Seemingly incomplete tolerancing, reality check needed
Seemingly incomplete tolerancing, reality check needed
(OP)
A customer's part will be milled from extruded bar stock. The profile of the extrusion is governed by a MIL standard. I believe the attached drawing of this profile dates from 1995. I am very interested to hear what some more experienced folks have to say about the GD&T used on this drawing.
In addition to your general thoughts regarding the tolerancing strategy, I'm keenly interested in the angled surfaces oriented with the basic 45 deg. dimensions. In my mind, these surfaces are free to pivot on the points of the inscribed rectangle. The small flats at the "points" of the angled surfaces would grow/shrink as the angle deviated from 45 deg. Likewise, the width of the top flat surface would grow/shrink.
Rightly or wrongly, I'm more hesitant than usual to call foul since this in an established government drawing. What say you?
In addition to your general thoughts regarding the tolerancing strategy, I'm keenly interested in the angled surfaces oriented with the basic 45 deg. dimensions. In my mind, these surfaces are free to pivot on the points of the inscribed rectangle. The small flats at the "points" of the angled surfaces would grow/shrink as the angle deviated from 45 deg. Likewise, the width of the top flat surface would grow/shrink.
Rightly or wrongly, I'm more hesitant than usual to call foul since this in an established government drawing. What say you?





RE: Seemingly incomplete tolerancing, reality check needed
Bigger context of that clip you posted:
Maybe some of the statements from MIL-W-13855D provide further control?
I do see what you mean about the angular basic dimensions being under defined. However, I think it falls mostly under 'good practice' that they don't deviate from 45 too much, and because that's mostly a "stock" dimension controlled by the dies and manufacturing process of the extrusion. The design and nature of this mounting interface also allows for minor deviation.
Additionally, since no one expects to be able to place any precision optics on one firearm, zero it, and then place it on another firearm and have it be zeroed accurately, there is no recognizable failure due to lack of interchangeability. It just has to fit the mount on the optic base, which are somewhat forgiving. I'm reasonably sure the general open-ended verbiage in the couple MIL documents cover extreme deviations from nominal, though. I'd be interested in hearing from someone more familiar with manufacturing and quality control in the context of this item though.
RE: Seemingly incomplete tolerancing, reality check needed
I would be much happier if those basic 45's were directly toleranced instead. I'm responsible for providing the cross-section specs to the extruders and I believe I'm going to directly tolerance those angles.
RE: Seemingly incomplete tolerancing, reality check needed
Now that I know what role you're in, I see the problem. You're in the one role where this drawing bites you in the rear! lol. What is a "stock" dimension to some, is a manufacturing/quality requirement for someone else...
RE: Seemingly incomplete tolerancing, reality check needed
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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
RE: Seemingly incomplete tolerancing, reality check needed
I'm also having trouble understanding their reasoning behind using the MMB modifier on C in the two positional tolerance frames. The parts that mate with the rail CLAMP onto it, often on the angled faces. The features controlled by these positional tolerances have no mating function; they must simply not interfere with the apparatus clamped to the angled faces.
Also, the top "surface" has a flatness tolerance. I'm not sure how to interpret this on a 2D profile. If they had noted that this flatness tolerance be held over a given length of extrusion, I could understand...
I am not aware of any official gauge drawing.
RE: Seemingly incomplete tolerancing, reality check needed
Nescius
Not sure what version this drawing refers to, however what I see is 'basically' hosed. (yes pun intended
Datum feature C is not a feature of size.
The Basic dimensions do not originate from a Datum Feature.
Pretty much a free for all interpretation.
RE: Seemingly incomplete tolerancing, reality check needed
For my drawing, I'm moving forward with an all-around profile of a line tolerance. The straightness and twist of the extrusion are controlled by themselves in notes, on a per foot basis.
RE: Seemingly incomplete tolerancing, reality check needed
Since the MIL-STD drawing shows the final product, not the raw material / extrusion, the flatness would (correct me if I'm wrong) only apply to individual flats between slots - not the continuous top surface comprised of n-surfaces.
RE: Seemingly incomplete tolerancing, reality check needed
"3.5 Profile. In this document, profile refers to the cross
sectional outline as depicted in Figure 1 which establishes a
dimensional tolerance zone extending the length, width and depth
of the considered feature."
...and this:
"5.1 Accessorv mounting rail Profile. Standard dimensioning
and tolerancing for the profile of the accessory mounting rail
for small arms weapons shall be as shown in Figure 1."
...and this:
"5.3 Lenqth of accessorv mountina rail. The length of the
accessory mounting rail is dependent upon the particular
application of Small arms weapon it is to be mounted on."
So, I really don't know what the .005 flatness applies to.
RE: Seemingly incomplete tolerancing, reality check needed
RE: Seemingly incomplete tolerancing, reality check needed
RE: Seemingly incomplete tolerancing, reality check needed
RE: Seemingly incomplete tolerancing, reality check needed
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
RE: Seemingly incomplete tolerancing, reality check needed
The 4X 45° BASIC angles, .108 BASIC dimension, and .748 +/-.002 dimension all combine to create four tolerance zones that the angled surfaces must simultaneously fall within. This requirement could be equivalently defined by making the .748 dimension basic, and applying a profile of a surface tolerance of ~0.001414 to each of the four angled surfaces (as a simultaneous requirement and without datum feature references).
Datum feature C is an irregular feature of size composed of the four angled surfaces. The maximum material boundary is clearly defined by the profile of a surface tolerance interpretation already described.
The .164 +0 / -.020 requirement creates another tolerance zone in a similar manner. This requirement could be equivalently defined by changing the dimension to .154 BASIC, and applying a profile of a surface tolerance of .020 to the top surface. Whether it should reference datum feature C at MMB or RMB is not clear, but RMB would be the conservative choice.
I'm not saying this interpretation is correct, or even that a correct interpretation exists. However, I do feel that this approach is fairly conservative and has some basis in the standards.
See ASME Y14.5-2009 Fig. 2-21 for an example where basic angular and linear dimensions combine with a toleranced linear dimension to create what is essentially a profile of a surface tolerance.
All this still leaves the question of what length the tolerances apply over. Note that the .617 width and the lower two angled surfaces extend continuously for the entire length of the rail. Per MIL-STD-1913 paragraph 3.5, the tolerance zones for these features would extend the full length also. This includes half of datum feature C, so you could argue that the other half (upper two angled surfaces) should be included as well.
A useful compromise might be to control the entire profile continuously (ignoring the slots) for some intermediate length between the slot pitch and the full rail length.
- pylfrm
RE: Seemingly incomplete tolerancing, reality check needed
My extrusion consists of barely more than the angled surfaces themselves, so I was able to "cheat" and use an all-around profile tolerance, .0015 inches. More than the calculated .001414213..., but I'm comfortable with it. If my extrusion was more complex and the 1913 was a mere feature on the whole, .0015 would've been far too tight to apply all-around. In that case, I'd have to roll up the sleeves and undertake some deeper GD&T wrangling.