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Guest yutyjytj

Geometric Tolerance

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Guest yutyjytj

i want to gain a better understanding of geometric tolerance dimensioning, does any one have any experience with this particular subject?

 

 

when do you actually require geometric toleranceing when drafting is it only for specific products?

there are a few terms i have come across e.g, plus minus, plus minus symmetric, limits, how do u actually use this?

 

if you have a nominal value dimension of 1.540 inch and the upper tolerance is 0.005 and lower tolerance is 0.000, what exactly does that mean?

 

any help would really be appreciated

post-24160-1264723337.jpg

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Guest rhymeswithpuck

I wouldn't claim to be an expert, but my understanding is that tolerancing is part of the reality of manufacturing and producing products. Because of various processes and materials used in manufacturing, there will be some variance in the sizing of a part within a range deemed acceptable by the designer.

 

The level of precision depends on the product's intended use, and what costs are allowable. The production of a surgical scalpel will demand much more precision than that of the butter knife you use in a diner, for a price. There are also standards - American Society of Mechanical Engineers (ASME) Y14-point-something, and International standards (ISO) that you can refer to.

 

So in reality, every product has tolerances associated with it, but I think it depends on the stage in the design process as to when they are needed.

 

As to your specific question, 1.540 + .005, - .000 means that when that part is produced, the actual dimension needs to be between 1.540 (1.540 - .000) and 1.545 (1.540 + .005) inches. This dimension may specify a hole that accepts a shaft, and so while a slightly larger space won't be a problem, if the hole is too small, the part is useless in the assembly.

 

Likewise, in the drawing you attached, the indicated length will be between 29.87 and 30.13 units.

 

That's just tolerancing. I think Geometric Dimensioning and Tolerancing (GD&T) is much more involved, dealing with positioning, angles, and finishes, ie questions like how round is the cross section of a circular shaft, how smooth is the glass in the windshield. There's a lot to it, and I think some people do this sort of thing all the time.

 

It comes down to awareness of uses of the product and the manufacturing methods available to you, in a practical and economic sense.

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Guest Karl Baxter

Basically it means, in that case that the part due to manufacturing is able to be 0.005 larger than the actual dimension, and still be an acceptable part, however the dim. can not fall lower than the dimension. This is important as other parts in an assembly will also have tolerances. In that part if the measurement was to fall lower than the dimension it would fail to be an acceptable part ie. it wouldn't join properly or something.

For example, if you were getting a metal plate with holes stamped, there would be slight tolerances on the holes in the

plate. If the holes fell within the tolerances then bolts/screws or whatever would still function properly and attach to other parts. If they fell outside the tolerances then the part would be unacceptable. It is common for a manufacturer to do an initial run of an agreed amount of parts and provide a report stating how each part performs regarding tolerances, the average etc.

I don't know if that makes sense or not but...

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For a basic overview do a Google search for GD&T and associated terms - there are tons of sites that cover this.

 

The reality is though, and speaking as a professional designer working in many industrial sectors for twenty years, is that I rarely need to worry about this. Back in the good old days of drawing boards we used to draw things out and tolerancing was usually added to most sheets. These days, 90% of the time I send 3D data straight from SolidWorks to a toolmaker in Asia with a simple drawing attached which has KEY dimensions highlighted. The main use of this is to ensure there is no scaling on the CAD import process.

 

I will also add critical dimensions as tolerances where necessary but this tends to be only on mating surfaces (like the fit needed on a shaft, or a tolerance requirement for a snap fit). What I have learned is that the more you tolerance up a drawing the more trouble you tend to have finding people to make the parts and the more expensive the parts are. Remember for a contractor, if a part is out of tolerance it is a reject, so costs money and reduces their profit on the job - so they need to build in a bit more margin for tougher jobs.

 

Most engineers, let alone industrial designers, couldn't properly tolerance up a part drawing or even understand half the symbols for GD&T (I've actually heard someone see the "eccentricity" note and comment that it was a bit odd to mention the eccentricity of the shape - "it looks like a good design to me - nothing odd about that"!). I'm certainly no expert.

 

My advice is unless you are a time served manufacturing engineer who knows what tolerances can be achieved under normal production conditions stick to overall dimensions, but add a note on any drawings describing critical areas of fit.

 

At the end of the day, tolerancing is like anything else. Expect trouble or costs (both) if you tolerance a dimensions as 175+/-0.0001mm, in the same way that you would expect problems trying to drill a 1mm Dia hole 200mm deep in steel! Both of which I have seen on drawings.....

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It all depends on your line of work and how close you are to the manufacturing side of things, but everything said above was correct.

 

Everything is built to a tolerance, so it's important to understand the basics. For example, if I have a plastic part that I want to print a logo on, and I find out the tolerance is +/- .37mm it may not be much, but ultimately in my design I may need to realize I can't print this tiny graphic in this small area without it potentially running off the edges, so I need to give myself a buffer zone larger than my tolerance.

 

Color matching is also another type of tolerance. When printing things in large quantity the vendor usually will set up a tolerance on the colors as well, so you need to understand those so you can say how far you can go before it becomes out of spec.

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Guest Bowl of Soup
For a basic overview do a Google search for GD&T and associated terms - there are tons of sites that cover this.

 

Start Here:

 

http://en.wikipedia.org/wiki/Geometric_dim...and_tolerancing

 

There are other standards, but Y14.5 2009 would be my preference (in north america).

 

The problem with the drawing that you are showing is this: the tolerances are too abstract (not fully defined). So the basic intent from the person drawing may be to manufacture within the dimensions (the upper and lower tolerances on the drawing). BUT there are a lot of assumptions that are not being communicated in this basic portion of the drawing you are showing...

 

For example, a better practice is to tolerance dimensions off of a datum, and then add surface and "parallelism" notation. Why are all these extras required...? simple, for assembly fit. For example, I bet the intent of the designer in that drawing is to have one surface perpendicular from the other, but allow the surface 'cut' within a certain "horizontal" tolerance. If the cut was made outside top and inside bottom (at a small angle), while respecting the tolerances, I do no think the design intent is maintained. This may seem trivial for simple parts, but it becomes significant very fast in assemblies.

 

Do not forget that tolerancing is intimately linked to the process that will be used, so tighter tolerances are necessarily going to cost more $$ to produce.

 

Cheers.

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