Significant Figures applied to GD&T 7

[cmwatson78]

Thank you so much for your replys, it has really helped me.
Im new at all this forum stuff so Im sorry if I keep asking questions wrong.


The reason for the question of significant figures was because, I was asked was to find out what the term was for when you divide two numbers with different decimal places. You cant divide them with out adding a zero to make them have the same number of places, so whats that called when you cant do that...... It was found that what he was trying to find was the term "significant figures."
So, my boss, is trying to apply the law of significant figures to a deminsinal drawing using GD&T. But, can you use that law? Or, can you only use tolerance? and when you have different decimal places you add onto the tolerance.

 

[KENAT]

cmwatson78, multiple threads on the same topic is generally not a great idea. It was probably reasonable to start a new thread over in GD&T as that is where the question probably fitted better but this one should have been a reply to one of your two existing threads.

thread404-195481 thread1103-195484

I think the answer to your question is no, but I’m still not really sure I understand correctly.

The number of decimal places has significance on most American Drawings as I detailed before. If you aren’t working to typical American Drawing standards the answer might be different.

At the end of the day if you are doing some kind of tolerance analysis you need to take into account the tolerance of each dimension.

So if you have the block tolerance I gave before and you have a stack of 1.3 + 1.200 the total worst case stack is = 2.5 +- .035

Does this answer your question?

What drawing standards are you working to, this may help with the answer.


KENAT, probably the least qualified checker you'll ever meet...

 

[rb1957]

cmwatson78,

i'm having trouble visualising what you're trying to do ... why would you need to divide two real numbers on a drawing ?

could you give us an example of the problem ?

 

[cmwatson78]

I have to see how much info I am allowed to give before I can ask. I work for a company who does not want to give away info. Let me find out and Ill ask again in a way that might be easier to understand.
Thanks again

 

[rb1957]

oh geez, we're only asking the dim'ns invovled are 1.0 and 0.200 ... you could make them up ... i just find it odd that you're dividing real numbers on a drawing

 

[handleman]

Significant figures have nothing to do with making a drawing. They are only applicable to the results of calculations whose inputs were gained by measurement, and they express the amount of certainty of the result. For example, if you measure the diameter of a circle on a piece of paper, you may be able to measure it to a certainty of 0.1mm. Let's say you measured 13.5mm. You can then calculate the circumference of that circle to be 13.5*pi. If you use a calculator to perform this operation, the result is 42.411500823462208719245685674273. Obviously, you do not know the circumference to this level of accuracy. You can really only be confident in its accuracy to about 42.4 mm.

This is basically the opposite of dimensions on a drawing. Every nominal dimension on a drawing is exact because you are defining it by the drawing. If the drawing says 13.5mm, it means 13.50000000000000000000000000000..... Tolerances on the drawing, whether they are defined by the title block, GD&T, or called out explicitly on the dimension itself, express how far away from the theoretical exact value the actual fabricated part can be.

 

[JsTyLz]

I think the poster is talkning about assembly tolerances. I would recommend statistical tolerances for the higher levvel assemblies iff (if and only if) your processes are verified to meet 1.67 Cpk. Otherwise just add up the tolerances and watch the final assembly tolerance blow up. Look up RMS method or sum of squares method for defining assembly level tolerances.

 

[fcsuper]

To add to handleman's comments, his statement about what a dimension means is established in the ASME Y14.5M-1994 standard.

Significant figures are fairly well described here:

http://en.wikipedia.org/wiki/Significant_figures

Trust me, you don't want to use this method on drawings.

Matt
CAD Engineer/ECN Analyst
Silicon Valley, CA
sw.fcsuper.com
Co-moderator of Solidworks Yahoo! Group