Machine Screw Diameters
By Gordon DeWitte
You Toolmongers have been holding out on me. How come no one told me — or, perhaps worse, why didn’t I discover before — that there are simple formulas for determining the nominal diameters and clearance holes of (Unified Thread Standard) machine screw sizes 0–14*? All those years I spent looking up that little table, or trying to find that plastic gauge thingy…
Specifically, the nominal diameter in inches for a size N machine screw is: .060″ + (N × .013″), and the clearance hole is: (N +4)/64. For example, a #6 machine screw has a nominal diameter of .060″ + 6 × .013″ = .138″, and its clearance hole is 10/64 = .156″
If you work with really small machine screws like #00 and #000, the formulas also work if you set N to -1 and -2 for screw sizes 00 and 000, respectively.
What other clever, or quick, or back-of-the-envelope calculations should I know about?
*and, yes, I know machine screw sizes 12 & 14 are rare; the next “standard” size above #10 is usually ¼-20.
Machine Screw Table [PDF]
Unified Thread Standard [Wikipedia]
September 22nd, 2009 at 1:38 pm
Harbor Freight has a plastic gauge for screws, bolts and nuts.
$1.99
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=39240
September 22nd, 2009 at 2:28 pm
Nice going, Gordon. You’ve blown our cover.
“You have not much future there. It will happen this way. You may be walking. Maybe the first sunny day of the spring. And a car will slow beside you, and a door will open, and someone you know, maybe even trust, will get out of the car. And he will smile, a becoming smile. But he will leave open the door of the car and offer to give you a lift.”
(– 3 Days of The Condor)
September 22nd, 2009 at 2:50 pm
Here is one that made me feel pretty dumb when I learned it-
Tap drill size (for approx 75% thread) = nominal size - thread pitch
so tap drill size for M6×1 = 5mm (.197)
for 1/4-20 = .250 - (1/20) = .200
This works for cutting taps, of course, not form taps, so you no longer need a tap drill chart.
September 22nd, 2009 at 3:52 pm
Don’t most drill indeces have this info stamped on them? I have never had to look it up. The bit and the information are always together.
September 22nd, 2009 at 3:58 pm
Screw number times .013 plus .060 equals diameter in thousandths.
September 22nd, 2009 at 4:04 pm
Oh, sorry, I didn’t read it and see that you’d already posted that, Gordon.
September 23rd, 2009 at 6:16 am
Why make life complicated? The nominal diameter of a size N screw in mm is N.
http://en.wikipedia.org/wiki/ISO_metric_screw_thread
October 1st, 2009 at 5:14 am
This calculation part is very tough actually.