geometric dimensioning 3

[pso311]

I work for a cabinetry manufacturer and am looking into ways to reduce tolerance stacks. Is there a need to switch to GD& T to gain the maximum tolerance allocation, or can it all be done with general dimensioning? Switching to GD & T will require extensive training, but if it is worth it, that's where were headed. I'm looking for some solid advice as to which direction we should take.

 

[swertel]

GD&T is used for exactly your situation. It allows the designer to tighten up some tolerances while keeping others loose (and less expensive to manufacture) and still have an assembly that fits together and functions correctly.

There are ways to get around using GD&T, but they may or may not be cost effective for your situation. I do tool design and because of the nature of our product, are standard tolerances are very tight, which lends itself to high cost tooling. We have to explicitly loosen tolerances on drawing face which adds time and cost to the design process. But, having default tight tolerances gets us away from having to use a lot of GD&T; there is still necessity for some.
Our in house machine shop can hit our default tolerances without problem. So, the main manufacturing cost goes up when we send the fab job outside and they see the default tolerance block. That usually ups the estimate immediately.

We also have other creative solutions such as matched machining and other "make at assembly" references. When doing one-off tooling, that works. It may not work if you want interchangeable parts.

--Scott

 

[TheTick]

GDT can help to reduce tolerance stackup by forcing more sensible choices in design definition. However, the effect of this will be limited.

However, there may be more to your tolerance picture. Calculating tolerance stacking is a statistical process, and not always as simple as just adding the tolerances between parts. Depending on your processes, you may be able to employ some statistical tolerance stack calculations to reduce your total stackup.

For instance, if there are two dimensions with tolerances of +/-1.0, the effective tolerance between them could be as low as +/-1.4, depending on the processes and quantities of parts.

I hope someone who knows more jumps in at this point. I'm looking for some resources on this very subject.

I may make you feel, but I can't make you think.

 

[McLeod]

As noted, GD&T can help make your tolerancing fit the function, but like any versatile tool its effectiveness depends heavily on the user. Proper choice of critical dimensions is essential to reducing tolerance stack-up. Manufacturing must also recognize the dimensions as critical, and adjust the process and tooling to suit. I highly recommend that ANYONE working with mechanical designs be trained in GD&T, but be forewarned: tolerance stacks that involve GD&T controls are a bit trickier to calculate.

Statistical tolerance stack calculations can be used to allow larger individual dimension tolerances, but only if the core assumptions are understood and handled. The method assumes a normal distribution, centered on the nominal value. A certain defect rate is expected, and can be estimated. Basically, if you don't have suitable SPC data for your parts, or if you have less than three stacking dimensions in the calculation, just use worst-case tolerancing (or risk suffering unacceptable defect rates).

The best resource I've found on these issues is the Dimensioning and Tolerancing Handbook (Drake, 1999). It includes chapters on GD&T, one-dimensional tolerance stacks, multi-dimensional tolerance stacks, and Six Sigma tolerance design. Amazon.com carries it, but if you get a copy, treat it gingerly - the binding on this edition is horrible, and mine is already falling apart.

 

[drawoh]

pso311,

The advantages of GD&T are as follows...

- The meaning of all the dimensions and tolerances you apply are explicitly defined by the standard. Presumably, the ISO standard, whatever it is, would do this too. You might be surprised by what the definition is, which is why you should get trained.

- It provides a simplified way to apply tolerances to complex geometry. I think the symbols provide a more convenient way to define your requirements as exactly as possible, which may allow you to back off some excessive tolerances.

Tolerance stack-up is more of a design and manufacturing issue, although it is discussed by the standard.

If you take all of your dimensions from the same datum, then conventional tolerances will not stack up. If you apply your dimensions feature by feature, they will stack up. With GD&D, you can apply dimensions feature by feature and use profile tolerances to control the geometry WRT a single set of datums, thus eliminating the tolerance stack-up ON YOUR DRAWINGS.

How are your parts jigged, fabricated and assembled? If your final as-manufactured dimension is the result of multiple parts or processes stacked togther, you have tolerance stack-up, GD&T or no GD&T.

JHG