dimension scheme

[PROEGUY]

i'm getting redlined on my dimensiioning scheme. i have always tried to dimension from a common edge (datum)and not use chain dimensions. the standard says that chain dimensions have the most tolerance stack while baseline/ordinate and direct dimensioning schemes have lesser tolerance stacks (AMSE Y14.5m-1994 2.7.1). any feedback on this to help convince this engineer that chain dims are not the way to go, other that giving him the spec to read for himself?
thanks for your time

Sr. Pro/E Mechnaical Designer

 

[ewh]

Show him the tolerance stackup. If a huge cumulative tolerance is acceptable to him, so be it.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[SDETERS]

Are The dimension you mention basic dimensions or rectangular dimensions?

If they are rectangular using +/- on them yes this will add to the tolerance stack up.

 

[TheTick]

What about hole patterns? Usually dimension one primary hole from edges and dimension rest from primary.

 

[PROEGUY]

it's 2 holes dimensioned from a common edge is the way i did it. he claims that using a base line (ordinate) dimension scheme has a greater tolerance stack than if one hole is dimensioned to an edge and the other hole is dimensioned to that hole

Sr. Pro/E Mechnaical Designer

 

[fsincox]

I assume you are not refering in any way to position tolerancing and basic dimensions.
Frank

 

[PROEGUY]

no, just standard dimensions +-.005

Sr. Pro/E Mechnaical Designer

 

[ewh]

he claims that using a base line (ordinate) dimension scheme has a greater tolerance stack than if one hole is dimensioned to an edge and the other hole is dimensioned to that hole

He is correct as far as the tolerance between holes is concerned.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[pmarc]

PROEGUY,

Please give us a simple sketch how you did this.

If you are following Y14.5 standard, the standard recommends basic dimensions usage together with position tolerance (eventually profile of surface callout). But this will also require some features of a part to be assigned as datum features.

Any +/- dimensioning scheme will create ambiguity here (even if you are dimensioning the centers of holes from common edge), because you never know from which feature measurements should be taken.

Only GD&T will help you to eliminate any misinterpretations of your drawing.

 

[MintJulep]

it's 2 holes dimensioned from a common edge is the way i did it. he claims that using a base line (ordinate) dimension scheme has a greater tolerance stack than if one hole is dimensioned to an edge and the other hole is dimensioned to that hole

He is correct. For this particular case.

 

[PROEGUY]

here's a PDF to show my scheme

Sr. Pro/E Mechnaical Designer

 

[ewh]

While the method you used does limit tolerance stackup relative to the edge, it increased it between the holes.
Since you specified +/- tolerances, discussion of basic dimensioning would be irrelevant.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[drawoh]

PROEGUY,

I used to use datum dimensioning systematically, because I knew that was the way the machine shops wanted it. On a complex part, you wind up with a simpler drawing.

Today, machinists go straight to CNC on most parts. Your dimensioning scheme probably is not as critical for them. Dimensioning to show functionality is much more feasible now, consistent with readability.

If you are using GD&T positional tolerances, as you ought to be, you may have a requirement that your hole pattern must be internally accurate, but its location, less so. In this case, datum dimensioning is not meaningful.

I would say that on a machined part, your dimensioning scheme is not critical. The machinist will locate everything accurately from the datum, regardless of how you apply the dimensions.

If your part is sheet metal, you can punch the holes accurately on the flat sheet, but your bends are not accurate. I would avoid datum dimensions.

For weldments and castings, you are going to have to think through all your dimensions. Machining drawings are easier than all the rest.

JHG

 

[PROEGUY]

ok. thanks for all of your inputs.

Sr. Pro/E Mechnaical Designer

 

[fcsuper]

drawoh is correct. The machinist work from their own datums (based on the set up within the cnc). You can try to assign those datums for the machinist using GD&T.

For such a simple pattern, I would default to the function as the guide for dimensions. In this case, I would agree with your engineer. The pattern's association to it constituants is more important than their relationship to the edge.

However, this line of reasoning should not be extended to include a long change of features where accumulation between features will cause the overall pattern violate design intent.

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

http://groups.yahoo.com/group/solidworks & http://twitter.com/fcsuper

 

[KENAT]

Agree that dimensioning scheme should be based on function first and foremost (not to say legibility, manufacturability... should be ignored) so typically for hole patterns that meet with patterns in another part, hole to hole dimensions are better if insisting on using +- tols. As mentioned though, pos tol would be better if this is the case.

Posting guidelines faq731-376

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

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