Fig 7-27 (Y14.5-2009) and datum feature long holes

[aniiben]

If straight bolts are going to be used into the holes of a part similar to the one from fig 7-27 (Y14.5-2009) and the thru holes are secondary datum feature ( as per the part function) how to specify in GDT language that the holes (as a pattern) are datum feature?

Lets say Surface C is primary (datum feature C) and the eight holes are, as a pattern, datum feature E, positioned within 0.5 at M at Surface C and also positined within 1 at M at Surface D, how to say the holes makes datum feature E

Ignore original datum A and B shown on the drawing for my question clarity.

We would like to use the holes as datum feature and allow more tolerance one one side (Surface D) versus the other (Surface C), but not sure how to say and show it correctly since the tolerance zone is conical.

 

[chez311]

aniiben,

Is this for an actual part/drawing or a purely theoretical question? I have to imagine that typically, at least in the example you are showing, the conical/tapered tolerance zone is provided to accommodate a manufacturing process like casting and not actually reflect function - as in your example the mating part is not conical but cylindrical (straight - round shank of a fastener). I figure you could simulate a conical feature - but I'm not intimate with the specifics and from the discussions I've seen on these forums it can probably get hairy pretty fast.

What might be a better option would be to utilize something like whats described in paragraph 4.11.6.3 of Y14.5-2009 to specify a boundary at whatever size is desired (presumably either the smaller sized boundary suggested by the FCFs or something even slightly smaller with clearance to it - as mentioned in this post

https://www.eng-tips.com/viewthread.cfm?qid=440127).

To me this would seem to better reflect the functional intent you described.

 

[aniiben]

It is not purely theoretical question, but a true design scenario.
Long bolts can be bent beyond their perfect form at MMC and we would like to know how to accommodate their extra form error -straightness- into the mating component (body part similar to the one shown in fig 7-27/2009).

 

[3DDave]

If the bolts take a larger volume than their perfect form would suggest then make the holes larger by the expected amount of diametral straightness tolerance.

 

[Sem_D220]

If the goal is to make the end of the holes where the bolts are expected to bend more accurate than the end through which they enter by use of non-uniform tolerance zone, another good reference in the standard may be para. 8.3.2 (Y14.5 2009) You could show the tolerance zone with phantom lines and basic dimensions and that way you can have just one feature control frame to which you can attach the datum feature symbol that you are going to associate with the pattern. Even though the concept appears at chapter 8 and the examples deal with Profile, I don't think the concept is restricted to Profile only and you probably can use it with Position (but additional opinions on this are welcome).

 

[chez311]

aniiben,

A few questions:

Is this component required to have tapered/conical holes due to a manufacturing process (like casting as I initially noted or injection molding, etc...) or is the introduction of this non-uniform tolerance zone purely for the purpose of accommodating form error in the mating bolts? And in what direction is the required/desired taper - ie: larger tolerance zone on the side of the head tapering to a smaller tolerance zone near the end of the bolt/threads?

Does the mating component which the bolts are utilized to mate this to have tapped holes where the bolt threads into or does it have through holes and a nut is utilized with the bolt?

 

[aniiben]

Chez311,

Is this component required to have tapered/conical holes due to a manufacturing process (like casting as I initially noted or injection molding, etc...) or is the introduction of this non-uniform tolerance zone purely for the purpose of accommodating form error in the mating bolts?
And in what direction is the required/desired taper - ie: larger tolerance zone on the side of the head tapering to a smaller tolerance zone near the end of the bolt/threads?

Does the mating component which the bolts are utilized to mate this to have tapped holes where the bolt threads into or does it have through holes and a nut is utilized with the bolt?

A1: for the form error
A2: larger TZ on the side of the bolts head, smaller TZ on the threads side
A3: cylindrical holes ( not conical) and no process requirements to have conical TZ.
A4: both cases described depending by the customer applications: either the bolts are secured with nuts either blind threads.

 

[3DDave]

How does having a larger tolerance zone help - please create a picture.

 

[Sem_D220]

3DDave, I think that the reasoning there is not that a larger tolerance zone will help, but that a smaller tolerance zone for the holes at the side where the bolts are expected to bend and potentially occupy more space, may prevent interference.

With that said, I'm not sure how effective this method is and would also like to see a diagram of it.

 

[pylfrm]

If straight bolts are going to be used into the holes of a part similar to the one from fig 7-27 (Y14.5-2009) and the thru holes are secondary datum feature ( as per the part function) how to specify in GDT language that the holes (as a pattern) are datum feature?

As shown in ASME Y14.5-2009 Fig. 4-26. Alternately, the datum feature symbol may be attached to the horizontal portion of the leader line for the hole pattern specification.

Lets say Surface C is primary (datum feature C) and the eight holes are, as a pattern, datum feature E, positioned within 0.5 at M at Surface C and also positined within 1 at M at Surface D

Will C be the only datum feature reference for the position tolerances applied to the pattern of eight holes?

For the other tolerances where you intend to use C as primary and E as secondary, will E be referenced at MMB?

If the answer to both of these questions is yes, then I don't see any issues with drawing interpretation. The datum feature simulator would be a pattern of conical pins tapering from diameter 12.0 at surface C to diameter 11.5 at surface D.

Another option would to be to dodge the datum feature issue entirely and rely on simultaneous requirements instead. It's generally possible to express an exactly equivalent scheme in this way.

I think that the reasoning there is not that a larger tolerance zone will help, but that a smaller tolerance zone for the holes at the side where the bolts are expected to bend and potentially occupy more space, may prevent interference.

That would indeed make more sense for the case of a mating part with threaded holes.

The taper direction shown in Fig. 7-27 might make more sense for a case where bolts and nuts are used to clamp a relatively thin plate with matching clearance holes against surface C. This assumes it's acceptable for the bolts to end up somewhat tilted.


pylfrm

 

[3DDave]

To the original question - refer to the holes as a datum feature the same as any other pattern of holes used as a datum feature. The taper does not affect the reference. The most convenient place for the datum feature symbol is on the horizontal leader line attached to the hole quantity and size call out. See figure 3-4(d) in '2009.

 

[Belanger]

Interesting question. Although I agree a tapering position tolerance doesn't affect how you label the datum feature, what if that datum is subsequently referenced in a FCF with the MMB modifier? What size would the datum feature simulator pins be?