How to calculate true position

What is the formula for calculating true position?

Thanks

"Car 54, where are you?"

This is a meaningless question. All positions are relative. They are relative to something. And, calculate it from WHAT?

I suggest that you go (back) to high school and pay attention in math and geometry classes.

I am sure this thread will be closed.

Edit: Well, I guess the term "true position" has a specific meaning that I was not aware of. I apologize if my remarks above were not appropriate. But then, it seems to me that this is a poor choice of terms for what is actually meant.

If I remember correctly it's 2 times the square root of xsquared + ysquared. You might want to google it to make sure.

In GD&T a true position tolerance defines a circular (diameter)tolerance zone around a theoretically perfect position.
the equation for a circle is x^2+y^2=r^2 where r is the radius of a cirle and x and y are coordinates of points on the circle.

The rest of the answer is left has an exercise by the reader.

Astrolabe or sextant in the old days, GPS today.
Bill D.
Modesto, CA, USA, North America, North West Hemisphere, Earth, Solar system, Sagittarius arm, Milky Way galaxy, The universe

GreenMachinist17 said:

What is the formula for calculating true position?

Thanks

Click to expand...

Usually when the print says put the hole here, I just put it there, thats how I "calculate" it. The formula would be using a Cartesian coordinate system. (basic XY graph)

R

EDIT: 'Cause your a newb, ask the right question you must young Padewan. There is no formula for it, it just is true or not true, try getting it there and if it doesn't work then post a question about that. BUT here's an Olive branch, if you are asking how to measure TP, divide the tolerance for TP by 3 and use that for you linear tolerance between points and Origin/Datum and that will get you really close.

Seek training in GD&T per ASME Y14.5M-2009.

Erich said:

In GD&T a true position tolerance defines a circular (diameter)tolerance zone around a theoretically perfect position.
the equation for a circle is x^2+y^2=r^2 where r is the radius of a cirle and x and y are coordinates of points on the circle.

The rest of the answer is left has an exercise by the reader.

Click to expand...

Your statement is not universally true.
The tolerance zone may be circular or rectangular depending on the callout. Read ASME Y14.5M-2009, 3.6, DEFINITION OF THE TOLERANCE ZONE.

Thank you for the input guys, I am sorry if my question offended anyone but I am a newb and it sometimes comes with the territory. I think people forget that at some point in time you did not know everything but you had to learn and perhaps relearn something's you haven't brushed up on in 10+ yrs.

I guess my question pertains to a hole location for example it's nominal is x30. Y30 within .2 true position, But it measures x30.011 and y 30.011. I want a way to fact check our older brown and sharp cmm. How would I calculate true position based on the given example

The equation explained. The X axis deviation from nominal squared plus the y axis deviation from nominal squared = a square root which is multiplied by two. With the numbers that you gave the true position is .0311

As mentioned, true position is the circular tolerance zone around a theoretically perfect point, in your case .100 or R.05.

If you inscribe a square, the corner at 45 deg will be your radius (.05)
Remember the 45 deg right triangle relationship, 1/1/SQRT2 ?

The quick and dirty way we used to calculate it was:
True position tol.(in radius) / SQRT2

In your case: .05/SQRT2 = .0354

So basically, if your measured results are within +/- .0354, you are within that box and within your true position tolerance.
If your measured results are outside that box, you will have to trig it out like johnb53405 said.

Better than calculating is to find the survey marker and use a certified chain and then measure. Yes some survey numbers are in Rods... so look that up .

Actually true position is most often a certain distance from a certain place so [place + or - distance = position] is the correct formula.

To confirm look it up in a book..If you cant find a book to confirm then write one and it will be a big help to everyone who can't find position,

A Rod is about 5.0292 meters and/or about 16.5 feet and/or 5.5 yards...They made it not an even number so the Metric/ imperial guys would not consistently argue about it.

The Rod was based on how far a Martian could throw his wife.. Yes a grouchy wife would throw her husband so the number was not actual be an assumed average.

Enough of this as I am going to Clair Mi, Camp Rotary for a week with the Boy Scouts..Yes we will use fishing rods...Yet another kind of Rod.

And, Yes we sleep in those old army tents spiders and all, and a raccoon if you have any food in the tent. Thankfully not yet a Bear..

Last year I got skunked and did not get top fish here.

clare mi boy scout camp - Google Search

Inspired by this thread, I did some searching on "true position". Here's one:

http://www.gdandtbasics.com/true-position/

According to this, "true position" is the theoretical position of the feature and then there is an allowable deviation from that "true position" which is specified as the diameter of a circle enclosing the permissible locations. I suspect you want a way for calculating that deviation from the "true position". That formula is given on that page.

Wouldn't that be what we previously called a tolerance but refined into a circle instead of a square? Or am I still missing something here?

EPAIII said:

Inspired by this thread, I did some searching on "true position". Here's one:

http://www.gdandtbasics.com/true-position/

Wouldn't that be what we previously called a tolerance but refined into a circle instead of a square? Or am I still missing something here?

Click to expand...

That's my understanding. What "true position" does is allow a tolerance to reference multiple datums. The farther you are from nominal on one datum, the tighter the remaining tolerance is on the other datum.

Teryk

extropic said:

Your statement is not universally true.
The tolerance zone may be circular or rectangular depending on the callout. Read ASME Y14.5M-2009, 3.6, DEFINITION OF THE TOLERANCE ZONE.

Click to expand...

Very true, but given the level of the OP's knowledge, I went with a less than complete answer.

When teaching division, the first problems don't have remainders. You don't want little heads to explode.

EPAIII said:

Wouldn't that be what we previously called a tolerance but refined into a circle instead of a square? Or am I still missing something here?

Click to expand...

The part you are missing is that GD&T allows you to describe a FUNCTIONAL part..
Plus or Minus tolerances on each axis make the manufacturing easy, but they
don't describe what is and is not functional.

Take the "True Position" thing.. And most times, at least for me, you will see it
with a little M in a circle, Max Material Condition..

That hole has to line up with or be within a certain distance of something else, either
on the same part, or on another part.. Lets say a clearance hole for a bolt..

1/4" bolt, .275 ±.010 clearance hole. Tolerance on the other bit lets say is .005 TP..
IF the engineer is willing to skirt the edges we would see. .275 ± .01 TP within .010, and
then we would get the Magical "Max Material Condition".

So a .265 hole could be .010 off TP.. If its .005 off in the Y, it'll still work if its
.0000 off in the X. If we had to describe this with simple ± XY tolerances, we would have to
be ±.0035, and the part would still be FUNCTIONAL, but would be scrap per the print.

Now lets say we drill that hole to nominal, .275... MaxMaterialCondition(MMC), we get a .010 BONUS.
We can be off .010 now in the Y, and ZERO in the X and still have a functional part.. We can
also be .006 off in the Y and .007 in the X and still have functional part... Much better than
the ±.0035 if we went with simple plus/minus tolerances..

Now lets take that hole to .285, we get yet another .010 of bonus on the TP.. And we can be even
further off...


Its pretty darn cool stuff when you made the parts and then do the assembly and can actually
see the GD&T in action.. And a lot of times it actually benefits the machinist, its just
more complicated...

I personally prefer it.. It makes the prints look scary, and the "value" of the part goes up, meaning
I get more money.. When in reality its actually easier to deal with than straight ± tolerances (mostly)..

Bobw said:

It makes the prints look scary, and the "value" of the part goes up, meaning
I get more money..

Click to expand...

This has always seemed to me to be "what happens" when I send out a dwg that has a goodly dose of GD&T feature control frames. I think most shops see all that geo tolerance stuff (even if it's dead simple and easy) and fill in a multiplier factor on the quote prep sheet. Sort of like when you buy something whose description includes the words "clean-room compatible". Makes the price go up 5X.

specfab said:

This has always seemed to me to be "what happens" when I send out a dwg that has a goodly dose of GD&T feature control frames. I think most shops see all that geo tolerance stuff (even if it's dead simple and easy) and fill in a multiplier factor on the quote prep sheet. Sort of like when you buy something whose description includes the words "clean-room compatible". Makes the price go up 5X.

Click to expand...

I agree, but there is a reality in the added cost, even when it is dead simple. IE; I interpret one thing as dead simple, Hole to Hole TP easy!! Now my part goes to Quality, my inspector feels that all GD&T needs to be done on the CMM (ya ya I know, but it's his department) ANY time a part needs to be checked that way, it takes longer. Like double. Now shop rate is not the same for QC as it is for Manufacturing BUT it's still there. And thats just one linear dimension/2 holes.

R

Thank you guys for helping me to better understand