FCF on Reference Dimensions?
FCF on Reference Dimensions?
(OP)
I recall coming across this before saying that there should not be a FCF on a dimension that is reference. I was trying to find where I saw it before and seem to not be able to find it. What is the rule with this?
I attached a sketch of what I'm talking about.
Thanks for the help.
I attached a sketch of what I'm talking about.
Thanks for the help.





RE: FCF on Reference Dimensions?
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: FCF on Reference Dimensions?
RE: FCF on Reference Dimensions?
In any case, I think it's a confusing practice at best. Why would a feature's size tolerance and geometric tolerance be shown in different sections of the drawing? The same thing goes for the datum feature's size tolerance and its datum feature label.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: FCF on Reference Dimensions?
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
RE: FCF on Reference Dimensions?
If this part an in-separable assembly then it is a new part and it can (should) include each necessary dimension Paragraph 1.4(c).
I know that this is not often the practice in design because assemblers claim that they cannot be held liable for sub-assembly "pass-thru" deviations... while the sub-assembly operation claims that the assembler introduced the deformation that caused the previously qualified feature specification to fail.
With in-separable assembly operations un-predictable things happen and when they do new specifications end-up on the assembly drawing despite all objections.
Paul
RE: FCF on Reference Dimensions?
RE: FCF on Reference Dimensions?
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
RE: FCF on Reference Dimensions?
Paul
RE: FCF on Reference Dimensions?
I tend to agree with PeterStock that it may be better to exclude the dimensions altogether, unless there are similar features that may cause confusion.
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: FCF on Reference Dimensions?
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
RE: FCF on Reference Dimensions?
RE: FCF on Reference Dimensions?
The MMC qualifier requires a size tolerance. Without a size tolerance, the MMC (and entire FCF) are invalid, regardless of what they are attached to.
RE: FCF on Reference Dimensions?
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
RE: FCF on Reference Dimensions?
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: FCF on Reference Dimensions?
Though, from a configuration control point of view that would potentially be a pain so having the part drawings to hand may be better.
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
RE: FCF on Reference Dimensions?
What is/was gained by using a FCF on the insert internal thread?
Was the insert a floating variety or fixed?
RE: FCF on Reference Dimensions?
Um, am I misunderstanding you? What was gained was that the required location tolerance is clearly stated. Position is used to take advantage of the increased tolerance from the circular tol zone and support its use on mating part.
Fundamentally, from an interface/function point of view what we care about is where the thread of the insert ends up, not where the OD or any of the other radial features on the insert end up. If you don't somehow state which dia the FCF applies to then there could be confusion or parts not fitting.
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
RE: FCF on Reference Dimensions?
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
RE: FCF on Reference Dimensions?
Serspec, Your statement is a prediction! If true then the assembly process may not introduce variation and the reason to omit the size tolerance would be an economical choice. There are however, other factors that contribute variation in assembly that smart designers often fail to predict... thermal influences, effects of stress on the sub-assembly pieces, effects of process influences (forces) on the sub-assembly pieces, and other unknowns. I have witnessed many design assumptions for in-separable assemblies go awry. That is why I cautioned that unpredicted variation typically results in feature controls that are added afterwards.
TheTick, I agree.
Paul
RE: FCF on Reference Dimensions?
Reference dimensions are provided on drawings to provide information to people who are not responsible for fabrication and inspection of the feature. There are a bunch of assumptions and issues here.