【转帖】dimension origins in the basic views

yang686526

dimension origins in the basic views
our machine shop prefers the dimensions to cascade from the upper left hand corner in the base 2d view of the part. but this does not appear to a basic standard.
in working with a number of defense contractors that have extensive documentation and procedures on drawing standards, i was surprised to see that this basic requirement, namely the basic origin of the dimensions is not specified. some research on the net of drafting schools and other standards also show confusion and a wide divergence on this very basic requirement. in an examination of various drawings and samples from technical schools and universities, i have seen the origin of the drawing in a basic view starting at the lower right hand corner, upper right hand corner, upper left hand corner or lower left hand corner. one manual from a technical school proudly stated "read up from the bottom" for the dimensions. another major aerospace company crowds the dimensions in starting from the lower right hand corner, creating an incredible mess between the projected views! others split the dimensions, essentially creating the classic error of chaining the dimensions, thus causing a potential huge tolerancing error buildup. overall, my impression is that a lot of technical schools and universities appear to be picking up tribal knowledge, ansi standards notwithstanding, (even tho the standards are quoted over and over again) and are repeating these standards without questioning their actual validity. so, after this lenthy statement, does anybody know of a document out there that specifically states in clear and unambiguous language the following: "the origin for the basic dimensions should be the upper left hand corner in a plan view of the part" or whatever?
i don't think you'll find this.  dimensions should be applied in a manner that will yield the required parts within the tolerances specified.  i think "dimension origins" are more of a function of a request from the manufacturing shop floor so they can reduce the number of set-ups required to make the part.
i think there is a world market for maybe five computers.
thomas watson, chairman of ibm, 1943.
johnfalky,
   when i started drafting, i asked machinists how they wanted dimensions presented on drawings.  they wanted everything to come from one corner.  they did not care which corner.  i was on a drafting board.  the machinsts were translating their milling machines manually, watching an led display for the coordinates.  i saw vernier scales in college, but not in a workplace.
   it makes sense.  you mount the part.  you locate zero, whereever it is, and you drill all the holes in one setup.  
   technology has come a long way since then.  i am on 3d cad.  the machinists are using cnc, even on one-off parts, and programming them from cad applications on their computers.  on anything even slightly complicated, they call me and ask for dxf files, and they use the cad geometry to program the cnc.  i suppose if someone is still out there on a drafting board, the machinists will use their cad program to generate the geometry they want.  
   a strict coordinate system is still convenient for manual dimension inspection.
   there are all sorts of fancy tricks in asme y14.5m-1994 for locating patterns to the part and separately to the other elements of the pattern.  defining each hole pattern separately, with composite position tolerances is a good idea on a sheet metal part or weldment.  often the hole patterns are oriented the same way, but they are on different faces or parts, and the  location of the overall pattern cannot be accurate.  it sounds like you do not have this problem.  showing each hole pattern separately does make clear what the part is for.  
   asme y14.5m-1994 allows just about all kinds of dimensioning.  what will your boss allow, what does engineering and manufacturing want?  your machine shop is probably on cnc now, so the ordinate dimensioning is less critical.  it all depends on what you want to accomplish.
                          jhg
i have to agree with drawoh on this.  the purpose of a detail drawing is to clearly define the part.  while starting in one corner may be a shop preference, it can quickly lead to a tolerance stackup between related features.  it is more important to dimension the part such that important relationships are maintained and tolerance stackup is kept to a minimum.
i agree with drawoh, too.
i have talked to machinists that want to setup the machine from one corner. i tell them that i don't care where they start from, as long as it meets the drawing.
chris
sr. mechanical designer, cad
solidworks 05 sp3.1 / pdmworks 05
chris,
that is totally correct.....as long as the part meets print.  as a designer we follow the design intent but knowing how the machinist is going to machine the part is up to them.  design intent is the ruling variable here....which should include manufacturability, assembly, and design.  the outcome would functional datums that work with the design at the part level and in the assembly.
best regards,
heckler
sr. mechanical engineer
sw2005 sp 2.0 & pro/e 2001
dell precision 370
p4 3.6 ghz, 1gb ram
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"there is no trouble so great or grave that cannot be much diminished by a nice cup of tea"  bernard-paul heroux

   actually people, i was being flexible about this.
   let us look at the specific example of a plate with three pitch circles on it, each one mounting a separate part, not particularly accurately.  
   an extreme fabrication solution is to make a cardboard pattern of each pitch circle, and use the patterns to punch the holes.  if you are willing to let the fabricator do this, you can specify composite positional tolerances as per asme y14.5m-1994, allowing the patterns to be off a bit.  your drawing will show the three pitch circles.
   on a milling machine, the machinist will do one setup, and everything will be located to the same datum with the same accuracy.  you can specify composite tolerances, but the fabricator cannot take advantage of them to work more efficiently.
   confronted with a set of pitch circles, the machinist should be able to model them on cad, and use this to set up his cnc machine.  often, the separate pitch circles make for a cleaner, more readable drawing.
   in the old days, i did the orthogonal dimensioning as the machinists requested.  it saved time and cost in the machine shop, and it meant the conversion was done once, only.  orthogonal dimensions are easier to inspect, even now.  
   strictly speaking, you do not care about pitch circles.  you care that the holes are located within a certain distance of the nominal position.  the gd&t positional tolerance makes the dimensioning scheme irrelevant.  
   just for fun sometime, work out the tolerances for a pitch circle with +/- tolerances on the diameter and angle.  your angle tolerances get ugly real fast.
                           jhg
  
if someone were to list the most common and most detrimental errors made on engineering drawings, somewhere near the top of both lists would be dimensioning schemes that cater to manufacturing methods rather than part functionality.  it's common because many manufacturing engineers think that the design drawings should cater to their needs, since they end up using them more than anyone else, and detrimental because frequently the scheme that manufacturing wants is nothing like the functional application.
quote:
an engineering drawingfff"> is an engineering document that discloses (directly or by reference) by pictorial or textual representations, or combinations of both, the physical and functional end product requirements of an item.
asme y14.24, types and applications of engineering drawings
quote:
dimensions shall be selected and arranged to suit the function and mating relationship of a part and shall not be subject to more than one interpretation
asme y14.5m-1994, §1.4(d)
quote:
engineering production drawings must not under any circumstances be prepared to accommodate a particular method of manufacture except as described above. ("for reference purposes only, unless such data are vital to end definition and engineering control of the product.")
quote:
engineering production drawings must not under any circumstances be prepared to accommodate a particular method of manufacture except as described above. ("for reference purposes only, unless such data are vital to end definition and engineering control of the product.")
i don't fully agree with this.  selection of tolerances is a design function, and proper design gives consideration to the likely method of manufacture.  this consideration can be a significant driver in the cost to produce a part or entire product.
some parts are obviously best suited for a particular manufacturing method, and i see no problem with tailoring a drawing to accomodate that method.
the part should be designed with machining practices and tolerances in mind, but not listed on the drawing. for example: you call out a thread "10-32 unc-2a", but not drill, tap, mill, turn, or whatever to describe the process.
a companies may cut a sheet with lasers. but the dwg should be setup so that it can be cut with any process, ie laser, water, saw, router, etc. but the part should be designed per company policies as if it were cut by laser.
my point is, design the part, create the dwg per drafting standards, but don't explain how to machine it.
chris
sr. mechanical designer, cad
solidworks 05 sp3.1 / pdmworks 05
"engineering production drawings must not under any circumstances be prepared to accommodate a particular method of manufacture except as described above. ("for reference purposes only, unless such data are vital to end definition and engineering control of the product.")" isn't that sort of a "circular statement" and frankly i find it highly incorrect, having worked in a manufacturing environment and as an inspector in a precision aerospace machine shop. to take this one step further, certain processes in certain drawings for certain industries should call out the processes required to generate the details of the part, as part of the process/concept to ensure that the item will be "per print" otherwise, the part will be manufactured based on assumption. in this day and age many technicians (in my case aerospace) are poorly trained, because their teachers are technically off the power curve. including processes in the drawing forces inspection standards to be accepted that might otherwise not be used at all. for example: if any of you called out a fastener hole bore dia with a certain tolerance, how is the tolerance to be achieved? with a drill bit? with a reamer? or, to take it one step further, how many of you have really ever drilled, reamed or machined the holes called out in your drawings??? do you know the effect of a worn reamer on the bore of a fastener hole? would you put more data in a drawing after having to interpret the drawing at the shop floor level? or maybe better data?
while designers and engineers cannot be the quality personnel on the shop floor or the machine operators, the assumption "that i'm done with the drawing, you folks do the rest" just does not work for me. too often i have seen situations where an otherwise superb drawing failed to yield an equally superb part. why? the shop floor was given liberal license in interpretation of the drawing. likewise, i take some exception with the non-relevance of the dimensioning scheme if gd&t is applied. while i fully understand that using applying these standards should result in a precise part within the specified parameters, a dimensioning scheme that is adhered to can only help to make such standards more consistent and easier to intepret, i.e using the standards to generate a sloppy drawing just doesn't make sense to me...