Dimensioning of a hole from tolerances

[mootu11]

Hi
I hope you guys can help.

I have two parts that should be bolted together with four M6 bolts. The tolerances of the bottom part are quite big and the holes for this part are fixed to Ø6. The tolerances of both parts are fixed due to manufacturing and lowering of costs.
I need to determine the size of the holes of the top part in order for them to fit together. I have attached a photo showing the tolerances of both parts.

I am in doubt how to calculate the size of the holes. For instance for the left side, is this the correct way?
Diameter = Delta_dim + Tol_dist + D_shaft= (211.5-210)+3.5 + 6 = 1.5 + 3.5 + 6 = Ø11

Thank you in advance

 

[drawoh]

mootu11,

Actually, no, you have not attached a photo.

This stuff is easy to work out from first principles. You have M6 bolt, hopefully with nuts passing through Ø6mm holes, which had darn will better be oversize. There is no allowance for movement in your clearance holes, making this what the standard calls the fixed fastener case.

Your bolts occupy space that is a function of their size, and of the positional tolerance of the clearance holes, in this case, BOLTDIA+POSTOLERANCE. The clearance hole in your part must not encroach on this space. Your smallest allowable hole is BOLTDIA+POSTOLERANCE+YOURPOSITIONTOL. If you want a larger, cheaper hole, you need to make it bigger.

--
JHG

 

[KENAT]

drawoh, look more closely the hole patterns in the 2 parts don't match up.

Still going back to first principle works but there is the extra mis alignment term.

Then there's taking into account variation in both axis at the same time.

So much simpler if the holes lined up and you used position tolerance.

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[mootu11]

KENAT, yes it would be a lot easier if the holes lined up, but it would be too expensive changing the setup now.

I'm not exactly sure what you are referring to when writing first principles, it may called something else in Danish? I can't seem to find anything when searching for first principle and GD&T

drawoh, it is showing a picture on my screen. Like KENAT said, there should be something taking into the account of the misalignment between the two parts, but is it just another term that's the difference in maximum and minimum distance?

 

[CheckerHater]

On the matter of misalignment: Did anyone notice that hole patterns are in shape of trapeze/trapezoid (depending on country)and the infinite number of such may be produced, so the concept of hole alignment is completely thrown out of the window to begin with?

Also, what is that magical process that can only produce +/-3.5? is such kind of "lowering cost" worth the trouble?

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[JNieman]

Your geometry is not fully constrained - you need more than 4 lengths to fully define and control a quadrilateral.

You say the tolerances in the bottom part are quite big but say the holes are "fixed" to a diameter of 6. What is their actual diameter? Surely it's larger to pass a M6 bolt. Are the tolerances large on the hole? The bottom hole size and location is a huge part of figuring out your top part geometry.

How do you propose to fasten these parts together, anyways, with such a misalignment? Are you thinking you can use gigantic holes and over-sized washers (or worse) to sloppily fasten them?

A little more information and it may be possible to solve the problem.

 

[drawoh]

mootu11,

I cannot see a picture here. I will have to try at home.

By "first principles", I mean draw it out and do the calculations from scratch. They are not complicated, and the exercise helps you understand everything. When it is me doing everything, I assume bolts are located exactly at nominal position, and that the holes' diameters and positional tolerances must enable them to clear those bolts. The OP's case sounds more complicated.

--
JHG

 

[drawoh]

Okay, I can see the photo here at home on Firefox. At work, neither Firefox or Explorer will show it. My security settings are getting interesting. If mootu11 has an evil plot, I am safe from it, at least at work.

The drawing can be solved if you assume the horizontal looking lines are horizontal. This is not a reliable assumption. If you have CAD, draw out the two diagrams, superimposed on each other. Enlarge the Ø6mm holes on the bottom by the positional tolerance. Draw holes on the top figure that encompass the bottom holes. Add the top positional tolerance to the top holes.

--
JHG