# fine thread vs. coarse thread



## mwkellner (Mar 9, 2010)

I know y'all can help me with this one. If all other things are equal which has more holding power in a vertical position from underneath, coarse threads or fine threads? In my mind I can think of reasons why both would be right. thanks for sharing your knowledge with me. Thanks.


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## dodgeboy77 (Mar 18, 2009)

A fine threaded (UNF) bolt is stronger as it's minor diameter is bigger than a coarse threaded (UNC) bolt. In other words, the threads aren't cut as far down into the bolt. They also are less prone to loosening up from vibration, etc. 

From The Engineer's Companion ( http://www.designnotes.com/companion/manual-2.html ):

<<_Unified national coarse. UNC is the most commonly used thread on general-purpose fasteners. Coarse threads are deeper than fine threads and are easier to assemble without cross threading. The manufacturing tolerances can be larger than for finer threads, allowing for higher plating tolerances. UNC threads are normally easier to remove when corroded, owing to their sloppy fit. However, a UNC fastener can be procured with a class 3 (tighter) fit if needed (classes to be covered later). 

Unified national fine._ _ UNF thread has a larger minor diameter than UNC thread, which gives UNF fasteners slightly higher load-carrying and better torque-locking capabilities than UNC fasteners of the same identical material and outside diameter. The fine threads have tighter manufacturing tolerances than UNC threads, and the smaller lead angle allows for finer tension adjustment. UNF threads are the most widely used threads in the aerospace industry._ >>

Bill


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## Hammer1 (Aug 1, 2010)

It depends on the materials and application. In general, size for size fine threads can be stronger. They have a larger center and more incline, which helps them to be stronger in tension and shear. That's provided the threads gain full grip. This isn't the case in soft materials like wood or man made wood products like particle board. To be equivalent to coarse threads, you often need longer length. The fine threads often foul in wood, if power driven they can strip. In most woodworking applications a coarse thread is more suitable. In shear, you may need to increase diameter compared to a fine thread. From underneath, in tension, the bigger the teeth, the better.


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## cabinetman (Jul 5, 2007)

mwkellner said:


> I know y'all can help me with this one. If all other things are equal which has more holding power in a vertical position from underneath, coarse threads or fine threads? In my mind I can think of reasons why both would be right. thanks for sharing your knowledge with me. Thanks.


If you are referring to screws, I prefer coarse threads. They work better in composite materials that have the tendency to crumble. The threads are sharp and the gullets are deep, and IMO, work better in wood in general. Screws that are piloted and countersunk, should be driven slow and just bumped in tight. Spinning screws in fast makes them hot, and prone to breaking. Driven slow helps to prevent over spinning.










 







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## mwkellner (Mar 9, 2010)

Thanks to all of you for your quick replies. I failed to mention that I was refering to wood screws. Thanks alot.


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## cabinetman (Jul 5, 2007)

mwkellner said:


> Thanks to all of you for your quick replies. I failed to mention that I was refering to wood screws. Thanks alot.


I was referring to screws not bolts for wood. The term "wood" screws could refer to the tapered screw, which I don't use or recommend.










 







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## GeorgeC (Jul 30, 2008)

cabinetman said:


> I was referring to screws not bolts for wood. The term "wood" screws could refer to the tapered screw, which I don't use or recommend.
> 
> 
> 
> ...


You just confused me. Are you referring to machine screws, which are used with nuts?

I guess I would call anything else that would be used to hold two or more pieces of wood (or particle board, MDF, etc.) together a wood screw.

????

George


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## cabinetman (Jul 5, 2007)

GeorgeC said:


> You just confused me. Are you referring to machine screws, which are used with nuts?
> 
> I guess I would call anything else that would be used to hold two or more pieces of wood (or particle board, MDF, etc.) together a wood screw.
> 
> ...


Wood can be tapped for threads to use machine bolts.










 







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## GeorgeC (Jul 30, 2008)

Oh I know it can be done, but why? Under what circumstances would you want to thread a piece of wood to accept a machine screw?

George


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## woodnthings (Jan 24, 2009)

*table saw insert plate*

For the leveling screws if you used hardwood or plastic you may want to thread the set screws into the wood. There is no easy way other than that. JMO  bill


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