"nominal"
"nominal"
(OP)
Is there much confusion, in y'all's experience, with what "nominal" means? I use it in the way that the word's really defined--"nominal 1/2 inch" means the part that's NAMED as a 1/2 size in the manufacturer's chart, when really the dimension might be 0.486".
Are people using it to mean "just about" or something like that?
Hg
Are people using it to mean "just about" or something like that?
Hg
Eng-Tips policies: FAQ731-376





RE: "nominal"
Some people seem to think "nominal" means "normal" or "usual." Others seem to think it means "at least" or "minimal."
RE: "nominal"
RE: "nominal"
A dimension can have a "Push Fit" or "Press Fit" tolerance of say +.001/+.003 showing that the actual nominal is NOT what is required.
RE: "nominal"
I agree with the others.
Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)
RE: "nominal"
Hg
Eng-Tips policies: FAQ731-376
RE: "nominal"
For example, a 1/2" pipe has a nominal size of 1/2" even though basically nothing about it is anywhere close to .500" "Half-inch" is its name.
Likewise, it is good form to use a nominal size in a dimensional callout for mating, fitted parts. This may sometimes require that it have a unilateral tolerance. So, for example you should say
.500" +.006/+.002
rather than
.504" +.002/-.002
This way it is easier to match it up with its mating part (which would also have a nominal size of .500, but a different tolerance) and figure out what kind of fit you have by looking at the drawings.
Remember: drawings should reflect design intent first. What's easier for a machinist to dial in is of secondary concern. He has a calculator, just like you.
Hope this helps.
Don
Kansas City
RE: "nominal"
RE: "nominal"
nom?i?nal? /?n?m?nl/ –adjective
1. being such in name only; so-called; putative: a nominal treaty; the nominal head of the country.
2. (of a price, consideration, etc.) named as a mere matter of form, being trifling in comparison with the actual value; minimal.
3. of, pertaining to, or constituting a name or names.
4. Grammar.
a. of, pertaining to, or producing a noun or nouns: a nominal suffix.
b. functioning as or like a noun.
5. assigned to a person by name: nominal shares of stock.
6. containing, bearing, or giving a name or names.
7. (of money, income, or the like) measured in an amount rather than in real value: Nominal wages have risen 50 percent, but real wages are down because of inflation.
8. Aerospace. performing or achieved within expected, acceptable limits; normal and satisfactory: The mission was nominal throughout.
9. Slang. done smoothly as expected: The space shot was nominal, proceeding without a hitch.
–noun
10. Grammar. a word or group of words functioning as a noun.
RE: "nominal"
RE: "nominal"
Remember: drawings should reflect design intent first. What's easier for a machinist to dial in is of secondary concern. He has a calculator, just like you.
Don, can I get you to come talk to our shop supervisor? He, and our machine shop, believe that the purpose of drawings is to make it easy for the machinists. We had a big argument about this a few days ago. My assertion that the purpose of a drawing is to define what a good part looks like falls completely on deaf ears.
If the shop had their way, dimensioning would be strictly baseline cartesian. They even bitch when we dimension hole groups center-to-center. Needless to say, it is not a professional shop. Most machinists are not ticketed, they were trained on the job. Part of the problem is they only know what they've learned here, and since it has never been a professional shop they don't know any better.
Still, I'd like them to stop arguing with me about it!
RE: "nominal"
For example, how can you show a specific tolerance between two holes (like locational dowel holes) using strictly ordinate dimensions? If it's necessary that the distance between two holes be +/-.0005", how is this represented in dimensions that all come from an arbitrary common point? Let's say you use four decimal places and that this means +/-.0005" in the default dimensions. That means that the first hole can be .0005 short and the second hole can be .0005 long and be in tolerance. But that locates them .0010 apart, which is no good.
The only way to get what you want is to cut the tolerance in half and mark both ordinate dimensions +/-.00025 from the reference zero. So measurements have to be twice as precise as necessary (and look ambiguous and confusing as hell to someone trying to read the drawing and determine what you're up to) just so the "machinist" can punch numbers in like a chimp instead of using his head just a little.
I've had machinists complain about unilateral tolerances too. Oh, crap! Figure it out, stupid! If you gave them a $10 bill they'd complain that it wasn't two fives!
Give me a *real* machinist any day. There's plenty of them still around. Those guys know what they're doing and understand why it's imperative that drawings reflect design intent. Let the rest of them sweep the floor.
Don
Kansas City
RE: "nominal"
IMO, all drawings need to ne dimensioned and toleranced properly as if it can be made almost anywhere in the world and not taylored to your own machine shop, like they wish.
I see the same thing happen with people in inspection. I tell all of them, "I follow ASME/ANSI/MIL standards. If you don't understand it, the manuals are stored in a certain area ... go read them, I'm not changing it unless I'm proved wrong (which happens to all of us). I'm not changing the tolerance because that is what works with the design.".
Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)
RE: "nominal"
It can also mean a very small or largely insignificant amount. For example a slab with some very light mesh has "nominal reinforcing".
RE: "nominal"
Machinists will prefer ordinate dimensions as they are clear and easy to use, they will save time during manufacture, if someone has to add and subtract a series of numbers it will take time and add cost to the product as well as increasing the likelihood of mistakes, which I am sure we all make.
Of course they are not always the best way to dimension a part from a functional point but should the machinists point of view not be considered in deciding that?
To have a I am right and they are only chimps who punch numbers into a machine is a very arrogant attitude, just as for machinists to have the I am always right and the designers are only a bunch of chimps who play on a keyboard all day is.
Surely good engineering as about producing a good product as cheaply as possible and all the steps involved in doing that be as easy as possible for the next department?
If all the guys on the shop floor look at a drawing and think oh dear it is one of his maybe people should look at what they do?
RE: "nominal"
This is a usage which has caused transatlantic confusion on more than one occasion. Brits don't use the word this way (even in an aerospace context). You get as far as saying something like the "Release Standoff was nominally 15 km..", and the US delegation starts smiling, ticking boxes and nodding appreciatively before you've even begun to describe the Horlicks the pilot really made of the trial.
A.
RE: "nominal"
ajack1,
Yes, the fabrication of a part should be taken into consideration, but the function of that part should still be primary. The key is to define it so that it will meet the intended function without added unnecessary expense to the fabrication. Good engineering is about producing a quality product as INEXPENSIVELY as possible while meeting all of the requirements of that product.
I was taught the same as ctopher that a proper drawing done to applicable standards will produce the same part regardless of where it is manufactured. If your shop has a problem with that, then provide training to them so that they can properly interpret those standards. If they still have a problem, then make sure that whoever creates your drawings also has that training. Encourage communication between the two parties, and they will find a solution that is allowed in the standards. This should be a win-win situation.
RE: "nominal"
ajack1,
"... I am right and they are only chimps who punch numbers into a machine is a very arrogant attitude ..."
I would never assume a machinist to be a chimp or anything less. I always work with machine shops to make sure I stay with their technology and have good communication. On the other hand, I design the part, they most of the time do not know the design or what the part does, so they are to make the per dimensioned. I encourage feedback from machinist's on drawings. If they find error, I will talk to them and change as needed.
I always create solid models at "nominal". I have sent the CAD files to CAM software in machine shops and have parts sent to me outside the tolerance because of misinterpretation of nominal.
I have a note on the dwgs stating nominal to be the target dimension, but not to exceed the indicated tolerance.
If the dim is .250-.500, then a part is created at .375 +/-.125. (exaggerated dim for easy understanding). I have had some shops tell me their people create models at max or min, then setup the machines to set tolerance. So, I recieve parts at max or min (nominally) which makes them out of spec.
All parts I have recieved from several countries in Europe or Asia are good understand nominal. (we just have problems with their materials, another thread).
Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)
RE: "nominal"
I agree with Tomfh, to my grasping nominal sizes, as opposed to actual, are used for purposes of general identification.
RE: "nominal"
Forever Young
RE: "nominal"
As regards the divergence from the thread.
I try to dimension:
1st Function/Performance
2nd Inspection (usually this should reflect Function/Performance so is almost a sub set of 1)
3rd Ease of Manufacture & other considerations.
This isn't to down play the importance of design for manufacture but an Engineering drawing should define the requirement, not how to get there (it’s in ASME Y14.5 and the like), so long as the requirement is reasonably achievable.
Ideally as Snowfire says for all but the simplest items there will an intermediary point between the drawing and the tool operator/assembler that explains/details how to achieve the drawing requirements. This may be a production plan/drawing(s), work instruction, detailed routing or potentially a computer program, I suppose under certain circumstances the CNC program could count as the intermediary step.
I thought manufacturing/process engineers were meant to perform this intermediary role. This in turn allowing the actual people carrying out the action to be 'down skilled' to unskilled labor. Seems in many cases places down skilled the labor without introducing enough manufacturing/process Engineers.
RE: "nominal"
http://www.howellpipe.com/howpspec.htm