Capability study on true position callout. What would be the upper and lower spec?

[eypon]

Hello all,

I have a question my customer is asking me to do a capability study on a True position call-out of 2.0mm in diameter. What would I input as the Upper and Lower Spec Limits?

Thanks

Eypon

 

[jrubio]

If the Customer does not set tolerances in the Customer Drawing, just establish you as Supplier the tolerances:

i.e: Dmax, Dmin. meassuring some parts you have.

 

[prabhudvp]

Hi Eypon

If the true postion tolerence itself is 2mm
it becomes max spec
zero is your lower spec

Prabhu

 

[jrubio]

In fact there are 3 grades of fredom.

The center coordenates:
X, Xmax, Xmin.
Y Ymax, Ymin.

D , Dmax, Dmin

That´s why many companies use this charasterist as Attributive and therefore to compare with a master. (If attributive var no capability is feasible according with the manual)

If you have to meassure a Capability for that var you have to meassure 3 vars.

¿Do this charasterist define your process, segurity, significant?.

No, Use other. more easier to meassure

 

[Jim Wynne]

Hi Eypon

If the true postion tolerence itself is 2mm
it becomes max spec
zero is your lower spec

Prabhu

It depends on the software, I think. In some instances, there is a difference between making no entry and entering a zero. The latter might cause errors. Most software has can explicitly handle unilateral tolerances, though. There's a good description of process capability formulae, including those for unilateral tolerances, here

 

[Trae1170]

Re: Capability study on true position callout. What would be the upper and lower spe

True Position as is all form tolerances are 99.9 % of the time unilateral, so you would only have a upper limit. The only time that I can think of where they may be a bi-lateral tolerance would be for callouts like .00002" - .0001" Flatness, I measure parts with this tolerance and it is bi-lateral. There are probably others but they are not the norm.

Like previoiusly stated, make sure the software or excel form you are using will calculate unilateral tolerances and you enter this information correctly (either enter a zero or leave the lower tolerance blank; on the form I use, you actually have to specify if it is a bilateral or unilateral tolerance and if you indicate unilateral it automatically blanks out the box for the lower limit). What type of gage are you using to check position, a fixture (attribute gage) or something like a CMM (variable gage)? In the positional dimensions that I inspect, I use a CMM. I am able to either enter the calculated position or I can use the X,Y & Z coordinate values it reports. The coordinate values are much more accurate, they tell the exact location, whereas the calculated positional value basically tells you what zone you are in, but because your zone is a circle you do not know exactly where you are at (EXAMPLE: .0002" position could be .0002" anywhere within that zone, whereas X=2.00 , Y= -.999 & Z=1.250 is an exact location.)

Thanks,

 

[Mezzaluna]

Re: Capability study on true position callout. What would be the upper and lower spe

it also depends how the tolerance is written. Is it a diametrical (Ø) true position tolerance? If so your the upper limit will be the old "two times the square root of a squared plus b squared" - unless I;m reading the question wrong that is. If so , apologies.

Mezz.

 

[tenaiwa]

Re: Capability study on true position callout. What would be the upper and lower spe

Hello,

Here we need to use 3 different laws. Normal law, Form defect law also known as folded Normal law and finally Rayleigh law:

a. Normal law:
this law can be used only for double limit characteristic, which mean characteristic which has a defined minimum and a defined maximum. For example: we can not use it for shape defect characteristics because they have only a maximum. The better we are the more our results will tend to 0, but also the more our Cpk will tend to 0, which would mean that our result is really bad...

To sum up: use Normal law only for dimensional characteristics (example: 55 +0.1/-0,05 or 61 +0,2 -> 61 +0,2/-0)

b. Form Defect / Folded Normal Law:

This is one variant of normal law.

for quick explaination:
for unilimit (maximum) characteristics.
when raw results of the measurement machine can be negative and are always positive after calculation.
Raw result are according normal distribution. When calculating the absolute value we some how fold the result on 0 axis.

for detailed information, please, look in internet: folded normal distribution

To sum up: use it for positioning and some of form defect characteristics (see enclosed file)

c. Rayleigh Law:

for unilimit (maximum) characteristics.
When even raw results can absolutely not be negative (Cylindric positioning, concentricity...).

for detailed information, please, look in internet: Rayleigh distribution