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Associate

Join Date: Jan 2011
Posts: 47

Applicable Dimensional Tolerance to Part Drawings

11/06/2011 6:02 PM

Hi, Currently here is what I have. We create 2d drawings to all alluminum castings we use. Any all the drawings have Title block displaying tolerance table. It says all dims with 4 decimal places carry tolerance of +/- 0.001, 3 places +/- 0.01, 2 places +/- 0.02 and 1 place +/- 0.04 But, it's too difficult to control the number of decimals. What do you guys do? What's your practice? Please share.

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Guru

Join Date: Oct 2008
Posts: 42294
#1

Re: Applicable dimensuonal tolerance to part drawings

11/06/2011 6:13 PM

I only use the number of decimals that I need to control the location/dimension of the feature in question.

If this is too difficult for you, become a manager.

You should buy a copy of something like this:

Standards Tuesday - Fundamental Rules of Dimensioning - The ...

Associate

Join Date: Jan 2011
Posts: 47
#2

Re: Applicable dimensuonal tolerance to part drawings

11/06/2011 11:09 PM

Lyn, That suggestion is actually not bad. I shouldn't delay more On that. On a second thought, I read on a vendors website about a different approach than controlling by numbers after decimal point. If the dimension is between 1 to 5 it holds a tol of +/- 0.005, if it was between 5 and 10 it was little more liberal like +/- 0.01 Some thing on this lines. Basically if the value is smaller tighter the tolerance. In case there was a special case then that dimension should be explicitly carry tolerance value. Any way, I want to know if anybody here use this method of tolerence application.

Guru

Join Date: Apr 2009
Location: Yellowstone Valley, in Big Sky Country
Posts: 6954
#3

Re: Applicable dimensuonal tolerance to part drawings

11/07/2011 8:36 AM

Percentage tolerancing is generally not acceptable, but this depends on the part.

Where unlisted on a machine drawing, tolerance is generally understood as:
1 decimal place (0.X): ± 0.2
2 decimal place (0.0X): ± 0.01
3 decimal place (0.00X): ± 0.005
4 decimal place (0.000X): ± 0.0005

If you loosen your tolerances, what happens when a part is max under (but within tolerance) and the mating female part is max over? Sounds like piston slap in the making it you are not careful.

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Guru

Join Date: Oct 2008
Posts: 42294
#4

Re: Applicable Dimensional Tolerance to Part Drawings

11/07/2011 9:51 AM

If you want to produce drawings that anyone can understand use this: [PDF] Teaching Geometric Dimensioning and Tolerancing in a ...

Associate

Join Date: Jan 2011
Posts: 47
#5

Re: Applicable Dimensional Tolerance to Part Drawings

11/07/2011 12:27 PM

Thanks Guys

Power-User

Join Date: Apr 2007
Posts: 414
#6

Re: Applicable Dimensional Tolerance to Part Drawings

11/07/2011 10:55 PM

That PDF was worth the price of admission. Thanks.

Guru

Join Date: Feb 2007
Location: Cairo, Egypt
Posts: 1734
#7

Re: Applicable Dimensional Tolerance to Part Drawings

11/08/2011 12:26 AM

I is very interesting to locate this code: ASME Y14.5 Dimensioning and Tolerancing - Engineering Drawing and Related Documentation Practices (227 Pages).

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Member

Join Date: Aug 2011
Posts: 5
#8

Re: Applicable Dimensional Tolerance to Part Drawings

11/08/2011 2:26 AM

Every cad package that I have had to use over the last 15 years has a default option for decimal places on dimensions. I usually only have to override two or three dimensions per print- along with a bilateral or max/min tolerance so the tool maker knows what is important and which way to hold the tolerance.

Commentator

Join Date: Dec 2006
Posts: 71
#9

Re: Applicable Dimensional Tolerance to Part Drawings

11/08/2011 7:36 AM

You are doing the drawings of castings. The tolerance table would need to reflect the process which would have greater tolerance range than your finished, machined, part from these castings. There is a standard for castings that may help you but I can not post it here ASME Y14.8M-1996. Another posting you mentioned the larger the feature the greater the tolerance range, this would be typical as the casting variation will increase with size. A while back I found a document on tolerances for steel castings by Texas Steel Company online and is very useful for steel castings. I can not find the url right now. Your alluminum castings will probably be a little different in variation than the steel but it will give you a guide to start with. I think it would be good to learn the capabilities of your casting vendor and get some help with reasonable and standard practice tolerances for your products from them. I would check with a few different resources though so you are not influenced by a lesser capable shop to help them meet your drawing specifications.

Power-User

Join Date: Apr 2008
Posts: 138
#10

Re: Applicable Dimensional Tolerance to Part Drawings

11/08/2011 8:18 AM

We have almost the same tolerance notes on our formats. The Tooling Design notes are: Fractions +/- 1/8, 1 plc +/-.060, 2 plc +/-.030, 3 plc +/-.015, 4 plcs +/-.005 and angles +/-1/2 degree. These tolerances are used as guide for the fab shop. Any detail that needs closer tolerances is detailed with it's own set of tolerances e.g. ( .500 Dia +.000/-.005).

Engineering uses only 3 tolerance numbers. 2 plc +/-.03, 3 plc +/-.010 and angles +/-.5 degrees.

In Tooling we use half of the engineering tolerances but also have freedom to apply our own tolerances as needed.

Anonymous Poster #1
#11

Re: Applicable Dimensional Tolerance to Part Drawings

11/08/2011 8:44 AM

The GD&T - we normally get into a bit of problem with suppliers since not many understand the GD&T terminology especially where the max material and min material comes in picture and that are the most effective tools of GD&T.

In your case though I expect the GD&T to be a perfect fit however in a lot of cases we go for the more inefficient linera dimensioning principles.

Based on the applications and the reference points most of the tolerances may be converted to teh standard ISO grades ege H6 or H7s and that usually (in most of the cases) takes care of the closer tolerance for smaller parts. But if it is one of in house or the vendors are knowledgeable enough go for GD&T, It will be worth it you will see the relaxed tolerances with respect to the linear dimensioning.

Power-User

Join Date: Apr 2008
Posts: 138
#13

Re: Applicable Dimensional Tolerance to Part Drawings

11/08/2011 11:50 AM

Yes, we also used GD&T.

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Power-User

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#12

Re: Applicable Dimensional Tolerance to Part Drawings

11/08/2011 9:09 AM

Try to use the loosest tolerance you can. the tighter they are - the higher the mfg. cost!

Associate

Join Date: Jun 2011
Posts: 48
#16

Re: Applicable Dimensional Tolerance to Part Drawings

11/08/2011 2:29 PM

I agree. I remember my plastics part design professor teaching this lesson. He showed an example of how you go from a 1¢ hole to a 10¢ hole to a \$1 hole. Just a demonstration to have the lesson stick with us not to over specify as there can be a huge impact on the bottom line, and potentially on manufacturability.

The tolerancing system and the tolerances that you can expect vary by material type and manufacturing method. In aluminum castings you will need secondary operations to get tight tolerances. In the plastics industry and the rubber industry percentage type tolerances are often used. Then tighter tolerances are added to the drawing only where they are required.

Guru

Join Date: Sep 2008
Posts: 692
#14

Re: Applicable Dimensional Tolerance to Part Drawings

11/08/2011 1:10 PM

The title blocks on standard drawing forms or on title blocks derived from CAD programs when designing aluminum castings should be corrected or made to conform with the "Standards for Aluminum Sand and Permanent Mold Castings" or NADCA (North American Die Casting Association) Standards or the corresponding European Standards.

Any dimension that needs more than 2 place accuracy in sand casting with less than a +/-.03" tolerance should be designated as requiring machining. The Engineering Series of the 15th Edition 2008 of the Aluminum Sand & PM give the various base tolerances plus the additional tolerances required based on many various features such as size, crossing the parting line, use of cores, projected area of pattern, core prints and areas between due to runners, gates and risers and so on.

The use of 3 or 4 place dimensions basically means that that area will need to be machined. Whether the tolerance must be held to +/- .001" or .0001" or something better or less is dependent on its use and mating parts and so on.

Proper design should be to produce both a casting drawing which has sufficient finish added to produce the finished part and should be as either a separate casting drawing or shown separately on a common drawing. In that case separate tolerance blocks should be used for each section, one for the casting and one for the machined part. Many designers will add a Note that all cast dimensions and tolerances will be IAW the applicable Standard.

There is nothing more frustrating than having a raw casting rejected because Incoming QA looks at the tolerance block and sees +/- .01 or +/-.003 on a casting that should have a minimum of +/- .03 or .06 or even more based on draft, finish or numerous other factors.

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Location: washington state
Posts: 28
#15

Re: Applicable Dimensional Tolerance to Part Drawings

11/08/2011 1:51 PM

Your drawing format sounds like it was for general drawings. Another drawing template should be created for the casting drawings with the appropriate tolerances fixed inplace of the decimal tolerances. Other templates (drawing format) for your machine drawings, assembly drawings, and then installation drawings. My drawings are usually in fractions.

Your checker or an appover should catch this mistake. It is good that you have noticed it. If you have a CAD manager, find out what template should be used. If your company does not have a CAD manager, sugest to them that it would be benificial to have one. Then volinteer for the position.

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