Threading 8.8 bar

Hi,

I have some 10mm threaded bar 8.8 class and was wanting to turn down a lentgh to 8mm and thread that 8mm part, ending up with a stud with the 2 diameters around 75mm long.

Would the newly threaded part (8mm) still be the same strenght of the original 8.8 10mm threads?

Thanks in advance

Interesting question, Paul

Two factors to consider [there may be more]

1. The original threads may have been rolled
2. The hardness may vary across the diameter [I know this to be true of some cap-head screws]
   

Not sure how much either of these contributes to the 8.8 rating though.

... hopefully, someone better-informed will be along soon.

MichaelG.

Can't comment on the strength, but the task will be a right PITA. My experience with 8.8 bar and bolts is that they machine horribly.

No, an 8mm thread is smaller than 10mm. The core diameter is smaller, 8mm is 6.466mm diameter, and the 10mm is 8.16mm diameter.

By the way, welcome to the forum, Paul.

Hi

The original M10 threads are definitely rolled and I would be threading the M8 via a Coventry die head, so cut threads.

The studs are not for myself so not sure how critical the strength needs to be at this moment.

Thanks

Hi, the 8.8 is a reference to the tensile strength, so the 8mm bit you make will be at the very best, that of an 8mm threaded rod. The only way that can be tested, is to make one and test it on a tensile testing machine, but unless to happen to have one or have access to one it's impracticable. A DIY way of testing, would be to compare it against a new 8.8 8mm piece of threaded rod and using a torque wrench till each one breaks.

Regards Nick.

.

Handy reference table here: **LINK**

https://www.westfieldfasteners.co.uk/Ref_Strength_Spec.html

[standard information, widely available]

MichaelG.

Ive done a quick DIY test on my threads and here is the findings

M10/8.8 rolled threads snapped at 130Nm

M8(cut threads) snapped at 90Nm

Also tried M8 A2/70 S/S rolled threads snapped at 70Nm

Not got any M8/8.8 rolled threads to compare!

I have read that the recommended torque for a M8 Bolt/Nut is 28.8Nm unless stated different by a manufacturer.

Looks like you’re pretty comfortable there, Paul

Thanks for sharing the results.

MichaelG.