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When use 3/8-32 instead of -24?

Bill Kahn

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#1
I need to thread an almost 3/8" steel rod for like 1/2" and thread into brass I will tap for it. It will be assembled once and not taken apart. Generally, when should I select 3/8-16, 3/8-24, and 3/8-32? For this specific application (steel into a shallow blind hole) what are the pros and cons? Thanks for any thoughts. -Bill
 

DAT510

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#2
Typically finer pitches are used when higher tensile strengths are needed. The greater thread pitch gives a larger surface area to disperse the load over and the Minor Diameter also increases (on external threads) as the pitch increases which helps to support the additional tensile loads.

One advantage of coarser pitches is they typically more durable (with regards to dings and the like). Coarser pitches are harder to cross thread. On Softer materials, coarser threads are also less prone to stripping.

If if was me, I use the 3/8-24.
 
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4gsr

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#3
Along with what DAT510 said, The extra fine series is generally for "fine adjustment devices" type of connections and where alignment is critical like used on telescopes and microscopes or such, and other applications. They do not do well in heavily loaded connections, or I should say, get the connection apart or screwed together. Requires some kind of never-seize lubricant to help from galling.
In the design work I do for a living, we use lots of different threads, and these threads are sometimes subject to high loads, 100K plus on a hollow member that may only be 2" in diameter. The common thread pitch we use is 8-pitch Stub Acme. Not so common is 10 and 12-pitch Stub Acme. We use very few vee threads unless they are on older tools and equipment, which we have to stay with what was used. The older vee thread pitches we used were 12, 10, and 8-pitch and once in a while a 16-pitch thread.
Anyways, depending who you talk to, an extra fine thread may be 12-pitch vee thread to them, where 4-pitch is a coarse thread.
 

4gsr

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#5
How big is "almost"?

A fine pitch will have proportionally less thread on an undersized shaft than will a coarse pitch.
I did not catch that! "almost 3/8" steel rod"

The rod has to be .375" to no smaller than .366" In diameter for a 3/8-24 thread. Can't be any bigger than .375" Just won't work, if it's bigger on the OD!
 

whitmore

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#7
I need to thread an almost 3/8" steel rod ... Generally, when should I select 3/8-16, 3/8-24, and 3/8-32?
You'd want 3/8-32 for an application with a ring nut for lightweight retention. If it's strength you want,
any pitch that allows more than two threads in the hole, will have roughly comparable strength
to any other. Pick a thread for which you have a bottoming tap for the blind hole, for the obvious
reasons. The more metal/metal thread contact, the better.

Ripping the threaded rod from the hole would shear a comparable amount of material regardless
of the pitch, if the tolerances aren't loose.
 
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