Dimension origins in the basic views
Dimension origins in the basic views
(OP)
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?
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?





RE: Dimension origins in the basic views
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RE: Dimension origins in the basic views
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
RE: Dimension origins in the basic views
RE: Dimension origins in the basic views
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
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RE: Dimension origins in the basic views
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
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RE: Dimension origins in the basic views
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
RE: Dimension origins in the basic views
If the CNC programmers on the shop floor need a drawing with coordinate dimensions or whatever, they should create one based on the engineering drawing. But it's the engineering drawing which defines the final requirements of the part. ASME Y14.24 refers to shop drawings and other types which supplement engineering drawings as "ancillary drawings" - and they are not strictly governed by any standards.
RE: Dimension origins in the basic views
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.
RE: Dimension origins in the basic views
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
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RE: Dimension origins in the basic views
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...
RE: Dimension origins in the basic views
When I make a drawing (yes, an engineer that still drafts), I do what I can to make it as easy to follow as possible. Sometimes there are features that have definitions that defy attempts at dimensioning with a nice grid.
Easy-to-follow dimensioning standards are nice for the manufacturing floor or inspection table, but they are no substitute for drafting literacy in any quarter.
RE: Dimension origins in the basic views
- M. Kendall, Engineering and Production Drawings
RE: Dimension origins in the basic views
johnfalky,
It seems that you feel that it is the responsibility of the drafter/designer to make up for any slack in the fabrication of the part. That just doesn't work for me. If I call out a hole that has a tolerance of +.0000/-.0001, it is the machinists responsibility to know how to achieve it. Yes, it may be an expensive feature, but the design requires it, or I wouldn't specify it. You can't blame the drafter/designer if the machinist uses a worn drill bit and the hole is out of tolerance. The same applies to tapped holes. I don't care how the machinist achieves it, it is not the purpose of the drawing to tell him the tap size, etc, only to specify the thread size, class and depth.
RE: Dimension origins in the basic views
In either case, if you apply GD&T positional tolerances, they mean exactly the same thing. If you apply composite positional tolerances, pitch circle dimensioning is clearer to the reader. If someone is using a manual mill or doing manual inspection, orthogonal dimensions are more convenient, and reliable.
Drawings do not exist independent of the reader. I do not understand the concept of an engineering drawing _and_ a manufacturing drawing of a fabricated part. It sounds to me like a strategy for causing transcription errors and partial revisions. To me, the engineering drawing is that one that shows that part_A attaches to part_B through a bunch of holes that line up.
JHG
RE: Dimension origins in the basic views
I might be in a slightly strange position in that the company I work for designs and manufactures tooling, special purpose machinery and components mostly for the automotive trade, however some work is still contracted out.
If we are after a casting we create a model and “add on extra” for areas that require machining, this is sent to a pattern maker and the foundry it will not have one dimension on it, obviously material and finish will be specified.
The same model will be modified before being sent to the CNC shop, the software we use recognises hole types and positional tolerances are tied up to hole type and machined faces are recognised by colour. It would be impossible to dimension the 3D forms, so why bother with the holes, what is the difference?
The same goes with profiles, if something is to be laser cut, wire eroded, flame cut or water jet cut why try to dimension something that can have many hundreds of Radii, angles, arcs and splines when you have a file? Sure give it a tolerance but dimension it?
RE: Dimension origins in the basic views
We send out many of our mold models in the same manner. Any dimensioned drawings are more for the benefit of our customers. Inspection still uses drawings also, but the dimensioning is limited.
On a related note, we have started receiving part models from customers that lack drawings and are fast approaching the ASME Y14.41 standard. Times are indeed changing.
RE: Dimension origins in the basic views
Because of the complexity of most shapes they are measured on a CMM from the model again no dimensions or drawings other than tolerances.
We do produce a few drawings for items that will be manually machined, so whilst not telling a machinist how to do his job we have to be very aware of how it will be made.
We simply could not survive working any other way; margins are far to tight to waste time.
RE: Dimension origins in the basic views
In most cases I agree. However, consider:
It is possible to make an entirely satisfactory engine block by machining it out of a solid block, but this is not very practical. Much more economical to use a casting. When designing a casting the designer or engineer needs to include provisions for draft angles. Perhaps there are areas were a parting line could not be tolerated. The design needs to consider the process.
Would you consider a bolt with cut threads for a con-rod big end? Probably not, you want rolled threads here. The process can be the difference between a suitable and unsuitable part. Do you want this information on the drawing?
Welding drawings need to specify (either directly or indirectly via WPS) the proper process. Do you want to risk that a stick welder was used on materials sensative to hydrogen embrittlement? Do you need preheat, limits on interpass cooling, post weld stress-relief?
Can you knurl something that isn't a turning?
RE: Dimension origins in the basic views
Quite right, hence the caveat in the drafting manual: "unless such data are vital to end definition and engineering control of the product". Molded parts need draft angles and acceptable gate locations, cast parts need porosity specifications, etc. Processes which directly control bulk or surface material properties or which require certain features to be designed into the part cannot be ignored on the engineering drawings.
However, baseline or chain dimensioning the design from one particular surface just because that's the easiest one from which to fixture and program one particular machine runs counter to the fundamental principles of GD&T. The part should be dimensioned and inspected according to its function and mating interfaces. Hopefully the machinists can use the functional datums to orient their location. If not, they'll have to maintain tighter control to avoid tolerancing stacking which renders the part useless.
RE: Dimension origins in the basic views
RE: Dimension origins in the basic views
Engineering- all critical specifications should have be referenced in the general notes of the drawing, and ISO procedures should have been followed regarding change control.
Manufacturing- parts should not have been made from a redlined drawing without approval (stamped and signed) from engineering.
Inspection- the "CAD design staff and management" didn't need to know what a proper installed rivet looked like, but your inspection personnel should have been trained to know the difference between accept/reject criteria.
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Thomas Watson, chairman of IBM, 1943.
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RE: Dimension origins in the basic views
Best Regards,
Heckler
Sr. Mechanical Engineer
SW2005 SP 2.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
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RE: Dimension origins in the basic views
If someone wants to list how to machine a part on a drawing (a side from special circumstances, like molding) then that person is not experienced enough to make drawings in the first place. IMO.
Sorry, not trying to knock anyone, just real world.
Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP3.1 / PDMWorks 05
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RE: Dimension origins in the basic views
Okay, then the real world interferes. What one does when the fabricator(s) are in-house/captive is different than what is required when everything is sub-contracted out.
There is no standard that specifies a corner or ??? for the dimension origin. There are too many variables for that to work in all cases and dimension origins should be chosen by the requirements of the part/design. That being said, there is usually a way to accomodate the desires of the machine shop AND the requirements of the part.
Generally, don't specify process on drawings. First, the engineer/designer/drafter is seldom the fabrication 'expert'. Second, someone will come up with a new, better way of making the part (stronger/faster/cheaper/etc) but can't use that method because the drawing specifies a different process.
A good example of this is "flow-drilling". I needed 36 holes with 1/2" diameter female pipe threads in an extruded 1/8" wall aluminum square tube (fluid manifold). My out-of-date machine shop knowledge said these threads were virtually impossible to achieve. The shop said "no problem" because they could "flow-drill" the holes. "Flow-drilling" extrudes the wall material into an internal boss, pierces it and creates threads thru the combined wall and extruded boss thicknesses. Neat!
Specify performance instead of process and you'll get what you want without "tying someone's hands". It doesn't matter how a part is produced as long as it meets the functional requirements (including delivery and cost).
Best regards,
RedPen
RE: Dimension origins in the basic views
Even if the fabricator does not look at the dimensions, the inspectors do. If the inspector rejects something, you need to clearly tell the fabricator what is wrong. There is also more than one way to move from scale CAD model to toleranced dimension.
I have been looking into having parts fabricated by processes such as Stereo Laser Lithography, where the fabricator would work off my 3D CAD model, rather than my drawings. Since the shapes are likely to be weird, we are going to have to work out an inspection strategy. Probably, I will have to design inspection fixtures of some kind. I am sure I am in for a long chat with someone.
JHG
RE: Dimension origins in the basic views
I have been thinking of that also. But not 'til next year.
Please let me know how it works out.
Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP3.1 / PDMWorks 05
ctopher's home site
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RE: Dimension origins in the basic views
RE: Dimension origins in the basic views
No one here has advocated specifying processes on drawings.
I do not like walking into a shop or even to a colleague's desk and finding my fabrication or dimension drawings covered in pencil marks due to somebody recalculating all my dimensions. Even if I am not responsible for an error, it is still an error.
For example, I was about to produce an outline drawing of a printed circuit board to go into a housing I was designing. I asked the PCB layout person if she could work in metric, and she said "Yes" in a thick eastern European accent. When I cruised past her desk a couple days later, I noticed that she had taken a pencil and converted all my dimensions into inches. Clearly, the CAD software she was using was limited in capability.
The question "Can you work in metric" has a different meaning when I am using SolidWorks. I went back to my desk, changed the PCB drawing and model to inches, I wiggled all the features so that they worked out to round inch values, then I re-issued my drawing.
No more pencil marks. No need to check pencil marks. The mechanical drawings work because SolidWorks makes everything line up.
Both of my drawings met all the requirements for an engineering document. One of them was very much more clear and reliable than the other one. The fabrication drawing of my housing is in metric, and the PCB hole locations are not round numbers, which is unfortunate. It was CNC machined, so it does not matter very much. Everthing lined up, so our communications worked, and the PCB fabricator and machinist did their jobs.
If I had wanted to keep my fabrication dimensions are round numbers, I could have changed my PCB drawing to dual dimensioning, or I could have just changed the units to inches. In mass production, I might have. In limited production, I judged that communicating with the electronics people was more important.
JHG
RE: Dimension origins in the basic views
What you say about inspection fixtures is correct, you will need them and certainly in the automotive trade the models will show the location areas and the locations (datums) beyond that nothing is dimensioned other than tolerances. A CMM is perfectly capable, with the correct software to inspect the part.
We have ISO 9000 and also we are audited by amongst others Ford, BMW, GM, Audi and Bentley and supply them all as first and second tier suppliers.
Working purely from models is now the norm in the automotive trade, certainly for panels and fascias.
If you do go down the road you are talking about in a years time you will look back at this and what you are now doing and laugh.
RE: Dimension origins in the basic views
Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP3.1 / PDMWorks 05
ctopher's home site
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RE: Dimension origins in the basic views
Very few dimensions are shown on automotive drawings. Mostly limited to hole diameters. Everything else is spec'd out by GD&T referenced to the CAD model. I'm quite surprised to see so much discussion on (old school) dimensioning and so little on GD&T applications.
My favourite drawing note: "Math data is the master, drawing is for reference"
Ok, actually my real favourite comes from Japanese drawings that state (in English), "In the case of any discrepancy between the English and Japanese text on this drawing the Japanese shall take precedence"
Not being able to read Japanese, I've never found a discrepancy.
regards,
Hydroformer
RE: Dimension origins in the basic views
That is not entirely true. We are involved in a military program providing parts for an unmanned aircraft, and we have no requirements for drawings. The files we receive from the customer (of parts we have to produce) have no drawings, and all tolerance callouts and related information is in the file with the solid model. We do not produce drawings of the finished part, as their files cover everything (build to "print").
Another customer of ours is producing a small corporate jet, and while they do required drawings, dimensions are kept to a minimum, mostly hole sizes, overall dimensions and geometric tolerances.
Prceedures, certs and related documentation are provided, but fully dimensioned drawings seem to be going the way of the buggy whip. The future is closer than you think!
RE: Dimension origins in the basic views
Cars and airplanes are full of complex curved surfaces. Conventional definition and inspection with dimensions on paper is difficult. Working directly from the model in CNC or with SLL is fairly easy. Using the CAD data to generate inspection fixtures is easy too.
Most of my stuff is orthogonal, and fabricated by sheet metal and machine shops. Given our production levels, 2D dimension drawings are still convenient and efficient.
Curved stuff looks neat though, so I am getting interested in SLL and in castings.
JHG
RE: Dimension origins in the basic views
I was suprised when we received some of these part packages. Not only are the formed and molded parts without drawings, but the orthogonal hardware is too, which made my job a little harder because I had to get some of that information on paper for our vendors. Sometimes I miss the days of square parts with round holes!