x2 - most lathes are metric the tap and dies are metric. It is much easier to simply work in metric, calculations to imperial is just extra work.Honestly, it is MUCH easier to stay with the measurement system of the tap & die.
In metric - a general rule is to make the tenon 0.1mm smaller than the nominal major diameter. So for your M14 x 0.8 x 3 die, make the tenon 13.9mm - and chamfer the end 2mm at 45 degrees so the die can glide on easier.
When you drill a hole to tap - use a drill bit that is diameter minus pitch: 14 - 0.8 = 13.2mm.
For the tenon - you can reduce the diameter a bit more if you feel comfortable doing so. I like to reduce it a little so the peaks of the threads are not sharp. It feels nicer, and because I polish the tenon before cutting the threads, the flat-tops are shiny so it looks nicer too.
TAP MAAT | SNIJTAP | ||
INCH | METRISCH | BOORMAAT | DECIMALE EQUIVALENT |
0-80 | 3/64 | .0469 | |
M1.6 X 0.35 | 1.25mm | .0492 | |
M1.8 X 0.35 | 1.45mm | .0571 | |
1-64 | 53 | .0595 | |
1-72 | 53 | .0595 | |
M2 X 0.40 | 1.60mm | .0630 | |
2-56 | 50 | .0700 | |
2-64 | 50 | .0700 | |
M2.2 X 0.45 | 1.75mm | .0689 | |
M2.5 X 0.45 | 2.05mm | .0807 | |
3-48 | 47 | .0785 | |
3-56 | 46 | .0810 | |
4-40 | 43 | .0890 | |
4-48 | 42 | .0935 | |
M3 X 0.50 | 2.50mm | .0984 | |
5-40 | 38 | .1015 | |
5-44 | 37 | .1040 | |
M3.5 X 0.60 | 2.90mm | .1142 | |
6-32 | 36 | .1065 | |
6-40 | 33 | .1130 | |
M4 X 0.70 | 3.30mm | .1299 | |
8-32 | 29 | .1360 | |
8-36 | 29 | .1360 | |
M4.5 X 0.75 | 3.70mm | .1476 | |
10-24 | 26 | .1470 | |
10-32 | 21 | .1590 | |
M5 X 0.80 | 4.20mm | .1654 | |
12-24 | 16 | .1770 | |
12-28 | 15 | .1800 | |
M6 X 1.00 | 5.00mm | .1969 | |
1/4-20 | 7 | .2010 | |
1/4-28 | 3 | .2130 | |
M7 X 1.00 | 6.00mm | .2362 | |
5/16-18 | F | .2570 | |
M8 X 1.25 | 6.70mm | .2638 | |
5/16-24 | I | .2720 | |
M8 X 1.0 | 7.00mm | .2756 | |
3/8-16 | 5/16 | .3125 | |
3/8-24 | Q | .3320 | |
M10 X 1.50 | 8.50mm | .3346 | |
M10 X 1.25 | 8.70mm | .3425 | |
7/16-14 | U | .3680 | |
7/16-20 | 25/64 | .3906 | |
M12 X 1.75 | 10.20mm | .4016 | |
1/2-13 | 27/64 | .4219 | |
M12 X 1.25 | 10.80mm | .4252 | |
M14 X 2.00 | 12.00mm | .4224 | |
1/2-20 | 29/64 | .4531 | |
9/16-12 | 31/64 | .4844 | |
9/16-18 | 33/64 | .5156 | |
5/8-11 | 17/32 | .5312 | |
5/8-18 | 37/64 | .5781 | |
M16 X 2.00 | 14.00mm | .5512 | |
M16 X 1.50 | 14.50mm | .5906 | |
M18 X 2.50 | 15.50mm | .6102 | |
M18 X 1.50 | 16.50mm | .6496 | |
3/4-10 | 21/32 | .6562 | |
3/4-16 | 11/16 | .6875 | |
M20 X 2.50 | 17.50mm | .6890 | |
45/64 | .7031 | ||
23/32 | .7188 | ||
M20 X 1.50 | 18.50mm | .7283 | |
M22 X 2.50 | 19.50mm | .7677 | |
M22 X 1.50 | 20.50mm | .8071 | |
7/8-9 | 49/64 | .7656 | |
7/8-14 | 13/16 | .8125 | |
M24 X 3.00 | 21.00mm | .8268 | |
M24 X 2.00 | 22.00mm | .8661 | |
1-8 | 7/8 | .8750 | |
1-12 | 59/64 | .9219 | |
M27 X 3.00 | 24.00mm | .9449 | |
M27 X 2.00 | 25.00mm | .9843 | |
1-1/8-7 | 63/64 | .9844 | |
M30 X 3.50 | 26.50mm | 1.0433 | |
1-1/8-12 | 1-3/64 | 1.0469 | |
M30 X 2.00 | 28.00mm | 1.1024 | |
1-1/4-7 | 1-7/64 | 1.1094 | |
M33 X 3.50 | 29.50mm | 1.1614 | |
1-1/4-12 | 1-11/64 | 1.1719 | |
1-3/8-6 | 1-7/32 | 1.2188 | |
>M33 X 2.00 | 31.00mm | 1.2205 | |
1-3/8-12 | 1-19/64 | 1.2969 | |
M36 X 4.00 | 32.00mm | 1.2598 | |
M36 X 3.00 | 33.00mm | 1.2992 | |
1-1/2-6 | 1-11/32 | 1.3438 | |
M39 X 4.00 | 35.00mm | 1.3780 | |
1-1/2-12 | 1-27/64 | 1.4219 | |
M39 X 3.00 | 36.00mm | 1.4173 |