More power is not always better. The wrong spindle size wastes money, exceeds your machine’s mechanical limits, and can overload your electrical supply. This selection guide uses the torque formula — Power (kW) × 9550 ÷ Speed (RPM) — to match spindle power and body diameter to your actual materials and processes.
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Jump to: Platforms · Torque formula · Material fit · Power supply · Selection matrix
100 mm vs. 125 mm: two platforms, two purposes
The body diameter is not just a size — it determines the spindle’s weight, torque capacity, mounting requirements, and electrical demands. Choose the platform first, then select the power level within it.
100 mm body · 3.0-3.5 kW — Compact high-speed platform
Medium routers
| Spec | Value |
|---|---|
| Weight | 10-15 kg |
| Power range | 3.0 / 3.2 / 3.5 kW |
| Torque | 3.2 / 4.0 Nm |
| Speed | 12,000-24,000 RPM |
| Clamp | 100 mm mount |
| Typical use | Routing, drilling, tapping, light profiling |
The 100 mm platform is the standard choice for mid-size CNC routers. It balances power, weight, and speed for the majority of routing, drilling, and light profiling work. The compact body fits most gantry machines without requiring Z-axis reinforcement. At 3.0-3.5 kW with 4.0 Nm of torque, it handles wood, plastic, acrylic, and light aluminum capably at 18,000-24,000 RPM. The lower weight (10-15 kg) keeps Z-axis acceleration responsive and reduces stepper/servo sizing requirements.
125 mm body · 5.5-7.5 kW — Heavy-duty torque platform
Production machines
| Spec | Value |
|---|---|
| Weight | 25-30 kg |
| Power range | 5.5 / 7.5 kW |
| Torque | 5.6 / 7.6 Nm |
| Speed | 18,000-24,000 RPM |
| Clamp | 125 mm mount |
| Typical use | Heavy profiling, mold work, steel cutting |
The 125 mm platform is built for sustained heavy cutting. The larger body diameter houses a more powerful motor with 5.6-7.6 Nm of torque — nearly double the 100 mm platform. This torque is delivered at lower RPM, making it suitable for large-diameter cutters, deep single-pass roughing, steel and iron machining, and production environments where the spindle runs continuously for hours. The trade-off is weight (25-30 kg) — the Z-axis must have adequate payload and braking capacity, and the machine frame must be rigid enough to control the additional mass during acceleration.
The torque formula: why power alone doesn’t tell the story
Two spindles can have the same power rating but behave completely differently at your cutting speed. The torque formula explains why.
Torque calculation: T(Nm) = P(kW) × 9550 ÷ N(RPM)
Torque is inversely proportional to speed. A 3.5 kW spindle produces 2.8 Nm at 12,000 RPM — but only 1.4 Nm at 24,000 RPM. A 7.5 kW spindle at a low 6,000 RPM delivers 12.0 Nm. Higher power matters most at lower speeds.
Treat the formula as an estimate, not as the final product rating. The calculation shows theoretical torque for a power and speed point. Rated torque on a product page is based on manufacturer data and rated operating conditions, so a simple formula back-calculation may differ. Final spindle matching should consider the product specification, actual load, usable speed range, VFD setup, and engineering confirmation.
| Power | RPM | Torque | Typical application |
|---|---|---|---|
| 3.5 kW | 24,000 RPM | 1.4 Nm | Light routing — plywood, acrylic, small engraving bits |
| 3.5 kW | 18,000 RPM | 1.9 Nm | Medium routing — MDF, softwood, 6-8 mm compression bits |
| 3.5 kW | 12,000 RPM | 2.8 Nm | Drilling and tapping — lower speed, higher torque demand |
| 5.5 kW | 18,000 RPM | 2.9 Nm | Heavy routing — hardwood, large-diameter bits at speed |
| 5.5 kW | 6,000 RPM | 8.8 Nm | Steel cutting — needs low-RPM torque; only 125 mm platform |
| 7.5 kW | 6,000 RPM | 12.0 Nm | Heavy steel / thick aluminum — maximum torque envelope |
What material and process demands what spindle?
Match spindle power to the material’s cutting resistance and your typical tool diameter. Over-spec wastes money; under-spec ruins parts.
Plywood / MDF / particle board
Process: Cabinet nesting, furniture parts, general wood routing Recommendation: 100 mm · 3.0-3.5 kW
Wood cutting at 18,000-24,000 RPM with compression or spiral bits requires speed, not torque. A 100 mm spindle at 3.5 kW handles 12 mm single-pass cuts in plywood without strain. The 125 mm platform adds weight and cost with no practical benefit for these materials.
Hardwood / solid timber
Process: Deep roughing, stair treads, large furniture components Recommendation: 100 mm · 3.5 kW for general; 125 mm · 5.5 kW for production
A 3.5 kW / 100 mm spindle handles most hardwood work. Upgrade to 125 mm / 5.5 kW only when running deep single-pass roughing with 12 mm+ diameter bits in continuous production — where the extra 1.6 Nm of torque prevents RPM sag under load.
Aluminum plate & profiles
Process: Profiling, pocketing, face milling, drilling Recommendation: 100 mm · 3.5 kW for light; 125 mm · 5.5 kW for heavy
Light aluminum work (thin sheet, small cutters, intermittent cycles) runs on a 100 mm / 3.5 kW spindle. When moving to thick plate, large-diameter tools, or production quantities, the 125 mm platform’s 5.6 Nm of torque at lower RPM prevents chatter and tool breakage during heavy engagement.
Steel / cast iron
Process: Face milling, drilling, boring, light profiling Recommendation: 125 mm · 5.5-7.5 kW minimum
Steel cutting requires high torque at low RPM (3,000-8,000 RPM). At 6,000 RPM, a 3.5 kW / 100 mm spindle produces only 5.6 Nm — below the threshold for effective steel cutting. A 5.5 kW / 125 mm spindle delivers 8.8 Nm at the same speed. For any steel or iron work, the 125 mm platform is the starting point.
Acrylic / HPL / composites
Process: Sign making, display fabrication, industrial routing Recommendation: 100 mm · 3.0-3.5 kW
These materials machine best at high RPM with sharp tooling. Cutting forces are low. A 100 mm spindle at 24,000 RPM with a single-flute or O-flute cutter produces clean edges without melting. The 125 mm platform is unnecessary — the additional weight can actually reduce Z-axis responsiveness for fine detail work.
Mold & die (tool steel / hardened)
Process: 3D contouring, fine finishing, long cycle times Recommendation: 125 mm · 7.5 kW with HSK40E
Mold machining combines long continuous cycles, high precision requirements, and hard materials. The 125 mm / 7.5 kW platform provides the thermal headroom and torque stability to maintain micron-level accuracy over 8+ hour cycles. Pair with HSK40E tool holding for the best surface finish on hardened tool steel.
Your power supply determines your spindle ceiling
This is a hard physical constraint — not a preference. The VFD’s input rectifier cannot cheat the laws of electrical engineering.
Single-phase 220V (residential / small workshop)
Limit: Maximum 3.5 kW spindle
A 3.5 kW spindle draws approximately 16-18A at 220V single-phase — at the practical limit of a standard 20A circuit. A 5.5 kW spindle would draw 25-30A, exceeding most residential breakers and requiring a dedicated high-current circuit. More critically, the VFD’s input rectifier bridge converts AC to DC: on single-phase input, the rectifier diodes carry the full current. At 5.5 kW single-phase, the input current can exceed 30A, overloading the rectifier and causing it to fail — often catastrophically, with arcing and fire risk.
Verdict: Suitable for 3.0-3.5 kW. Do not attempt 5.5 kW or above on single-phase.
Three-phase 380V (industrial workshop)
Limit: Any power level up to 7.5 kW
Three-phase power distributes the current across three conductors, reducing the per-phase current by a factor of √3 (approximately 1.73×). A 7.5 kW spindle at 380V three-phase draws approximately 14A per phase — well within standard industrial circuit ratings. The VFD’s input rectifier operates with three-phase rectification, spreading the load across six diodes instead of four, with lower ripple current and higher reliability. Our 5.5 kW and 7.5 kW spindle configurations require three-phase 380V input.
Verdict: Required for 5.5-7.5 kW. Works for all power levels.
Spindle selection matrix by application
Find your scenario in the left column. The right columns give the recommended power, body size, and electrical supply.
| Application scenario | Power | Body | Supply | Notes |
|---|---|---|---|---|
| Hobby / DIY CNC router, intermittent use, wood and plastic only | 3.0-3.2 kW | 100 mm | Single-phase 220V | Basic or Model A series. 12,000-18,000 RPM is sufficient. |
| Small cabinet shop, daily production, plywood and MDF | 3.5 kW | 100 mm | Single-phase 220V | Model A. 24,000 RPM for clean edges. Ceramic bearings for reliability. |
| Sign making, acrylic fabrication, light aluminum engraving | 3.0-3.5 kW | 100 mm | Single-phase 220V | Model A. High RPM (24,000) more important than torque. |
| Aluminum profile machining, medium production volumes | 3.5-5.5 kW | 100 mm or 125 mm | Single or three-phase | 3.5 kW / 100 mm for light work. 5.5 kW / 125 mm for heavy profiling with three-phase power. |
| Hardwood furniture production, deep single-pass roughing | 5.5 kW | 125 mm | Three-phase 380V | Model A or B. The torque reserve prevents RPM sag in heavy cuts. |
| Steel component machining, mold cavity roughing | 7.5 kW | 125 mm | Three-phase 380V | Model B or C. HSK40E recommended for precision steel work. |
Three rules that prevent expensive mistakes
These warnings address the most common errors buyers make when choosing spindle power and size.
Warning: Do not run a 5.5 kW+ spindle on single-phase 220V
A VFD can accept single-phase input and produce three-phase output — but the input rectifier bridge still carries the full single-phase current. At 5.5 kW on 220V single-phase, the input current exceeds 30A. The rectifier diodes — typically rated for 25-30A in a standard VFD — will overheat and fail, often with arcing that destroys the VFD and creates a fire hazard. If your workshop only has single-phase power, your spindle ceiling is 3.5 kW. Do not be tempted by a higher-power spindle with a “single-phase capable” VFD — the rectifier math does not change.
Caution: Check your Z-axis payload and braking before upgrading to 125 mm
A 125 mm spindle weighs 25-30 kg — nearly double a 100 mm spindle at 10-15 kg. If your Z-axis uses a stepper motor without an electromagnetic brake, the spindle WILL drop when power is removed. Even with a brake, the additional mass increases inertia during acceleration and deceleration, which can cause position errors, missed steps, or servo following errors. Verify your Z-axis drive sizing with the heavier spindle before purchasing.
Caution: 100 mm and 125 mm use different mounting clamps
Upgrading from a 100 mm to a 125 mm spindle requires replacing the spindle mounting clamp, which typically means modifying or replacing the Z-axis mounting plate. The bolt pattern, clamp height, and overall clearance envelope are different. Budget for the clamp, potential Z-plate modification, and realignment time when planning a platform upgrade.
Frequently asked questions
How do I calculate the torque I need for my application?
Start with the formula: Torque (Nm) = Power (kW) × 9550 / Speed (RPM). Use it as a selection estimate for theoretical shaft torque at a given power and speed, not as a replacement for the product’s rated torque data. Product-page torque values come from manufacturer data and rated operating conditions, so they may not match a simple back-calculation exactly. Determine your cutting speed (RPM) from the tool manufacturer’s recommendation for your material and cutter diameter. Then calculate the torque required to remove the material at your desired feed rate and depth of cut. As a rule of thumb: wood routing at 18,000 RPM needs 1.5-2.5 Nm; aluminum profiling needs 3-5 Nm at 12,000-18,000 RPM; steel cutting at 4,000-8,000 RPM needs 6-10+ Nm. If your calculated torque exceeds the spindle’s rated torque, go up a power level and confirm the final match against the product specs, load, speed range, and engineering review.
Is a higher-power spindle always better?
No. Higher power means a larger, heavier spindle that may not physically fit your machine. The Z-axis must carry the weight, the gantry must be stiff enough to control it, and the electrical supply must support it. A 7.5 kW spindle on a lightweight gantry router designed for 3.0 kW will cause deflection, vibration, and poor surface finish. Match the spindle to the machine’s mechanical capability, not just the power number.
Can I use a 5.5 kW spindle but limit the VFD to lower power?
You can program the VFD to limit output current, which effectively caps the spindle power. However, this does not solve the single-phase input problem — the VFD’s input rectifier still sees the full input current demand during acceleration and heavy load transients. If you have single-phase power, purchase a 3.5 kW spindle and run it at its rated capacity. A de-rated 5.5 kW spindle is heavier, more expensive, and risks VFD failure with no performance benefit.
What is the practical speed range for different materials?
Wood and plastic: 18,000-24,000 RPM with sharp carbide tooling. Aluminum: 12,000-18,000 RPM — higher speeds risk melting; use coolant mist or air blast. Brass: 8,000-15,000 RPM with sharp tooling. Steel: 3,000-8,000 RPM — this is the critical range where the 125 mm platform’s torque advantage is decisive. Cast iron: 2,000-6,000 RPM with appropriate carbide grades. Below 3,000 RPM, confirm that your VFD can maintain stable output without overheating the motor at low frequency.
Do I need ceramic bearings at higher power levels?
All Model A, B, and C spindles from 3.0 kW to 7.5 kW use ceramic hybrid bearings. The bearing specification is driven by speed, not power — ceramic bearings are required above 12,000 RPM regardless of power level. The Basic series (steel bearings, 12,000 RPM, 3.2 kW) is the exception. If you need more than 3.2 kW, you are automatically in the ceramic bearing range.
Quote the Power Class That Fits Your Machine and Production Load
Use your machine specs, typical materials, and electrical supply details to quote the correct power level and body size so the spindle fits mechanically and electrically.
Request a Power-Matched Quote · Browse power-range models
Also see: ATC spindle selection guide · Model A vs Model B