What does 4-40 mean when I'm looking to buy some machine screws?

February 2, 2006 11:24 PM

What does 4-40 mean when I'm looking to buy some machine screws? How big is that in real units?
posted by anonymoose to Grab Bag (13 answers total)

Best answer: did google not work for you?
posted by kcm at 11:33 PM on February 2, 2006

I just bought some 4-40 screws at the hardware store a few weeks ago to install a new fan inside my CPU. It is a measure of size - how big, I don't exactly recall, but the boxes at the store on the screw aisle were clearly labelled with 4-40.

I'm also thinking it's a measurement of diameter only, with different lengths available. Could be wrong.
posted by mdonley at 11:35 PM on February 2, 2006

Response by poster: Google did not, but thanks for the link.

What are major and minor diameters?
posted by anonymoose at 11:39 PM on February 2, 2006

major being the outside of the thread, and minor being the outside of the shaft.
posted by kcm at 11:41 PM on February 2, 2006

...and in almost all practical cases the "major and minor diameters" are completely academic and not useful in real life. The tap drill and clearance hole drill sizes are what actually matter if you are building something. They tell you the size of the hole you need to make if you intend to use a tap (i.e. create threads to accept the screw) or if you want the screw to pass through something unobscructed. If you're just talking about identifying the type of thread then "4-40" or perhaps "#4 course" (as opposed to 4-48 which would be #4 fine) is all you need, and everyone should know what you're talking about.
posted by Rhomboid at 12:34 AM on February 3, 2006

unobstructed
posted by Rhomboid at 12:34 AM on February 3, 2006

coarse
posted by benzo8 at 1:57 AM on February 3, 2006

Why not have major and minor? Because threads per unit extension are vital. Screws will either not fuction in the wrong thread-per-inch hole or they'll strip or they'll retap the hole in a way that the right size won't work anymore.

Think of it like stairs. You can specify a staircase's rise and run, but if there aren't enough steps over the distance you can't walk up it.
posted by plinth at 6:12 AM on February 3, 2006

Major and minor diameters are only useful numbers if you're cutting threads on a lathe. (Or, I suppose, if you're making a tap or die.) For most purposes, the clearance and tap-drill diameters are what you're interested in.

Note that screws develop their maximum holding power with three full threads of engagement. Extra length of engagement does not provide any increase in strength.

To learn all there is to know about screws, fasteners, and metalworking, get a used copy of Machinery's Handbook.

And if you are creating a tapped hole for a #6 screw, where you get to choose the thread pitch, for God's sake, use a 6-40 tap and screw. 6-32 is the worst thread form on the planet, and responsible for more broken taps and stripped threads than any other.
< /pet peeve>
posted by Kirth Gerson at 6:49 AM on February 3, 2006

Well 6-32 does have the advantage that a full turn will bring the screw head 1/32" closer to the object. -32 and -16 threads are very useful for making adjustable jigs where the objects being jigged are measured in fractions of an inch.
posted by Mitheral at 11:32 AM on February 3, 2006

Best answer: I recently did a blog post on this subject, 4 is the diameter, 40 is the number of threads per inch. To convert a screw size in number to diameter you multiply the number by .013" and add .060". So a number 4 screw is 4*.013 + .06 = .112". You now know something that even most machinist don't.
posted by 445supermag at 12:21 PM on February 3, 2006

Mitheral, an 8-32 or 10-32 screw has the identical quality, and enough beef in the minor diameter that you're much less likely to break a tap making the threads. The only time I used that effect, it was better to use 20 TPI, for 0.05" increments.

Supermag, you're right, I didn't know that.
posted by Kirth Gerson at 7:49 PM on February 3, 2006

As an aside, as an example of Kirth's point about fine thread sizes breaking less taps, rifle scopes are mounted with 6-48 (#6 extra-fine), probably because recievers are fairly hard and gunsmiths really don't want to break off a tap in someone's custom rifle.
posted by 445supermag at 12:21 PM on February 4, 2006