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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? $$\begin{align} \text{Diameter} &= \text{Delta}_{dim} + \text{Tol}_{dist} + D_{shaft} \\ &= (211.5-210)+3.5 + 6 = 1.5 + 3.5 + 6 \\ &= Ø11 \end{align}$$

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    $\begingroup$ I'm highly skeptical that any type of manufacturing other than "squint at it and whack it with a hammer" will have such huge dimensional tolerances. What is going on here? $\endgroup$ – Carl Witthoft Jul 13 '16 at 11:51
  • $\begingroup$ Make a scale drawing if the lower part with all the areas tha may be hole marked, draw a margin (tolerance of upper part) around these areas. Scale the drawing 1:1 so you can put proper nuts and washers on the paper and see how it fits. $\endgroup$ – mart Aug 12 '16 at 10:07
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Tolerancing is about stacking the min and max tolerances of each part, if you take the left side as you said, you have 211.5+-3.5 and 210+-1: Bottom = min 208 max 215 Top = max 211 min 209

If you take the worst case scenarios, which is the min - max you get 6mm at one end of the tolerances and and 3mm at the other between the 2 holecentres, so the holes can be as much as 6mm offset.

So with 6mm variation on a 6mm hole, i think your top plate will need at least a 12mm hole to fit every time...

you could make that smaller but you'd have to change the dims on on of the plates, if you can.

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The most straightforward solution to this problem is to drill the holes in the second plate at the time of assembly, using the first plate as a template.

Having said that it really does sound like there is a manufacturing issue which needs to be addressed if you have such large tolerance on a part this small. Even with the most basic manufacturing process I can't imagine that this can't be improved on without undue cost. Even just making a template for marking the hole centres with a marker pen should improve matters substantially, bearing in mind that having to drill 12 mm clearance holes for an M6 fastener and having to obtain and use oversized washers is going to add a significant amount of faff to the whole process.

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