Iso dimensioning of holes and shafts 6

[benensky]

Hi,

I am new to this forum. Hope you can help me.

We were bought by a European company and I work with ISO drawings at times and they tolerance there shafts 10 -2/-4. I have never seen double negative dimensioning. In the USA I was taught that if the fit of a 10unit shaft is -2 to -4 to draw it at 8 units and tolerance it at 0/-2. Likewise for holes. Is this standard for ISO drawings to double negative or double positive tolerances on features?

I would like to get this right since soon we are to draw to ISO standards and the guidebook they gave us is not really clear on this.

 

[CoryPad]

You need to review ISO 286-1 ISO system of limits and fits -- Part 1: Bases of tolerances, deviations and fits & ISO 286-2 ISO system of limits and fits -- Part 2: Tables of standard tolerance grades and limit deviations for holes and shafts. These standards show tolerances using double positive, double negative, symmetrical bilateral, and nonsymmetrical bilateral.

The ISO standards are available here:

http://www.iso.org

Here is a site that shows values for typical shaft tolerances:

http://www.roymech.co.uk/Useful_Tables/ISO_Tolerances/ISO_286_2s.html

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[benensky]

Cory,

Thanks for your reply. I would review those standards if I had them. However I do not and the chances I will be supplied them are pretty slim. That is why I posted here.

If you feel my post is inappropereate please let me know and I will not post here any more.

-Mike

 

[prohammy]

Here's a couple of little things that you may find helpful

http://www.tecnologix.net/en/tools/passungsrechner/passungsrechner_v05.html

http://www.roymech.co.uk/Useful_Tables/ISO_Tolerances/ISO_LIMITS.htm

Kevin

“Insanity in individuals is something rare, but in groups, parties, nations and epochs it is the rule” Nietzsche

 

[CoryPad]

benensky,

I don't understand why it would be a problem for you to get the tools you need to do your job. I would imagine that this is the type of expense that would be expected when companies are purchased by other companies, especially when they do not share nationalities. The prices for these standards are modest for commercial enterprises. In fact, your new corporate parent may have access to these documents.

As far as appropriateness, I suppose you are reacting to my signature. You will notice that several users use this link in there signatures. It is not directed at you solely, just there to promulgate site rules.



Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[gearguru]

The ISO fits are also in Machinery's Handbook; a good selection of them also in any bearing catalog.
The ISO 286 is obviously the most complete source of the information.

 

[benensky]

prohammy - thanks for the links! I understand fits and how that works. What I am asking about is how they are drawn and toleranced on mechanical drawings. For example, if a 1" diameter shaft's fit calls -0.001"/-0.002” I draw the shaft at 0.999" in diameter and tolerance it 0/-0.001". It seems the European drawings for a 25mm diameter shaft with a fit that is -1mm/-2mm are drawn oversized at 25mm diameter and then double negative toleranced at -1mm/-2mm as the fit charts read. What I am asking is this the correct or preferred way according to ISO standards.

Cory - I agree with you, but I hope you understand that they have given us the Mechanical ands Metal Trades Handbook and expect us to work form that and that only. The modest expense for standards as you and I understand is to them exorbitant and overpriced. I cannot change that and need to work within the limitations I have. Thanks for your concern and attention to my question.

 

[prohammy]

Ben,

Have a look at the third last post in this thread

Kev

http://www.mcadcentral.com/proe/forum/forum_posts.asp?TID=33613&KW=tolerance

Kevin

“It is a mathematical fact that fifty percent of all doctors graduate in the bottom half of their class." ~Author Unknown

"If two wrongs don't make a right, try three." ~Author Unknown

 

[HgTX]

I have a profoundly stupid question.

What is double negative tolerancing? To me tolerances are how far away from the designed dimension one can go, in either direction. Looks like for something designated as 10 (-2, -4), actually building it at 10 would be incorrect. Why would one set up a drawing such that the given dimension is not the target dimension?

Hg

Eng-Tips policies: faq731-376

 

[benensky]

Kev - Thanks, posted a similar question on the thread you recommended to me.
Hg - I wonder the same thing. However I have been drawing to ANSI standards my whole life and accustom to it. For the designers at the European company that is common practice. It may seem odd to us but it is the correct way to draw to them and it may be the correct way to draw when drawing to ISO standards. I want to know if it is to standard, if it is a legacy style within Germany/Europe or if they are just too lazy to draw to size.

BTW - where are you from Hg?

 

[HgTX]

I'm in the U.S.

So...why pick 10 -2 -4? Why not 100 -92 -94? Why not 0 +8 +6? Why on earth would the nominal measurement not be a number you actually want the part to measure?

This is starting to remind me of eyeglass vs. contact lens astigmatism prescriptions...only it makes less sense.

Hg

Eng-Tips policies: faq731-376

 

[benensky]

Hg - Because the tolerancing charts say for a 10mm shaft with "X" fit is -2mm/-4mm. Or do you get that and you are just being hypothetical? A point, which I agree with, but some times the determining powers are not correct but have made a decision. A decision which is easier for me live with than fight.

 

[prohammy]

OK, here we go.....

Take shaft size dia 20mm

Take hole size dia 20mm

both nominal dimensions, agreed

Here's an example of when you use double negative tolerancing. Imagine shaft goes into first hole as a press fit

Shaft dia20 (H7) dia 20 +0.021/+0.000 - this is done across its whole length to save cost
Hole Dia20 (p6) dia 20 +0.035/+0.022

but shaft goes through second hole as a loose running fit then the fit is d6 ie dia 20 -0.050/-0.066





Kevin

“It is a mathematical fact that fifty percent of all doctors graduate in the bottom half of their class." ~Author Unknown

"If two wrongs don't make a right, try three." ~Author Unknown

 

[HgTX]

"Because the tolerancing charts say for a 10mm shaft with "X" fit is -2mm/-4mm. "

Apparently I don't understand what that means. I told you it was a profoundly stupid question

Does that mean that you want your shaft to be between 6 and 8 mm in diameter? Or does it mean something completely different?

Cuz if I'm given a diameter dimension of 10 mm +1 -0.5, that means it's to be between 9.5 and 11 mm. Are we talking about something completely different here?

Hg

Eng-Tips policies: faq731-376

 

[CoryPad]

HgTX,

For your example, yes the part must measure between 6 mm and 8 mm. This is a nonsensical example, but it does illustrate the point.

The reason you don't understand is because you have been told about the idea of NOMINAL diameter.

Say you have a shaft and a hole that must engage each other. Say the NOMINAL diameter of both is 10 mm. Now, you can have a wide range of fits - clearance, location, interference, force. To achieve these various fits, you adjust the shaft and/or hole size as needed. For various interference/force fits, you may need the hole to always be smaller than 10 mm and the shaft larger than 10 mm. Then, you can specify fits that result in double negative and double positive tolerances. For example, the hole may be 10 mm -0.1/-0.2 mm and the shaft may be 10 +0.1/+0.2 mm.

If you were looking only at a single part, the double negative type of dimension doesn't make a lot of sense. But when establishing a complete system of limits and fits, this makes a lot of sense.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[HgTX]

Ah. So this is a custom particular to tolerances that apply to two things that must fit within each other, where neither can be exactly the "nominal"? Would this double negative or positive tolerancing happen elsewhere, where there is only one absolute dimension to worry about, like, say, the dimensions of a test specimen?

But my example isn't nonsensical--it's quoted from the discussion above.

Hg

Eng-Tips policies: faq731-376

 

[CoryPad]

It is only for fits (two objects).

The original poster used

if the fit of a 10unit shaft is -2 to -4

Which was a generic example.

10 mm -2/-4 mm is nonsensical.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[benensky]

Hg – Sorry to confuse you I just used “X” as a hypothetical or “nonsensical” fit. It looks like you now understand or am I wrong and you need me to give you a new example.
Cory – What I was taught was that “nominal” meant the dimension you would like to see the feature made at in a perfect world where you could machine to a +0/-0 dimension with no additional cost. In the ISO shaft and hole fits terminology “nominal” means a number you start with and add your upper or lower deviation, then add or subtract your fundamental tolerance, to get your minimum and maximum size of your shaft or hole. The nominal dimension of the shaft can but does not need to be within the minimum or maximum size of the shaft or hole.

Cory – are you doing most of your drawings to ISO or ANSI specifications or a mixture?

Getting back to my original question. I have been researching it more and it looks like shafts and holes are drawn at the ISO nominal with a tolerance class attached. For example a 20s6 shaft is drawn at 20mm and dimensioned as Ø20s6. Likewise, if they do not specify by tolerance class they would dimension it Ø20+0.035/+0.048 but the tolerancing by tolerance class is the preferred method. This seems odd to me since I draw to see if things will fit together and want to see clearances and interferences between parts when drawn. The ISO system on the other hand, everything is drawn line to line and so that it is quicker to draw and it depends on the designer to assign fits to each part.

 

[CoryPad]

I work for a multinational company based outside the USA, so we use ISO.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[gearguru]

We discussed the ISO system of fits couple times. The basic "philosophy" behind it is something like this:
Two parts fitting together have the same nominal size.
The tolerance LOCATION relative to this theoretical basic size is given by a letter of the alphabet. Capital letters are used for internal features (holes) and small letters for external features (shafts). The number behind the letter gives the tolerance SIZE.
Now: The golden rule in ISO fits is, that one of the parts is in H (or h) tolerance. It is so, because H hole (h shaft) has the maximum material condition size (mallest hole, largest shaft) equal the basic size. Having one of the parts H (h) we can assign the desired fit using the other letters in alphabet. If the tolerance on the SECOND part has a letter "a" (A) to "g", then the fit has clearance. The smallest clearance is created using the H/g fit; the largest with the H/a fit. If the interference is created using the letters to the right from H: so the H/z creates the largest interference, H/m the smallest. H/j and H/k can have clearance or interference. Actual size of those clearances/interferences is controlled by the numbers following the letters.
The complete fit description can look like this:
Dia 50H7/g6. Because I know that "g" creates guaranteed clearance, the shaft - in this case dia50g6 - has to have DUAL NEGATIVE TOLERANCES.
This system is handy when designing, without too much calculations and with some experience one can pick the appropriate fit for his parts. The poor guy who has to machine it, needs the charts and needs to find the actual sizes for those parts, unless we show those sizes on the drawing.
Next advantage: if one uses standard ISO fit and preferred size, the ready made gages for those fits are available - it means no special tool is needed. Also to replace the parts made using this standard is easy around the world. An example can be the ball/roller bearings. That's why the bearing catalogs also contain the selection of ISO fits used to locate the bearings on the shafts/in the housings.
By the way - the JS and js fits are the same as the "plus minus" tolerances.
The preferred hole (H) is used most often (if change in fit is needed, it is easier to change the shaft...)
I hope this "article" brings some light on the logic behind the ISO fits.