Standard Drawing Tolerances

[phlyx]

This is more a general engineering question but since we use Inventor I'm thinking this is a good place to start this conversation.  My previous employer did all Imperial (inch) dimensioning and my current employer does all metric but they both have the same issues.  When we do a drawing our standard title block has defined decimal places = +/- tolerance.  For example (metric):

X.X = ±0.25

X.XX = ±0.12

X.XXX = ±0.06

X.XXXX = SEE DIM TOLERANCE

Problem with this all encompassing system is that if I want something dimensioned 10.25mm but I only want to call out ±0.25 I need to either put 10.3, 10.2 which neither are what I want, or 10.25 ±0.25 which takes a manual edit.  We design and build customer automation machinery so our schedules are very aggressive and don't really have the time to manually edit every dimension.  And we normally dimension with ordinante dimensions so each one would have to be addressed individually.  But if we leave everything 2 decimal places it can increase the cost of the parts we're having made.  Often a single part could have dozens upon dozens of dimensions. 

My previous employer used what we called the "carrot system" where we inserted a little pointing down triangle symbol so we could call out (in inches) everything to 3 decimal places and just plop the carrot down where we wanted.  For example a dimension we want held to one decimal place tolerances:

      ▼

12.125

That way the dimension is not rounded off to 12.1 but is only held to the tolerance of a 1 place decimal.

Just curious how everyone else does this as this debate has been going on for decades.

Thanks

[rayessle]

We use something similar but the tolerances on the drawing sheet are covered with a note stating U.N.O. (unless noted otherwise). We would then put the actual tolerance on the dimension if it deviates from this.

Ray Esslemont

[jtylerbc]

At both my previous company and my current one, we use "default tolerances" as well.  All of this example is in inches.  At my current company;

.X isn't included

.XX = ±.030

.XXX = ±.010

At my current company, which deals mostly in steel fabrication with some limited dabbling in machined components, we also added:

Fractional = ±1/16"

At my previous company, it was similar, except we didn't use fractional.

Something like 1.13 usually actually meant 1.125 ±.030.  Obviously that means you can't tell the difference between something that is 1.125 and something that really should be 1.130.  If you're being nitpicky about it, it is definitely somewhat unclear.  But looking at it practically, if that difference of .005 matters on the nominal, do you really want it to vary from that by ±.030 anyway?

At my previous company (where the machined tolerances were much more heavily used), .XX and .XXX was usually the difference between an as-cast and a typical machined dimension.  Critical machining such as bearing seats and sealing surfaces often had individual tolerances applied.

[phlyx]

Thanks for the feedback but I think this comes down to a time and expense issue.  Could call out X.XXX (for inch dimensions) on everything and be safe but for overall plate dimensions that could be very expensive where a saw cut edge would do.  And calling out 2.4" where you really wanted 2-3/8" but rounded can be an expensive oops.  2.4" would be ±0.03" which means you could end up with 2.43" where you wanted 2.375", that would be 0.055" off from nominal.  But editing the dimension to read 2.375" ±0.030" would be very time consuming for large drawings.   Well, even for small drawings.

[Anonymous]

Rounding off to achieve the tolerance is not a good idea.
Our customers do that and it leads to inaccuracies and drawings that don't match the model.
Bite the bullet and add the tolerance when neccessary.

[PaulMunford]

Could you create a second dimension style? That way you only need to select all the dimensions to change and select a new style.

[phlyx]

Was checking drawings for another engineering and the one on my desk right now has over 115 dimensions on it.  Making the assumption it takes me 10 seconds to add a tolerance (having to look at what it is, figure out what tolerance it needs to be, and editing the dimension), if I had to add tolerance to half the dimensions that would add 10 minutes to the detailing process for that one part.  The machine this is going on has 160 individual detailed parts which would add around 27 hours to the design process.  With overhead that's around $4,000 added to the engineering cost of the machine as well as around 4 days to the design process.  Not sure I could justify being that much over budget and late because we were biting the bullet.

Makes that carrot technique sound really good.  Plop down carrot sketched symbols probably less than a second each.

[Anonymous]

You could set up a dim style as Paul suggests and change them pretty quick.

[Curtis_W]

Hi phlyx,

I haven't read all the replies, but when I used to deal with this I would change one dim, then right click on it and choose Copy Properties, and apply the same tolerance to all dims that needed it.  These were mostly machined aluminum parts, and I found the exercise of doing this did have the benefit of making me slow down and consider the way that part was to be machined, so that I didn't overlook something. 

I don't know if any of that applies to your world, but I found it provided just the right amount of "slow down and think about this" without it being too slow or tedious for me at the time.

I hope this helps.
Best of luck to you in all of your Inventor pursuits,
Curtis
http://inventortrenches.blogspot.com

[rayessle]

@phlyx

re: the carrot symbol.

how would you attach the symbol to the correct digit in the dimension? If your design changed size and the dimension moved could your symbol now be over the wrong digit?

Ray Esslemont

[phlyx]

Didn't say the carrot system was perfect but it did resolve the rounding off issue.  Dim styles would help things but one of the problems there is we dimension 90% of our parts with Ordinate Set dimensions. All the dimensions in an Ordinate Set follow on dimension style.  You can right mouse click on a single dimension and change that particular number's style but you can't do that by copying dimension styles.

Also the ±0.005 or whatever added to a load of dimensions can really clutter up the drawing to the point of making it illegible. 

Still searching. 

[Frederick_Law]

We try to relax tolerance on x.xxx to 0.015 or 0.020.  Only add tolerance if it need more precision.

We'll put "stock" on dimension for raw material size.  So machinist won't try to machine it down.

Its down to what tolerance your produce usually need.  There is not much use of +/-0.005 in a fab shop but very important in cnc/machine shop.  Problem is we're both.

The next problem is if anyone in the shop will look at the title block.  Most will just "assume" x.xxx to be +/- 0.005.

[swalton]

We like to assign the tolerances in the modeling stage, and just display the model dims in the 2d prints.  Inventor has some silly limits with this workflow, so we have to plan our modeling around the 2d print.  I'll have to try this with an ordinate dimension workflow.

Is it possible to move away from ordinate dimensions on the prints?

Doesn't someone have to review and assign tolerances anyway?