I know that this was all discussed in that other thread, but maybe here's a better way to summarize. There are (at least) three ways to dimension this:
[ol][li]Dimension it with GD&T: Six dimensions, all basic, e.g. [6X 60°] with some geometric tolerance on the holes.[/li]
[li]Dimension it with a conventional tolerance, 6 angles: Six dimensions, e.g. 6X 60.0°±.5°[/li]
[li]Dimension it with a conventional tolerance, 5 angles: Five dimensions, e.g. 5X 60.0°±.5°, and one undimensioned gap[/li][/ol]
With the first method, nothing weird happens. Each hole is located according to the tolerance zone specified by the geometric tolerance.
With the second method, because you've specified six angles, you have to measure six angles—you can't assume that if five are good, the part is good. For example, with actual angles of 60.1°, 60.1°, 60.1°, 60.1°, 60.1° & 59.5°, every angle conforms to the ±.5 tolerance and the part passes. But with 60.2°, 60.2°, 60.2°, 60.2°, 60.2° & 59.0°, only five of the six angles are valid, and the sixth is invalid. In effect, by calling out ±.5°, you're saying that you'll accept that deviation on any given angle,
so long as the entire system is still self-consistent. Therefore,
on average, an angle's actual deviation from nominal will be much less than .5° in a conforming part.
With the third method, five angles are specified, so you don't care what the sixth is (for purposes of inspection). As long as they all pass, the part passes. This means that 60.5°, 60.5°, 60.5°, 60.5° & 60.5° is valid, but if you measure the remaining angle, it's 57.5°. If that situation is unacceptable for your application, don't use this dimensioning method!
EidenC said:
If a welder welded all of the gussets going around the plate at 60.5 Deg apart, he would be way out of tolerance when he welds the final gusset on. The dimension would be 57.5 Deg between the last gusset and the first.
It looks like your drawing follows method 2, so that implies that the welder should definitely avoid placing the gussets at 60.5° from the previous one, because he'll quickly discover that he can't make all of the tolerances work. The assumption that because the tolerance is ±.5°, he could err by .5° on each dimension, is faulty. He can err by .5°, but only if he makes up for it in the other dimensions.
EidenC said:
Does this mean that the welder has to find all of the angles theoretically perfectly and he has +/- 30' off that?
No. If he does that, he might put the first at -0.5° and the second at 60.5° (relative to the origin). The angle between them is 61.0° > 60.5° (out of tolerance).
So, the bottom line is that any of the three methods are potentially valid, but they're not equivalent. In particular, method 2 isn't overdimensioned, but it may be confusing (because the six dimensions cannot simultaneously and individually take advantage of a full .5° in tolerance).
PeterStock said:
Or leave the angle dimension off and just state "EQUALLY SPACED".
With regard to the notation "EQUALLY SPACED", I think that would be unclear. How do the tolerances apply? Does that imply method 1, method 2, or EidenC's example (where you determine the theoretical lines 60° apart and apply tolerance from those points)? Or worse, does it override the tolerances (only perfect 60° angles are allowed, within the ability of the inspection gauges)?