Positional similar to ASME Y14.5M fig 4-8 1

[KENAT]

I’m helping an intern dimension a part which has a datum scheme similar to figure 4-8 of ASME Y14.5M-1994.

Say the 9.2 hole has a couple of ‘key’ (male) features spaced 180° in line with B-C.

To dimensions the keys I was going to use a scheme similar to 4-6. To do this I’d make the 9.2 hole datum D, to ensure they are radial to that hole.

1. Does this make sense, it has fairly tight tolerance/functional requirements so I think this may be better than relating it back to ABC.

2. Should my tertiary datum (for orientation) just be B or should it be B-C? I’ve looked at 4-9 & read the paragraphs and am leaning to B-C but want to make sure I’m right.

I know I’ve been asking a lot of questions lately, probably more than I’ve been answering but I appreciate the help. (One of my departments goals for the year is apparently to give everyone at least 1 training so I’m hoping mine will be in slightly more advanced GD&T so I can quit bugging you all so much!)


KENAT, probably the least qualified checker you'll ever meet...

 

[fcsuper]

I'm tired, and have evidence already on eng-tips that I shouldn't be answer questions in this state, but I'll give a tiny bit of input that may or may not even be close to what you are asking.

1. If you wish to emulate Fig 4-6, then yes. Otherwise, I don't know cuz i don't know the intent/fuction of these features.

2. 4.5.7 says it all. Well, not really. It differs back to 3.4.2, which establishes a pretty strict rule about using multiple datum. I don't know if there are two different instances where to use them, or if one is the rule, and the other is the application. I know this isn't what you asked. I guess I'm kinda asking for what it meant in the standard for multiple datum features. However, if I'm reading 3.4.2 right, B-C wouldn't be allowed since B-C consists of two features base on surfaces that have nothing in common, unless the new datum formed from those datums is the plane created by their two axes. Am I off base here? I think I'll be learning from anyone's answer.



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[Xplicator]

Have you considered a profile on the hole/key feature?

 

[KENAT]

Thanks,

The keys locate into grooves into a cylindrical item with key-ways to orient it.

I don't see how 3.4.2 forbids B-C in this case, it's mainly talking about how to show it in the FCF. I essentially want to invoke the datum plane C shown in the bottom left of 4-9.

Profile might be an idea, although the scheme as is works quite well except that I want to make sure I have the orientation properly addressed.

KENAT, probably the least qualified checker you'll ever meet...

 

[Xplicator]

Either composite profile or profile w/position (Fig. 6-19) should more than achieve what I believe your looking for.

 

[KENAT]

Sketch for clarification.

Very simplified sketch of part showing the datums. There are a bunch of other features too.

B & C locate on dowel pins and align this part to other, A mates to the others. There is another hole patter that relates to D which I haven't shown.

The keys & diameter are critical, the small corner releifs aren't.

Thanks for the help.

KENAT, probably the least qualified checker you'll ever meet...

 

[drawoh]

KENAT,

I do not see a problem with adding a fourth datum. If you need to locate something to your 9.2mm hole more accurately than to the main datums, then you should specify the hole as Datum_D. Datum_D provides centreing. I would use Datum_B or Datum_C to control rotation. Perhaps someone out there can visualize a fixture that uses B-C to control rotation, without the additional location control.

It is hard to go any further than this without understanding what your part does. There are all sorts of valid approaches. What must they accomplish?

JHG

 

[KENAT]

Thanks drawoh, I answered 1. for my self by looking through some training material I had and am confident in adding the datum D.

It's Q2, getting the keys aligned correctly I was struggling with. I wasn't sure just B (or C) defined a datum for this.

KENAT, probably the least qualified checker you'll ever meet...

 

[CheckerRon]

OK, I would go with position to datums A,D, and B-C for clocking, for your keys. Hole "D" probably being positioned relative to A,B, and B-C.
The tolerance values look hugh, but they are arbitrary numbers?

 

[Xplicator]

If the dowel holes share equal function, use of pattern datum is the most robust and encouraged. Make the pattern "B", addressing your secondary/tertiary datum and locking the part.

The feature in question here is not a hole it is commonly referred to as an "irregular feature of size", while others may call it a bounded feature. In some respects it reminds me of a spline, but we won't go there.

I sense some hesitancy in applying the Profile(all around) combined with Position and invoking "Boundary" and even possibly @ MMC. I would imagine that this is because of the corner reliefs. If this is the only reason, look at para 6.5.3 and Fig. 6-12. As you can see all you will need is a local note, overriding the spec in regards to the reliefs only.

Now once you apply this aforementioned callout, remember to basically define this "bounded feature" from the axis and tie the axis to the pattern datum "B". You can make this a datum "C". Now reference the other hole pattern you mentioned w.r.t. A|C|B and any other feature that should be tied to it.

I hope this helps.

 

[CheckerRon]

After reading what Xplicator has just posted, I like it and would consider it, especially if the tolerance size tolerance on the hole and keys needs to, or can be the same. It may take a little verbal ustification, but should be clear.

 

[CheckerRon]

My mind and fingers just can't get together. Guess one should re-read their post before posting.

 

[KENAT]

Thanks Ron. Yep, arbitrary and I slipped a decimal in my rush - oops. It's not the real drawing though. I think ABC makes sense for other features on the real part if applied like in 4-8 & 4-9.

Xplicator, at the moment the dowels are equal function but that may change, one of them may become a slot just for clocking. I thought about a hole pattern but I don't think it supports some of the other features I need to dim as well.

I nearly called them splines or tabs but couldn't decide what would be less confusing, hence the sketch.

As to hesitancy with profile, I’m actually fond of profile and can see how it could be applied but many round here and our machine shops aren’t great with GD&T. In particular the intern landed this task has no knowledge of GD&T, wasn’t meant to be doing drawings so we didn’t give him even basic training, has limited supervision from anyone who knows what they are doing and I’m not scheduled to look at the drawings in detail for a long time if at all. It’s also a fairly busy drawing and a rush job.

So all this will compound to probably make it a nightmare, and anything I do will probably make it worse so I’m damned either way.

I’m off Monday and will spend a bit more time looking at your suggestion but it may be too late to implement by the time I get to work on Tuesday.

I appreciate the help though, this kind of thing comes up here with some frequency so I may be able to get it right next time!




KENAT, probably the least qualified checker you'll ever meet...

 

[Xplicator]

It will still work if it changes to a hole slot config. One hole becomes the secondary and the midplane of slot width is tertiary. Then the use of composite or perhaps even multiple single segment profile of Surface could be employed, again I'm not sure of your application so it is again merely speculation as to what you could do.

You will soon realize that I believe everything and anything can be controlled by three symbols - profile of a line, profile of a surface and position. However, a firm understanding of GD&T's remaining eleven symbols needs to be there before use of these three only can be used effectively.

This sounds as if this probably won't take place here but thats OK cause if someone takes anything away from this, it was worth it.

 

[KENAT]

Thanks Xplicator

KENAT, probably the least qualified checker you'll ever meet...