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Drafting: Basic dimensions

Drafting: Basic dimensions

Drafting: Basic dimensions

Hi everyone!

I am having some troubles with reading drafts, and I hoped someone here could help me out. When defining a positional tolerance (let's say a hole center), one must define it with respect to some reference features (namely datum planes or surfaces), and define also some basic dimensions. Those basic dimensions are theoretically exact dimensions with no tolerance associated. Ok, so far so good. However, they must have an allowed tolerance in order to be manufactured.

My problem is that we just bought some measurement equipment, and I want to know if I can measure those basic dimensions with our equipment. I have gone through ISO 2768 (both parts), ISO 1101 and DIN 7168, but didn't find anything useful besides a comment saying "tighter tolerances shall be applied to these basic dimensions".

Drafting is not my area of expertise, so I don't know whether I should control those basic dimensions in a dimensional control or skip them.

Best regards!

RE: Drafting: Basic dimensions

In ASME Y14.5 I have found the following comment (section 2.3.1 d):

"Where basic dimensions are used, associated tolerances contain the number of decimal places necessary for control." And the following figure shows a ΓΈ0.15 positional tolerance associated to a 25 mm dimension. Does that mean that to control that 25 mm dimension I need 0.01 accuracy or 0.001?

RE: Drafting: Basic dimensions

the usual tolerances, typically in the title block, are ...
if the dim'n is to 1 deciminal place, then the tolerance is +- 0.1",
2 places, +- 0.03"
3 places +- 0.01"

Quando Omni Flunkus Moritati

RE: Drafting: Basic dimensions

bibayoymicabayo forum1103: Drafting Standards, GD&T & Tolerance Analysis may get you a more detailed response, or if it's an inspection question then even forum286: QC, Inspection & Testing engineering.

Sorry rb1957 but if basic dimensions are being used then the block tolerances like you suggest have nothing to do with it.

The tolerance on those basic dimensions is given by the geometric controls in the feature control frames - that is the position tolerance or surface profile etc. The basic dimensions give the perfect target location, the geometric control then says how far the feature can deviate from that location.

bibayoymicabayo when referencing Y14.5 you need to say which year version as there can be differences.

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RE: Drafting: Basic dimensions

KENAT, thanks for the tip on the forums, I'll dig to check if there is any useful information there.

Regarding the Y14.5, it is '09 version.

RE: Drafting: Basic dimensions

Basic dimensions should be the theoretical exact location, the tolerance is how far away you can go from the basic and still have the part work. The tighter the tolerance, the more it will cost to produce. It makes no sense to apply a tolerance that you don't have the capability to measure. DYou didn't say what materials or what equipment you have to measure with, but keep in mind that temperature plays a big part in beain g able to measure a part. I know the standard talks about temperature and humidity controlled conditions, but how many machine shops still don't have AC.I recall one shop I worked in had parts that were calling for +-.0005 on an aluminum part. Just with the temp change from machining it to putting it in the cleaning tank it would grow more than the .0005.

RE: Drafting: Basic dimensions

The material I am working with is Titanium gr.5 (Ti6al4V), and the equipment is a Faro Gage arm (http://www.faro.com/en-us/products/metrology/measu...). It supposedly compensates for the temperature changes, since you select the material you are working with in the software.

The thing is that the equipment software allows me to define the nominal position of the features I want to control, but it makes me insert a tolerance (and does not allow it to be 0). Thus I don't know whether that has any influence in a positional tolerance or not, for example. But I guess this is a question to ask to the equipment support service...

RE: Drafting: Basic dimensions


If your software doing a pass/fail test? If so, the tolerance is absolutely necessary. If you have basic dimensions, look for feature control frames.

I do not know how your machine or its software works. You should read the ASME Y14.5 standard and understand how datums are defined. If a datum is a plane, I would hope you could rub your cursor over the surface, and the program would figure out where the datum was. You can read up on datum targets, and tell your drafters how much you like them.

If your machine "knows" the datums, the type of tolerance and the value, it should be able to tell you if the feature passes.


RE: Drafting: Basic dimensions

Drawoh, is that the proper way to develop a Y14.5M compliant datum structure with CMM?

I thought to do it properly you had to derive it from an inspection fixture (granite plate etc.)to better simulate the 3/2/1 contact points.

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RE: Drafting: Basic dimensions


I don't know. According to ASME Y14.5, if the datum is applied to a flat plane, the datum plane is defined by the three points sticking out the furthest. You would have to find these points by rubbing the face with your probe, and letting the software work out where the points are. What fun it will be if the part is concave. If you are confident that the part is flat, you can tap three arbitrary points.

Datum targets create several opportunities to make all of these easy to implement.


RE: Drafting: Basic dimensions


As long as the face is not concave. A fixture works nicely with datum targets.


RE: Drafting: Basic dimensions

Might want to try posting this question to the following forum...

Drafting Standards, GD&T & Tolerance Analysis

Regards, Wil Taylor

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