CAD data records correspond to nominal geometry?

[SSezer]

thread1103-341758

Hello,

I have just read the post titled "Do you use B4.2 a lot?". I wanted to add some information to the page. But not able to due to thread was closed. If it is possible, It would be better to move this post under the thread.

There is a discussion on one of the user's following statement; "At least nowhere in the standards it is said that they shall be done in the nominals."

I couldn't researched entire ISO and ASME standards for related concern. Yet I know at least one standard exactly mention about it. 'DIN 16742 Plastic Moulded part tolerances' says that Moulded part drawings or CAD data records correspond to the nominal geometry.

https://files.engineering.com/getfi...file=cad_data_represents_nominal_geometry.jpg

 

[drawoh]

SSezer,

Let's start by agreeing on terminology. We have a 40mm shaft passing through a hole with an ANSI RC5 fit. I am using my Machinery's Handbook here...

Shaft Nominal diameter: 40
Shaft ISO tolerance: 40 e7
Shaft Max Diameter: 39.949
Shaft Min Diameter: 30.924
Shaft Median Diameter: 39.936

Hole Nominal diameter: 40
Hole ISO tolerance: 40 H8
Hole Max Diameter: 40.041
Hole Min Diameter: 40
Hole Median Diameter: 40.020

If you are submitting a drawing, you present this as some form of dimension and tolerance. There is more than one way to accomplish this in CAD. If the fabricator is working from your scale CAD data, your method matters.

For machining and sheet metal fabrication, I prefer to model at nominal size. SolidWorks allows me to apply the ISO tolerances. If I change my nominal shaft diameter from 40mm to 60mm, the parametric model updates the hole, then all the tolerances on both drawings update automatically.

If I am rapid prototyping this thing or otherwise working off the CAD model, I need to model everything at median diameter. This works for castings, metal or plastic. This might be critical for inspection by CMM. I am not familiar with them. From my point of view as the designer, this process is a pain in the ass. If I want to change the shaft size, I must manually look up the tolerances, work out the median diameters, update both models, and manually update the tolerances on both drawings.

Your fabricators must understand your scale drafting procedure, and they must trust you to follow it at all times.

Is there an industry standard that specifies one of these processes and not the other? Definitely, you need a company standard.

Could there be a note on the drawing specifying the CAD modelling?

--
JHG