Welcome to Engineers Edge!
Well, you having thought about this in detail.... I'm tempted to just tell you to call me; however you did post this on the forum.
First, a little about me to qualify my answers...
I’m an old guy, 34 years engineering and design, mostly in aircraft, spacecraft and scientific hardware.
Last eleven years I have been a DFM and GD&T consultant and trainer – Google me
Authored four books on dimensioning and tolerancing (GD&T) and one book Design for Manufacturability.
You can feel free to buy one or all here --> http://www.engineersedge.com/catalog/index.php/cPath/21 --sorry about that folks....
Oh ya, I’m certified by ASME at a Senior GDTP – since 1997 and have an FAA Airframe and Powerplant Lic.. Educated too..
First, I agree that read-able tape measure increments (fractions) for tolerances and distances (Feature of Size) are a great choice for weldments. This is good practice from a DFM and dimensioning tolerancing point of view - works great.
The latest and greatest ASME Y14.5-2009 GD&T standard specifically says that that “US Customary linear units are the decimal inch”. But, it is understood that this is a general standard meant to cover the needs of many industries, so corporate standards are often utilized.
Often corporate dimensioning standards are supplements based on an existing standard (e.g., ASME, ISO) with additions or exceptions described. Typically, corporate supplements include four types of information:
• Choose an option when the standard offers several ways to specify a tolerance.
• Discourage the use of certain tolerancing specifications that may be too costly for the types of products produced in a corporation.
• Include a special dimensioning specification that is unique to the corporation.
• Clarify a concept, which is new or needs further explanation from the standard.
In other words – manufacturing and design need to get together and agree to what is in the best interest for your organization. Everyone should have a say – need keep competitive in this world.
Designing to common or preferred decimal or fractional increments is a good DFM practice for about any thing engineered. This will allow manufacturing to use standard or stock sizes without additional processes, such as milling a thickness. However, in engineering and design we often get stuck using odd non-preferred sizes, etc. due to design constraints. Design should try really hard to use preferred sizes.
As far as rounding up or down to whole numbers this should be specified in your company standards and or on the engineering drawing and understood by both design and manufacturing. Rounding up from .25 to .30 could in some cases cause a fit and stacking problem at end-item assembly or weldment. No simple answer, just that everybody needs to be on the same page.
For a standard tolerance block, you can do anything that fits your applications there are no hard and absolute rules.
For example, you can have a tolerance for a linear distance and a tolerance that applies between two fractional sizes…
You might post what you think you want and we can go from there…
Ok, it’s late here and I got a grandparents breakfast tomorrow morning…