I know, it sounds weird, but I have a print that calls for Parallelism of the Center line of the part to 2 Datums, both of which are perpendicular Cylinders on the right side of the part. What would be the best way in dimensioning it?
Can you post a snippet where we can see the tolerance requirement and the datums?
I can't post a snippet of the print, but I do have a crude drawing if that will help explain things. The center line is made up of 5 Cylinders on the top side. The 2 datums are Cylinders on the right side of the part. I put arrows to illustrate which way the cylinder would go.
I'd say that is a wrong callout. Can you get clarification from the designer? If the designer can explain what their intent is, maybe you can advice on a GD&T tolerance that fits the purpose better.
I mean, what should this centerline through the 5 cylinders be constructed from? The top or bottom of each cylinder or their centerpoints? Theoretically you could create a plane using the start points and end points of A and B and that would be used as datum for this centerline. But it makes no sense, really.
I'd say that is a wrong callout. Can you get clarification from the designer? If the designer can explain what their intent is, maybe you can advice on a GD&T tolerance that fits the purpose better.
I mean, what should this centerline through the 5 cylinders be constructed from? The top or bottom of each cylinder or their centerpoints?
I really think what they are trying to achieve is parallel to the face on that side (which it passes for), but they are trying to make it more difficult by going off of the 2 dowel holes.
Construct midline from A & B cylinders & align to that. Create a line from the 5 holes. Check for perpendicularity. I believe the only way for parallelism to be checked is to project circles from A&B cylinders onto the face, construct a line between them & see if pc dmis will accept parallelism between A-B line & line that you constructed from those 5 circles. Once again brilliant engineering at work. What do they teach & how do they teach at those fancy expensive engineering schools? I've only have had problems with those newbie engineers who think they know it all; I can't count how many times I got into fights with them over their bs ideas, totally unworkable GD&T callouts & tolerances for plastics that require millions of dollars worth of super precision measuring technology.
You don't have a positional callout for the five cylinders?
Sure do. The True Position callout for the 5 Cylinders is to 3 datums, the primary datum being the face on the side with the Cylinders. The other 2 datums are the ones we're talking about above.
Construct midline from A & B cylinders & align to that. Create a line from the 5 holes. Check for perpendicularity. I believe the only way for parallelism to be checked is to project circles from A&B cylinders onto the face, construct a line between them & see if pc dmis will accept parallelism between A-B line & line that you constructed from those 5 circles. Once again brilliant engineering at work. What do they teach & how do they teach at those fancy expensive engineering schools? I've only have had problems with those newbie engineers who think they know it all; I can't count how many times I got into fights with them over their bs ideas, totally unworkable GD&T callouts & tolerances for plastics that require millions of dollars worth of super precision measuring technology.
I think it will have to end up being something along these lines. (No pun intended) I might also check parallelism to the face and give them that dimension as well.
