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I have two disks to bolt with. 4 centrally located holes on each with Pitch circle diameter P and one hole at center of disc. If standard bolt diameter is B +/- b, what should be each hole diameter if tolerance of drilling hole is +/- h.

For the load rating I got M6 bolts. Their dimensions are 5.85+/- 0.1 mm. And PCD is 10 cm, and I have to bolt it via 4 bolts symmetrically placed and one in center. Drilling of holes on pitch circle is of tolerance 0.5 mm, while central hole tolerance is 0.1 mm. Now, what should be the hole diameters?, so as all four bolts get into and proper fitment occurs.

Thanks.

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  • $\begingroup$ If your pitch diameter is 10cm, then if you are working to 0.001mm the centres should be between 4.9995 or 5.005mm but the bolt diameter themselves should be according to the function or load applied. $\endgroup$
    – Solar Mike
    Commented Jun 17, 2018 at 15:48
  • $\begingroup$ @SolarMike For the load rating I got M6 bolts. Their dimensions are 5.85+/- 0.1 mm. And PCD is 10 cm, and I have to bolt it via 4 bolts symmetrically placed and one in center. Drilling of holes on pitch circle is of tolerance 0.5 mm, while central hole tolerance is 0.1 mm. Now, what should be the hole diameters?, so as all four bolts get into and proper fitment occurs. $\endgroup$
    – Raj Arjit
    Commented Jun 17, 2018 at 17:56
  • $\begingroup$ Then plan accordingly... $\endgroup$
    – Solar Mike
    Commented Jun 17, 2018 at 17:58
  • $\begingroup$ @SolarMike: sorry, I am new on SE, so had pressed enter and it had occured as comment earlier than I intended. $\endgroup$
    – Raj Arjit
    Commented Jun 17, 2018 at 17:59
  • $\begingroup$ The best way to accurately locate the two plates is to use locating pins - then the bolts are only used for holding them together... $\endgroup$
    – Solar Mike
    Commented Jun 18, 2018 at 8:16

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Let's take worst case: 5.85+0.1mm = 5.95 bolt, 0.5mm offset off the center when drilling on one plate, -0.5mm offset on the other plate, and -h error in dimension of the hole. For the plates to be guaranteed to align, the hole would need to be at least 6.95+h mm diameter. And that leaves at least a millimeter of play, which is bad. For the central hole you'd get away with 6.15mm+h but that's still over 0.3mm of play.

A much better approach is to align and temporarily bind (clamp) the disks together, and drill the holes through both at the same time. That way whatever error in hole placement is made on one, it's duplicated exactly on the other so there's no misalignment. Make the holes 5.75mm-h (assuming h is in the ballpark of 0.1mm or less) and expand them using an M6 thread cutter (tap) to fit the bolt's thread and hold the bolt snugly. And as soon as you have one hole complete (drilled and threaded) install the bolt and tighten it, before drilling another hole; this way any play/error introduced initially by the clamps will be nullified in consecutive bolts.

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