Not matter how many time I have seen this I still don't really understand what they are asking for. I have a drawing with the callout of TRUE R.125. What does the TRUE mean?
As Kulpa posted, it means that the view of the print is not square to the radius itself, the print view is angled away from the radius.
So if you set the part on a comparator EXACTLY like viewed on the print where the callout is, you would not be able to measure the radius correctly.
In the pic from the standard, the radius is that angled surface.
If you put the angled surface flat, you can measure the 20mm radius.
If you put the bottom there on the comparator and shine light over the angled surface, you're gonig to get some complex curve, but not a radius, and definitely not 20mm.
Usually it is done with some fillet, and the engineer doesn't feel like making a custom print view to tolerance the fillet, so they find somewhere you can see it cattywampus and put TRUE after the callout to compensate for the 2D view of the 3D geometry.
As Kulpa posted, it means that the view of the print is not square to the radius itself, the print view is angled away from the radius.
So if you set the part on a comparator EXACTLY like viewed on the print where the callout is, you would not be able to measure the radius correctly.
In the pic from the standard, the radius is that angled surface.
If you put the angled surface flat, you can measure the 20mm radius.
If you put the bottom there on the comparator and shine light over the angled surface, you're gonig to get some complex curve, but not a radius, and definitely not 20mm.
Usually it is done with some fillet, and the engineer doesn't feel like making a custom print view to tolerance the fillet, so they find somewhere you can see it cattywampus and put TRUE after the callout to compensate for the 2D view of the 3D geometry.
