Understanding screw sizing

[Swami805]

I've been buying screws by the set on eBay and I've gotten to the point where it would pay to buy in bulk for the ones I use the most. I bought a gauge but I'm not sure which numbers I should be using when ordering?  To measure length is it the total length from tip to tip or length of threads? For example I have a screw in the gauge in the pictures, what would that size be?
Thanks in advance

[PacRat]

It's 5-40. The length is measured from under the head to the tip except for a flat-head which is measured for the full length or from the mating surface to the total depth (this is true for oval-heads also). The thread length is another dimension but for reels we typically use all-thread.

If you look at McMaster-Carr you can reference the correct dimensions even if you don't purchase from then. When you click on the screw you are interested in there will be "Product Details". Open the details and you will get a CAD drawing with all the dimensions and information so that you will know how to specify the correct fastener.

[Swami805]

Perfect. Thank you!

[Gfish]

The number 5 the Swami pictured, it's 40 threads per inch. What about the diameter of 0.125"? Is it the space between the very outermost part of the shaft(the "peaks of the threads") or the diameter minus the thread space that you could measure at the very bottom of the screw?

[jurelometer]

The number 5 the Swami pictured, it's 40 threads per inch. What about the diameter of 0.125"? Is it the space between the very outermost part of the shaft(the "peaks of the threads") or the diameter minus the thread space that you could measure at the very bottom of the screw?

5 is also a diameter.  Under 1/4 inch, sizes are used instead of fractions of an inch.

.125 is the nominal size.  If you were cutting male threads on unthreaded rod, this is the size stock that you could use.  The actual outer diameter of of the male threads will be a bit smaller.

-J

[Gfish]

The number 5 the Swami pictured, it's 40 threads per inch. What about the diameter of 0.125"? Is it the space between the very outermost part of the shaft(the "peaks of the threads") or the diameter minus the thread space that you could measure at the very bottom of the screw?

5 is also a diameter.  Under 1/4 inch, sizes are used instead of fractions of an inch.

.125 is the nominal size.  If you were cutting male threads on unthreaded rod, this is the size stock that you could use.  The actual outer diameter of of the male threads will be a bit smaller.

-J

Ah, so after using a die to cut threads onto a 0.125" rod, the max. Outside diameter would be slightly less than 0.125"? Come-on Mr. Engineer, use 0's and units, ha,ha!

[jurelometer]

The number 5 the Swami pictured, it's 40 threads per inch. What about the diameter of 0.125"? Is it the space between the very outermost part of the shaft(the "peaks of the threads") or the diameter minus the thread space that you could measure at the very bottom of the screw?

5 is also a diameter.  Under 1/4 inch, sizes are used instead of fractions of an inch.

.125 is the nominal size.  If you were cutting male threads on unthreaded rod, this is the size stock that you could use.  The actual outer diameter of of the male threads will be a bit smaller.

-J

Ah, so after using a die to cut threads onto a 0.125" rod, the max. Outside diameter would be slightly less than 0.125"? Common Mr. Engineer, use 0's and units, ha,ha!

Yeah, it has to shave off a little.  If the male and female threads have  exactly matching  dimensions, you would not be able to screw them together.  The allowed gap range is specified by the thread class. 

Classes go from 1-3 (looser to tighter).  Only Class 2 is ever really used - unless you are getting into some pretty demanding situations like aerospace stuff, where Class 3 might be specified.

The inch system is just kind of weird.  Units below an inch are standardized on powers of two for a divisor ( half, quarters, eighths, etc.). This can cause problems when other specific sizes are needed, most frequently in smaller sizes.  There is the number ranges for thread diameter, and the unrelated overlapping number and letter range for drill bits.  I think that the number series for drill bits matches to the steel wire gauge number range and the letter series is a smidgen over the corresponding number size.  If you look up the thread size and class on a thread chart, you will see a corresponding tap drill size, often as a letter or number.

You can see how this was an ad hoc system that evolved based on need.

Another reason why the rest of world has gone metric. But we here in the USA are a stubborn bunch.

-J

[oc1]

Reel manufacturers have their screws produced in bulk to their specifications.  There specification is seldom a standard size.  It's always been that way.  I think they make their screws a bastard size just to control the parts distribution.

If you find something really close in a standard size you may be able to make it work.  However, the female and male threads will be buggered-up just enough that the original screw will not longer fit properly.

A century or so ago you had to be careful taking a reel apart to insure each screw goes back into it's original hole.  The manufacturing tolerances were not as good back then and a screw that was ever so slightly out of spec would be too loose or tight enought to deform the female threads.

[jurelometer]

Reel manufacturers have their screws produced in bulk to their specifications.  There specification is seldom a standard size.  It's always been that way.  I think they make their screws a bastard size just to control the parts distribution.

If you find something really close in a standard size you may be able to make it work.  However, the female and male threads will be buggered-up just enough that the original screw will not longer fit properly.

A century or so ago you had to be careful taking a reel apart to insure each screw goes back into it's original hole.  The manufacturing tolerances were not as good back then and a screw that was ever so slightly out of spec would be too loose or tight enought to deform the female threads.

My experience has been different.  I have made both inch and metric threaded parts for Penn, Daiwa, Shimano, and several fly reel manufacturers.  I used  standard sized taps, and they all fit very well with the OEM screws.
 

Some of the older Penn parts used less common thread sizes, so it sometimes takes a bit of hunting for taps and dies, or even a special order for the more oddball but still standard stuff like buttress threads for gear sleeves on the big reels.  I can't think of a reel screw that was not a reasonably common thread size.

The reel part side of the business is not particularly profitable, so it does not make sense to put more expensive proprietary thread sized screws into each reel that they sell.  I only know of one reel company that made the actual nuts and screws (Abel), and these were still standard sizes.

I don't work with the old stuff like Steve does, and I wouldn't be surprised if part of assembling a mid-priced 1920s reel at the factory included trying a couple different screws at times to get one that fit right.

-J