multi segment question

[thepete]

I have a disk with 5 holes in it.

The hole center is sloppy big, and largely uncritical. The 4 tapped holes, I want located around the center hole.

The tapped holes are not critically spaced to the center hole, but they do need to be critical in regard to the 4 holes in the pattern. (ie, the position of the pattern can be sloppy, but the position of the pattern itself needs to be tight.)

The question is... did i do the position tolerance right? I'm struggling with the differences between composite and mulit-segment, and also with judging when to add a datum into a multi-segment. I don't see where adding a reference to B would do anything, and I don't see the need for a reference to A unless I wanted to tighten it up from the rule 1 (and I don't).

-Pete

http://i.imgur.com/VOFPY94.jpg?1[/IMG]]

 

[3DDave]

Only if whatever aligns with the holes nominally doesn't have to mate with the face of the part.

What's the ATAN of .02/.105? That's the angle you are accepting between the axes of the holes and -A-.

 

[Tunalover]

Questions/Comments:
a. Why is it necessary to call out pilot holes for the threaded holes? The threaded hole call out alone is sufficient. Let the fabricator decide what pilot hole to use (it generally depends on a number of variables that the fabricator must take into account; don't tell him what to use! He knows best. Besides you provided no tolerance on the size of the pilot hole anyway. Why make life harder than it has to be?).
b. Why say "THRU"? It's understood that unless otherwise specified the hole is through the part.
c. If the metric thread is coarse and the class fit is 6H, the pitch and class fit are not required i.e. "M3.5" alone is sufficient. Are these metric coarse threads?
d. In the composite feature control frame, the tolerances are not stated as diametrical tolerances although I'm sure that's what's intended.
e. I think in the lower segment of the feature control frame you want to include reference to datum A.

This part requires a composite feature control frame as you have shown.




Tunalover

 

[CheckerHater]

Agree about pilot holes. Unfortunately several places simply won’t risk reconfiguring SolidWorks and it appears that it is what OP is using.

“It’s understood that unless otherwise specified the hole is through the part.” – Citation needed. This forum is full of “THRU” threads that are leading nowhere; so there is no final agreement.

On metric thread – see enclosed. If OP is going by American standards (and it looks like it) then nothing is implied with metric threads.

Diametrical tolerances won’t hurt. So will reference to A in lower segment.

To add to the nit-picking, Basic 90 DEG angle is usually implied.

 

[ewh]

THRU is added only if the callout would be ambiguous without it. If it is obvious that the hole is through the part, THRU is redundant.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[CheckerHater]

THRU is added only if the callout would be ambiguous without it. If it is obvious that the hole is through the part, THRU is redundant.

I understand that. I just don't see the section, not even dash lines. What makes this particular callot unambiguous?

 

[thepete]

You guys got a bit off the question. I have good answers for most of them, excepting maybe the lack of hidden lines (sorry).

But at the risk of getting further off topic, can we stick to the intent of the original question? Does the feature control frame do what I want?

 

[thepete]

Not that I don't appreciate the other insight, I do, but intent of the original post seems to have been lost - except for 3DDave, whose post I'm still taking in.

 

[axym]

thepete,

Here are some comments:

-The current drawing has multiple single-segment feature control frames (not a composite feature control frame).
-The first segment controls the pattern shape and spacing, perpendicularity of each hole to A, and location of each hole to B (all within 0.020).
-The second segment refines the pattern shape and spacing within 0.005. In most cases, the perpendicularity of the holes needs to be refined along with the pattern shape and spacing. So A should probably be referenced in the lower segment.
-The difference between multiple single-segment FCF's and composite FCF's are in special rules for the lower segment of a composite FCF. Any datum features referenced in the lower segment will control rotational degrees of freedom only, and not location. This would have no effect in this case, as the only applicable DOF's that A controls are rotational DOF's anyway. The rule of Simultaneous Requirements is also waived for the lower segment of a composite FCF, but this would not apply in this case as there are no other FCF's.
-I don't see the need for a composite FCF in this application.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Tunalover]

In my post I said "This part requires a composite feature control frame as you have shown." That was in error as the figure actually shows two single-segment feature control frames. My mistake. My recommendation is that you use a composite feature control frame (with diametrical positional tolerance zones).



Tunalover

 

[thepete]

My memory clearly sucks on this matter, I'm going to study this a bit this weekend. I'll leave you with a picture with a few refinements, and showing both a composite and two-single segment tolerance.

3ddave,
I get your point about the angle. you've convince me that adding A to the segment is useful (the difference between 11 degrees and 3 degrees normal to A). Perhaps it should be even closer tolerance than .005 to -A-, but we'll leave it as-is for now.

tunalover,
Yes, to the diameric intent on the tolerances. It sounds like you're advocating the composite fcf (on the right). As I said, time to study up a bit.

thanks,
P

 

[axym]

thepete,

The two alternatives result in identical requirements in this case. I would go with the two single segments - it's simpler, and the special properties of the composite FCF have no effect.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Tunalover]

CheckerHater,
Thanks for correcting me on the metric thread call out. It is indeed the ANSI standard (prevailing US) way to fully state the size, pitch, and class fit for both fine and coarse series metric threads. I've been using the abbreviated ISO standard call outs for coarse metric threads for 12yrs or more on drawings claiming conformance to ASME Y14.100!

As for CAD systems automatically placing pilot hole call outs with screw thread calls out I strenuously disagree with that practice; it clutters the drawing and unnecessarily requires something of the fabricator when s/he should have the leeway to make his/her own choice! I'm no machinist but I tend to think that a hard difficult-to-tap material may require a larger pilot hole than what a CAD system will spit out!



Tunalover

 

[Belanger]

An aside to Evan -- if showing one position symbols or two is equivalent in this case (and I agree they are), then I'm curious why you vote for using the two single segments. As I read the standard, they seem to go with the composite FCF in these cases, unless there is a specific need to isolate/refine location-related datums on the bottom.
Is it because it's less tempting for people to read the bottom tolerance as doing only perpendicularity?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[axym]

John-Paul,

I agree that Y14.5 presents composite FCF's as the first option for multi-level "pattern location" and refinement" tolerances, and multiple single-segment as the second option. I wasn't thinking that people would read the bottom segment as controlling only perpendicularity, although I agree that it would be possible to misinterpret the "orientation only" property in this way. It's more that there just seems to be more general confusion over composite FCF's, in my experience anyway. So I recommend using single-segment FCF's where possible and use composite FCF's only where necessary.

In Y14.5-2009, the first composite FCF example is shown in Figure 7-38 and described in section 7.5.1.1. This example has only the primary datum feature A repeated in the lower segment. The text describes the interactions between the PLTZF and FRTZF in great detail, which is fine. But it turns out that the descriptions would be identical if two single-segment FCF's were used. The only difference that the composite FCF makes is that the lower segments would not be subject to simultaneous requirements, but this is not mentioned. So (to me) the reader is left wondering what the difference between composite and multiple single-segment FCF's actually is, and why one would be used over the other. It is only in the next example, in which the secondary datum feature B is repeated in the lower segment, that the orientation-only property of the FRTZF makes a difference.

The next examples of composite FCF's that have additional counter-intuitive details that, to me, add to the confusion. One is that the same sequence of datum features can be repeated in both segments, and the other is that sometimes extraneous datum features are included only for the purpose of maintaining the proper sequence. Let's face it, this section gets really confusing. Multi-level pattern controls are difficult in the first place, and mixing in the special rules of composite FCF's at the same time just adds to it.

After all of that, the standard presents multiple single-segment method as an alternative technique for pattern location. I have found that the whole thing is easier to get across to a class if the multiple single-segment technique is described first. Then the special properties of composite FCF's can be discussed, as an additional option when location refinement is not needed and/or simultaneous requirements are not needed. That's my two cents, anyway.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Interesting, Evan -- I was just nudging you to explain why the standard takes the approach that some would think is more confusing.

I tossed in the theory about perpendicularity in the FRTZF because -- as you have probably encountered -- when teaching this in a class, someone always asks why the FRTZF controls hole-to-hole position. They see datum A and think that the lower segment only does perpendicularity.
Perhaps using two single segments is better because at least it forces people to deal with the position symbol again, conveying that it involves location of some sort as well. But to me that would be the only benefit of deviating from the standard's recommended practice.

For another aspect of the possible confusion (re: composite vs. two single segments), here's a recent thread that toward the end got into a theory where one vs. two symbols implies simultaneous vs. separate gaging:

http://eng-tips.com/viewthread.cfm?qid=362764

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems