Dimensional Tolerance values

Dimensional tolerances should ALWAYS be stated on a drawing. At least that was what I was taught many years ago...

As a general guide though some people use the number of decimal places to indicate the tolerance, but even then it should be stated on the drawing....

John.

Look in the title block, It should have a box called "TOLERANCE UNLESS OTHERWISE SPECIFIED: .XX +/- (something) and .XXX +/-(something else) ANGLES +/- (something)"

With no specified tolerance, the drawings are worthless.

If you can accurately assume tolerance values, no drawings are needed.

I see that you all miss the point. I did say if it is not mentioned in the drawings, did I not? Well, many times it DOES happen, or am I the only unfortunate one who has come across it? Also, as an example, AWS D1.1 does give its tolerance limits on straightness, camber etc but not on shop fabricated assemblies which need to fitted at site. Then again I had said QGPC had a superb document which can be used by the design engineers as a guideline in this aspect. Now, with all this response fron CR4 forum I am more determined to intensify my search for that lost QGPC document.

Thanks all the same.

I recognise the situation, very unfortunate we don't all live and function in a perfect world with the luxury of turning back work because the data is incomplete.

The tolerances vary a great deal depending on the sort and make of valve, and the parts involved. i.e. a small Kammer control valve would have tolerances so tight I couldn't envision getting parts from anyone but Kammer. A Fisher 6" E Series control valve would be dimensionally no challenge in terms of tolerances (exact material composition, treatment and hardfacing if any would be another issue)

If you come across your QGPC document I would love to see it. Presumably it relates to the type valve for which you are to make parts?

Thanks bud, but it refers only to the fabricated items and I am afraid, does not cover valve dimensions.

With best regards,

Nachi

I'm not aware if this may be a hint.

When tolerances are not explicitly declared on drawings, a set of general tolerances should apply. These tolerances are defined in dedicated standards, for example ISO 2768-m or , in Italy, UNI EN22768-2 (I don't know other national standards).

The UNI document can be bought and downloaded from http://www.saiglobal.com/shop/Script/Details.asp?DocN=NSAI05640_0706&TDetailsCatalogue:tvRefs:node0_Expand=false#TDetailsCatalogue:tvRefs:node0

- It is not something with which I'm familiar, but might be a good fallback. When you mention "it refers only to the fabricated items" - are you saying the document you are currently expected to work from is merely an assembly drawing or diagram - perhaps exploded to show the parts?

I was forgetting... sorry: the applicable standard (if any) must be stated in the drawing (e.g. general tolerances according to ISO xxx).

Othrwise... I can't say any more....

Do you have a Machinery's Handbook for reference? Is there a tolerance grade callout on the drawing? The handbook has a section covering tolerances and you may be able to find a table that will fit your process. Why won't your client guide you in their needs for your fabrication? I work in microns for tolerance ranges and other machined parts might have 5.0mm range (just put a hole in this area). Perhaps there was some tolerance blanket value given in your original bid package. If I were you I would go back to the client and have them give you documented tolerances for each print. Without that how can you be sure that they will not pay you for your work claiming that your fabricated components don't meet their design intent. You also open your self up to hugh financial liabilities if the structure fails later.

You don't tell us what business you are in. I have worked most of my life in manufacturing and the tolerance was always spesified or there was a generat tolerance in the title block.

I now work as a checker for a consulting engineering company and there are a lot of drawings where there are no tolerances. Usually at the beginning of the project there is a drawing with general notes and tolerances or it is covered in a specification document that the equipment is purchased with. The drawing has to refer to the correct specifications. If ISO or DIN specs are called up then it must specify which tolerances they want because there are tables with fine, medium, course and very course tolerances.

Generally speaking consulting engineering companies are not providing detail shop drawings. If you want to use their drawings in the shop then you need to add the required information because the person on the shop floor does not have all the extra documents.

If you can't find the tolerances on the drawing then get back to the engineer or designer who did the drawing. You have an obligation to produce parts that work. If the tolerances are such that the parts are very expensive then get back to them and find out why. Often parts are over toleranced because the designers have no idea of how it affects the cost.

Good question - what business?

I assumed you were charged with making replacement parts for an existing valve. That would be a line where you are expected to make something with inadequate information. No fun at all flying blind.

I have seen that some times tolerances are deliberately omitted by the designer because they do not want anybody else to copy the manufacturing of their product. It might make sense to some in this competitive world as every bit of information might take you ahead of the competition .

I am thinking that the drawing is using GD&T according to ASME Y 14.5 which, makes use of basic dimensions.The tolerance being determined by the geometric feature. That is a very popular way of working in the industry. I hope that can help you.

Most Fab shops have a tolerance of +/- 1/16 of an inch for all welded structures Ive seen +/- 1/32 on machine pedestals but in 15 years thats it for welded structures. That said we have made fixtures and such that had to be within .002 But that was done after by a machine shop.

My dear wgh71,

Thanks for your contribution. But honestly, no fab shop can undertake any weldment work to such a stringent tolerance limits. It is simply not possible. FYI : I am talking about steel structure sub-assembliess welded-fabricated at fab sho for site installations. Machining is not involved here at this stage.

Thanks.

My dear Nachi ......

I personally Fabricate and weld machine pedestals up to 8'6" wide and so far 28' in length at 38" high with cross bracing and sometimes mid level framing made of 3 to 5" tubing with 1/4 to 1/2" wall. And i hold +/-1/16 of an inch in all dimensions across the entire structure. I then use heat to straighten it further and keep an average of +/- .012 when complete. Failure to keep at least +/- 1/16 of an inch is a problem with competence on the part of the fabricator. Our shop does extend up to a 1/8 tolerance for all structural ironwork however in regards to QC. That number comes from the amount of play in the holes compared to the bolt size i.e. a 3/4 bolt in a 13/16 hole. from hole center to hole center that leaves you 1/8of an inch play.