Basic numbers

What Are Basic Dimensions?


Basic dimensions describe the theoretically exact location, orientation, size, or profile of a feature or datum target. Because basic dimensions are theoretically perfect dimensions, there are no tolerances associated with them. A basic dimension is noted as a dimension with a box around it and must mathematically relate back to the Datum Features. In ISO standards, basic dimensions are called TEDs – Theoretically Exact Dimensions.

Because basic dimensions refer to theoretically perfect positions, when chain dimensioning basic dimensions, there is no tolerance accumulation.

Basic dimensions must always include a geometric control to ensure that there are no dimensions with a zero tolerance. Therefore, a basic dimension can be thought of as a nominal dimension where the geometric tolerance sets the tolerance range. Basic dimensions are always used to locate a feature using Position.

To better understand basic dimensions, let’s look at an example. Figure 1, which is discussed in the video, shows a part containing a pattern of four cylindrical holes. Basic dimensions of 15 and 30 are used to locate the true position of the holes from Datum Features B and C. We know that these are basic (thus, perfect) dimensions because this is indicated by the dimensions having a box around them. The geometric tolerance for the holes, shown in the feature control frame at the upper right hand of the drawing, defines the tolerances for both the position and size for the pattern of holes

https://www.gdandtbasics.com/reporting-basic-dimensions/
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Thank you, just as I thought also. Information much appreciated.

While Basic dimensions don't have tolerance, it is usually worthwhile to report their value. The shop needs to know how to adjust their equipment to produce good parts. With position, there's a vast difference between a hole pattern where all the holes are out the same direction and distance and one where the holes are scattered. Profile has the same type of issue. Is the shape good but shifted or is the shape running all over the place.

Technically you don't have to report the Basics, but practically you should.

Thank you. Yes, we usually report the measured for Basic with no tolerance, just deviation. This all stemmed from how to report Profile. We had a feature that had ALL AROUND profile and what I did was take hit points all around, created a Feature Set with those points, then report Profile of that set. Does that sound like a correct way to report Profile of a line ALL AROUND.

Thanks again,
Greg

Might want to turn on graphical and text reporting in the Profile report, especially if the part is out of tolerance. Graphics make it easy to understand where the out of tolerance area is at. Text gives adjustment data. Personally, I don't think that you can reduce Profile down to a single useful number.

I'm gonna start an argument, Basic numbers do have a Tolerance, but not to themselves, they are tied to a Feature Control Frame and that perfect number can fluctuate with the Tolerance given, even when you call out a position, Those hard numbers are given and results usually differ, but if they differ to much away from that Basic number, and fall out of the Tolerance in your FCF then well you might have wondered a little to far from your basic number. So I would argue they do have a tolerance, just not to themselves, you can't just say +/-.005 or anything like that. Also Basic Numbers are not necessary to record, But If you are scanning it's best practice to make sure the CAD model Matches Your BASIC numbers, Don't always assume your Engineer did it correctly.

NEVER assume your Engineer did it correctly.

Fixed it for you....

At my shop we normally assign an internal tolerance. If a feature has a T.P. of .030 and it is a 2D location, the basic X and Y dimensions are +/-.010. That way, if both are within .010, the T.P. is .030 or less. When reporting to the customer, it is what it is and no tolerances listed.

Thanks King, You like how I twisted that though, I gave those poor old Basic Numbers Tolerances but not really.

At my shop we normally assign an internal tolerance. If a feature has a T.P. of .030 and it is a 2D location, the basic X and Y dimensions are +/-.010. That way, if both are within .010, the T.P. is .030 or less. When reporting to the customer, it is what it is and no tolerances listed.

If some of your machinist don't know the formula for Position, Just Tell them to take your highest number that's off from there basics, times it by 2, take there smallest number and divide it by 2, add the 2 together and it isn't perfect but it will get them real close to the actual position.