Thread holding power - NC vs. NF

[pmerritt]

Hi Guys,

Would a 1/4-20 or 1/4-28 thread have more holding power. I am repairing the treads that hold my collet chuck in a south bend mill (see my thread in machine modifications). The original 5/16-18 threads are damaged and my idea is to make a plug to fit the damaged threads that will be held in with some red loctite. The plug will have 1/4" threads, hence the question... NC or NF? I realize that there are other ways to skin this cat, but this seems to be the best I can come up with without making things overcomplicated.

As always, thanks for the input,

Peter

 

[mu38&Bg#]

The minor diameter on an external 5/16-18 thread is less than 1/4". The plugs won't work. Have you considered helicoils to repair the thread?

Finer threads allow greater fastener cross section increasing the ultimate strength of the faster. You shouldn't be anywhere near the ultimate strength for this application. Is there a difference in loosening over time or in use? I don't know.

 

[pmerritt]

The minor diameter on an external 5/16-18 thread is less than 1/4". The plugs won't work. Have you considered helicoils to repair the thread?

Crap, I forgot to check that. Helicoils could work but the spindle is HARD so drilling out and cutting the threads for the helicoil will be difficult to say the least. Has anyone used the Permatex thread repair epoxy? It says it is good for up to 128 ft. lbs. which should be more than adequate.

Thanks again,

Peter

 

[MachineTom]

NC would have greater purchase in the parent material, BUT fine thread would exert a greater force against the collet for a given torque value. Also and this is the likely reason that NF was used in the design, the set screw was a larger cross section as the minor diameter is larger with NF thread. this prevents splitting of the setcrew from frequent use. If the spindle is soft enough to be tapped, use a sleeve type thread insert, as the threads are stripped, no loctite will hold in a plug, so you need to open up the 5/16 and get tight threads. If you want to do a Kludge type repair, run a 8mm x 1.0 tap through the hole its .003 bigger, that is not what I would do. But it could work, the sleeve type insert not the wire type would be the best repair.