Deciding Coarse Or Fine Thread ?

[BillWood]

Good Morning,

Was wondering how does a machinist decide what tpi thread to use for a particular job.

Currently in my head I see things as follows

Fine threads - used for both measuring tools and fastening i.e. at 40tpi 1 full turn = 25 thousandths so fairly easy to graduate a dial into thousandths when screwed onto a 40 tpi thread

Fine threads - also used when a thin bolt is needed, intuition and common sense indicates that a 10 tpi thread on a 1/8" diameter bolt is just not going to work

Coarse threads - used for fastening rather than measuring and used on thicker stock - i.e. 10 tpi on a 1" diameter bolt seems ok to me. It would takes aaaaages to tighten up big 2" diameter bolts holding an iron bridge together at 40 tpi.

And there must be some sort of grey area "in between"

The reason I ask is that I have made a drawbar for a chuck backstop and I noticed that in George Thomas book he recommended 32 tpi thread at the back end. See photo. I get the impression George Thomas was a perfectionist and paid great attention to tiny details.

My drawbar is 0.740" diameter and this seems to be exactly right to cut 10tpi and I just happen to have a spare 10pi nut just sitting around doing nothing.

At 32tpi would there be a lot more contact area therefore more friction therefore lower probability of any vibration loosening the thread ?

Pondering on these things makes me ask the following questions -

Any good web sites / previous threads / books that cover the logic behind the tpi and tooth depth for a threaded bar i.e. any thumb rules - for example, tooth depth must be <10% of stock diameter ?

For a secure nut on a drawbar would you go for more rather than less tpi ? Given the time I've put in so far I dont want to ruin this backstop at the last minute.

Bill

 

[T Bredehoft]

The potential fail point of a thread is the imaginary cylinder of the pitch diameter. A fine thread has a larger pitch diameter than a coarse thread, consequently a greater area of potential shear. So when this is the only consideration, fine pitch is the pitch of choice. Another consideration might be speed of removal/attachment, which would dictate a coarser pitch.

 

[Ulma Doctor]

the more threads per inch present will prevent fasteners from backing out better as well as having more thread surface area in contact.

edit:
tom was writing while i was still thinking...
sorry for the echo

 

[The Liberal Arts Garage]

Please also consider the unit strength of both the male and female threads. The
unit strength. For instance, threads in an aluminum engine block will have lower
Pull- out strength with a fine thread than a coarser one; a good compromise
would be, for instance, a HELICOIL insert. A subject of much discussion in
technical literature. You should read up on it.

 

[mikey]

Hi Bill, as I'm sure you've done, there are all sorts of information on coarse vs fine threads. Having used backstops similar in concept, I would also go with a finer thread. In addition to being stronger and less prone to loosening from vibration as Tom and UlmaDoc have said, it will require less torque to tighten down on the stop itself; this reduces the tendency for the stop to move as you lock everything down. It will also be stronger given the thinner wall section you have.

If this was my project, I think I would go with at least 28 tpi. Screwcutting a nice nut with a Class 3 fit should be simple enough and you can knurl it like GHT would do.

Good project by the way. This sort of device has really saved my bacon more than a few times.

 

[joshua43214]

Just to toss another thing in about fine threads. There is a certain amount of radial slippage in all the components as the screw is tightened. Because fine threads move less linear distance per rotation, they do a better job at getting washers seated in and the screw head running true.
Coarse threads will have a tendency to impact the surface and not allow the bolt head to slide around in order to center up. This can result in vibration causing the preload on the screw to be lost when the screw centers up, and then the screw coming loose. Coarse threads are really good in situations where there is a tendency to over torque the bolt, like in construction.

 

[4GSR]

Go with 3/4-16 UNF thread on your draw bar. That's the fine thread for 3/4" diameter. You can easily find a nut for 3/4-16 thread. 3/4-32 is not a standard thread pitch for 3/4" diameter. I think you will find 32 tpi may gauld on you from use. I have a 1/2-20 UNF thread on the upper end of my draw. So far for 12 years still holding up.

 

[dave2176]

Don't want to hijack the thread but I don't understand something on this subject. If fine threads are superior for alignment on tightening and resisting loosening under vibration why would engines (automotive) be built with coarse threads in critical locations like heads and bearing caps? Just hoping to understand.
Dave

 

[John Hasler]

Don't want to hijack the thread but I don't understand something on this subject. If fine threads are superior for alignment on tightening and resisting loosening under vibration why would engines (automotive) be built with coarse threads in critical locations like heads and bearing caps? Just hoping to understand.
Dave

IIRC the threads on the headbolts on all my tractors are UNF.

 

[Uglydog]

Please note the attached excerpt from Machinery Handbook 28th Edition. If you don't have one, consider picking one up.
Old ones are cheap. Well, not the really old ones. Fascinating reading. Seriously!
Of interest are the few sentences which highlight everything written above, with the interesting addition of some information about metal stock selection.

Daryl
MN