# Please help me understand True Position



## KJW (Dec 20, 2012)

I know the formula is (Square root of 2(X Deviation squared + Y Deviation squared) = True Postion) and have read several pages on the net but I still don't know if I understand fully or I am trying to make it more complicated than it is? My boss told me to learn true position so I need to get it figured out.

Are X & Y just the Tolerance of each axis? If so I think I already have it figured out otherwise I am missing something.


Thanks ahead of time for the help.
Kris


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## Tony Wells (Dec 20, 2012)

Kris, one way to think of it is comparing a square tolerance zone with a circular zone. In simple Cartesian terms, an X-Y tolerance forms a square zone. For an equivalent fitup tolerance, both parts that assemble (or more than two) benefit from having a system that will allow you to have a greater error in one axis IF you have a smaller error in the other. If you are exactly spot on in one axis, you can have the maximum error in the other axis. As you move away from spot on on one axis, the allowable error in the other axis shrinks. I can scan a page or two from the older book I have that has a pretty clear explanation on it and email it to you if you like, or perhaps post it here if anyone else is interested.


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## KJW (Dec 20, 2012)

I think I need to see an example and have it explained to understand. I don't know where the X & Y numbers come from, Are they basic demensions? 

For example there is part that measures 2" x 2" x 2" and gets drilled with a centered .687 hole.

.6565 would be the measurement from the edge of the hole to the side (Y) and end (X) of the part.  1" to center of hole.

That would get a perfect .687 hole centered at the true position.  I know I am missing something and what I wrote probably does not make sense but hopefully it shows my train of thought at the moment.

Thanks for the help Tony.

Kris


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## Tony Wells (Dec 20, 2012)

OK, let's examine your example. On a 2 x 2 x 2 cube, the coordinates for the hole would be 1.0000 from one side, and 1.0000 from adjacent side. The old way was to either bind you with the title block +/- 0.010 (or now 0.005) or a non-title block tolerance of +/0.002 on both axis. This gives you a placement zone that is square that limited to a deviation of a maximum of 0/.002 error in either axis. But since you'll see this type of tolerancing many times on round parts, and holes where things bolt together, it doesn't make sense to have a Cartesian tolerance system. If you drew a circle, centered on the true position of the hole (no errors considered) at the radius that included the corners of that theoretical square, you'll see it has more area than the square. Now consider a circle inside that square. Now you have a smaller zone, right? The are in the corners is not included now. On the larger circle, you can see that as you move away from the centerline of one of the axis, you have to adjust your position to follow the arc and stay within the circle. But it allows you to be further out on one axis that a Cartesian method, provided you are closer on the other axis. As long as both parts use the same tolerancing system and tolerance zones, you actually gain room for error and still have a workable part.

Clear as mud now, eh? I'll walk to the shop in a bit and see if what I have is worth scanning.


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## Tony Wells (Dec 20, 2012)

Your boss may be testing you now. Simply put, True Position is there the print says the feature should be _if everything were perfect_. Of couse, that's kind of a theorectical thing, so along with the statement of True Position, a print will give you a tolerance zone, in one of several ways.


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## KJW (Dec 21, 2012)

After reading through the calculator website again I finally think I get it and don't know why it took me so long to understand. The X & Y #'s are not just the offsets but rather the difference between the basic dimentions and the produced part. I was trying to do the math by just using the offsets. I am sorry for being confusing and I appreciate your help Tony. Let me know if I am wrong but I think I got it now.

I don't think my boss was testing me but rather giving me ideas of what to study online first. I told him I have been reading a lot on the web and watching youtube videos on GD&T. The other day in a meeting only a few people in the shop were able to hold up there hand when asked who knew GD&T. I think a few of those guys  said they only knew certain parts of it. One of the QC guys said the shop does not use it that much but one of the bosses said it is important for as many people in the shop to learn it as possible. I would like to be one that knows atleast some of it.

The boss also told me that someday I will be writing programs for the Viper so he must think I have some ability other than just running a mill or lathe. The guy that was on the Viper before I started will be starting back on the Viper again at the beginning of the year, I don't know where I will go from there but I think it will probably be another mill for a while.

This guys youtube videos  http://www.youtube.com/watch?v=dtlLIllpl1w have tought me a lot about GD&T but I know I have years of studying to go. I have been taking notes from the videos and plan on going over the notes over and over until I understand it all. So far I am only on the beginning of video 3.

Thanks again for the help. 
Kris


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## Tony Wells (Dec 21, 2012)

It's really just a different, and usually more generous system of establishing a tolerance zone. It is actually pretty involved when you look at the whole system, but it all will make sense if you take it a step at a time. When I first was exposed to it, I was intimidated, but didn't have any choice but to learn it. One of my old bosses who had been in the machine shop business since the early 60's called me into his office once and showed me a print we were bidding on. It had several features toleranced at 0.0000 TP. ????? How can that be, he asked. Well, in the feature control frame (the box around all the symbols and numbers used in GD&T) there was this little "M" in a circle. I knew, but he didn't know, what it meant. Actually, there was quite a bit of tolerance on those features. As I recall, it was 0.030 at MMC. He was about to no-bid the part.

I found a link for you:

http://webtools.delmarlearning.com/sample_chapters/goetsch_ch12.pdf

If you haven't already found this one. I will try to scan a few pages from my book this weekend.

I think you're using the term "offset" in a way different to what I am used to. To me, an offset is a compensation you enter in the control to make up for the tool not being exactly where, or how large, it actually is. Sort of kin to cutter compensation. I think as you are using it, more properly it should be "deviation". That signifies the difference between print and part. You're not likely to hit the dimension exactly, nor have the means to verify that you did. At least to an extensive degree. True positioning is a way to express how far you may deviate from the exact dimension in a way that controls the fit and function of a part in the most generous fashion.


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## jgedde (Dec 21, 2012)

Great link on GD&T Tony!  Thanks!  What book is that a chapter from?

John


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## Tony Wells (Dec 21, 2012)

John, I believe it is excerpted from this book:

http://www.delmarlearning.com/Browse_Product_Detail.aspx?CatID=40069&ISBN=1111129827

Delmar Learning has a ton of stuff available online that is really good quality educational materials.


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## Wreck™Wreck (Jan 26, 2015)

I "sort" of like the GD&T tolerance method for the most part, however for a simple part it often just makes the drawing more cumbersome. Young Engineers seem to enjoy adding complexity wherever possible, it also makes the drawings look all the more incomprehensible to the average person.

I made a lathe part today that would have been well suited to this method. Aluminum round 2 1/4" thick, 11.00" (+ - .01) OD with a  bore 1.770" (+ - .005) deep at *8.5000" +.0000 -.0001* diameter, this is what the drawing actually had written on it, in 4 decimal places.


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## Tony Wells (Jan 26, 2015)

That's pretty funny. AL isn't stable enough for splitting tenths, IMO. I'd bet there is 0.010 clearance on the mating part. 

Who is going to inspect this bore?


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## Wreck™Wreck (Jan 27, 2015)

Tony Wells said:


> That's pretty funny. AL isn't stable enough for splitting tenths, IMO. I'd bet there is 0.010 clearance on the mating part.
> 
> Who is going to inspect this bore?



No one. Especially not the person who will be whining about the cost.


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## nobog (Jan 28, 2015)

I have found that in the "CNC" world we live in now, True Position is less relavent. It's supposed to give you "extra" tolerance but most inspectors really have no idea how to apply it and simply accept the part.  Many prints are laid out wrong as the drafters love to reference TP to A-B-C because that's how they've seen it before.

I get where and why its used but in the industries I have worked it has not caught on - much like the metric system.

Jim


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## T Bredehoft (Jan 28, 2015)

I had a job one time, when some metric machines were being installed in our plant.  I was working night shift, I came in and the foreman showed me this tooling part, said something to the effect that this pocket has to be .078740 deep. No tolerance given.  It was an area  about 1 1/2 by 2 inches. When I received it it was about .05 deep. It took me about half an hour to find out that 1) the instructions had been passed by two or three individuals,  2) they  had originally been "made it about 2 millimeters deep." Again, no tolerances given. The instructions had translated from German to English as "make this pocket exactly 2 mm deep". Turns out it was only for clearance so a part could move without hitting the bottom. Using an end mill I got it within .010 of 2 millimeters and everyone was happy.  Sometimes accuracy is relative.

Edit to correct typo.


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