Power draw versus cutting force: sizing the spindle for heavy roughing

Technical FoundationConsiderationNovember 06, 2026 · 1,788 words · 8 min read

If roughing is light or intermittent, the standard spindle is the safer buy and the premium will not pay back.

Most shops that rough steel for more than about a third of spindle-on time will find a high-torque spindle is the lower seven-year cost, but only if the machine is a bottleneck with extra work waiting. If roughing is light or intermittent, the standard spindle is the safer buy and the premium will not pay back.

This article compares two plausible spindle choices for a vertical machining centre in a DACH or UK machine shop. Choice A is a standard 15 kW spindle. Choice B is a high-torque 22 kW spindle. The comparison runs over seven years.

Two plausible spindle choices

Choice A: a standard spindle rated at 15 kW peak, with about 120 Nm continuous torque at 800 rpm and an ISO 40 taper. This is a common layout for general milling, small-batch work and occasional roughing.

Choice B: a high-torque spindle rated at 22 kW peak, with about 200 Nm continuous torque at 800 rpm. It may use the same machine frame but a larger motor, a heavier bearing pack, and a stronger spindle taper — usually ISO 40 with a reinforced drawbar or HSK-A63.

The purchase price difference is typically between €12,000 and €25,000. The exact price depends on the machine builder and options. All numbers in this article are typical industry ranges, not published Gree CNC specifications. Confirm the torque curve and price against the official catalogue.

Power draw, cutting force and torque: what you actually size

Buyers often compare spindle power in kW. For heavy roughing, that is the wrong first check. Power draw, cutting force and torque are related but not interchangeable.

• Power draw (kW): the electrical power the spindle motor takes from the mains, measured at the machine’s power meter.
• Cutting force (N): the mechanical load at the cutting edge. It pushes against the tool tip and spindle shaft.
• Torque (Nm): twisting force on the spindle. Torque = cutting force × tool radius in metres.
• Duty cycle: how long the spindle can hold a load without overheating. S1 means continuous. S6 40% means 4 minutes loaded and 6 minutes rest within a 10-minute cycle.
• Base speed: the rpm at which the spindle motor changes from constant torque to constant power. Below base speed, torque is constant and power falls with rpm. Above base speed, power is constant and torque falls.

A 15 kW spindle motor with a base speed of 1500 rpm delivers 95.5 Nm continuous torque from 0 to 1500 rpm. At 800 rpm, its continuous power is only about 8 kW. The 15 kW number applies at base speed and above. If a buyer reads “15 kW” and assumes 15 kW is available at 800 rpm, the mistake leads to spindle stalls, alarm shutdowns or shortened motor life.

Heavy roughing in steel with an 80 mm diameter shoulder mill can easily need 140 Nm to 200 Nm of continuous torque. Tool radius is 0.04 m. At 3,500 N cutting force, required torque is 3,500 × 0.04 = 140 Nm. A standard 120 Nm continuous spindle will alarm or overheat on a long roughing pass. The high-torque spindle at 200 Nm holds the cut within its S1 rating.

Seven-year cost model: assumptions

The table below shows the base case. It assumes one machine, 2,000 spindle-on hours per year, 40% of that time in heavy roughing of steel, and the same finished output volume for both choices.

Parameter	Choice A: standard	Choice B: high-torque	Source/status
Peak spindle power	15 kW	22 kW	Typical class; confirm official catalogue
Continuous torque at 800 rpm (S1)	120 Nm	200 Nm	Typical class; confirm torque curve
Purchase price	€85,000	€103,000	Example for 1 m VMC class; quote dependent
Roughing share of spindle-on time	40%	40%	Assumption
Cycle time reduction on roughing ops with B	0%	15%	Field estimate, not catalogue data
Average roughing power draw	11 kW	14 kW	Estimated from field data; may vary
Non-roughing average power draw	3.0 kW	3.0 kW	Estimated; confirm with energy log
Electricity price	€0.18/kWh	€0.18/kWh	DACH/UK industrial average
Annual roughing tooling cost	€8,000	€7,360	Assumption; B gives 8% lower tool cost
Annual preventive maintenance cost	€650	€850	Typical range; confirm service plan
Residual value after 7 years	€22,000	€26,000	Typical used market range, not guaranteed

The premium for Choice B is €18,000. That figure is a typical range, not a fixed quotation.

Seven-year cost result, before throughput value

The table below sums seven years of costs for the base case. It excludes finance interest, inflation, downtime and toolholder change costs.

Cost item over 7 years	Choice A	Choice B	Difference B–A
Purchase price	€85,000	€103,000	+€18,000
Electricity	€15,624	€16,531	+€907
Roughing tooling	€56,000	€51,520	-€4,480
Preventive maintenance	€4,550	€5,950	+€1,400
Residual value at year 7	-€22,000	-€26,000	-€4,000
Net 7-year cost, fixed workload	€139,174	€151,001	+€11,827

With no credit for saved machine hours, Choice A wins. The high-torque spindle costs about €11,827 more over seven years. Its energy and maintenance cost a little more. Its tooling and residual value recover only part of the premium.

What changes if the machine is a bottleneck

Most heavy roughing shops buy more spindle capacity to cut cycle time. In the base case, Choice B cuts roughing time by 15%. That reduces total cycle time by 6% because roughing is 40% of the cycle.

Same output now takes 1,880 hours per year instead of 2,000 hours. Saved time is 120 hours per year. If the machine is a bottleneck and the shop can sell those hours, each saved hour has a net margin. A typical DACH/UK net margin after variable costs such as tooling, energy and coolant is €45 per machine hour. Confirm this with your own accounting.

Saved margin value = 120 hours × €45 = €5,400 per year. Over seven years, that is €37,800.

Adjusted net seven-year cost for Choice B = €151,001 - €37,800 = €113,201. Choice A stays at €139,174. Choice B is €25,973 lower over seven years.

Simple payback on the €18,000 premium:

• Annual operating cost difference for the same output: Choice B saves €640 in tooling, pays €130 more for electricity and €200 more for maintenance. Net saving = €310 per year.
• If the freed 120 hours are sold at €45 net margin per hour, Choice B adds €5,400 per year of margin. This margin already includes the variable costs of the extra production.
• Total annual cash benefit = €310 + €5,400 = €5,710.
• Payback = €18,000 / €5,710 ≈ 3.2 years.

If the machine is not a bottleneck, the €5,400 margin value is zero. The €310 annual operating saving is far too small to recover the premium. The standard spindle then remains the lower-cost choice.

Limits of this comparison

This model does not cover every shop. It makes three large assumptions:

1. The high-torque spindle actually reduces roughing cycle time by 15%. This depends on part geometry, tool length, workpiece material and CAM programming. A higher torque spindle cannot cut faster if the machine structure chatters.
2. The shop can sell the saved hours. If the machine is not a bottleneck, saved hours have no cash value.
3. The same toolholders fit both spindles. If Choice B requires HSK-A63 holders and your shop owns only ISO 40 tooling, add the cost of new holders. That can be €5,000 to €20,000 depending on the tool magazine.

When not to choose the high-torque spindle

If your heavy roughing requires more than 250 Nm continuous torque, or you use long tools with a length-to-diameter ratio above 5:1, a 40-taper vertical spindle may be the wrong machine structure. You may need a 50-taper spindle or a horizontal machining centre. A high-torque motor cannot fix a frame that lacks rigidity.

For aluminium, high-speed milling, or workpieces where roughing is less than 20% of spindle-on time, the standard spindle is often the better buy. The high-torque option usually spins slower and costs more, with little cycle time gain in light cuts.

Decision rule

Use this rule after you have the official S1 torque curve and your own part data. Ask your Gree CNC distributor for the S1 torque curve of each spindle option.

• If required continuous torque at your roughing speed is more than 70% of the standard spindle’s S1 torque, and roughing is more than 30% of spindle-on time, and the machine is a bottleneck, choose the high-torque spindle.
• If required torque stays below 60% of the standard spindle’s S1 torque, or roughing is less than 20% of spindle-on time, choose the standard spindle.
• If you are between these bands, calculate the exact cutting force from your CAM simulation or a dynamometer test. Do not decide from peak kW alone.

What to check before you buy

• Official S1 continuous torque curve from 0 rpm to maximum rpm, for each spindle option.
• Duty cycle rating for your longest roughing pass. Long cuts usually need S1, not S6 40%.
• Spindle taper and drawbar force. Confirm that your existing toolholders fit and that the drawbar force is high enough for heavy cuts.
• Toolholder interface cost if you must move from ISO 40 to HSK-A63.
• Machine frame rigidity for your longest tool assembly. A high-torque spindle cannot prevent chatter on a weak structure.
• Actual power draw from your current machine on the same part. Use the machine’s energy log or a power meter.
• Bottleneck analysis: is this machine limiting throughput, or does it have idle time every week?
• Residual value forecast for your local used market. Treat any forecast as an estimate, not a promise.
• A cutting trial with your own tooling and workpiece. Ask the machine supplier to run the part, not just a standard demo.

The comparison is not about peak kilowatts. It is about how much continuous torque you need at the spindle speed where you actually rough, and whether saved cycle time has cash value in your shop. If both conditions hold, the higher-torque spindle often pays back in about three years. If they do not, the standard spindle is the lower-risk choice.