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Tractor Talk Discussion Forum

Coarse vs Fine Thread ?

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Got a Question

07-03-2005 19:37:34




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I'm sure this has been asked before here but here goes. What is the advantage and disadvantage of each of the two basic thread pitches, fine vs coarse threads. Let's say for a standard 3/8" bolt with nut for example.




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Pharmall

07-04-2005 07:13:22




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 Thread Size Data in reply to Got a Question for you !, 07-03-2005 19:37:34  
third party image

I can e-mail this as a PDF file if anyone would like it.



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Pharmall

07-04-2005 07:15:07




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 Tap Size Data in reply to Pharmall, 07-04-2005 07:13:22  
third party image

I can e-mail this as a PDF if anyone would like a copy.



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dr.sportster

07-04-2005 06:57:21




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 Re: Coarse vs Fine Thread ? in reply to Got a Question for you !, 07-03-2005 19:37:34  
Generally speaking coarse threads would be used in a casting where fine would tend to strip out easier.Fine is good to pinch two parts together where a nut backs up the assembly.A helicoil in an aluminum casting is stronger than a machined thread in the same casting.On a Harley XR750 every thread in the crankcase comes helicoiled from the factory.



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txgrn

07-04-2005 05:36:29




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 All of the above er ah below! in reply to Got a Question for you !, 07-03-2005 19:37:34  
I enjoyed that and it's amazing how many reasons you would use one vs the other.

And yes, I too have problems finding the lost nut!

Mark



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Galen

07-04-2005 04:41:34




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 Re: Coarse vs Fine Thread ? in reply to Got a Question for you !, 07-03-2005 19:37:34  
Fine thread are more annoying...8^). Seems like every time I encounter one, the nut that was just on it disappears!



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Pharmall

07-03-2005 23:49:47




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 Re: Coarse vs Fine Thread ? in reply to Got a Question for you !, 07-03-2005 19:37:34  
And, of course, ;^) there are coarse, fine, and extra-fine standard thread sizes in the Unified Thread Series. This is the standard that was first agreed to by the UK, US, and Canada. The flywheel nut on a Briggs & Stratton engine is an extra-fine thread (guess how I found that out!).

The only thing the others haven't mentioned is that coarse threads are used for softer or more brittle materials (bronze, brass, aluminum, cast magnesium, cast iron, plastic). Coarse threads are more resistant to stripping, and are still useful when some corrosion or slight damage to the threads is present.

There are lots of other specialty thread systems. The farther back you go in time, things become less standard. A machinist with a good lathe and some experience can cut just about any thread they like, and some did, just out of spite. A lot of things got standardized in WW II.

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NC Wayne

07-03-2005 21:40:55




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 Re: Coarse vs Fine Thread ? in reply to Got a Question for you !, 07-03-2005 19:37:34  
A fine thread gives more surface area in contact in the threads giving a higher coeficient of friction between the mating parts and making the connection more resistant to loosening. For a given size fastener the smallest diameter, which is at the root of the threads, is larger for a fine thread (because the threads aren't cut as deep) than a course thread making the whole fastener a bit stronger. When you mix the two threads on a stud basically it's the differences in the two threads that makes the connection less prone to loosen than either one of them used alone.

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Don-Wi

07-03-2005 21:16:16




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 Re: Coarse vs Fine Thread ? in reply to Got a Question for you !, 07-03-2005 19:37:34  
The biggest place where fine thread comes in handy is when you have a thin peice of metal you want to use a bolt on. With fine, you get more threads in the metal than the same size in coarse.



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Davis In SC

07-03-2005 19:52:55




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 Re: Coarse vs Fine Thread ? in reply to Got a Question for you !, 07-03-2005 19:37:34  
In Theory, the fine thread is stronger than the coarse.. But fine threads are much more sensitive to wide manufacturing tolerances..... A few thousandths oversize on the hole size is less critical , if hole is to be tapped with a coarse thread. Also, we have almost standardized on fractional sizes being coarse... 1/4-20, 5/16-18. 3/8-16, etc. Smaller sizes such as #10 are still found in fine & coarse.....

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Steve Crum

07-04-2005 05:18:06




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 Re: Coarse vs Fine Thread ? in reply to Davis In SC, 07-03-2005 19:52:55  
In practice I'm told, a fine thread is no stronger than a course thread. The machinist that I have do the precision tool and die work for the shop explained this to me. The primary advantage fine threads have over course threads is their capability of acheiveing and maintaining a true torque value within a tolerable plus or minus percentage. ie; if a course thread bolt at 100 ft.lbs backs off 10 degrees you could conceiveably lose 10% of the torque value and the bolt would be at 90 ft.lbs torque.
If a fine thread bolt were to back off the same 10 degrees you may only lose 2 ft.lbs. torque so the bolt would still be at a more acceptable 98 ft.lbs. torque.
The fine thread gives you more capability to fine tune the torque to exact values making the net parameter closer to the true torque value.
I'm sure you engineering types have a more precise and easier way of explaining this, this is just how it was explained to me so I could understand it.

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Joe(TX)

07-05-2005 09:30:36




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 Re: Coarse vs Fine Thread ? in reply to Steve Crum, 07-04-2005 05:18:06  
Not correct. In paractice it is stronger.
A fine thread bolt has a larger cross section through the threads and more area in thread shear(axial). That is why the torque values are higher. It will carry more load in tension and shear through the thread area.
Fine threads are less tolerant to dirt and to poor thread quality.



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Nebraska Cowman

07-03-2005 19:43:44




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 Re: Coarse vs Fine Thread ? in reply to Got a Question for you !, 07-03-2005 19:37:34  
You can get a fine thread tighter with the same amount of wrench pressure. And where a coarse thread bolt will twist off a fine thread under equal torque would stretch.



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Brian G. NY

07-04-2005 15:38:10




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 Re: Coarse vs Fine Thread ? in reply to Nebraska Cowman, 07-03-2005 19:43:44  
If you think of the thread as a road around the bolt you can see that the fine thread is not as steep as the coarse thread so the travelling is easier on the fine thread "road", hence more leverage and a tighter "draw" with the same amount of torque.



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