Positional Tolerances

[drawoh]

I am looking at some drawings that call up positional tolerances of [⌀]0.01mm. I am trying to make sense of a design where I do not have all the information, and I am preparing new drawings. Will a machine shop take that number seriously? What is the tightest positional tolerance I can call up in a jobbing shop?

--
JHG

 

[3DDave]

How big is the part? Isn't that number +/- 0.0002 inches? Let the part warm or cool and you can hit that requirement for a fraction of a second.

Whoever makes the machine to verify that has a similar problem.

 

[cr7]

At RFS? As 3DDave already asked, how much are features apart?

 

[drawoh]

3DDave and cr7,

The holes are 4mmH7, and they are 98mm apart.

--
JHG

 

[ctopher]

.01mm is .0004in.
A good machine shop can hold that tolerance. But, it depends on your design if you need to be that tight.

Chris, CSWP
SolidWorks
ctophers home

 

[Nescius]

A good machine shop can hold that tolerance. But, it depends on your design if you need to be that tight.

I agree. Absent any other information, that number is unremarkable, not crazy at all.

That said, your application may still satisfy the very depressing "Triad of How Engineering Often Actually Works":

1. We don't actually need such a tight tolerance.
2. The vendor has never actually met this tolerance.
3. We've never actually confirmed the parts meet the tolerance.
4. Use the parts.

It's quite fitting that the triad actually has four elements.

 

[drawoh]

Nescius,

I think someone is doing precision assembly by using two round holes to locate on two round pins. I am trying to explain to people that this is the Wrong Way. What I see on the drawing are precision seating holes for what I assume are pins. There may be a surprise waiting for me.

--
JHG

 

[Nescius]

That's another, equally depressing, "Triad of How Engineering Often Actually Works".

1. Mystery design, nobody knows why.
2. Tolerances tight to make it function.
3. Money, manpower, and sanity expended for the next 20 years.

then...

4A. It was all a waste of time, or
4B. It was legit, design implodes if you even think about changing it.

 

[flash3780]

Is there a material condition modifier? That position tolerance without any bonus would be extremely difficult to achieve.

That said, it's not uncommon to have zero positional tolerance at the maximum material condition, where the position error tolerance is only related to the size of the hole.
A larger bolthole for could be out of position more than a smaller bolthole and still fit together.

Some folks balk at the zero position tolerance, so many designers will put a small tolerance at MMC rather than zero.

 

[Wuzhee]

Ø4 H7 with |pos|Ø0 at MMC| is absolutely doable, but for basic assembly? Unnecessary. I did Ø0.01 in several occasions, and how did they measure it? With calipers.
For me, Ø0.02 is precision already. No need for tighter, only if reeeallyyy justified.