Open Tolerance for Drawing

That would be ISO standards...

http://www.iso.org/iso/search.htm?qt=mechanical+drawing&searchSubmit=Search&sort=rel&type=simple&published=on

http://www.eng-tips.com/faqs.cfm?fid=1044

The company producing the drawings chooses the standards they wish to use based on their needs. Some companies adopt ISO/ANSI standards, some companies make their own standards.

We make our own standards, based on the ANSI standards... but it will be different from one company to the next.

No right or wrong...

Don't get too caught up in the "standards" unless you need to comply with some national/international standard, and it is your task to ensure compliance.

Follow exactly what the drawing says, and you will save your own backside.

Thanks Guru!!

I understand but the thing is we have to set the standard for our need and i just want to make sure that what is in there should be correct and as per proper standard. Ok can you help me with this. Can you tell me as per ISO or ANSI for plastic parts (ABS, PP) what open tolerence comes in title box of drawing sheet.

Talk to the person designing, and the person to make the part. Some parts require close tolerances for all their dimensions, others have few, even just one which is critical. The fewer close tolerances, the cheaper & easier the part will be to produce.

Yes, what u r saying in here goes in the drawing but not in drawing template (what i need). So once it has been fixed it will remain same (for plastic components) for all the drawing irespective of the criticality.

You are right, the tolerances in the drawing template are fixed & dimensions are controlled according to the number of decimal places. It is up to you to decide what these tolerances should be & when to override them by having a specific tolerance shown on the drawing dimension.

Quoting from Fundamentals of Engineering Drawing, by Warren J. Luzadder: "Tolerances; Necessary tolerances may be expressed by general notes printed on a drawing form or they may be given with definite values for specific demensions, where the part has been dimensioned. When epressed in the form of a printed note, the wording might be as follows: ALLOWABLE VARIATION ON ALL FRACTIONAL DIMENSIONS IS +- 0.010 UNLESS OTHERWISE SPECIFIED." Hope this helps!

The tolerances shown in the drawing title block are generic values listed on all of your prints and may look something like this. Decimal +/- .005 Fraction +/- 1/64 Deg +/- 1 Deg. Unless otherwise specified.

What is important is that there is input from the engineer, the producer and the end user. They all should have an input on what dimensions are critical and must be held to a close tolerance, and which dimensions are less important. Too close a tolerance on and unimportant dimension causes excess production costs. Too loose a dimension on an important dimension causes problems with form, fit, function, safety, quality and assembly issues. Specify tolerances that are important to the item on the drawing.

Be as relaxed as you can on unimportant dimensions and as tight as need be, for important dimensions, and you will have a quality item at the best possible cost.

Very often the boilerplate in the tolerance blocks of pre-printed drawing forms are based on machined parts. This can create problems if these forms are used for other items such as raw castings which are governed by a different set of tolerances such as the "Standards for Sand and Permanent Mold Castings".

Very often castings which are to be machined all over will be rejected by incoming QA inspectors on the basis that they do not meet the numbers in the tolerance block with +/- .005" tolerance requirements. It is a common oversight by many draftsmen not to change those numbers or at least to put an over riding Note specifying the correct casting tolerances or the reference to where the tolerances are contained. Casting tolerances can vary depending on the method of manufacture, i.e. sand, permanent mold, investment or die castings. Plastic injection parts I assume have their own requirements based on that industries Standards.

Over specifying tolerances can lead to many excessive costs. The part designers should be specifying the "finished" part requirements whereas the raw or rough part will only need more open tolerances to insure that one can get to the finished part with the rough part supplied.

Also get familiar with ASME Y14.5

http://www.asme.org/products/codes---standards/dimensioning-and-tolerancing

The Y14.5 standard is considered the authoritative guideline for the design language of geometric dimensioning and tolerancing (GD&T.) It establishes uniform practices for stating and interpreting GD&T and related requirements for use on engineering drawings and in related documents.

dhanishd, There's lots of information here but I get the impression you didn't get the answers you were expecting. Are you looking for someone to specify actual general tolerance values that would be suitable for your plastic parts drawings. I ask because in both of your follow up posts you emphasized "plastic parts" and "what I need".

Yes, you are correct I am exactly looking for that, If you have any information which can help me then it would be really great!!

dhanishd, as Nigh pointed out you're asking for answers that people can't give you. Nobody knows what your designing, how it's made, who will make it, how it will be used, etc, and all these factors play a role in how you establish general tolerances. For example we specify +/-.005 = .xxx, +/-.015 = .xx, +/-.030 = .x for typical machined steel components. One of our client's specifies +/-.010 = .xx. Both are valid so what's the standard?... In my industry both are valid as "standard" for machined components.

I don't work with plastic moulded parts so that's as far as I can guide you.

Just curious but are you a student?

Thanks TerraMan, no i am not a student but it was my curiosity to get deeper and to know how in most of industries it works. Because we don't have any specific vendor and some time our parts are made in different part of globe and in all that case our drawing template would remain same. So it is very difficult to know whether the tolerance specified in template is too tight for them or they are ok with that. Because too tight tolerance means cost. So that's why I just wanted to know is there any proper standard because I cant change my drawing templates from vendor to vendor.

You can specify a looser tolerance overall that all parts must be held to and then use specific tolerance dimensioning to specify additional tolerances that may apply to certain parts or even to specific areas of parts that need to be tighter than overall.