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Threading 8.8 bar

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paul berry 408/07/2020 13:06:21
6 forum posts

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

Michael Gilligan08/07/2020 16:43:36
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15891 forum posts
693 photos

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.

Stuart Bridger08/07/2020 16:50:05
457 forum posts
26 photos

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.

old mart08/07/2020 16:50:05
1829 forum posts
148 photos

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.

paul berry 408/07/2020 16:51:41
6 forum posts

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

Nicholas Farr08/07/2020 18:20:32
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2318 forum posts
1137 photos

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.

Michael Gilligan08/07/2020 18:38:31
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15891 forum posts
693 photos
Posted by Nicholas Farr on 08/07/2020 18:20:32:

Hi, the 8.8 is a reference to the tensile strength […]

.

Handy reference table here: **LINK**

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

[standard information, widely available]

MichaelG.

paul berry 408/07/2020 20:16:04
6 forum posts

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.

Michael Gilligan08/07/2020 21:14:55
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15891 forum posts
693 photos

Looks like you’re pretty comfortable there, Paul yes

Thanks for sharing the results.

MichaelG.

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