Setting up Datums, and controlling the orientation of a datum simulator or measurement

[Nereth1]

Hi all,

I have a fairly simple problem I think, but I am new to this and I think I am missing something basic.

I have a simple tube that I am trying to control the length of. Its ends are cut to a certain perpendicularity to its own axis (I have a good set of datums that allow me to define a datum axis along the length of the part). They are not necessarily cut parallel to each other.

How do I set up a feature control frame to measure length along the length of the axis? If I put a datum on one end and set up a position tolerance on the other, relative to that datum, then the datum simulator will end up taking up the angle of the lack of perpendicularity of the end it is on, and therefore the tolerance zone will do the same thing on the other side. That throws out the measurement.

Is there a way to reference the datum axis in order to 'measure along it'?

I definitely feel like I am missing something basic in the theory of setting up Datums for a part. I think I will be taking Y14.5 home with me over the weekend.

 

[dtmbiz]

Why Datum C ?
Basic dimensions need to be related (traced back) to Datum(s) (DRF)

 

[CheckerHater]

And basic dimensions on this figure are traced back to what?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[pmarc]

CH's idea with profile wrt A-B only (without referencing to C) applied to both sides of the part holds water.

As an alternative to additional runout controls, two perpendicularity callouts wrt A-B can be used.

 

[flash3780]

CheckerHater,
I guess that the figure you provided is illustrating the use of "all around" profile. Since they're treating the entire shape as a single profile with an all-around tolerance zone, only a single datum reference is required to establish perpendicularity. I don't think that's appropriate in the case of the pipe.

 

[dtmbiz]

CheckerHater
Nice... good example to get one's attention.

My position would be that the shape example that you posted is just that, a shape that is related to itself via basic dimensions and back to Datum A, a planer surface feature. That shape would have a 90 degree implied basic angle to datum feature A (a planar feature, presuming there is another view to confirm that).

Not so without Datum C. Yes Datum A-B is an axis with the implied 90 degree basic to the end surfaces. However, in practicality the axis is a line and IMO not as reliable to serve solely as a stable or clear DRF without Datum C. Datum plane A could easily be qualified with a flatness control and stable for setup.

Datum A-B doesnt clearly show the relationship to the end surfaces regarding a physical DRF setup. Like for instance the end surfaces would have 2 parallel plane tolerance zones for each end.
Where would the DRF setup locate those tolerance zones? At the end point of the Datum A-B axis? A point location?

What is the physical stop to measure from? More than likely the datum simulators to establish Datum A-B would be hindering and/or preventing access to those points.

Where is the measurment taken from, a physical feature? Shouldnt it be taken from a datum simulator?

For simplicity and clearity I would have Datum C ( I do believe my Datum C would have been at the bottom of the C-bore; likely the mating surface).

Figures in the standard have a disclaimer not to be considered complete. Personally I believe another datum(s) for clarity as to where to position the shape in a physical DRF setup would be good to add.

Theoretically you have a point to be considered.
IMO, in practicality it would be much more clear and practicle to add Datum C. Avoids confusion.

 

[CheckerHater]

I am not really in the mood to start a lengthy discussion.

With Profile tolerance zone you know where the body of the part can and cannot be. That is usually enough to devise a gauge - there is no need for "physical stop to measure from"

And in practical world it would be more clear NOT to specify Profile - just toleranced length of the pipe combined with Total Runout requirement for the ends.

Also I don't believe the figures in the Standard are "incomplete" to the extent of concealing the truth.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Nereth1]

I just want you guys to know I am still reading this thread religiously. Especially the part on profile tolerance without the full Datum frame. I'm not sure who to agree with right now so I'm hoping it goes on for a little bit longer for me to gather more info and form an opinion, as otherwise I will not know what to take from it.

I do agree that in the practical world it would be clearer not to specify profile. In fact in reviewing the part we tightened up the perpendicularity tolerance (the ends will have to be milled or machined for the bore, depending on the process the suppliers chose, so cleaning up the ends for perpendicularity is not going to be an issue), which has thus reduced the importance of ensuring you measure along the axis of the pipe. As such that length tolerance has been replaced with a directly toleranced length of pipe.

This was in combination with speaking to some potential suppliers here about the clarity of the symbology (we have to be careful about scaring off shops, as we have an exceptionally weak manufacturing industry here so not all shops are used to GD&T, not that I'm in a particularly strong position myself), and that one tolerance was the one that was flagged as "not sure why that is there". I explained the reasoning and meaning, but for every one that raises it with me there will be another that didn't understand it but doesn't want to ask.

 

[CheckerHater]

we tightened up the perpendicularity tolerance, ... which has thus reduced the importance of ensuring you measure along the axis of the pipe

This is exactly what I mean by "practical" - when you specify nice finish on the ends of pipe, possible ambiguity in defining "length" becomes irrelevant.

But just like Nereth1 I am curious to see more opinions

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[dtmbiz]

- From the purposes ASME Y14.5 came into being over the years regarding dimensioning and tolerancing for commonality, practice, understanding, cost savings concepts, etc; for me has proven to be defined by who is wielding the checker pen and how much will it cost the company.

A man convinced against his will is a man that remains unconvinced....

Somebody famous said that

 

[flash3780]

...we tightened up the perpendicularity tolerance, ... which has thus reduced the importance of ensuring you measure along the axis of the pipe...

Hopefully your part tolerance is stackup driven. You could be making a more expensive part than necessary if you are arbitrarily tightening tolerances.

 

[Nereth1]

We judged it incredibly cheap to drop the tolerance on that one as the process to machine it is already going to be there as part of other processes - for example the edges are chamfered and just inside is a bearing bore. In all likleyhood, either it's a facing tool added to an already custom boring head, or it's another (fairly quick) motion on a lathe that is already right there.

As such, the actual justification was driven by "why risk it when it's so easy to get right" more than a serious investigation on how far we can push the tolerance. Our presses touch that surface during assembly and we don't particularly want to find out what happens if it isn't quite square. There's an arrangement of seals pressed in there as well and it seizes up quite easily when the wrong combinations of misalignment and mispositioning takes place.

Maybe not the perfect justification for tightening a budget, but a finite run of parts means finite resources to optimize the design.