BOUNDARY for noncircular features

[SpaciouS]

Hi,

I’m having a discussion about using the ‘BOUNDARY’ condition on a slot, and I am having trouble figuring out its benefit. I’ve read this section ('94), SEVERAL times and still don’t fully understand if one theoretical boundary is better than the other, particularly between;
(a) In Terms of Surface of a Feature
(c) In Terms of Boundary for an Elongated Feature

I understand the theoretical boundaries have different shapes, such as a rectangle vs. identical shape, but is there a benefit to using BOUNDARY? The only thing I could come up with is when BOUNDARY is invoked; the ‘clocking’ of the slot about true position is essentially tighter, since the bundary is longer (not sure if that’s right).


Thanks,
Sean

 

[powerhound]

The corners of your tolerance zone are not sharp, they follow the radius of the slot.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
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[SpaciouS]

Yes that's true but does that create a differnt result? Is there a benifit to using BOUNDARY?

Thanks,
Sean

 

[drawoh]

SpaciouS,

Are you talking about positional tolerances?

You need your part to work. Right now, we do not know what it does. Let's assume it clears a screw.

The alternative to a profile all around the slot, is a width and length with ±[ ]tolerances and with positional tolerances on both features of size. The advantage of this approach is that the length-wise size and positional tolerances are probably way less critical than the width-wise size and positional tolerances.

A profile tolerance controls the form of the radii at each end. It may be neater and more readable on your drawing. If the part is being machined, there is no way for your fabricator to take advantage of the sloppy tolerance in the one direction.

If your slot is facing a bent sheet metal edge, then you need the sloppiness.

JHG

 

[SpaciouS]

I'm using Positional tolerances for a hole/slot configuration which mates to pins for location and alignment of a lens mount.

Example I'm referring to in the '94 standard is FIG. 5-47, page 143. The topic of non-circular features starts on page 139 and covers 3 different types of boundaries:
(a) Surface of a feature
(b) Center Plane of a Feature
(c) Boundary for an Elongated Feature

I'm just wondering what, if any, benefit there is in stating "BOUNDARY" beneath the FCF.

Thanks,
Sean

 

[KENAT]

Does the position of boundary take advantage of MMC bonus tol better?

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(probably not aimed specifically at you)
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[Belanger]

The advantage to boundary is that it cuts right to the chase: it creates a protected area where the rim of the slot cannot penetrate, thus ensuring your function. That's the main advantage.

(FYI, if you're using the 2009 standard, you don't have to say BOUNDARY on the print; it's automatic if using position on an irregular shape with the MMC modifier.)

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

http://www.gdtseminars.com

 

[SpaciouS]

Belanger,

That's true the 2009 standard states that the term BOUNDARY maybe placed beneath the FCF but is is not required. That's actually one of the reasons why I'm seeking this answer.

If you don't have to define which boundary you want to evoke than the results of either must all be the same in the end right?

And it's up the inspector, let’s say, to determine which method he want to inspect to?


Thanks,
Sean

 

[Belanger]

That's just it; there is no difference (which is why the new standard says the word is unecessary).

You listed three tolerance options:
(a) Surface of a feature
(b) Center Plane of a Feature
(c) Boundary for an Elongated Feature

(a) and (c) are identical. And (b) will also serve the same purpose, but the mental image of the tolerance has bee shifted to a theoretical center axis/plane.

Think about a regular, cylidrical hole. We often talk about position tolerance around the axis, but if the MMC modifier is invoked, then you can talk about the "virtual condition" of the rim of the hole rather than the axis -- it's two ways of discussing the same thing.

Now step over to an elongated hole (slot). You are welcome to say that the tolerance zone resides around the axis, but I may have to scratch my head a little to derive the axis of a slot. So in that case, it makes sense to think of the tolerance zone as residing around the boundary, which is simply the "virtual condition" of the slot. Voila, no real difference!

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

http://www.gdtseminars.com

 

[SpaciouS]

Thank you so much, that's make perfect sense to me.

I was mentored, by a senior GD&TP, in using the term BOUNDARY (pre 2009), and at that time I assumed there was a better result that came from that. But as I continue to scour through these standards on my own I realize, from time to time, I'm not grasping the concept fully.

I can now check this one off the list


Thanks,
Sean

 

[drawoh]

Belanger,

I am curious as to what BOUNDARY actually controls, the radii at the ends?

The OP calls up an interesting problem -- locating on pins.

When I call up a clearance hole for a screw, I assume the part is locating by something other than that screw. The clearance hole's MMC (MMB?) state must be a hole centred on the screw, large enough to clear the screw plus its positional error. The BOUNDARY condition of a hole or slot is meaningful.

Most locating pin holes are very accurate. The clearance is way less than the positional tolerance. Pin hole and slot location is more repeatable than accurate. You have a hole perhaps Ø6.02/6mm located to a true position of Ø0.2mm. You can work out a minimum outline that will not be encroached, but it has no useful meaning.

JHG

 

[Belanger]

For your first question, JHG, the boundary idea controls the location of the entire slot, radii and all.

I get what you mean about a locating pin's hole being very accurate for size but less so for location. That's because it is the "driving" factor. So you're asking what benefit there might be in discussing its "virtual condition" or outline/boundary (MMC minus position tol).
Well, if the hole is being positioned (0.2, in your example), then somebody apparently cares about the effect of a mislocated hole.

But this is the sticky part of the situation you describe: What is important is not the inner boundary of the hole, but the outer boundary of the pin that plugs into the hole. You may have to do a sketch, but if that hole moves around, the pin goes with it, creating an outline, but on the outer "locus."
So interaction of hole size and hole location is still -- indirectly -- of value.
(And, in case my post isn't already confusing enough, the idea of a press-fit locating hole would also warrant a projected tolerance zone.)

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

http://www.gdtseminars.com

 

[drawoh]

...
(And, in case my post isn't already confusing enough, the idea of a press-fit locating hole would also warrant a projected tolerance zone.)

There is no difference between a press fit hole and an accurate clearance hole. In either case, the hole locates the fastener, as you intend when you design such a thing. If both tolerances are Ø0.2mm, then your located part is positioned to Ø0.4mm.

JHG

 

[fcsuper]

Also, BOUNDARY is understood for slots, according to ASME Y14.5-2009.

Matt Lorono, CSWP
Product Definition Specialist, DS SolidWorks Corp
Personal sites:
Lorono's SolidWorks Resources & SolidWorks Legion

 

[Belanger]

"There is no difference between a press fit hole and an accurate clearance hole."

No comprendo. If it's a clearance hole, doesn't that mean it has clearance?
Therefore, a clearance hole won't compel the pin to take on the same orientation.

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

http://www.gdtseminars.com