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Decimal places

Decimal places

For a cylindrical feature, is a diameter tolerance of +/-.001" the same as +/-.0010"? I always said no. That the tolerance with 4 decimal places would require a measuring instrument with better discrimination (resolution) to measure. Any thoughts?

I often see general tolerances in the title block specified as .XX +/- .010, .XXX+/-.005, .XXXX+/-.001. Is this in violation of Y14.5 (can't look up the section right now) stating that the tolerance should have the same number of decimal places as the nominal for English units.?

RE: Decimal places

My understanding would be +-.001 is the same as +-.0010 since they both mean +-.00100000000000000000000000000... but I have a tiny niggling doubt in the back of my mind.

Per ASME Y14.5M-1994 section 2 the decimals should match so yes your example tol block is a bit naughty.

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RE: Decimal places

I would say yes as the tolerance is a maximum deviation. if the measurer has a resolution of .0005 and it reads .0005 off nominal then he is in tolerance. I would expect that the posible measurement error plus the deviation from nominal should be less than or equal to my tolerance. I think it would be up to the machineist to decide if he wants to use better measreing equipment or make the part more exactly to compensate for uncertainty in the measurement.  

RE: Decimal places

+/-.001 & +/-.0010 are the same. You would still use a measuring instrument with .0001 resolution.

The title block tolerances are set by designer, so, even though it should match, what they are really saying is what the tolerance is for .xx & on.

Harold G. Morgan
CATIA, QA, CNC & CMM Programmer

RE: Decimal places


   As per ASME Y14.5, ±.001=±.0010.  Also, it is explicitly legal to place tolerance notes on the drawing title block.  The ASME standard explains what they mean.    


RE: Decimal places


I wasn't questioning the legality of having tolerances in the title block for directly dimensioned features. Its the tolerance not matching the number of implied decimal places in the nominal that's a pet peeve. I would write .XXXX+/-.0010" to be in compliance with Y14.5.  BTW, I also see this issue in the body of the drawing where someone specs .0105+/-.001". It's just my colleagues being lazy I guess and not setting the tolerance default in SolidWorks to have the tolerance "Same as Nominal". Sometimes my anality (if that's a word) shines through.


RE: Decimal places

I agree with the others.
I think it is good practice to be consistent. Use ".XXXX +/-.0010". This way there is no question.

SolidWorks 10 SP4.0
ctopher's home
SolidWorks Legion

RE: Decimal places

That what you get for using Solidworks, Pro/E requires the number of dec places for the dim and tol to be the same.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

RE: Decimal places


Your concern is right, there is an important factor in measuring is discrimination. A rule of thumb used in the industry for discrimination is the 10% rule. This rule states that we should always make sure that the measuring instrument's discrimination consumes no more than 10% of the total tolerance of the dimension being measured. For example, a part that has a total tolerance of .001" (or ±.0005") should be measured with an instrument that discriminates to at most .0001". Another example is that a part that has s dimension with a ±.005" should be measured with an instrument that discriminates to .001". It is important to note that, if 10% of the total tolerance is a discrimination that does not exist, we should use the next finer discrimination.


RE: Decimal places

It has nothing to do with SolidWorks, it's the text in the title block. Anyone can type in the decimal places.

SolidWorks 10 SP4.0
ctopher's home
SolidWorks Legion

RE: Decimal places


   The only sin here is the less than optimal formatting.  The title block note about decimal places only applies to dimensions that have no tolerances or other geometry control.  

   The ±10% rule applies to the tolerance, not to the number of decimal places.   


RE: Decimal places


On my last job with Pro/E the tolerances in the default were those set by the model. I would have liked the abiity to use something other than the same decimal places at times, but the Pro/E setup prevented it.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

RE: Decimal places

1994 ASME Y14.5m pg 25

All limits are absolute. Dimensional limits, regardless
of the number of decimal places, are used as if
they were continued with zeros.

12.2 means 12.20...0
12.0 means 12.00...0
12.01 means 12.010...0

To determine conformance within limits, the measured
value is compared directly with the specified
value and any deviation outside the specified limiting
value signifies nonconformance with the limits.


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