Tolerance

Thanks for clearing my doubts.

If that was the intent then the designer needs a spanking.

While I am not a professional draftsman, I have never seen a tolerance specified in that way.

A more typical way to express that would be 10.02 +0.03/-0.00 or 10.05 +0.00/-0.03 or 10.03 +0.02/-0.01, etc.

Which leads me to believe that the post by Raynee has incorrectly framed his question or he did not correctly interpret the drawing.

What you are specifying would be standard. There are occasions where special circumstances require special designations in tolerances.

Let's assume his tolerance is in regards to an inside diameter of a bore, that requires something else to be able to slide through constantly. Anything less than +.02 is too tight and anything over +.05 gives too much play.

OP's already stated it's a dimension of a pocket. But same thinking applies.

In my early days at Boeing , we could not put +/- on a print dimension. It was noted porm, which caused all kinds of confusion with our suppliers.

If I was building tooling with these requirements, I would shoot for the tolerance that would allow wear. For example if this was tooling to make a hole, I would aim for the maximum tolerance to provide room for tool wear to the lower tolerance.

Interesting.

This must be a common difference between a fabrication tolerance specification and a measurement tolerance specification. I'm use to always seeing a tolerance being a positive and negative value that once in a rare while might not have a mean value of zero but always one limit negative and the other positive.

It does make sense to simplify some of the documentation by specifying in this manner but I would've expected the paperwork to show in your example 10.035 ± 0.015 or the limit numbers you presented.

It's good to see other approaches.

It's a commonly-used method of dimensioning for fit - e.g. a hole to accept a nominal 10mm dia shaft could be 10 +0.02 +0.05 on the manufacturing drawing, with the shaft being e.g. 10 -0.02 -0.05.

I believe you have it. Like you said, possibly it's a fabrication print going off the part print. Not normal for us machinist-type folks, but I see it often in fabrication plants. Some places don't want prints with differing dimensions floating around. Even though most here understand that it's the same numbers, it can be confusing to others.

It is after all, just numbers.

Hi,

Thanks for your reply. But I was actually referring to a dimension in a square pocket

What is the difference in using ++ tolerence & +- tolerence. Any advice please.

Presumably something has to fit into the pocket.There really is no difference in the end result, provided it was either e.g. 10 +0.02 +0.05 or 10.035 ±0.015,; sometimes designers (and machinists) find it easier to use the "++" method, especially for holes/pockets etc. (and "--" for the mating parts).

I don't think anyone here, but you, can truly answer the question.

You should have a tolerance block on the drawing or at least know the context of the dimension and reason for the somewhat odd form of the tolerance.

Also in some cases even the thermal expansion of the material, and the thermal expansion of the mating part, if any, may come into play over the service temperature of the part.

One would also assume that a "pocket" and a "peg" to fit that pocket would be dimensioned differently, as well. One inside (pocket), one outside (peg).

I think we need more information.

Many assumptions are being made here without the benefit of any reference point.

(ASME) Y14.5-2009, ISO/TC 213, ISO 8015 and 5459, DOD-D-1000B, shop practice and seat of the pants drawing preference.

OP needs to supply more information.

Because it's easier to understand very quickly without thinking about it that the desired finish must be larger. Specifying a +/- of the said example (10.35) will make some pull out a calculator.

And what if the measure is 10 or 10.01? It is not acceptable?
I think it refers to -0.02 +0.05. If it does not so I see no sense
WP

"And what if the measure is 10 or 10.01? It is not acceptable?" - correct. As the OP has stated, the tolerance refers to a dimension of a pocket. Something presumably fits into this pocket; we do not know the tolerance to which this mating object was manufactured, but provided than it is less than 0.02 larger than its base dimension it is guaranteed to fit. This would not be the case if the tolerance of the pocket were -0.02 +0.05 ( or, indeed, if the finished size were e.g. 10 or 10.01).

Rather than (to continue the assumed datum of 10) 10mm (+0.02 +0.05), it would be better represented as 10.02mm (-0.00 + 0.03).

Perhaps a review of your copies of ISO 8015 and 5459 is in order.

As a Tool and Die Maker, I have to say that I never ran across a double + tolerance. Though I know that some draftsmen happen to make up their own tolerance specs. without regard for standards.

Typically we would get the prints that had dimensions with a Plus / Minus detail.

I.E. 10 mm +.02 / -.05 or as a few others have pointed out in this case maybe

10.035 mm +- .015 mm

or sometimes 10.02 mm to 10.05 mm and it cannot be outside the specified dimensions?

Without actually seeing the prints then we are still stabbing in the dark!

There are standards as you say but designers are a lazy bunch (speaking as one myself) & will just use the tools they are given. In a CAD package you can tolerance using standard hole basis or shaft basis sizes eg. H7, g6 etc. & the CAD will dimension the part as a ++ or -- tolerance.

Thank you for the info.

The cad package I use is an older version and doesn't add the tolerances, and most everything I draw up is fairly simple so it's quicker to just do it by hand.

I have seen double + and double - tolerances. I do not dimension that way myself but it is very common for shafts in particular. That way you can order a TG&P shaft and call it 6" Ø and know that it will be 6"Ø -.0002/-.0005 so it will fit in the 6"Ø bearing bore.

I would have called this out if it was 10 for example as 10.035 +/- .015 or 10.02 +.03/-.00

Your drawing should also list the drawing number of the part that will fit into that pocket. Look at that drawing, too, and see if it will provide insight towards interpreting the pocket drawing.

That is a pretty loose tolerance. I suspect it was specified as such to make it easy for a semi skilled fabricator to produce it without need for a lot of precise measurements. He would work toward the +0.05 limit, but if his skill was not top notch, he might end up with +0.03, which would be ok. He could consider the job done. Stating ±0.015 and he would be forced to work more closely, increasing his chances of going out of limits.

Although never explicitly stated, I think the tolerances in the post are millimeters not inches (0.02mm= .0008", 0.05mm= .0020") I'd call that pretty tight.

For all we know this might actually be in Sumerian cubits. I doubt it but it sure is fun to be absurd.

Regardless of what unit is actually used, this fabrication tolerance question should apply to any unit. With this being a pocket dimension it does kind of make sense that this should be a little bit bigger than the casual dimension of whatever. Particularly when what goes in this pocket will likely have a dimension of a little less than whatever.

I was going to bring up the same point, but I didn't want to confuse the issue any more than it already has been

.03 mm is only .001 inches. That is not a loose tolerance!

.3 mm is .011 inches. Now that's loose

When I machine a part, I try my best to hit the center of the tolerance because of thermal expansion or contraction.

When I first saw this double + tolerance I questioned its use and was told by a very experienced engineer who came from an aerospace background that not only is it used for regularly for shaft/hole fits but is very common in the aerospace industry. Having never worked in it myself I can't confirm this though.