Determining tolerance limits

[3DDave]

I figure there are several ways to determine tolerances for dimensions:

1) Use whatever was on the last drawing someone else did - seems very popular; see title block tolerances.
2) Get QC data for the last several hundred or thousand similar features, picked from an inexpensive process that isn't totally sloppy, calculate a 3 to 6 sigma value and tweak the design accordingly.
3) Get a stress report showing likely levels of force to push parts into place or stresses and deformations induced due to variations in manufacture and choose dimensional variation limits that are acceptable.
4) Make up some numbers because manufacturing will usually use a process that has a 1 sigma chance of making to print and will ask you to open the tolerances anyway, including then moving to a crappier process and asking to loosen them again, while reminding you you held back unnecessarily the first time.
5) Look at the assembly costs and choose values so the parts need no manual alignment, even though the manufacturing estimates are a little higher, saving big in assembly time and fixtures.
6) Dart board.
7) Ouija board.

Just for fun, rank these in order of desirability and then in order of likelihood. Feel free to include other methods that have worked, or might be expected to work better.

 

[MikeHalloran]

I almost immediately adopted the most basic GD&T symbology, mostly for its clarity and its brevity. Machinists just 'get it', whether they read English or not.

I tend to avoid using GD&T for a whole drawing, or supporting its adoption as a mandate, because, of all the people I have met in five decades in the workplace, the ones who are most aggressive about GD&T adoption are the ones who understand it the least.

A very rude tirade awaits the next yahoo who tries to lecture me about the wonderful new concept of 'bonus tolerance'.


I also get a kick out of outfits that educate their workforce on the subject, exactly once, and continue to abuse their low wage apprentice minions into leaving at the first opportunity, then don't lift a finger to train their rent-a-bum replacements. ... and expect the education to continue doing good, when all the educated minions have long since gone.


Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

I'll simplify my response to this:
"If you need a calculator to read and use a drawing, then the drawing is wrong."



Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

Mike,

Ever hear anyone say "I know that's what the drawing says, but that's not what it means"?

My remark in another post about D&T being a form of programming stands here as well. Would one replace all the trained programmers with non-programmers and expect software development to continue smooth as silk?

 

[Belanger]

If you don't need to use a calculator, couldn't the drawing be overdimensioned? Sounds like there would be a lot of ref dims.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[ewh]

I agree with JP... often a design requires definition in such a way that the machinist may need that calculator to suit his workflow without affecting part function.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[KENAT]

I'll vote for functional requirements bounded by the need to fall within relevant process capabilities most of the time.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Tenkan]

my default is to use the biggest tolerance the design allows even if the process is capable of a fraction of that.

lightweight, cheap, strong... pick 2

 

[cjccmc]

In my experience: 1) Use whatever was on the last drawing someone else did - seems very popular; see title block tolerances.

Then use a fancy term like "heritage design" to justify and all heads will nod in agreement ;-)

 

[fsincox]

My experience is the same as cjccmc, they will look at you strange and think "OH, No, another troublemaker" if you don't.
Frank

 

[3DDave]

When a former high-school teacher turned QC head suggested to me that engineers had a bucket of tolerance that they were simply unwilling to share I considered one case this way:

To increase the location tolerance of a hole in a plate, one has to make the hole bigger. The larger the hole, the larger the washer needs to be to prevent falling in. The larger the hole, the thicker the washer needs to be to keep from being pulled in. The thicker the washer, the longer the bolt. All assuming the washer material stays the same.

The hole costs more to make, the washer and bolt cost more to buy, the weight of the item increases, possibly increasing shipping or operating costs, and the more likely the plate or the item attaching to the plate will be located farther from its ideal location.

 

[drawoh]

3DDave,

The hole costs less to make, due to the sloppier position tolerance.

In a machine shop, unless you specify something very tight, the hole will be positioned to whatever tolerance the machine tool provides, regardless of how sloppily you specified it. When you go to sheet metal and weldments, things get interesting.

--
JHG

 

[3DDave]

drawoh, generally I agree with the idea, but details of the cost implementation don't suggest a solid savings.

A larger hole costs more to machine than a small one.

There's more effort in material removal, it usually takes longer, and there is more waste material to handle. This cost is on a per-hole basis.

Setup, which affects location tolerance, is usually on a per feature-group basis. Unless only one feature is being created, the setup cost divided by the number of features is what might be added to the cost of creating the hole.

There is offset between decrease in setup cost and the cost of material removal. It's not a simple savings or holes would be a foot across for .250 hardware**. In the shop I was dealing with, the machine tool was often some guy with a pistol drill and a piece of chalk.

I forgot further factors. To make up for the larger holes, there has to be more edge distance so the overall size of the part is larger. To maintain clearance with other parts, the overall assembly gets larger. Because the assembly is larger sections have to increase due higher moment loads. Now the whole thing is heavier.

<rant***>So sure. Let's just bump the .250 clearance hole from .281 to .375 because the drill wanders away from where the chalk mark was made. After all, engineers have a bottomless bucket of tolerance that is jealously guarded for no good reason. The last 'Yes' cost a few hundred engineering and stress hours to make all the related changes and make sure the product wouldn't fail and to add alignment requirements to the assembly instructions to account for the extra clearance. The money came from profits that could have been spent on developing a new product, but instead went to cover the cost-savings of not buying a center punch and a caliper or a brush to sweep the chips from the fixture before shoving the next part almost in place.</rant>


**I wonder how jarring it is for metric readers to come across dimensions expressed that way.

***The MEs and QA/QC this is directed at should know who they are, but they won't recognize themselves.