When buying a machine tool, table size, axis travel and price come up easily. The spindle torque diagram usually never comes up at all — because reading it is rarely taught. Yet that single diagram tells you more clearly than anything else which jobs your machine will handle comfortably and which it will struggle with.
First we explain how a spindle torque diagram is read, what a rating such as "11/15 kW" actually means, and how to find the speed band where your machine works most efficiently. At the end of the page there is a tool where you enter your machine's details and see typical torque values and the optimum working band.
What do the two numbers in the catalogue mean?
Machine catalogues normally give motor power as a pair of numbers: 7.5/11 kW, 11/15 kW, 18.5/22 kW. The two numbers describe different duty ratings, and both are worth knowing.
| Notation | What it means | When it applies |
|---|---|---|
| First number (11 kW) | S1 — continuous duty | The power you can use without interruption through a whole shift. Production planning should be based on this figure. |
| Second number (15 kW) | S3 / S6 — short-time duty (25–40% ED) | Available in bursts of 15–30 minutes. Useful for short roughing passes or difficult sections. |
So if you run continuous production, planning around the lower number is more realistic. The higher number is a ceiling, not something you can always reach.
Beyond that, neither number tells you the torque directly. And torque is what actually removes metal.
Same power, different torque: why the drive layout matters
Torque at the spindle depends not only on motor power but on how the motor is coupled to the spindle. The same 11/15 kW motor can deliver very different torque at the spindle nose depending on the drive type.
Typical values compiled from spindle torque diagrams in the market:
| Drive type | Typical max speed | Base speed | Spindle torque (S1) |
|---|---|---|---|
| Geared head — low gear | 3,500 rpm | ~400 rpm | ~235 N·m |
| Belt drive | 8,000 rpm | ~1,150 rpm | ~87 N·m |
| Direct drive | 12,000 rpm | ~1,500 rpm | ~70 N·m |
| Geared head — high gear | 6,000 rpm | ~1,500 rpm | ~67 N·m |
| Built-in motor spindle | 15,000 rpm | ~2,500 rpm | ~42 N·m |
| Belt drive — high speed | 10,000–12,000 rpm | ~4,000 rpm | ~24 N·m |
| Electrospindle | 24,000 rpm | ~9,000 rpm | ~11 N·m |
The spread is striking: the same 11 kW motor gives roughly 235 N·m through a geared head in low gear, but only 24 N·m on a high-speed belt-driven spindle. Roughly a tenfold difference.
The reason is the basic relationship between torque and speed. A gearbox lowers the speed and multiplies torque mechanically. A high-speed belt drive does the opposite: to gain top speed it pushes the base speed upward, which reduces the torque available at low speed.
For example, on Haas's 60 hp geared BT50 spindle the low gear is rated at roughly 529 N·m, while the same machine in high gear drops to 163 N·m. That difference does not come from the motor — it comes from the gear ratio.
How motor torque reaches the tool
Torque produced at the motor shaft (Tm) passes through two factors before it reaches the tool: the transmission ratio (i) and the mechanical efficiency (η). Net torque at the spindle output is:
How the three drive architectures affect this equation:
| Drive type | Transmission ratio (i) | Efficiency (η) | Character |
|---|---|---|---|
| Geared head | 2:1 – 4:1 (low gear) | ~85–90% | Drops the speed, multiplies the torque. Below 1,000 rpm it gives 2–4 times the torque of a belt drive. The standard answer for heavy roughing. |
| Belt drive | 1:1 – 1:2 | ~92–98% | Passes the motor's torque curve through almost unchanged. Synchronous (timing) belts run above 95% efficiency; V-belts drop to about 92% because of slip. |
| Built-in / direct drive | 1:1 (fixed) | ~98–99% | No mechanism in between; the torque curve is purely the motor's own characteristic. There is no way to multiply torque mechanically — unbeatable at high speed, limited in low-speed heavy cutting. |
This table also explains the torque differences in the previous section: a geared machine's high torque comes not from its motor but from the transmission ratio (i>1). A built-in spindle's torque depends directly on the quality of the motor winding — there is nothing in between to multiply it. This is why looking only at the kW figure on the nameplate gives you incomplete information; the same kW in a different architecture is a completely different machine.
One point worth noting: on machine catalogue diagrams the horizontal axis is spindle speed, not motor speed. The transmission ratio is therefore already built into the curve, and in practice manufacturers do not subtract a separate efficiency loss on top of it. For that reason the calculator below does not apply an extra η factor to the result either. The efficiency figures above are there so you can compare drive types with each other.
How to read a torque diagram
Every spindle torque diagram has two regions. The break between them is called the base speed (or knee point). That single point is the most important information on the diagram.
1. Constant torque region — below base speed
From zero up to base speed the motor holds its maximum torque. Torque is constant, but power rises linearly with speed:
The practical meaning is this: below base speed you cannot reach the motor's full power. On an 11 kW machine with a base speed of 1,150 rpm, running at 400 rpm leaves you with roughly 3.8 kW. The motor is perfectly healthy — it simply cannot deliver more than about a third of its power at that speed.
2. Constant power region — above base speed
Once past base speed the motor reaches full power and power stays flat. Torque, by physics, falls in inverse proportion to speed. A 22 kW class built-in spindle running to 15,000 rpm gives roughly 84 N·m at its 2,500 rpm base speed, but under 15 N·m at maximum speed; on many spindles the power curve is also derated at the top end, so a real diagram may show something closer to 5 N·m.
The place where you use your machine's power most effectively is just above base speed. That is the band where torque is still high and power has reached full capacity. When choosing a cutter diameter, the real goal is to place the cutting speed (vc) inside that band.
Taper size and cutter diameter
The spindle taper limits the cutter diameter you can safely use. The values below are not a hard standard; they are widely accepted approximate upper limits compiled from tooling manufacturers' application guides and shop practice. For a firm limit, check the catalogue of your toolholder and spindle manufacturer:
| Taper | Recommended maximum cutter diameter (Dc) | Typical use |
|---|---|---|
| BT/ISO 30 | 50 mm (with low ap/ae) | High speed, light machining |
| BT/ISO 40 | 100 mm | General purpose — the most common |
| BT/ISO 50 | 160 mm | Heavy roughing |
| BT/ISO 60 | up to 250 mm | Gantry / heavy industry |
Two effects appear once these limits are exceeded:
- The bending moment grows. Cutting force acts on the spindle nose through a lever equal to the cutter radius. As diameter grows that lever gets longer and the same force produces a larger moment, which is carried by the front bearing set.
- Vibration risk rises. Large diameter cutters run at low speed, and combined with a wide radial engagement they create torque fluctuation. That leads to chatter and, in turn, to premature insert breakage.
A shop-floor example: the advantage of a smaller diameter
At a die-making shop, blocks of roughly 700×500 mm were being face milled, removing about 3 mm of stock in total from each block. It was a repeat job, so every minute of cycle time went straight into cost.
The shop started with a Ø120 face mill. Intuitively that seems reasonable: the wider the cutter, the more surface covered per pass. The results, however, were not what they expected. Insert consumption was high, and the spindle bearings had been replaced twice in two years.
Looking at it through the torque diagram made the picture clear. In tool steel, the Ø120 cutter was turning at roughly 400 rpm at a suitable cutting speed. The machine's base speed was around 1,150 rpm. In other words the spindle was working in a region where it could not reach full power — only about 3.8 kW of the 11 kW motor was usable. On top of that, Ø120 was above the widely accepted 100 mm limit for BT40. Whenever a deeper pass was attempted at that power limit the motor bogged down and chatter started, so depth of cut had been reduced to 1 mm and feed per tooth to about 0.10 mm.
The fix was not a new machine — it was a smaller cutter. With a Ø40 face mill the speed moved up into the base speed band, feed was increased, and a single 3 mm layer replaced three 1 mm layers.
Before — Ø120 cutter
- Depth of cut (ap): 1 mm × 3 layers (reduced because of chatter)
- Speed: ~400 rpm — below base speed
- Usable power: ~3.8 kW / 11 kW
- Machining time: ~40 minutes (vf ≈ 320 mm/min)
- Insert consumption: high
- Bearings: 2 replacements in 2 years
After — Ø40 cutter
- Depth of cut (ap): 3 mm — single layer
- Speed: ~1,200 rpm — inside the base speed band
- Usable power: 11 kW / 11 kW
- Machining time: ~11 minutes (vf ≈ 1,200 mm/min)
- Insert consumption: down 60%
- Bearings: 3+ years without trouble
The Ø40 cutter did not pull more torque from the machine. The machine's torque was identical in both cases. What Ø40 did was move the spindle into the speed band where that torque is actually usable. Torque did not increase — access to it did.
This is also the basis of Sandvik Coromant's "light and fast machining" approach: instead of a large diameter with a shallow pass, use a smaller diameter, a deeper pass and a higher feed.
Note: the times, feeds and power figures in this example are representative calculations for the scenario (tool steel, vc = 150 m/min, kc ≈ 2,400 N/mm², Ø120 z=8 / Ø40 z=4, ae ≈ 75% of diameter). Run the numbers for your own part with your tooling supplier's data.
Questions worth asking when buying a machine
Working envelope figures — table size, travels — obviously matter. But deciding on size alone can leave you paying for capacity your parts never needed. Motor and spindle capacity deserves as much attention as machining capacity.
When you request a quotation, it is worth asking for:
- The spindle power–torque diagram. Not just a kW figure — the curve itself. S1 and S3 curves should be shown separately.
- What is the base speed, in rpm? That single number tells you most of what the machine is suited for.
- Drive type: geared head, belt, direct drive or built-in?
- Taper: BT40 / BT50 / CAT / Capto? This sets the cutter diameter you can use.
- Spindle manufacturer and bearing arrangement. Bearing type, number of rows and cooling method.
- The torque your own part needs. The tool below gives you a starting figure for that.
Find Your Machine's Torque Band
Describe your machine — see typical torque values and the optimum working band
If the spindle power–torque diagram appears in your machine manual or in the technical documents supplied by the builder, enter the two values below. You then get a calculation specific to your machine instead of averages.
- Base speed: the speed at which the torque curve stops running flat and bends downward. On real machines it is usually between 300 and 2,000 rpm; built-in spindles and electrospindles sit higher.
- S1 torque: the constant torque value up to that knee point. On charts it is labelled "Continuous", "S1" or "60min S3 40%".
- Unit warning: if the value is in ft-lbs, multiply by 1.356 (e.g. 390 ft-lbs = 529 N·m).
- On geared machines there are two curves (Low Gear / High Gear). For roughing, enter the Low Gear values.
- If only power (kW) is given instead of torque, leave the torque field empty — it is calculated from the base speed.
Cutter diameters that suit this band — guidance only
The table below shows which cutter diameter falls into your machine's optimum speed band for different materials. The aim is to find the diameter at which the cutting speed (vc) suits the material and the resulting spindle speed sits inside the torque band. Remember: cutter diameter is chosen first from the width of cut — Sandvik Coromant recommends a cutter 20–50% larger than the radial engagement (ae). This table is not a final answer but a starting point that shows the best diameter from a power point of view; factor in the width of cut and your tooling supplier's cutting data before deciding.
| Material | Suitable vc | Recommended cutter diameter | Speed (at that diameter) | Power access |
|---|
Choosing the diameter from the table puts your machine in the speed band where it can use its full power. The "Speed" column shows the approximate spindle speed to program with that diameter and cutting speed. "Power access" tells you what percentage of the motor's continuous (S1) power you can reach at that speed; 100% means the speed is above base speed and the full power is at your disposal. That means both deeper passes and less vibration. Set depth of cut (ap) and feed according to your tooling supplier's recommendations for that particular insert.
You can obtain your machine's actual torque values and working range from its builder. Ask for the spindle power–torque diagram and base the exact calculation on that curve. This tool gives you a starting point so you ask the right question and move in the right direction.
Checked against a real catalogue
To see whether the calculation above actually holds, we read the Fanuc αi spindle power–torque diagrams in the catalogue of one vertical machining centre series, one by one, and compared every value with the formula. It makes a good test case because belt-driven, direct drive and geared options all appear within the same machine family.
| Spindle | Base speed | Continuous (S1) power | Torque in catalogue | Formula result |
|---|---|---|---|---|
| Belt drive · BT-40 · 8,000 rpm | 1,500 rpm | 7.5 kW | 47.7 N·m | 47.8 N·m |
| Belt drive · BT-40 · 10,000 rpm | 1,875 rpm | 7.5 kW | 38.1 N·m | 38.2 N·m |
| Direct drive · BT-40 · 12,000 rpm | 1,500 rpm | 7.5 kW | 47.7 N·m | 47.8 N·m |
| Belt drive · BT-50 · 4,500 rpm | 1,125 rpm | 11 kW | 93.3 N·m | 93.4 N·m |
| Belt drive · BT-50 · 6,000 rpm | 1,500 rpm | 11 kW | 70.0 N·m | 70.0 N·m |
| Geared head · BT-50 · 4,500 rpm | 1,125 rpm | 15 kW | 127.2 N·m | 127.3 N·m |
| Geared head · BT-50 · 6,000 rpm | 1,500 rpm | 15 kW | 95.4 N·m | 95.5 N·m |
The result is unambiguous: every torque value in the catalogue lands within 0.1 N·m of 9550 × kW ÷ base speed. The same check was run on the 30-minute (S3) curves; they matched exactly as well.
Four practical conclusions
1. The catalogue does not subtract belt or gearbox losses separately. The horizontal axis of these diagrams is already spindle speed, so the transmission ratio is baked into the curve. That is why the tool above applies no extra efficiency factor either; it uses the value you read off the diagram as it is.
2. Base speed is the number that determines torque. Both BT-50 machines in the catalogue have an 11 kW motor. Yet one gives 93 N·m at a base speed of 1,125 rpm while the other gives 70 N·m at 1,500 rpm. A third of the torque, and all of it comes from base speed. Same kW on the nameplate, different machine.
3. Maximum speed does not set the base speed. A common mistake is to assume that "if the spindle reaches 10,000 rpm, the base speed must be high too". In the catalogue the 8,000 rpm belt spindle has a base speed of 1,500 rpm and the 10,000 rpm one 1,875 rpm. Raising the top speed by 2,000 rpm lifts the base speed by only 375 rpm. Even on a 10,000–12,000 rpm belt-driven spindle, base speed is still below 2,000 rpm.
4. BT-50 spindles give more torque at the same power. The belt ratio is larger and the base speed sits lower. That is why, when you pick a belt drive in the calculator above, the taper you choose changes the result.
One BT-50 spindle in the same catalogue showed two ranges on its diagram: in the low range the base speed was about 320 rpm and the torque 326 N·m, with the range change at roughly 1,200 rpm. If your machine has a geared head, look for two curves like this on the diagram. Calculate the low range for roughing and the high range for finishing separately — a single torque figure does not describe such a machine.
The values above were read from the catalogue of a single machine family. Other builders use different belt and gear ratios, so these base speeds should not be treated as valid for every machine. The preset drive types in the calculator are likewise typical values tuned to this and similar catalogues. If you have your own machine's diagram, always enter it — the tool then stops estimating.
Formulas used (Sandvik Coromant convention and basic engineering):
Power–torque relationship: P = T × n / 9550 · Output torque: Tc = Tm × i × η · Cutting speed: vc = π × Dc × n / 1000
Spindle speed: n = 1000 × vc / (π × Dc) · Net power: Pc = (ap × ae × vf × kc) / (η × 60 × 10⁶)
© Mentor CNC — This tool is intended for training and preliminary assessment. Before applying any cutting parameters, verify them against your tooling supplier's recommendations and your machine's actual torque diagram.