Tolerace understanding.

[dimitp]

Hi, i am completely new to this forum and i have decided to join and ask people for help. I'm to embarrassed to ask my colleagues so i will ask you. I have a simple problem that has been on my mind for a while now. see pic below:

http://files.engineering.com/getfil...0d-a457-78b71a7810d7&file=20150112_220607.jpg

Now, what i dont seem to understand is this. If i put tolerances acc to iso 2768 corase the first hole will be 5,3mm the second hole will be 7,6mm and the last hole will be 7,9mm as a maximum value.

If those holes are to be lined up with holes underneath on another part with the same tolerance and fastened with a 2,9 mm self tapping screw that requires a 2,4 mm hole with -0 +0,1 tolerancee the part will be wrong.

The only way i can fix it is by setting the tolerances to +0,03 and -0 which is way to tight. How am i supposed to think when placing tolerances? i always end up with these very tight tolerances?

I would be so grateful for any replies.

Thank you in advance.

/D

 

[CheckerHater]

Your question is a bit confusing.

Normally you would open up bigger hole to compensate for misalignment, not make hole tighter.

Also, ISO 2768 is not intended for positioning holes. I suggest you use datums and Position controls.

Even if you insist on using 2768, the tolerance group for general machining will be "medium", not "coarse".

If you could provide little more insight into what you are trying to do, it may be easier to find the solution.



"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[MintJulep]

As you have sketched things the first hole location is controlled by the distance from the edge +/- a tolerance.

The second hole location is controlled by the first hole location +/- another tolerance.

And the third hole location is controlled by the second hole location +/- a third tolerance.

Start by using a dimensioning scheme that has only one tolerance for each hole location.

 

[CheckerHater]

Good point MJ,

Although, once you dimension all holes from the edge, that will increase between-the-holes tolerances



"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[dimitp]

Thank you for your reply.

I understand the confusion sorry about that. A self tapping screw of 2,9 mm requires a 2,4 mm hole in a 2 mm aluminium sheet.

Let me try to reexplain.

I am Placing one aluminium sheet on to a second aluminium sheet. These two sheets are 24 mm long and y mm high. They will be fastened to each other with the aforementioned self tapping screw.

In the first sheet i add the tol acc. to ISO 2768-1 m ( i changed it from c to m to clarify).

This means that the last hole will be 17,3 mm at max. tol. from the edge of the sheet.

If the other sheet is produced at the minimum tolerance the last hole will be 16,7 mm form the edge.

This means that the misalignment will be bigger than the hole tolerance ( witch for self tapping screws in this application are H12 = +0,1 -0 mm)
) and the sheets will not fit each other.

Now the only way to make this right is to tighten the tolerances between the holes down to +0,03 -0 witch is very tight in my understanding of what a normal machine shop can produce at a reasonable price. (its lower than 2768-1 f).

Now to make it worse, if i want the edges of the sheet to be flush i cant use the medium tolerances acc. to 2768-1 witch would give me :24,2 mm at maximum so this must also be taken in to account. This means that i have to tighten the tolerances between the holes even further.

How should i think when placing tolerances like these, what am i thinking wrong?

PS.
If i instead place each dimension from the edge of the sheet i would end up with a wider tolerance as well
acc. to 2768-1 m. At maximum this will give me 7,2mm, 5,1mm and 5,1 mm. So the first hole will be wrong.

I'm thankful for any help.

 

[CheckerHater]

Now, let us separate the issues:

1. Function: 2 parts to be aligned flush by the edge; mounting holes to align well enough to allow assembly.
2. Manufacturing: if your shop cannot produce parts accurate enough, no drawing will help.
3. Drawing: is there a way to dimension the part so tolerances do not accumulate? Yes there is one.

I already mentioned that 2768 is not intended for positioning holes. Are you familiar with GD&T symbology and using Feature Control Frames (FCFs)?



"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[drawoh]

dimitp,

You are showing us a standard tolerancing problem. When you apply [±][ ]tolerances to a stack of dimensions, the last position becomes very inaccurate. This is why you use ordinate dimensions. If each dimension comes from the same datum, you only have one relative location.

This is also why you use GD&T and positional tolerances. Regardless of how you apply your dimensions, everything is located to the same datums.

--
JHG

 

[KENAT]

Over reliance on ISO 2768 for positioning features such as holes can be problematic.

As others mention, for matched hole patterns using appropriate GD&T i.e. position control, is typically more appropriate.

Alternatively, for your situation you could consider match drilling though it comes with its own limitations & issues.

Posting guidelines faq731-376

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

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