Basic Dimensions and General Feature Control Frame

Your BASIC dimensions are your theo dimensions that will then be controlled with a FEATURE CONTROL FRAME. You are correct as they have no tolerance, thats the function of the FCF. A general note on a print contolling "profile" would be for any dimension / feature not specificly dimensioned on that print. As in the old "REFER TO CAD MODEL FOR PART GEOMETRY" .

The way I do things for our engineering group, and keep in mind, I'm inspecting my own company's parts...I don't report basic dimensions, per se. If a hole pattern is defined on the drawing using basic dimensions, and then "located" with a feature control frame, I use Exact Meaure, define the datum features, and set up the FCF in PC-DMIS, then use text only reporting, and accept the default (out-of-the-box) report template. Now, the ExactMeasure will show where each hole is along the axes, will show the size it measured, and then report any datum shift if there is a composite FCF. I do not go back and use the location dialog box to report the basic dimensions themselves. I let the FCF and Exact Meaure do all the heavy lifting. Simply because, if there is any modifier on the feature, (MMC, LMC) each hole has to be treated differently, and will have a different tolerance, based on those modifiers.

This sounds to me like a simple miscommunication where someone said one thing, it has passed through a couple of other people, the words have been changed just a little bit, and now it doesn't really make sense.

IF you have surfaces on the print that are dimensioned with basic dimensions AND the form, location, or orientation of those surfaces is not toleranced directly with a feature control frame, THEN the surface profile feature control frame in the general tolerances block will apply to those surfaces.

That is the only way that I can make any sense out of the request that you have received.

This sounds to me like a simple miscommunication where someone said one thing, it has passed through a couple of other people, the words have been changed just a little bit, and now it doesn't really make sense.

IF you have surfaces on the print that are dimensioned with basic dimensions AND the form, location, or orientation of those surfaces is not toleranced directly with a feature control frame, THEN the surface profile feature control frame in the general tolerances block will apply to those surfaces.

That is the only way that I can make any sense out of the request that you have received.

+1

This sounds to me like a simple miscommunication where someone said one thing, it has passed through a couple of other people, the words have been changed just a little bit, and now it doesn't really make sense.

IF you have surfaces on the print that are dimensioned with basic dimensions AND the form, location, or orientation of those surfaces is not toleranced directly with a feature control frame, THEN the surface profile feature control frame in the general tolerances block will apply to those surfaces.

That is the only way that I can make any sense out of the request that you have received.

That is exactly what they are telling me. Every surface that is not noted by a feature control frame falls to the general notes Profile tolerance. Then they tell me to take the tolerance in the profile FCF in the general notes and apply it to the basic dimension. So if the Basic Dim is 3.00 and the general note profile tolerance is 0.030, then the basic dim would get a +/- 0.015. Is this correct?

thanks

sam
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No, it would get Profile of 0.030 which is not same as +/-0.015

Then they tell me to take the tolerance in the profile FCF in the general notes and apply it to the basic dimension.

You can't really get away with thinking about GD&T concepts in the same mindset as coordinate dimensions, which is, it seems to me, what this statement is implying.

You need to think about GD&T in terms of a zone within which the feature must reside. The basic dimensions tell you where the middle of the zone is(assuming that you have an equal bilateral tolerance, which is the most common situation.)

Imagine that you are making a drawing and you must use only the basic dimensions provided plus implied parallelism/coaxiality and implied perpendicularity(if the basic dimension would be 0 or 90 it doesn't need to be specifically called out) to create the shape. Once you have created whatever shape the basics describe you must offset it to create the zone that the feature must reside in. The offset value is equal to half of the value shown in the feature control frame.

So if the number in there is .030 you would offset the shape that you have drawn by .015 on each side. This gives you the tolerance zone that the measured feature must fit inside. If you make a few sketches you will soon realize that offsetting a shape by .015 on each side is not the same thing as applying a +/- .015 tolerance to all of the basic dimensions that went into creating that shape. The result could be the same in a few limited circumstances but generally it will not be the same.

If these concepts are new to you, or you haven't been exposed to them for some time, then a GD&T course will be worth it's weight in gold. There is a ton of information available online from sites like Tec-ease and companies like Effective Training International have published some great books and self study courses.

Profile vs Tolerancing:
http://www.tec-ease.com/gdt-tips-view.php?q=231

No, it would get Profile of 0.030 which is not same as +/-0.015

+1

Well, that is what I thought the response would be. I have been to GD&T class and was taught that basics had no tolerance. The class never mentioned the "general profile tolerance" being applied to basics... so i was not sure. Feels good to get one right for a change!

Thanks
Sam