Limit style tolerancing vs bilateral
Limit style tolerancing vs bilateral
(OP)
We are in the process of writing drafting standards, and one of the debates is in the method of tolerancing. The majority of our old drawings had ± tolerancing, with some limit style tolerances. The group working on the standards is heading towards standardizing on limit style dimensioning only. The reasoning is that when you are inspecting a part, you only care if you are outside the limits, so the nominal does not matter. The debate comes on the question of design intent and the cad geometry. A part with a .4998+.0001-.0005 tolerance has a different design intent than a .4999/.4993 toleranced part.
Looking for feedback.
Looking for feedback.





RE: Limit style tolerancing vs bilateral
"Wildfires are dangerous, hard to control, and economically catastrophic."
"Fixed in the next release" should replace "Product First" as the PTC slogan.
Ben Loosli
CAD/CAM System Analyst
Ingersoll-Rand
RE: Limit style tolerancing vs bilateral
RE: Limit style tolerancing vs bilateral
However...
Most machinists/operators/inspectors do not care about your intent. They also don't care much for mental math and would prefer the easy way out.
The fact remains that if your part doesn't function over the entire range of the tolerances, then something is wrong.
RE: Limit style tolerancing vs bilateral
"I think there is a world market for maybe five computers."
Thomas Watson, chairman of IBM, 1943.
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: Limit style tolerancing vs bilateral
Is it necessary to rigidly mandate this?
You should format the information for whoever is going to use it the most. That standard changes depending on production levels, inspection requirements, and the need for engineering to monitor the process.
JHG
RE: Limit style tolerancing vs bilateral
RE: Limit style tolerancing vs bilateral
RE: Limit style tolerancing vs bilateral
RE: Limit style tolerancing vs bilateral
You can design for intent, but machinist will go for the nominal or mean. Years ago, our company standarized on limit dimensions. We create our models at nominal. All of our machinists and vendors understand it and makes it easier to import into CAM. I like bilateral in some cases, but we don't uses it unless a customer requires it ... it is rare.
Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP3.1 / PDMWorks 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716
RE: Limit style tolerancing vs bilateral
If the intent of the _document_ is to show what dimensions are to be measured, and what limits the measurements must lie within, then limit style dimensioning is appropriate.
Yes, one of our customers maintains a separate set of inspection documents, with simplified geometry, and limits shown only for the dimensions to be measured, and with tabular space reserved for each item in a set of samples. A copy of that drawing becomes the inspection record for that lot.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: Limit style tolerancing vs bilateral
The same goes with unilateral vs. bilateral size tolerances. If a unilateral tolerance most clearly delineates the product function, then USE IT!
Tunalover
RE: Limit style tolerancing vs bilateral
Best Regards,
Heckler
Sr. Mechanical Engineer
SW2005 SP 3.1 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
NIVIDA Quadro FX 1400
o
_`\(,_
(_)/ (_)
"There is no trouble so great or grave that cannot be much diminished by a nice cup of tea" Bernard-Paul Heroux
RE: Limit style tolerancing vs bilateral
I do find some of the comments odd about let the machinist sort out the trig, surely everyone should aim to make the next guys job as easy as possible, for example items are dimensioned you do not have to add up half a dozen dimensions and then subtract two to find a size.
We do seem to be becoming an “It is easier if someone else does it” culture.
RE: Limit style tolerancing vs bilateral
RE: Limit style tolerancing vs bilateral
My understanding is as a rule of thumb you aim for 7/10ths of the limit, where if you are piercing or drilling for example you go towards top limit as tool “dulling” brings the part towards mean. If you are turning for example you go towards bottom limit as in this case “dulling” makes the shaft larger and towards mean.
You then have a control limit so all parts are around mean. I do realise there is more to it than that but like I say rule of thumb.
RE: Limit style tolerancing vs bilateral
If, for example, you have a core pin on an injection molding tool that you know will wear quickly, you'll want to target the size near maximum so that as it wears, you'll get more useable molding cycles from the core. If you set your target as the mean of the spec limits, you'll make it easier to build the core, but you lose life cycles.
RE: Limit style tolerancing vs bilateral
I still like this system but everywhere else I worked at they used limits, probably for the reasons stated by Mr. Tick.
Often times there is confusion of what the nominal dimension is. For example, if a length is 5.000 minus .002, some drafters and checkers use 4.999 as the nominal and you can go into a cad layout and find the part drawn as 4.999 instead of 5.000.
Another thing they do, and I think it is an ANSI standard, is to put the largest number on top if it is a limit dimension. If it is a bore, the top number is not the one you want to shoot for. I suppose it makes sense to most people, though.
RE: Limit style tolerancing vs bilateral
RE: Limit style tolerancing vs bilateral
.4998+.0001/-.0005 = .4996±.0003 = .4999/.4993 = .5000-.0001/-.0007, exactly.
The fabricated part must not exceed .4999" diameter and must not fall below .4993" diameter. The shop's job is to accomplish this somehow. Nothing in the above formats indicate anything about how the shop is to achieve the tolerance. If the fabrication process drifts, the fabricator will set it towards the end of the dimension range that provides maximum production between setups. If the fabricator likes to show off and can achieve the tolerance easily, they will fabricate as close to MMC as they can.
Statistics have nothing whatsover to do with this. As per ASME Y14.5M-1994, you can indicate statistical tolerancing and allow the dimension to open up a little, accordingly. If you do not do this, the part must fall between .4999 and .4993. All of the dimension formats above are unambiguous.
JHG
RE: Limit style tolerancing vs bilateral
Chris, Sr. Mechanical Designer, CAD
EngJW (Mechanical)
Drawoh-
drawoh (Mechanical)
EngJW,
ajack1 (Automotive)
The last two posts are interesting. In America is it still a recognised standard to tolerance holes and shafts with numbers and letters say H7 for holes and h7 for shafts?
ctopher (Mechanical)
yes
ewh (Aerospace)
Yes, it is still a recognized standard, but not used in all industries.
ajack1 (Automotive)
I guess I must be doing that wrong then. I figure that the whole concept is you call the hole or shaft up as a nominal size say 25mm even although it may have a plus/plus limit or a minus/minus limit so it could never be 25mm. The modelling system I use also has the option of adding a H7 for instance to “round” figures but not if you have made the shaft say 24.997.
EngJW (Mechanical)
Ajack1- I don't think you are wrong. I would make it 25mm and let the drawing handle the tolerance, as Drawoh also has said.
drawoh (Mechanical)
ajack1,
SolidWorks 05 SP3.1 / PDMWorks 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716
RE: Limit style tolerancing vs bilateral
The approach that I like, for the example you show, is do my cad designing at an easy .500 because at that point I am more concerned with arranging the parts than what their tolerances will be. At the detailing stage, with Autocad or Solidworks you can select the tolerance method that the company likes and just plug in the right values to make it come out. The part will still be .500 in the model.
RE: Limit style tolerancing vs bilateral
Where were you for thread404-101081? :)
I prefer to model to nominal dimension too, rather than to MMC. It simplifies top down design in SolidWorks, and it gives me more control over the tolerances on the drawing. If I want to switch from an RC4 fit to an RC5, all I do is change two tolerances on the drawings.
JHG
RE: Limit style tolerancing vs bilateral
If so do you still model or draw to mean, even although the hole might have limits of +0.0003 to +0.001?
RE: Limit style tolerancing vs bilateral
yes
Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP3.1 / PDMWorks 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716
RE: Limit style tolerancing vs bilateral
Yes.
RE: Limit style tolerancing vs bilateral
I guess this system pre-dates CAD and I should rethink the way I work.
RE: Limit style tolerancing vs bilateral
If later on you decide to change the tolerance, you just change it on the drawing and would not have to go back and change your layout or model.
RE: Limit style tolerancing vs bilateral
ASME Y14.5M-1994 gives you the option of using the ISO tolerance codes to specify metric tolerances. They are very clear about the metric.
eg. 30 f7.
This will not work in any CAD sofware I know of, unless you model to nominal size.
JHG