FIFTY LINETurn G-code
RU
Online generatorOnline calculatorsReference

UNC and UNF thread chart for CNC lathes: TPI, pitch, depth and G-code

· ~11 min read

Most thread charts are made for tapping: size, TPI, tap drill. On a CNC lathe you need different numbers - how deep the tool goes, what the minor diameter is, how to split the depth into passes, and what to put in the G-code. This chart covers the common UNC (coarse) and UNF (fine) sizes from #4 to 1 inch with exactly those values.

Contents

  1. How to read an inch thread size
  2. Formulas
  3. UNC chart
  4. UNF chart
  5. Splitting the depth into passes
  6. G76 example: 1/2-13 UNC in LinuxCNC
  7. Single passes with G33
  8. Practical notes
  9. Inch threads in Turn G-code

How to read an inch thread size

1/2-13 UNC-2A means:

  • 1/2 - nominal major diameter, 0.5 in. Small sizes use numbers: #10 is 0.190 in;
  • 13 - threads per inch (TPI). The pitch is 1/13 in = 0.0769 in = 1.954 mm;
  • UNC - Unified coarse series, UNF - fine series. Both have the same 60° profile as metric threads;
  • 2A - tolerance class: A is an external thread (bolt), B is internal (nut); 2 is the common class, 3 is tighter.

Formulas

The profile is 60°, the same as ISO metric, so the formulas are the same. P is the pitch, D the major diameter:

Value Formula 1/2-13 UNC
Pitch P 1 / TPI 0.0769 in (1.954 mm)
External thread depth h 0.6134·P 0.0472 in
External minor diameter D − 1.2269·P 0.4056 in
Internal thread depth 0.5413·P 0.0416 in
Internal minor diameter (bore) D − 1.0825·P 0.4167 in

The external depth 0.6134·P is the usual shop value for a full profile. The standard gives every diameter as a tolerance range for each class, so the final check is always a thread ring or plug gauge, or the mating part. In practice the major diameter of a bolt is turned a few thousandths under nominal, and the bore for an internal thread is made at or slightly above the internal minor diameter.

UNC chart

Coarse series. Diameters and depths in inches; the mm columns are the same values converted, for metric-minded machines.

Thread Major Ø, in TPI Pitch, in Pitch, mm Ext. depth, in Ext. minor Ø, in Int. depth, in Int. minor Ø, in Ext. depth, mm Ext. minor Ø, mm
#4-40 0.112 40 0.0250 0.635 0.0153 0.0813 0.0135 0.0849 0.39 2.07
#6-32 0.138 32 0.0313 0.794 0.0192 0.0997 0.0169 0.1042 0.49 2.53
#8-32 0.164 32 0.0313 0.794 0.0192 0.1257 0.0169 0.1302 0.49 3.19
#10-24 0.190 24 0.0417 1.058 0.0256 0.1389 0.0226 0.1449 0.65 3.53
1/4-20 0.250 20 0.0500 1.270 0.0307 0.1887 0.0271 0.1959 0.78 4.79
5/16-18 0.3125 18 0.0556 1.411 0.0341 0.2443 0.0301 0.2524 0.87 6.21
3/8-16 0.375 16 0.0625 1.588 0.0383 0.2983 0.0338 0.3073 0.97 7.58
7/16-14 0.4375 14 0.0714 1.814 0.0438 0.3499 0.0387 0.3602 1.11 8.89
1/2-13 0.500 13 0.0769 1.954 0.0472 0.4056 0.0416 0.4167 1.20 10.30
9/16-12 0.5625 12 0.0833 2.117 0.0511 0.4603 0.0451 0.4723 1.30 11.69
5/8-11 0.625 11 0.0909 2.309 0.0558 0.5135 0.0492 0.5266 1.42 13.04
3/4-10 0.750 10 0.1000 2.540 0.0613 0.6273 0.0541 0.6417 1.56 15.93
7/8-9 0.875 9 0.1111 2.822 0.0682 0.7387 0.0601 0.7547 1.73 18.76
1-8 1.000 8 0.1250 3.175 0.0767 0.8466 0.0677 0.8647 1.95 21.50

UNF chart

Fine series. Same columns.

Thread Major Ø, in TPI Pitch, in Pitch, mm Ext. depth, in Ext. minor Ø, in Int. depth, in Int. minor Ø, in Ext. depth, mm Ext. minor Ø, mm
#4-48 0.112 48 0.0208 0.529 0.0128 0.0864 0.0113 0.0894 0.32 2.20
#6-40 0.138 40 0.0250 0.635 0.0153 0.1073 0.0135 0.1109 0.39 2.73
#8-36 0.164 36 0.0278 0.706 0.0170 0.1299 0.0150 0.1339 0.43 3.30
#10-32 0.190 32 0.0313 0.794 0.0192 0.1517 0.0169 0.1562 0.49 3.85
1/4-28 0.250 28 0.0357 0.907 0.0219 0.2062 0.0193 0.2113 0.56 5.24
5/16-24 0.3125 24 0.0417 1.058 0.0256 0.2614 0.0226 0.2674 0.65 6.64
3/8-24 0.375 24 0.0417 1.058 0.0256 0.3239 0.0226 0.3299 0.65 8.23
7/16-20 0.4375 20 0.0500 1.270 0.0307 0.3762 0.0271 0.3834 0.78 9.55
1/2-20 0.500 20 0.0500 1.270 0.0307 0.4387 0.0271 0.4459 0.78 11.14
9/16-18 0.5625 18 0.0556 1.411 0.0341 0.4943 0.0301 0.5024 0.87 12.56
5/8-18 0.625 18 0.0556 1.411 0.0341 0.5568 0.0301 0.5649 0.87 14.14
3/4-16 0.750 16 0.0625 1.588 0.0383 0.6733 0.0338 0.6823 0.97 17.10
7/8-14 0.875 14 0.0714 1.814 0.0438 0.7874 0.0387 0.7977 1.11 20.00
1-12 1.000 12 0.0833 2.117 0.0511 0.8978 0.0451 0.9098 1.30 22.80

Values are calculated from the formulas above and rounded to 0.0001 in / 0.01 mm. Check critical parts against the standard tables or a gauge.

Splitting the depth into passes

The full depth is never cut in one pass. With a constant depth per pass, each pass removes more metal than the last, so the depth is reduced as you go. The common rule is constant chip area: depth after pass n = h × √(n / N), where N is the number of passes.

1/2-13 UNC external, h = 0.0472 in, 8 passes:

Pass Total depth, in Diameter, in
1 0.0167 0.4666
2 0.0236 0.4528
3 0.0289 0.4422
4 0.0334 0.4333
5 0.0373 0.4254
6 0.0409 0.4183
7 0.0441 0.4117
8 0.0472 0.4056

Add one spring pass at the final diameter to take out the springback. The number of passes depends on the material, the insert and the rigidity of the machine - coarse threads in stainless on a light lathe need more, brass needs fewer. Your insert maker’s catalog is a good starting point.

The thread calculator builds this pass table for any size in the charts above, nuts included.

G76 example: 1/2-13 UNC in LinuxCNC

External 1/2-13 thread, 0.75 in long, inch mode G20, radius mode G8 (X is the radius). Zero is on the face and the centerline; the major diameter is already turned.

G8 G18 G20 G90 G40      (radius, XZ plane, inch, absolute)
G97 S300 M3             (constant RPM)
G0 X0.3 Z0.2            (drive line: radius 0.3 = D0.6, 0.2 in off the face)
G76 P0.076923 Z-0.75 I-0.05 J0.008 K0.0472 R2 Q29.5 H1 E0.08 L2
G0 X1 Z1                (retract)
M5
M30
  • P0.076923 - the pitch, 1/13 in, with enough decimals (see Inch threads in LinuxCNC on why 0.0769 is not quite enough on long threads);
  • I-0.05 - the thread peak is 0.05 in below the drive line, at radius 0.25 (Ø0.5); negative I means an external thread;
  • J0.008 - first pass depth, K0.0472 - full depth from the chart;
  • R2 - constant chip area, Q29.5 - flank infeed;
  • H1 - one spring pass, E0.08 L2 - a smooth run-out at the end.

At S300 the Z axis moves at 300 / 13 = 23 in/min - check that your machine can do that comfortably. This is a teaching example: before cutting, check the G7/G8 mode, the zero and the tool offset, and run it once “cutting air” with extra X clearance. All G76 parameters are explained in CNC lathe threading in LinuxCNC: G33 and G76.

Single passes with G33

The same thread without G76 - each pass written out with G33 in diameter mode G7, using the diameters from the pass table:

G7 G18 G20 G90 G40
G97 S300 M3
G0 X0.6 Z0.2
(Pass 1)
G0 X0.4666
G33 Z-0.75 K0.076923
G0 X0.6
G0 Z0.2
(Pass 2)
G0 X0.4528
G33 Z-0.75 K0.076923
G0 X0.6
G0 Z0.2
(... passes 3-8 down to X0.4056, then a spring pass at X0.4056)

Longer, but every pass is visible, and the same code with G32 and the pitch letter your control expects runs on controls that do not have G76.

Practical notes

  • Run-in. Start 2-3 pitches off the part (0.2 in for 13 TPI) so the Z axis is up to speed when the tool reaches the metal.
  • Major diameter. Turn the bolt a few thousandths under nominal - a thread on a full-size blank is tight and has a sharp crest.
  • Internal threads. Bore the hole to the internal minor diameter from the chart (not to a tap drill size), cut with increasing diameter, and make sure the boring bar clears the bore during the retract.
  • Inch thread on a metric machine. Either switch the program to G20, or stay in G21 and use the pitch in mm: K = 25.4 / TPI. Both are covered in Inch threads in LinuxCNC.
  • Pipe threads (NPT, NPTF, BSPT) are tapered and have their own charts - see Tapered threads on a CNC lathe.

Inch threads in Turn G-code

Turn G-code has a mm / inch switch for all cycles: in inch mode the program outputs G20 and every value in inches. The Threading cycle takes the diameter, pitch, length, number of roughing and finishing passes, run-in and run-out, splits the depth across the passes, and shows them in the 2D simulator before you get the code. Every pass is written out with G33 or G32 and the pitch letter K, I or F - to match your control.

You can try the inch mode right now in the free online G-code generator (OD turning cycle, no sign-up), or download Turn G-code - 15 days free, no key needed.

← All articles