Form vs Cut threads in 304SS

[mjk]

I have a part that will be from 1/2" thick 304SS
The threads are 3/8-24 thru and once assembled will be used as a means of adjustment as often as a weekly basis.
This "nut" will be split on one side with a 6-32 as a clamp screw, released before the adjustment is made.

Although a gauge may not know the difference of the finished thread, I'm debating on whether to cut or form tap the 3/8-24 threads? The set screw that is the "adjuster" is black oxide steel

 

[eKretz]

Formed threads in stainless will probably be a little tougher since stainless steel tends to harden with cold working. Either way will probably be strong enough. (Not sure if the strength is what you're asking about... There wasn't really a clearly formed question there). In terms of size and finish they should be pretty close if you use the same class of tap. Form tap might have a slight edge on finish if you use the right lubricant.

 

[mjk]

I'm not worried about strength so much as which would be less prone to galling.
This isn't a high torque situation but the thread form can not be changed in order to match/maintain interchangeability

 

[eKretz]

In that case, probably the form tap. Also for extra insurance use a good anti-seize lubricant and you should be golden. If there's not a high load there's usually little danger of galling with stainless fasteners in my experience unless corrosion starts in the threads and the fastener is forced through it. If you're really concerned, since you say it's not highly stressed you could always use plastic or brass etc. for the adjuster if possible instead of the black oxided steel.

 

[Zeuserdoo]

I would also say go with the formed thread, the cold worked surface will be less prone to galling. Another thing to keep in mind, is to never screw in a black oxide screw dry. As long as it has some lubrication it should work fine.

 

[mjk]

The black oxide set screw is a "component" after modification(can't change to maintain interchangeability with older assemblies)
The "nut" used to be a detail in a bronze casting
The finished part will now be a SS304 welded assembly instead of the bronze casting

Oh....and lubrication has to be nearly food grade/non-toxic

 

[Goff]

Sometimes, when form tapping stainless or other work hardening material, using a bottom form tap instead of a plug form tap will work better. The theory is that with fewer threads doing the forming, there is less work hardening of the threads and less stress on the tap.

 

[eKretz]

LocTite makes a "food grade" anti-seize IIRC.

 

[DanASM]

I would say my concerns have already been addressed above. Breaking through will be where the material will be harder from cold working.

3/8 should have plenty of meat on it to not break on you. I use the expensive OSG form taps specific to Stainless. I form tap 316 SS all day.