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I am quite new to CNC machining and want to design two parts which should be accurately aligned with each other. I searched online and many people suggest the use of two dowel pins to align the parts, rather then using the screw holes directly. For the 1,5x5 mm, m6 tolerance pins (spaced 20 mm apart) which would fit my application, H7 tolerance holes are suggested for a "typical" transition fit. However, I failed to find any information on the required positional tolerance of the holes to ensure the parts can be assembled properly.

Can anybody provide me with a pointer or some explanation on how I would go about figuring out the position tolerance for the holes? Any help would be highly appreciated, thank you!

EDIT:
Thank you for all the suggestions so far. I added a simplified drawing to better clarify my design intent. I want to ensure that when the two parts are assembled (by lightly pressing two pins into the left part, sliding the right part over those pin -- preferably by hand --, and fixing the parts together using M2 screws) the distance between the 4mm H7 hole on the left part and the 12mm H7 hole on the right part is within 11mm +/- 0,05mm (considering the position tolerances of those holes as well as the position tolerances for the alignment pins).

Simplified drawing

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  • $\begingroup$ you could make the holes on both sides in one operation. $\endgroup$
    – joojaa
    Jan 16, 2023 at 20:37
  • $\begingroup$ That probably is actually the best solution. Not sure how to tell the manufacturer that though! $\endgroup$
    – raisyn
    Jan 16, 2023 at 21:38
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    $\begingroup$ Tell them what to do, not how to do it. You can give context to help them but try not to micro-manage them if you are not involved in the manufacturing chain. $\endgroup$
    – DKNguyen
    Jan 17, 2023 at 3:29
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    $\begingroup$ "which should be accurately aligned" -- my "accurately" might be +/- 3mm. Your "accurately" might be 0.03mm. Edit your question to give a number -- include the overall size of the part, too -- +/- 3mm on a 30m diameter part is going to be harder to achieve than 0.03mm on a 10mm part. Think about what you really need -- newbies tend to overspecify for accuracy, which drives up machining costs. Or they underspecify, which drives up reject and replacement costs. $\endgroup$
    – TimWescott
    Jan 17, 2023 at 16:46
  • $\begingroup$ Thank you for this suggestion, I edited the question to make it more clear on what I want to achieve and what I mean by "accurately". $\endgroup$
    – raisyn
    Jan 17, 2023 at 22:59

2 Answers 2

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I was told that you could only rely on pins to parts in fixtures to within a 0.001" or 0.002" repeatability which works out to be 0.0254mm 0.0508mm.

Accurate hole position and accurate hole diameter/shape are two different things.

If you need accurately located holes you should bore. Follow with a reamer if you also need accurately shaped and sized holes. Or if you need accurately shaped and dimensioned holes but not location you can ream but not bore.

You can technically bore for proper diameter and circularity without needing to ream but it can be tricky to dial in a boring head to your proper diameter...lots of test holes and measuring. It can also be tough to dial in the speed and feed get a good finish compared to reaming.


Do you actually want a transition fit across multiple fixed pins? Sounds like trouble. I don't think you will ever get the parts to mate. Ever try to simultaneously align two holes with two pins where you actually have to tap them to get them in? Good luck. Any lateral misalignment or tilt will prevent success when you need to tap the parts. Even if you somehow have perfect initial alignment it won't work since you will never be able to tap both pins squarely, identically, and simultaneously.

Some clearance fits for just one hole are so close that you can't mount them by hand because you can't push square enough so it jams so it seems like an interference fit. But if you have a jig or arbor press that can push square, it slips on with nearly no force. Your multiple transition fit holes is worse.

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  • $\begingroup$ That sounds about right. Use the dowels to line up a stepped circular collar or some other land. The layers of a hydraulic motor (say 6 layers) are located with 1.5 mm dowels. Then the whole sandwich is through-bolted with 6 8mm bolts. You don't want those pins to be loaded ever - you'll never get the thing to seal if they get pinched. $\endgroup$
    – Phil Sweet
    Jan 17, 2023 at 1:07
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H7 for 1.5 mm hole is 1.500+.001-.000 mm so you'll need a reamer

m6 for a 1.5 mm shaft is 1.505+/-0.003 mm , hopefully you are buying ground silver steel pins, good ones are better than that -about half that is easily available.

So from those numbers you can work out the possible limits of location of each centreline, and so the maximum errors in the distance between them.

Obviously the press fits are no issue, the worst case is a small pin, large hole, with 0.008 mm diametral clearance, or 0.004 mm radial.

The other hole/shaft could see the same error (unlikely), so the maximum variation in centre to centre location would be 0.008 mm, so there's no point in specifying a tolerance for position tighter than that. Which is good, I doubt typical XY tables can hold much better than 0.04 mm.

Tolerance schemes need to be defined in view of the intended use. As you can see this one is all over the place given the unstated accuracy of the XY table.

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  • $\begingroup$ Mhm, I looked up that m6 for a 1.5 mm shaft is between 1.502 and 1.508, giving a max. clearance and max. interference of 0.008 for the H7 hole. $\endgroup$
    – raisyn
    Jan 16, 2023 at 21:36
  • $\begingroup$ Sorry, yes i missed a zero out in the m6, but not in the maths $\endgroup$ Jan 16, 2023 at 21:43

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