Tolerances on Mech Drawings
Tolerances on Mech Drawings
(OP)
Many years ago when I took a HS course in mech. drawing, the correct way to show dimensional tolerances was to have the same number for the + and - eg., if you want something between 7 and 9 in. long you used 8+/- 1 (I can't figure out how to get the + directly above the - here).
But the foreign co. (Italian) I'm working at has a weird (to me) way of showing them. They'll use anything from 7 +2/-0 to 9 +0/-2. They've even used (I am NOT making this up!) something like 6 +1/+3!!! This is not a small company, but a fairly large but old manufacturer of BIG equipment.
Anyone seen anything like this before?
But the foreign co. (Italian) I'm working at has a weird (to me) way of showing them. They'll use anything from 7 +2/-0 to 9 +0/-2. They've even used (I am NOT making this up!) something like 6 +1/+3!!! This is not a small company, but a fairly large but old manufacturer of BIG equipment.
Anyone seen anything like this before?





RE: Tolerances on Mech Drawings
There are plenty of reasons why you would want to use unilateral tolerances, but most of them come down to ease of reading or ease of checking. If you have a hole that's specified as x -0/+tol, and a shaft that's specified as x -tol,+0, then you can easily see that they will have, at MMC, an exact match in size.
RE: Tolerances on Mech Drawings
8 +1/-1 is not necessarily the same as 7 +2/-0, or 9 +0/-2.
The problem with tolerances is that nobody bothers to consider the root of the word. Tolerance comes from tolerate. The whole point is to define how much ERROR from the ideal case can be tolerated by the design.
I often hear "Well the parts don't go together right because there was an accumlation of manufacturing tolerances." WRONG! The purpose of actually thinking about, and correctly defining tolerances is to ensure that the inevitable manufacturing ERRORS will not result in parts that don't go together.
rant.mode = false
RE: Tolerances on Mech Drawings
See ANSI 14.7 for correct notation.
RE: Tolerances on Mech Drawings
This way of tolerancing makes it easier for the designer do define the min. clearance/interference.
Say we have a shaft 50mm dia and we want a bore locate on it with the min. clearance of 0.1mm and the machining total tolerance for both parts 0.05mm.
The "metric way" to dimension it would be:
Shaft dia 50 +0.0/-0.05
Bore dia 50 +0.15/+0.1
Locating smallest bore 50 +0.1) on largest shafts (50 +0) shows immediately the min. clearance.
The dia 50 in this example links the two parts together - it is usually called the "nominal" size for both of them.
This tolerancing style probably originated in the ISO system of fits( like 50H7/g6), what is used for decades and simplifies design, manufacturing and checking (if used properly).
Again - the main reason for doing it is to give to BOTH parts the same nominal size and using the tolerances define the fit. And for the ISO fits - there are standard tools, gages waiting for the smart user, it means less expensive tha the "special" gages/tools if no standard is used.
RE: Tolerances on Mech Drawings
One problem though is how do you represent the geometry with a tolerance (such as 50 +0.15/+0.1) with a CAD/CAM system?
Most CAD designers would model a shaft with the nominal diameter of 50. But this would be out of tolerance if the NC programmer only used the CAD geometry.
RE: Tolerances on Mech Drawings
A sketch can tell someone what you WANT.
A drawing tells someone what you WILL NOT ACCEPT.
;---
Asymmetrical tolerances are a shorthand way of saying that you'd prefer the feature be at the nominal location, but the design can tolerate (there's that word again) more varaition in one direction than another.
;---
Symbolic tolerance ranges are a shorthand way of dimensioning a joint for a particular fit, without going to the bother of calculating the numbers for every joint. It makes a lot of sense, though I never got used to it.
;---
I've gone to the trouble of trying to include tolerances in CAD geometry. It doesn't take a very complex part to drive you crazy, trying to figure out which line is the one you want to snap to, connect, etc. The closest I've come in practice is to model tapped holes as a cylinder of the tap drill diameter, and clearance holes at their nominal drill size.
As for modeling a feature where the desired size is not on the nominal, i.e., the tolerance range does not include the nominal, I'm not sure what to do. If you model the desired size, you have to override the dimension to show the nominal on a print, and if you model the nominal, the programmer has to work more carefully. Either way is likely to cause an error sometime. Modeling the nominal is slightly less likely to cause production of bad parts, but overriding dimensions carries risks of its own.
On the other hand, the number of times I've toleranced a part like that could be counted on the fingers of one hand, and it confused the hell out of everyone who saw the print. If you want to know when your part actually hits the shop, that's one way to do it; the phone will ring for sure.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: Tolerances on Mech Drawings
RE: Tolerances on Mech Drawings
My copy of Engineering Drawing by Thomas E. French, McGraw Hill, published 1941, shows +/+ tolerances. They describe the practise as acceptable, but not as clear as limit tolerancing. They discourage making the workman do arithmetic. That makes the practise legal here in North America at least back to WWII.
With CAD, there are several ways to model something to scale and apply dimensions and tolerances. The nominal dimension, max and min allowance format makes clear what someone did. If you are working on someone else's drawings, you could get a nasty surprise.
JHG
RE: Tolerances on Mech Drawings
For the CAD representation: in our company the models are always created using the mean dimensions (average between the upper and lower limits) even if they are dimensioned different way. This helps if the models are used for rapid prototyping or castings etc. Again - the drafter has to be trained how to calculate the mean dimension. In case of confusion - and ISO fits are confusing for most of them -they ask your truly...
RE: Tolerances on Mech Drawings
Enjoy!
http://www.tecnologix.net/en/tools/passungsrechner...
RE: Tolerances on Mech Drawings
http://www.tecnologix.com/en/tools/fset_.html
but the site closed down a while back.
Does anyone know any other similar sites out there?
RE: Tolerances on Mech Drawings
about acad or not, but for some
reason in order to get the plus
sign to appear, I would enter
1.00000001 for the plus tolerance
and 1 for the minus tolerance.
Then it would show the plus and
minus in the tolerance in the
dimension.
RE: Tolerances on Mech Drawings
ISO fits are explained in the Machinery's Handbook. The ANSI fits are based on ISO tolerance values, and are also based on the Machinery's Handbook.
There ought to be a macro button on this site that automatically generates the reply "Refer to the Machinery's Handbook!".
JHG
RE: Tolerances on Mech Drawings
RE: Tolerances on Mech Drawings
There are a ton of them, %%C for the diameter symbol, %%D for degrees, the list goes on.
I gotta admit, I never heard or seen +/+.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Tolerances on Mech Drawings
An engineering drawing exists for depicicting the end-item; if a dimensioning and tolerancing style emphasizes the end-item's functional requirements while requiring the machinist to do some calculations then that's OK.
The drawing is an ENGINEERING tool that is used for inspection and for clearly protraying the end-item's functional requirements.
To be fair, though, the draftsman should never go out of his way to make a drawing that requires the machinist to figure things out!
Tunalover
RE: Tolerances on Mech Drawings
Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP2.0 / PDMWorks 05
ctopher's home site
FAQ559-1100
FAQ559-716
RE: Tolerances on Mech Drawings
I was quoting my 1941 drafting book. I collect old books, especially engineering and history.
The book actually is correct in the context of their discussion. If I am working on a drafting board, the clearest way to express a tolerance is to show limits...
1.253/1.251 DIA
This is as clear an instruction as you can send to a fabricator, and it meets engineering requirements.
In my next paragraph, I was somewhat vague, but the point is that with CAD, we use the scale drawing/model, and there are several ways of causing the display above. This could be nasty if your job is to model and dimension the mating part.
1.250 +.003/+.001 DIA shows the real diameter of the scale model. It shows that I applied the ANSI fits and/or the ISO tolerances entirely with the dimensioning tool's tolerance feature. At the preliminary phase of the design, I even use the ISO tolerance codes because they are independant of the dimension. I have not the nerve to send these out to the fabricator.
I tend to change diameters to limit dimensions when I finalize the drawings. The toleranced dimensions are used mostly by the fabricators and inspectors. Engineers can get at my CAD model.
JHG
RE: Tolerances on Mech Drawings
There is nothing wrong with reporting 1.253/1.251, just as 1.252 dia +/-0.001, 1.251 dia +0.002/-0.000 or 1.253 dia +0.000/-0.002. Depending on the circumstances of the design, the designer would make a consideration on what is theoretically needed to what is practically possible. This in itself takes years of experience, although I have seen enough raw talent to have faith in the practice of engineering, as a skill and an art.
CAD drawings should show maximum material. That way if buddy is having a bad day on the machine and overshoots the lower number, he has an upper target to meet knowing the piece is not dysfunctional. Should he violate the limit, then I expect a visitor.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Tolerances on Mech Drawings
http://www.mmsonline.com/articles/0305cnc.html
Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP2.0 / PDMWorks 05
ctopher's home site
FAQ559-1100
FAQ559-716
RE: Tolerances on Mech Drawings
max material condition. The shop insisted that we
draw to mean dimensions. The shop won. Both make
sense.
RE: Tolerances on Mech Drawings
In this example the shaft is supposed to be 50mm diam. So, I would just specify:
shaft: 50.0 +/- .025
bore: 50.15 +/- .025
This would also make sense, since the machining error is "random" and can be "+" or "-". I assume this is why why fabricators would prefer mean tolerances.
Largest shaft (50.0+.025) in smallest bore (50.15-.025) shows min clearance of .1 as expected.
RE: Tolerances on Mech Drawings
I prefer to model at the exact nominal dimension. The shaft and the hole are modeled at 1.250"DIA or 50mmDIA or whatever. On my arrangements or assemblies, I make a note indicating an ANSI fit like RC4 or LC5, depending on my requirements. Lately, I have been marking the ISO tolerance codes on my dimensions, making the tolerance independant of the current dimension. Modeling MMC requires me to know what tolerance I am using, and I am not always sure of this until late in the design. The ANSI and ISO tolerances vary with diameter. At the end of the job, I fix everything to make it readable to the shop, since they are the primary users of the drawing. I prefer to send drawings out to the fabricator, and I send DXFs if they ask me. I have sent SolidWorks models to sheet metal shops.
A corporate standard is desirable here.
JHG
RE: Tolerances on Mech Drawings
In this example the shaft is supposed to be 50mm diam. So, I would just specify:
shaft: 50.0 +/- .025
bore: 50.15 +/- .025
This would also make sense, since the machining error is "random" and can be "+" or "-". I assume this is why why fabricators would prefer mean tolerances.
Largest shaft (50.0+.025) in smallest bore (50.15-.025) shows min clearance of .1 as expected.
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I would agree with the above. I've been producing programs for milling and turning for over 10 years in the UK and we get drawing from all over the world.
We always standise them to nominal with +/- tolernaces.
It is far easier to control in production for especially on milling out thin materials where we cut out circuits in one pass.
Regarding +/+ tolrances we see these a lot on features such as push fit holes for electronic components, often drill diameters are specified 1.8mm Dia +0.02 / +0.5
RE: Tolerances on Mech Drawings
"I heard someone say that the drawing needs to minimize how much arithmetic the fabricator has to do. NO! That is part of the fabricator's job and it is expected that he will have to do it."
"At the preliminary phase of the design, I even use the ISO tolerance codes because they are independant of the dimension. I have not the nerve to send these out to the fabricator. "
"To be fair, though, the draftsman should never go out of his way to make a drawing that requires the machinist to figure things out!"
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We used some moderately complicated power drawbars from a German company. They called out their required fits as H7/g6, etc. They also included a table translating those designators into real numbers.
I'll bet that saved a few re-works and non-value-added discussions about the difference between g6 and G6.
Well worth it.
RE: Tolerances on Mech Drawings
For us, modeling/dimensioning a nominal value with either +/+ or -/- tolerances is the shortest path to rejects and scrap. When the modeled geometry and/or dimensioned print show nominal values outside of the allowable tolerance range we often get what we modeled/drew instead of what we wanted.
RedPen
RE: Tolerances on Mech Drawings
RE: Tolerances on Mech Drawings
I've always found Zeus to be hepful when quoted H&/g6 type fits they have the tables inside.
http://ww
RE: Tolerances on Mech Drawings
It is OK to specify H7/g6 - you only have to reject a part once and then everybody knows. Sounds harsh - but heck, my time costs more than the guy on the floor and hence I only need to communicate my design intention, in this case a close sliding fit.
Each workshop is different. You first have to understand how a part will be made, then dimension it. What works for one workshop costs time in another - they have different tools and techniques.
Drawing is the way we talk to the guys on the floor. While the correct way is to give only the minimum dimensions, the best way is give some reference dimensions and mark them as such - especially for fabrications.
If you get things done on contract you better tollerance properly or you will have no comeback. However, use realistic tollerances - or it can cost you heaps.
RE: Tolerances on Mech Drawings
As far as how to show the tolerance formatted as you have asked depends on the cad system you use. I have SolidWorks. In the drawing mode it allows me to choose various methods of displaying the tolerance. You should choose according to your company standard if applicable, or whatever method most clearly shows your requirement for deviation.
RE: Tolerances on Mech Drawings
For instance, lets say I wan't a cylinder assembly with a nominal bore of 10". I would dimension a class RC3 precision slip fit intended for a nominal diameter of 10 inches like this:
10.0 +.0020/-.0000" for the hole,
and 10.0 -.0025/-.0037" for the shaft.
This lets the machinist know immediately what the target assembly dimension is, in this case we are going for a nominal bore of 10.0".
The method that you are accustomed to would dimension these parts like this:
10.0010 +/-.0010" for the hole,
and 9.9969 +/-.0006" for the shaft
This would likely confuse our machinists as to what the intended result would be.
RE: Tolerances on Mech Drawings
RE: Tolerances on Mech Drawings
Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP3.1 / PDMWorks 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716
RE: Tolerances on Mech Drawings
The reason for this is they are a standard so any shaft or bearing or anything from any manufacturer will give you a press fit, slide fit or whatever is required.
They are all based around a nominal size and the tolerances change as the nominal shaft or hole size changes. So to use your example it may do what you want but the hole would be in a different set of limits to the shaft, i.e. one is 10-15mm and one is 6-9 (the exact figures may not be correct) so your sizes would not fit say a h7 manufactured by another company.
Why change a standard that works perfectly well?
RE: Tolerances on Mech Drawings
I guess I don't understand your point. As in the example above, I have a 10" nominal bore and need a class RC3 precision slip fit. This calls for an H7 hole and an f6 shaft.
What is the difference if I dimension the hole 10.000 +.002/-.000" and the shaft 10.0000 -.0025/-.0037
vs
the hole 10.001 +/-.001" and the shaft 9.9969 +/-.0006"?The range of an acceptable part is the same, it is easier to accurately model, and those numbers mean the same thing anywhere in the world (as long as they understand inches;))
It shouldn't matter where I have it made, if it meets those tolerances, it will have a class RC3 slip fit and will work.
RE: Tolerances on Mech Drawings
I am on ewh's side.
Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP3.1 / PDMWorks 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716
RE: Tolerances on Mech Drawings
Maybe I am just being grumpy, I got home tonight to find my house had been broken into.
RE: Tolerances on Mech Drawings
Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP3.1 / PDMWorks 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716
RE: Tolerances on Mech Drawings
I'm just going along with the times. The classification system for fits is excellent for design. However, in today's world, we have to document that design using solids on a computer. It is much easier to see how a design works when you have the parts modeled at a size they will actually exist, and document it as such. A +/+ or -/- tolerance adds confusion to this situation, i.e. a 10" nominal shaft should be modeled as it would be created, 9.9969 +/-.0006".
Sorry to hear about the break in. That's enough to make anyone grumpy.
RE: Tolerances on Mech Drawings
If I was asked to dimension ouside of the standard it would require me to do a few calculations to convert the standard to a different system. This in itself could introduce error, especially towards the end of the day when I too am tired and grumpy
RE: Tolerances on Mech Drawings
RE: Tolerances on Mech Drawings
None of this really matters if everyone is on the same system, but becomes more of a problem when information goes through a translator, as text and annotation seem to be two of the things they (especially IGES) do not handle that well. As far as I am aware no system allows you to model intent, only text does this.
I wonder if we will have new standards in a couple of years as 3D becomes more and more the norm, probably driven by the largest supplier I guess Catia or Solid works or whether this will still be being debated in a couple of years time?
RE: Tolerances on Mech Drawings
Curious on your opinion. For example, if a part is dim at 9.877 +000/-.002, would you model it at 9.877 or 9.876? I would use 9.876.
Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP3.1 / PDMWorks 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716
RE: Tolerances on Mech Drawings
I hope that softwares and translators will support ASME Y14.41-2003 DIGITAL PRODUCT DEFINITION DATA PRACTICES in the near future. As it is, one of our customers (a major aeronautics company) has sent us files that approach ASME Y14.41, and almost all of the part info is in the 3D file. What isn't in the part file is in text files. All of the annotations and dimensions were converted to curves, and there are no drawings. They look quite confusing (and intimidating) at first, until you understand how they are structured.
I don't see this standard getting a huge following for quite awhile, but it can streamline design and manufacture.