please explain how to apply mmc to a internal thread. I do understand it should be to a pitch diameter, but I’m still very confused how it works. I really appreciate all your explanations and in advance thank you so much for your hard work to teach us.

[@Gdandtbasics]

Thanks for the comment! Applying MMC to the pitch diameter of an internal thread is simply applied by adding the MMC modifier symbol to the feature control frame however this is extremely hard to inspect. One would have to use the maximum pitch diameter allowed for that thread type using the data listed in ASME B1.1. This is why we recommend adding "MINOR DIA" for internal threads so that a simple pin could be used to engage the minor diameter in a fixed functional gage.

[@ernest.g]

Great video

[@alikahaei9305]

What I understand from the video is that True Position is the specified coordinate or in other words the nominal position of where the feature should be in theory but our product owner (who is a mechanical engineer) says True Position is the measured position. So, which one is it? Note that the answer can finally prove that software engineers are indeed smarter and more precise than mechanical engineers so I'm looking forward to the answer. 😂

[@Gdandtbasics]

As a mechanical engineer myself I would have to surrender a "loss" for "our team". The ASME Y14.5 - 2009 Dimensioning and Tolerancing Standard reads directly as "Basic dimensions establish the TRUE POSITION from specified datums and between interrelated features. A POSITIONAL TOLERANCE is indicated by the position symbol, a tolerance value, applicable material condition modifiers, and appropriate datum references placed in a feature control frame." We see this misinterpretation of verbiage often. The important part is to understand that the position symbol establishes a tolerance zone centered at the theoretically perfect location and the measured axis or midplane must fall inside that zone.

[@pardo94]

God bless you :)

[@sarge7526]

Thank you for this explanation

[@Gdandtbasics]

You are welcome!

[@MA-ns5ov]

Thanks for this. Metrology Laser tracker / Stationary CMM tech here.

[@Gdandtbasics]

Glad it was helpful!

[@danishshafiq8008]

Hi there, Wonderfully explained. Just a little question. By position tolerance, we're specifying the centre of that axis in a circular position tolernace zone (if we consider looking at the part from top view). But don't we have to use other controls like perpendicularity or angularity to control the inclination of the center axis within that cylindrical tolerance zone?

[@Gdandtbasics]

You are definitely thinking correctly, however one thing most people forget is that Position will ALSO control orientation of the hole to the primary datum referenced in the feature control frame. For instance if a hole is perpendicular to the primary datum plan, the position tolerance would be first controlling it perpendicular to datum A and then likely located to the following datum references. However, you can ALSO refine the position tolerance and hold the hole to a tighter orientation that the original position tolerance by adding an additional feature control frame below the position that only controls perpendicularity, angularity, or parallelism as you mentioned.

[@trexinvert]

Good question. If you "actually" measure a part with the hole, you will need to place the part down on an inspection plate, right? That face of the part should correspond to a drawing indication for [-A-], right? Then when you get a pin gauge, you will place it not only through the hole but "sitting" on that [-A-] surface, correct? Thus, you just guarantee that pin is perfectly perpendicular, now the hole that it goes through can be rough and slanted in which case that forces a smaller pin gauge to fit. Do you see how that happens, the pin "must sit" perpendicular, but if the hole is not perfect form then the "best" pin will end up being slightly smaller. However, it's always perpendicular. Then once, you find the best fit pin, you can proceed to measure the x(measured) and y(measured) from the datum x and y edges. Then add the radius(pin gauge diameter /2) to get the center point measurement of that particular measured hole. At this point, we are 50% there and...I hope this helps and you can figure out the rest.

[@tonyarc9455]

Ugggg.... why cant they just give me REAL tolerances!?? THIS should be THIS diameter.. etc... I dont get it! A FEATURE should be within a certain tolerance! For GODS SAKE!

[@jacencko011]

But there IS a tolerance and it IS real. On face value it can seem really confusing and then when a bonus is involved it seems even more confusing lol. It’s all about how the parts will ultimately function when assembled when they are not “perfect”. May I ask what exactly you find confusing about it? Do you know what the formula for true position is?

[@oberweiss7190]

If you do not include a datum, can you locate a hole with respect to its true position as defined in a solid, using position tolerance without datum reference frame?

[@Gdandtbasics]

Thanks for you question, the position symbol must always include datums. The true position only exists with respect to datums and would be under defined with out datums.