Nominal and Tolerance Differences 1

[Bewler]

I'm designing a part and I had a question about additional cost but I'm not sure if this really affects my part.

One tolerance on the part is originally called out as .095+- .002
I asked my suppliers if the true nominal value could be called out without additional cost, .0948+-.002.
They responded and said there's no cost associated as it doesn't buy any additional tolerance and they can measure to 4 decimals, however, doesn't this add .0002 of tolerance adjustment? Wouldn't the .0948+-.002 limit the part to .0968 and .0928 rather than .097 and .093 or would the .002 be rounded from .00249 and .00195 so the min and max are actually +-.00249?

Simply put, does a .xxx callout mean suppliers can round from +-.xxx49 back to .xxx?

 

[chez311]

Bewler,

I would recommend you take a look at a thread which parallels your exact question (

https://www.eng-tips.com/viewthread.cfm?qid=453654).

I will repeat what I said there, if you follow ASME Y14.5-2009 section 2.4 it has the below verbiage which essentially states the tolerances provided on the print are absolute limits, out to infinite decimal places - a measurement of 0.0001 over a x.xxx (3 decimal place) tolerance is nonconforming. In the 2018 edition this is section 5.4 and is almost identical. I am sure ISO has similar provisions.

Considering the tolerance applied (+/-0.002) your supplier is probably already utilizing measurements out to four decimal places to ensure that their process is in control and within your tolerance zone taking into account measurement uncertainty/repeatability. Quite honestly they probably won't change anything in their process for a 0.0002 nominal shift with the same size tolerance zone.

 

[Bewler]

Thanks Chez311, that's very clear and to the point and exactly what I was looking for. My company doesn't have the ASME Y14.5 notes so I have no reference for these items though its called to that spec on the print.