Large pipe taps can require substantial spindle torque. The selected figures below are historical test results for specific taps and materials, not a universal horsepower chart. Use them to understand how test conditions change the load; do not use them alone to size a machine or choose a motor.
Selected test results
The source reports net power for large nominal pipe taps run in reamed holes. Most listed trials used cast iron; two marked cast-steel trials used a different material. The tap construction, metal thickness and speed are shown because a horsepower value without those details invites a misleading comparison.
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Within the selected 2 1/2-inch examples, the cast-steel test reported 9.14 hp at 38.5 rpm, compared with 5.15 hp for cast iron at 40 rpm. The source notes that cast steel required nearly twice the power in these trials. That does not mean every cast-steel tapping job needs exactly that ratio: the material grade, hole preparation, tool geometry, lubrication, engagement and machine control all matter.
Power and torque are not interchangeable
Rotational power is related to torque and speed. For a given power, lower RPM means higher torque at the spindle. A large tap may therefore challenge the gearbox, tap holder or spindle even when a motor’s nameplate horsepower seems adequate. Starting, reversing, synchronized feed and stopping loads also matter. The test table reports net cutting power and excludes the power used to run the machine without load.
Do not size a drive by matching a historical net-power number to a motor nameplate. Transmission losses, duty cycle, overload capacity, spindle speed range, reverse capability, rigidity and tap-holder limits must be considered. For production work, use the tap manufacturer’s application data and the machine builder’s rating. For one-off work, a qualified machinist should verify the setup, tool and workholding before cutting.
Why the values vary
Tap torque changes with work material, hardness, thread engagement, pitch, tap style, cutting-edge condition, lubrication and chip evacuation. A worn or damaged tap can raise torque; a chip-packed flute can bind and break the tool. A forming tap is not a drop-in substitute for a cutting tap: it displaces material rather than removing chips and requires the manufacturer’s specified pilot-hole size and suitable material ductility.
OSG describes its XPF forming tap as a product-specific low-torque design and lists material and size limits for that series. Such a claim applies to that tool family and its stated conditions, not to all forming taps. The maker’s own cutting data should control a current job. The published metric and SAE tap-drill chart helps with pilot-hole selection; it does not predict tapping horsepower. For pipe-thread context, see common pipe-fitting types.
Use the test table responsibly
Keep the historical test conditions attached to each value. The source describes reamed holes and inserted-blade high-speed-steel taps and distinguishes the cast-steel rows. It also warns that harder cast iron or taps without suitable relief may require more power. The examples stop at selected entries and should not be interpolated into a complete sizing model.
Before a large-tap operation, identify the exact thread, tap type, material specification, hole diameter, engagement length and depth. Confirm the spindle can deliver the required torque at the chosen speed and that the holder can safely transmit it. Set the feed to match the thread pitch, use the recommended lubricant and provide a method for chip control. Stop if torque rises unexpectedly, the tap chatters or the machine loses speed.
Related thread references
For a tap-and-die set overview, see the tap-and-die buying guide. Use that page for product selection, this reference for test-load context, and the applicable tap maker’s data for the actual operation.
Historical test values: Machinery’s Handbook, 31st edition, “Power Required for Pipe Taps,” printed page 2177. For current tap selection, consult the tap maker’s specifications, such as OSG XPF specifications.
