Tek-Tips is the largest IT community on the Internet today!

Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!

Register Log in
  • Congratulations TugboatEng on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Hex hole as a datum feature 1

Diametrix

Diametrix

Aerospace
Joined
Jan 31, 2023
Messages
54
Location
US
I know you can use a square hole as a datum feature - put a single dimensions with a square symbol and attach a datum symbol to it. Can you do the same for other geometries as well? With the square you define two planes of symmetry which form an axis at their intersection. With other geometries they either form multiple axes or they don't form a plane at all (like in a case of a triangular hole). I understand the intent here with the gauge pin used to establish the datum, but I'm not sure that it jibes with the standard.

Roller.jpg
 
Sort by date Sort by votes
Seems to me like it would be acceptable were it not for the gage pin bit, which I don't understand why they would ever include that. Just makes it confusing and ambiguous. Datum C as sets of parallel planes makes sense. Like creating a datum from the length or width of a rectangular slot.
 
Upvote 0 Downvote
It looks like the design intent is for the hex drive to define the axis of datum C, from which the runout of the 3.0" diameter can be measured. While that can be inferred, there's probably clearer ways to define it. Otherwise, I see no issues with the concept of applying the datum to the hex flats, but that specifically makes more sense to me when angular orientation is important.
 
Upvote 0 Downvote
Doug Hunter said:
It looks like the design intent is for the hex drive to define the axis of datum C, from which the runout of the 3.0" diameter can be measured. While that can be inferred, there's probably clearer ways to define it. Otherwise, I see no issues with the concept of applying the datum to the hex flats, but that specifically makes more sense to me when angular orientation is important.
Click to expand...
I'm not altogether clear on how it works. With 6 sides we define three center planes, which for the real parts will not intersect at the same axis. So we would have 3 separate axes. Which one do you use as the datum axis? In addition, what happens if the hole has odd number of sides, can you still define it like that?
 
Upvote 0 Downvote
Since the datum feature reference callout is RFS, then an expanding plug would be used and the center of that plug is the datum axis. Unless the mating part is also self-expanding the installed condition isn't going to match whatever inspection reports.

It would be equally useful to declare the outer diameter (OD) as a datum feature and have three pairs of surfaces with a position tolerance controlling them to the axis of OD datum feature. This would indirectly control runout due to the offset of the surface parts from the axis of the contracting cylinder around the OD of the part. Instead of runout, the OD would be controlled with cylindricity geometric characteristic control.
 
Upvote 0 Downvote
Diametrix said:
I'm not altogether clear on how it works. With 6 sides we define three center planes, which for the real parts will not intersect at the same axis. So we would have 3 separate axes. Which one do you use as the datum axis? In addition, what happens if the hole has odd number of sides, can you still define it like that?
Click to expand...
I think that the CMM will do a best fit to determine the center axis.
 
Upvote 0 Downvote
The datum feature symbol is aligned with the .753+.000/-.002 width of the irregular hexagon. So, if this drawing is to be interpreted by any internationally known standard on dimensioning and tolerancing such as the ones by ASME or ISO, datum C is the center plane of this specific internal width. It is unrelated to the other faces of the irregular hexagon, and the fact that there is a GAGE PIN note shown separately doesn't change the interpretation either.

There is no datum axis, and a Runout requirement relative to a center plane is meaningless.
 
Upvote 0 Downvote
Burunduk said:
The datum feature symbol is aligned with the .753+.000/-.002 width of the irregular hexagon. So, if this drawing is to be interpreted by any internationally known standard on dimensioning and tolerancing such as the ones by ASME or ISO, datum C is the center plane of this specific internal width. It is unrelated to the other faces of the irregular hexagon, and the fact that there is a GAGE PIN note shown separately doesn't change the interpretation either.

There is no datum axis, and a Runout requirement relative to a center plane is meaningless.
Click to expand...
Never mind. I agree about irregular feature of size. However, irregular feature of size can still be a datum feature. The .753+000/-.002 dimension is applied to the distances between all matching parallel surfaces of the hexagon based on the HEX note, which creates a set of three center planes as a datum C. What am I missing?
 
Last edited:
Upvote 0 Downvote
Diametrix said:
What is an "irregular hexagon"? Are you trying to say that the hexagon is an irregular feature of size? Based on 14.5Y definition it is a regular feature of size. The .753+000/-.002 dimension is applied to the distances between all matching parallel surfaces of the hexagon based on the HEX note, which creates a set of three center planes as a datum C. What am I missing?
Click to expand...
Diametrix, are there other features on the part which define angular orientation? I'm guessing not because it looks like a simple threaded plug. Otherwise, if datum C is only 1 pair of opposing flats (per Burunduk, and as I also initially thought), you'll never know which of the 3 pairs it is. 3 people could inspect the part and set it up differently on the 3 different pairs of flats. I agree that the HEX note adds something and indicates that the linear dimension applies to all three pairs of flats. I think that the gauge pin note also implies that the axis of the pin should be used as the datum, but that might not align with the "official" interpretations according to the standards.

There is another potential problem here. It appears that the hex is broached into the part after a 0.810" pilot hole is drilled. This means that the flats will not be complete. The middle material will be missing. Only the 6 vertices/corners will have the actual flat. This works fine for driving the part by a hex key, but it does not facilitate inspection of the part's flats. It will be challenging to register any gauge confidently to the flats. Any gauge pin used will not touch the 6 hex flats (theoretically), but will actually touch the pilot hole diameter. It appears to be drawn with a different expectation, as the gauge pin diameter is aligned with the flats. Is there a section view available on the center axis of the part?

Is it correct that you are not the designer of the part, but rather the inspector of the part?
 
Upvote 0 Downvote
Doug Hunter said:
Diametrix, are there other features on the part which define angular orientation? I'm guessing not because it looks like a simple threaded plug. Otherwise, if datum C is only 1 pair of opposing flats (per Burunduk, and as I also initially thought), you'll never know which of the 3 pairs it is. 3 people could inspect the part and set it up differently on the 3 different pairs of flats. I agree that the HEX note adds something and indicates that the linear dimension applies to all three pairs of flats. I think that the gauge pin note also implies that the axis of the pin should be used as the datum, but that might not align with the "official" interpretations according to the standards.

There is another potential problem here. It appears that the hex is broached into the part after a 0.810" pilot hole is drilled. This means that the flats will not be complete. The middle material will be missing. Only the 6 vertices/corners will have the actual flat. This works fine for driving the part by a hex key, but it does not facilitate inspection of the part's flats. It will be challenging to register any gauge confidently to the flats. Any gauge pin used will not touch the 6 hex flats (theoretically), but will actually touch the pilot hole diameter. It appears to be drawn with a different expectation, as the gauge pin diameter is aligned with the flats. Is there a section view available on the center axis of the part?

Is it correct that you are not the designer of the part, but rather the inspector of the part?
Click to expand...
Correct, it's not my drawing. As for the flats of the hex, the view that I sent is somewhat misleading. Think of a set screw with a hex socket. Here is an isometric view.
1748611010774.png

No angular orientation dimensions on this drawing. It seems ok though as is. Parallel sides are defined by implied zero and vortices are sitting on the center line. I suppose the shape form can be misinterpreted based on the fact that the angles are not defined explicitly. However the note HEX means a hexagon, which by definition means a six-sided polygon with internal angles equal to 120 deg.
 
Last edited:
Upvote 0 Downvote
Since the drawing clearly says "HEX," I think it's clear that the datum is intended to be created from the full six sides, not just the two flats being pointed to.
In the ASME Y14.5 standard, Figure 7-3 shows two interesting examples at the bottom of that graphic. Although they are rounded, not hex or rectangular, could there be a way to extend those concepts to the hexagon? The datum could then be a combo of an axis and a plane, much like their example (f).
 
Upvote 0 Downvote
A hexagon could be Irregular.
The irregularity could be per-design, at a level that is not visually detectable. Since there is a single dimension over flats without indication such as 3X, the definition is lacking.
 
Last edited:
Upvote 0 Downvote
Burunduk said:
A hexagon could be Irregular.
The irregularity could be per-design, at a level that is not visually detectable. Since there is a single dimension over flats without indication such as 3X, the definition is lacking.
Click to expand...
Suppose we were dealing with a square peg/hole: If it had a similar dimension of .753 +/- xxx and the word SQUARE, would you also say that its definition is lacking?
I wouldn't. (I know there's a square symbol, but I'm just posing this question to keep it analogous to the HEX question.)
 
Upvote 0 Downvote
Gentlemen , hex would be broach just as internal spline. The other features would be machined to it
 
Upvote 0 Downvote
I have a set of drill bits and screw driver bits where the axis of rotation is controlled by a hexagonal portion of the item; as I mentioned before, when used with an adjustable drill chuck it aligns the axis of the item with the axis of the drill very closely. When used in a hex-drive adapter of fixed dimensions there is noticeable wobble.
 
Upvote 0 Downvote
a hex hole can be held .006 inch true position, and can be held to a registered diameter. adjustable hex is basic hex collet. and can be very precise. because it is adjustable.
 
Upvote 0 Downvote
Garland23 said:
Suppose we were dealing with a square peg/hole: If it had a similar dimension of .753 +/- xxx and the word SQUARE, would you also say that its definition is lacking?
I wouldn't. (I know there's a square symbol, but I'm just posing this question to keep it analogous to the HEX question.)
Click to expand...
A square is a regular polygon, more specifically a regular quadrilateral, by definition. A hexagon could be regular or irregular.
 
Upvote 0 Downvote
The observation of a hex adapter to hold a drill bit is that only 3 faces are in contact with the typical chuck, meaning that there are two separate datum features on a hex that is used in a chuck, each essentially a triangle, from which each has a related inscribed cylinder and therefore an axis.

A solution would be to specify the two sets of surfaces that form the two triangles as separate datum features and specify the total runout geometric characteristic tolerance, with separate feature control frames to each of them, to ensure that whichever set was used in a chuck the result would be usable.

I appreciate that some 3rd world nations have problems with workmanship, but even the videos of Paskistani dirt-floor factories show they can turn out product of sufficiently good quality to be useful.

Otherwise one can over-specify with layer upon layer of additional requirements, such as a profile of surface geometric characteristic tolerance, refined with parallelism geometric characteristic (at least 3 datum features and 3 Feature control frames, 6 of each if flatness is not used) and further refined with flatness or toss in a dynamic profile of surface geometric characteristic tolerance.

The dynamic profile of surface geometric characteristic tolerance will be overkill and not advantageous if a 3 jaw chuck would be used.

The dynamic profile of surface geometric characteristic tolerance would not help if there is a hex recess drive as the part will simply rotate with clearance and jam against only the three most prominent corners.

The dynamic profile of surface geometric characteristic tolerance would help if this is to be a crimped or otherwise heavy interference fit.
 
Upvote 0 Downvote
A hexagon has one axis only if it is made perfectly. A CMM will fake an axis that does not represent the way an imperfect hex interacts with mating parts unless the hexagon is made perfectly, but if it's made perfectly there's no need to check it.
 
Upvote 0 Downvote
You must log in or register to reply here.

Similar threads

nmine
  • Locked
  • Question Question
Datum on hole pattern
Replies
6
Views
3K
Burunduk
Burunduk
CTengIS
  • Locked
  • Question Question
Irregular Feature Of Size as Datum Feature on CMM
2
Replies
20
Views
9K
CTengIS
CTengIS
M
  • Locked
  • Question Question
Primary Datum Parallel to Hole Axis
Replies
9
Views
3K
pmarc
pmarc
supergee
  • Locked
  • Question Question
threaded hole depth feature of size?
Replies
16
Views
5K
Burunduk
Burunduk
tN.gineer
  • Question Question
Cylindrical feature, datum symbol placement, in line vs not in line with dimension arrow
Replies
15
Views
10K
Burunduk
Burunduk

Part and Inventory Search

Sponsor

Back
Top