What RPM for a 1-3/8-6 tap?

RPM = 3.82 × SFM ÷ D with D = 1.3750 in. With a general-purpose HSS tap that is about 190 RPM in 6061 aluminum (70 SFM) and 60 RPM in mild steel (20 SFM). Premium coated taps on a rigid CNC can run faster; see the second table.

What is the feed rate for a 1-3/8-6 tap?

Feed per revolution equals the pitch: 0.1667 in (1 ÷ 6 TPI). IPM = RPM ÷ 6, so at 190 RPM the feed is 31.7 IPM.

How fast can I run a 1-3/8-6 form (roll) tap?

OSG lists 65–115 SFM for form taps in aluminum alloys, which is 180–320 RPM for 1-3/8-6. Form taps need a larger hole: about 1-19/64 (1.2969 in).

What speed for a 1-3/8-6 tap in stainless steel?

General-purpose HSS taps: 10–20 SFM, or 30–60 RPM at 5.00–10.0 IPM. Keep the tap cutting and use 60–65% thread to cut torque.

1-3/8-6 tap speeds and feeds

A 1-3/8-6 tap feeds one pitch per turn: 0.1667 in (1 ÷ 6 TPI) per rev. With a general-purpose HSS tap, start near 190 RPM and 31.7 IPM in 6061 aluminum, or 60 RPM and 10.0 IPM in mild steel.

UNC thread · major Ø 1.3750 in · tap drill 1-7/32 (1.2188 in) for a cut tap, 1-19/64 for a form tap.

1-3/8-6 tapping speeds by material

Cut tap = general-purpose HSS tap. Form tap = OSG form (roll) taps. RPM rounded to 10; IPM = RPM ÷ 6. Start at the low end.

1-3/8-6 cut and form tap speeds
Material	Cut SFM	Cut RPM	Cut IPM	Form SFM	Form RPM	Form IPM
Aluminum 6061 / 7075	70–90	190–250	31.7–41.7	65–115	180–320	30.0–53.3
Free-machining / mild steel (12L14, 1018)	20–40	60–110	10.0–18.3	50–115	140–320	23.3–53.3
4140 alloy steel (annealed to <35 HRC)	20–30	60–80	10.0–13.3	50–85	140–240	23.3–40.0
304 / 316 stainless	10–20	30–60	5.00–10.0	15–40	40–110	6.67–18.3
Tool steel (A2, D2, O1, H13, P20)	15–25	40–70	6.67–11.7	8–25	20–70	3.33–11.7
Gray / ductile cast iron	15–30	40–80	6.67–13.3	Not listed	Not listed	Not listed
Brass / bronze	60–100	170–280	28.3–46.7	Not listed	Not listed	Not listed
Titanium (6Al-4V, CP)	5–15	10–40	1.67–6.67	8–15	20–40	3.33–6.67
Plastics and composites (acetal, nylon, G-10)	50–70	140–190	23.3–31.7	Not listed	Not listed	Not listed

Not listed = no form-tap value in the cited tables (OSG lists no form tap for cast iron; brass and plastics are not in its form-tap table).

Premium coated cut taps on a rigid CNC

OSG's starting speeds for its premium HSS-E cut taps (A-Tap family and the titanium/stainless taps it names) are higher than the general-purpose row. Use them only with that class of tap and rigid tapping.

1-3/8-6 premium cut tap speeds (OSG)
Material	OSG row	SFM	RPM	IPM
Aluminum 6061 / 7075	Aluminum alloys	70–120	190–330	31.7–55.0
Free-machining / mild steel (12L14, 1018)	Carbon steels <35 HRC	80–120	220–330	36.7–55.0
4140 alloy steel (annealed to <35 HRC)	Alloy steels <35 HRC	35–65	100–180	16.7–30.0
304 / 316 stainless	Stainless 300 & 400 series <35 HRC	15–75	40–210	6.67–35.0
Tool steel (A2, D2, O1, H13, P20)	Alloy, tool & die steels 30–40 HRC	15–30	40–80	6.67–13.3
Gray / ductile cast iron	Cast iron	50–120	140–330	23.3–55.0
Titanium (6Al-4V, CP)	Titanium alloys <35 HRC	15–20	40–60	6.67–10.0

Worked example: 1-3/8-6 in 6061 aluminum

Surface speed: 70 SFM (low end of 70–90 for a general-purpose HSS tap).
RPM = 3.82 × SFM ÷ D = 3.82 × 70 ÷ 1.3750 ≈ 190 RPM.
Feed per rev = pitch = 0.1667 in (1 ÷ 6 TPI).
IPM = RPM ÷ TPI = 190 ÷ 6 = 31.7 IPM.

G84 tapping code for 1-3/8-6

Rigid tapping, aluminum numbers from the example, inch (G20), depth 1.5 × D. Check your control's manual before running.

Fanuc rigid tap (M29)

(1-3/8-6 TAP - RIGID)
G20 G17 G40 G80 G90
T5 M06
G54 G00 X0. Y0.
G43 H05 Z1.0 M08
M29 S190           (RIGID TAP MODE; M-CODE SET BY PARAMETER)
G98 G84 Z-2.06 R0.1 F31.67
G80 M09
G91 G28 Z0.
M30

Haas rigid tap

G20 G00 G90 G54 X0. Y0.
G43 H05 Z0.1 M08
G84 Z-2.06 R0.1 F31.67 S190   (NO M03 NEEDED)
(PECK: ADD Q = PECK DEPTH; J2 = RETRACT 2X, SEE SETTING 130)
G80 G00 Z1. M09

Floating / tension-compression holder

Without rigid tapping, run G84 with the spindle on (M03 S190) and the same F31.67 feed, but hold the tap in a tension-compression holder: it absorbs the overrun when the spindle stops and reverses. Leave the holder room to float at the bottom, so program the depth a little short and follow the holder maker's setting.

Peck tapping (Q) clears chips in deep or blind holes in stainless and 4140; skip it in aluminum unless chips pack.
Left-hand 1-3/8-6-LH taps use G74 instead of G84.
If the control caps rigid-tap RPM, lower S and recompute F: F = S ÷ 6.

Notes for this size

1-3/8-6 needs real torque: check that your spindle makes enough torque at 60 RPM (a gear-range spindle may need its low range). Thread milling is often the safer choice in tough material at this size.

4140 alloy steel (annealed to <35 HRC): Prehardened 4140 (28–32 HRC) is at the top of this band: use the low end and 50–65% thread.
304 / 316 stainless: Work-hardens: never let the tap dwell or rub, and use a tap made for stainless. 60–65% thread cuts torque sharply.
Tool steel (A2, D2, O1, H13, P20): Values are for annealed or prehardened stock. Tap before heat treat whenever you can; hardened tool steel needs a thread mill or a carbide tap.

Hole and tools for 1-3/8-6

Tap drill, cut tap 75%: 1-7/32 (1.2188 in)
Form tap drill (65%): 1-19/64 (1.2969 in)
50% thread drill for hard or gummy stock: 1-17/64 (1.2656 in)

Tap calculator, prefilled for 1-3/8-6 1-3/8-6 tap drill size Tap drill chart

Common questions

What RPM for a 1-3/8-6 tap?: RPM = 3.82 × SFM ÷ D with D = 1.3750 in. With a general-purpose HSS tap that is about 190 RPM in 6061 aluminum (70 SFM) and 60 RPM in mild steel (20 SFM). Premium coated taps on a rigid CNC can run faster; see the second table.
What is the feed rate for a 1-3/8-6 tap?: Feed per revolution equals the pitch: 0.1667 in (1 ÷ 6 TPI). IPM = RPM ÷ 6, so at 190 RPM the feed is 31.7 IPM.
How fast can I run a 1-3/8-6 form (roll) tap?: OSG lists 65–115 SFM for form taps in aluminum alloys, which is 180–320 RPM for 1-3/8-6. Form taps need a larger hole: about 1-19/64 (1.2969 in).
What speed for a 1-3/8-6 tap in stainless steel?: General-purpose HSS taps: 10–20 SFM, or 30–60 RPM at 5.00–10.0 IPM. Keep the tap cutting and use 60–65% thread to cut torque.

Nearby sizes

Sources

Productivity Inc. tool catalog, Tech Info - Taps: General Purpose & Other Tap Speeds (pp. 405-416)
OSG USA, A Brand & Premium Tooling Recommendation Guide Vol. 4 (Threading and Drilling), EDP 800313CA-V4, 2026
Haas Automation, G84 Tapping Canned Cycle (Group 09), mill G-code reference
Thread dimensions and tap drills: Machinery's Handbook, 31st ed.; ASME B1.1 Unified Inch Screw Threads.

SFM ranges are copied from the tables above; RPM and feed are computed for 1-3/8-6 (D = 1.3750 in). Speeds are starting values: your tap maker's data, coolant and machine come first.

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