dimensioning to hidden line of internal thread major diameter 1

[denden2013]

Facing the circular profile of an internal thread on a detail drawing, to which line, (A) the solid line minor diameter or (B) the hidden line major diameter, should a thread callout/dimension arrow lie on?

Early in career, I dimensioned to the solid line by the basic idea of not dimensioning off hidden lines but I remember having been corrected about eight years ago with enough supporting material to place the leader arrow right on the hidden line (major diameter).

What is a good reference to support whichever convention?

Thanks!

 

[KENAT]

Assuming ASME stds I was going to point you to Y14.6 but don't see it in there from a quick look, nor in 14.5 from a real quick look.

If you don't get the answer you want in this forum considering posting over in Drafting Standards, GD&T & Tolerance Analysis with a link to this one.

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[Cockroach]

You always dimension to the minor diameter of Box threads, and major diameter on Pin threads. So the answer is "that solid line" on the Box detail.

Regards,
Cockroach

 

[denden2013]

Link

Link

Could not find international standards but the two links above provide a good start ESPECIALLY the second one (from "wisc.edu")which, on page 8, reflects a practical approach wherein an arrow points to, or lies on, the last operation performed. Reference to thread callouts in the first link ("morainevalley.edu") may be found on section IV.

With the above findings, I, personally, will now maintain pointing to the (hidden) major diameter just as when I was first corrected going back eight years.

Thanks to all who gave their input in this thread.

 

[MintJulep]

I'm not sure of the answer, but will complicate things by pointing out that on physical internal threads the minor diameter is not a physical feature.

 

[rb1957]

if you're tapping a hole, wouldn't you drill the hole first, then tap it ?

sounds to me like dimensioning the hole and show the tap ... by drawing note tagged to the hole axis ?

Quando Omni Flunkus Moritati

 

[ornerynorsk]

I will concur with denden2013. Another method that I have been seeing a lot in the past 10 to 15 years is a single leader attached to a dot that is placed on the thread of a partial section view, with the thread detail and class fit called out.

Notwithstanding GD&T, I strongly suspect this varies by the type of industry, and many companies have their own drafting culture in which conventional protocol is thrown out to simplify or otherwise enhance detail that is more relevant to their internal workings. I have worked at companies in which the drawings, if submitted to a vendor for RFQ, would be the equivalent of sending them an ancient Greek manuscript, assuming that they are not fluent in that, as well, but were perfectly adequate for internal usage when taken in conjunction with work instruction, setup notes, machinists notes, etc.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[DiegoLGraves]

For what it's worth I was reviewing Caterpillar's latest release of geometric tolerance interpretation, 1E0012 when I saw Kenat's post over in the drafting standards forum. Cat has several reference figures with tapped holes. For internal threads, some point to the inside solid line, some to the hidden outside line. And of course some are unclear, fuzzy, or connect so close to the centermark that I can't tell where the arrow tip is.

In Cat's 1E0500 spec for inch threads the one clear figure shows the callout arrow to the inside solid line.

Again, for what it's worth. Diego

 

[denden2013]

Or another way to look at it is IF there is no governing body/standard (e.g. ANSI) specific about such detail, consideration is to be taken if the (educational) institution or (manufacturing) company has its own by which a drawing detail is to conform. Otherwise, either way should be acceptable and any unwritten preferences cannot render another incorrect.

 

[hydtools]

With no governing standard, I would select the feature which can be touched. For tapped threads that would be the finished minor diameter. Inspection is difficult when done to hidden features.

Ted

 

[denden2013]

Ted / Hydtools,

Just as in that one school's standards (earlier link) pointing to the "last operation," however opposite is your approach, it's a good/valid point. Again, not necessarily is there an incorrect way to do it.

Associating either the minor or major diameter with thread inspection, however, just may detract from the convention of the pitch diameter being of primary importance over the two other diameters. Inspecting to either minor & major diameters would not necessarily validate a thread's conformance to specifications; thread gages or the three-pin method oftentimes objectively validate threads/fits.

 

[hydtools]

Are specifying a thread and wondering where to attach the thread specification leader? The axis or center depending on the view.

Ted

 

[Cockroach]

I'm not sure why this is such a problem. I think there is a severe lack in the understanding of the machining process. You want to let the machinist know what is required by specifying a dimension on the drawing. Do you really think he will make a cut to some hidden line representative of a major diameter on a Box or minor diameter on a Pin specification? Do you really want to chance having a machinist stop, pull out a math book and start computing depths to said threading in order to get to the minor or major diameters? Give your head a shake.

Attached is an example for both the external and internal threading specification required to a recent piece. Never had a complaint from the shop, I use the three wire method of pin measurement to use as a gauge on the box threads. This gets around the added cost in buying material for GO and NOGO gauges and then machining them to a four digit accuracy and sending them out to get hardened. If that's what you need to do because you work for some large corporation, that's fine, they have the deep pockets to cover that cost with added QC infrastructure.

It's not rocket science guys. Get a decent technical book from the store on machining and learn the trade. It really lends itself well to mechanical engineering, particularly when you can jump on the lathe and show the guys how to cut the thread yourself.

Regards,
Cockroach

 

[KENAT]

Depends in part if you want to follow the relevant industry drafting standards (at least ASME) in which case it's more about legal contract defining what you'll accept than telling machinist how to make it.

General consensus seems to be that tapping drill size information doesn't belong on the drawing per section 1.4e of ASME Y14.5M-1994.

Likewise from a standards compliance point of view the sample drawing you give doesn't control coaxiality of the various diameters as required by 14.5M. Chances are most of the time parts will get made fine anyway however if they're wrong there is case law that suggests the machine shop would not be held liable and you'd have to pay for non functional parts.

However, all this is getting a bit off topic from the OP. The question is arguably a slightly pedantic drawing standards issue that it's hard to imagine having significant real world impact. forum1103 is perhaps a better venue for that type of question.

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[jbeckhou]

@Cockroach. I couldn't have said it better.

Petrotrim Services

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[denden2013]

Just reaffirming Kenat's first paragraph, or actually, the entire thought for that matter, that it is a matter of compliance--to what is set forth in POs/contracts.

Would caution, however, on Cockroach's approach. Hearing no complaints from a machinist would explain the adequateness of a spec and/or of the machinist's trade skills but there's always the underlying, and oftentimes overlooked, cost of inappropriately spec'd parts, not to mention the cost of rejecting non-conforming material.

 

[Cockroach]

But that is not the Machinist job, Denden2013, does he not make the part to print? Adequate ness of specifications, relavence to industry code, cost consideration of the part, etc is MY job, the engineer. Review of MTR and acceptance of that material is MY job, the engineer, not the machinist.

The shop makes the part to my specifications, that is what they get paid to do. I QC the part in order to prove the piece is to print. The MTR and QC print form part of the travel package traced to the piece.

You need to understand the process.

Regards,
Cockroach

 

[denden2013]

Engineers specify/design--to industry and/or company standards or run the risk of being misinterpreted if one exercises otherwise and it is then that machinists oftentimes come back with questions when ideally there should not be any.

Machinists/fabricators interpret engineers' works and physically produce parts to/by industry standards. Uncoventional specifications will prompt questions here or allow for any misinterpretation; over-specification on the other hand may propmt unecessary procedures and raise costs. It is also here where drawings which have only been internal to a company would get tested whenever sent outside or when parts are outsourced.

Quality Control, not necessarily Engineering, measures/verifies the parts produced for conformity/compliance to the specifications set forth by design/engineering AND/OR by any Procurement or Quality Assurance PROVISIONS set forth in Purchase orders or Contracts--without any assumptions of its own as to part technicalities (form/fit/function).Problems arise, or operations are compromised, when this function is often mixed up with engineering and design. About the only time it is ideal the engineer/designer measures its work is at the development stage. Subsequent controls/measurements should serve as checks independent of design/engineering to reinforce objectivity in the process.

An effective Quality Assurance/Management System (QMS) would have provisions that allow for effectiveness and seamless transitions of the different stages, from the opening of a project or conceptualization, or from the receipt of an order or contract, to developing and producing parts, all the way out to delivery of finished goods.

Anyway, this is way off topic now. All it was was a matter of conventions and compliance.

Thanks, everyone.