MJ thread to M thread interface? 2

[Andros]

I currently have a situation where a design engineer has specified a seriers of MJ threaded holes MJ2; MJ3 and MJ4 in an aluminium chassis and has specified M2; M3 and M4 screws as the mating fastener. My understanding of the MJ thread profile is that the root radius is increased (relative to the standard M designation) in order to reduce stresses in the threaded joint, and have never come across the deliberate mixing of thread forms before, niether have any of the other designers I have questioned on the subject, although the designer concerned claims that it is a standard practise to mix MJ and M, does any one else out there have any experience of or information pertaining to the correct application in this instance?

 

[vonlueke]

Your described scenario is OK. Here's why. Internal thread root radius is not appreciably increased in any of the following thread profiles; only external thread root radius is increased--slightly in UNR, and more so in MJ and UNJ. In fact, the MJ and UNJ external thread root is so shallow (thus increasing bolt fatigue strength), it requires the truncated crest of the MJ and UNJ internal thread specification.

As such, M, MJ, UN, UNR, and UNJ thread profile compatibility is as follows, where "incompatible" in the following table means "interference occurs between nontruncated internal thread crest and shallow external thread root."

External Internal Compati-
thread   thread   bility
-------  -------  --------
  M       M       compatible
  M       MJ      compatible
  MJ      MJ      compatible
  MJ      M       incompatible

  UN      UN      compatible
  UN      UNJ     compatible
  UNR     UN      compatible
  UNR     UNJ     compatible
  UNJ     UNJ     compatible
  UNJ     UN      incompatible