drawings with dimensions to theoretical intersections and metrology

[Aaron96]

When I get a print from a customer with obtuse or acute bends I immediately look to see how they dimension the bend locations. If they are dimensioning the intersection points I request them to change to the center of the bend radius. The intersection points are not a physical location, they are in space and cannot be controlled. In order to get these measurements accurately you need to use a CMM. With a gage pin and a height gage I can measure where the center of the bend is. Does anyone know of an ANSI spec, or something from ASQ that agrees with this? I know dimensioning to intersection points is a commonly accepted practice, but it makes it difficult when trying to actually make parts. I believe this is a point where what is easy on paper isn't so easy to reproduce in the real world. Any insight will be appreciated.

 

[Aaron96]

I did some more investigating and maybe this falls under ASME Y14.5M?

 

[KENAT]

Y14.5 M is the dimensioning standard, not inspection and emphasizes dimensioning on functional requirements not 'ease of inspection'.

Tolerancing to virtual sharps is supported in section 1.7.2 of the 1994 version of ASME Y14.5M.

If you asked me to change my dimension scheme from intersection points I'd probably say no but might vary depending on the exact tolerance scheme.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Aaron96]

I understand per Y14.5M that method of dimensioning is supported. As a designer isn't the cost of making the parts you design considered? If a dimension or tolerance scheme does nothing but add cost why would you do it?

I have dealt with customers that didn't want to change the drawing. When that happens I send them a price for making the parts to their drawing, or a no quote. I also send them a price if they change the drawing to the scheme I am asking for. If they want the cheaper price they change the drawing. If not, find someone else to deal with the headache of chasing dimensions that are in space and can't be controlled.

This is one of those area's that just because it's easy on paper, it doesn't translate to the real world. Centers of bends and radii can be found. Intersection points are theoretical and therefore are open to inconsistencies. As a manufacturing engineer that's what I'm trying to eliminate.

 

[SnTMan]

Aaron96, first, low volume and high volume are different of course. But when detailing low volume parts that do not have especially tight tolerances and without knowing where they might be made, I much prefer dimensioning to virtual sharps. I would not even call out the bend radius unless it is functionally important.

The intersection can easily be found and measured with a couple of straight edges or scales, the center of the bend, not so much.

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[KENAT]

Trouble is your dimensioning scheme that makes it easy to measure may not actually capture the functional requirements as well, and might even make me have to tighten tolerances to make sure parts are good in some situations.

Once you dimension to the center of the bend radius the size tolerance on the radius starts to play into the overal size, if overall size is critical then I have to tighten the other tolerances accordingly so while the part may be easier to measure it can become harder to meet required tolerance.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[SnTMan]

KENAT, I probably should not have entered this discussion in the first place

I was talking about parts where you need one or two made, you need an angle or a channel formed, and about all you are interested in is the leg dimensions and angle.

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[KENAT]

SnTMan - I was responding to Aaon96 not your post - sorry if that wasn't clear.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[SnTMan]

Oops, further proof that I should have abstained

The problem with sloppy work is that the supply FAR EXCEEDS the demand