Router and VFD Spindle Overview

Routers

A hand held router may be used for a CNC table to cut wood, plastics and even Aluminum. Routers are easy to understand and relate to since most woodworkers already have an assortment of them in their shop. Here is a list of qualities I would assign to routers:

  • Their power can vary considerably:
    • Less than 1 HP (under 5 Amps at 120 V AC US) for a palm router that would work well for desktop inlay work.
    • Over 3 HP (15 Amps at 120 V AC US) for a router that can cut through plywood sheets or do other heavy cutting jobs.
  • They use collet and locking nut to hold the router bit in place. Standard collet sizes are 1/4" and 1/2" which match the 1/4" and 1/2" shank sizes of router bits. There are many router bits to choose from at local lumber yards.
  • They have speed ranges between 8,000 RPM and 30,000 RPM, that is easily adjusted with the turn of a dial on the router body itself. Some routers will have soft starts, where they gently accelerate up to speed, and others don't.
  • Many routers have removable bases, which allows you to slide the router's cylindrical body into a router mount which will snug down around the body of the router.
  • Routers are air cooled. This means that there is a fan on the shaft that blows cool air over the motor to keep them from over heating. This also makes them loud.

For a hobbyist, a router is a cheap and great way to get into CNC work. Most routers in the US will work on a 120 V AC, 60 Hz, 15 Amp, single phase circuit that should be available in almost every basement or garage.

In the simplest of setups, the operator will simply turn the router on manually and adjust the speed on it, and then start the GCode program. The GCode will not control the router whatsoever.

In a slightly more advanced setup, the electronics package will include a relay that will switch the 120V AC power on and off to the router, and the operator will place M3 commands (to turn the router on) and M5 commands (to turn the router off) in the GCode. The operator would still need to adjust the speed of the router manually, but in a lot of situations it is left at a constant speed for an entire job. This just makes it nice from the standpoint that the router will shut off at the end of the job.

In a more advanced setup, something like the SuperPID could be used to control the speed of the router too. This relies on the GCode program issuing S commands to specify the desired speed, and then the SmoothStepper generates a PWM signal that tells the SuperPID how to control the router. This is an okay upgrade path, but I would not recommend going with a router and SuperPID to begin with. Why? Spindles are more capable than routers, but also more expensive. By the time you are going to buy a larger router and a SuperPID, it is worth it to just jump up to a spindle.

 

Spindles and their VFDs

Spindles have much more flexibility and power than a router, thanks to the VFD (Variable Frequency Drive), or inverter, which allows for precise speed control of the spindle. The Spindle is simply the motor and tool holder, while the VFD is the electronics that controls the spindle. Here are some of the benefits of a Spindle and VFD pairing:

  • AC motors are hard to start and take about 5 times more current to get moving compared to what they need to keep moving (this is why your lights sometimes dim you start a motor, due to the voltage droop).  This starting current heats and stresses the windings of the motor, which will shorten it's lifespan. A VFD will start a motor with no voltage at 0 Hz. The VFD will then accelerate your motor by increasing the voltage and frequency while monitoring the magnetization in your windings (by watching the current consumption).
  • The previously mentioned controlled start up of your motor by the VFD will help to extend the life of the motor, but will also reduce the voltage droop on your supply circuit. This voltage stabilization is a good benefit, but it comes at the cost of adding a lot of electrical noise on your power line.  For this reason you need to add filters and use good grounding techniques to mitigate the VFD noise.
  • A normal AC motor will run at just one speed, think of a table saw or the circulation fan for your house's HVAC system. The VFD will allow your motor to run at a slower speed, which will consume less current and be cheaper to run. Or in the case of a CNC system, the VFD will let your spindle run at the optimal RPMs for your bit, material and feed rate.
  • A VFD can have a torque limit, which will monitor the current consumed, and prevent situations where the motor cannot spin freely.
  • A VFD can operate in reverse for cutting material in a different direction, or for backing a tap out of a hole.
  • VFDs will allow for a controlled deceleration or a soft stop. If necessary, you can also equip a brake on your spindle.

 

VFD Power

A VFD is controlled by the user or the CNC electronics to determine what speed the spindle should be running at.  The VFD takes the power from the wall and changes the power to operate at a different frequency (3 phase output, but at an analogous output voltage to the input voltage) and this output frequency determine the spindle's RPMs. It is important to match your spindle head with the VFD that will be controlling it.

A common VFD output frequency range is 0 Hz to 400 Hz. Multiply that frequency by 60 seconds in a minute to determine the resultant RPMs:

  • 0 Hz * 60 seconds / minute = 0 RPM

  • 100 Hz * 60 seconds / minute = 6,000 RPM

  • 200 Hz * 60 seconds / minute = 12,000 RPM
  • 400 Hz * 60 seconds / minute = 24,000 RPM

Low end VFDs have power ranges from 1.5 kW (2 HP) to 7.5 kW (HP). (Please keep in mind that HP ratings between routers and spindles are somewhat inconsistent.) However, the INPUT power varies significantly and you need to know what you have available BEFORE you buy the VFD and you need to make sure that you purchase the correct model.

Here are the input power details you need to consider before you select a VFD:

  • Does the VFD take 1 phase (single phase) or 3 phase input power? Typically only industrial buildings will have 3 phase power, homes will have single phase power.
  • What is the power frequency? The US runs on 60 Hz AC power and Europe on 50 Hz (and planes on 400 Hz). Most VFDs will be able to handle 48-63 Hz so this should not be an issue.
  • What is the input voltage?
    • 110 V (or 115 V or 120 V ) AC, which is common in US houses.
    • 220 V to 250 V AC, which is common in Europe and can sometimes be found in US garages.
    • 380 V AC or higher.
  • How much current will the drive consume and what is your supply circuit capable of: 15 Amps, 20 Amps, 30 Amps 50 Amps?

One important thing to note is that Power is determined by Voltage x Current. 20 Amps at 120 V is 2.4 kW just like 10 Amps at 240 V. However, efficiency is determined by power losses and power losses are calculated by the square of the current consumed. A 20 Amp draw will have 4 times the power losses compared to an equivalent 10 Amp draw. Running a motor or VFD at a higher voltage will have lower power losses and therefor either deliver more power or cost less to operate.

A typical US house or garage will have 120 V (sometimes called 110 V or 115 V) AC 60 Hz power. Your breaker may be rated for 15 Amps or 20 Amps, but you need to make sure that there is nothing else running on that circuit or that you sum up all of the currents and don't exceed the maximum. Some garages may have multiple 120 V circuits, or even a 220 V circuit that may have 50 Amps. However, almost all residential power is single phase and not 3 phase.

A final note for this section is that if you ever need 3 phase power for a single machine and don't want to pay the expense for 3 phase power, a rotary phase converter can generate 3 phase output power from a single phase input source for a much smaller up front investment.

 

VFD control

In the CNC world VFDs have three (3) common control types:

  • 0 V to 10 V analog voltage control.  While this may just be a potentiometer knob that comes with some VFDs to adjust the speed, more commonly there is a Break Out Board (BOB) that will produce the 0 V to 10 V analog output voltage. The GCode will have a S command (i.e. S6000 or S24000) which will tell the SmoothStepper what the commanded speed is for the spindle. The SmoothStepper will then produce a PWM signal for the BOB, and the BOB will then convert that into analog output voltage of 0 V (stopped) to 10 V (full speed), or anything in between. The VFD will see that voltage and then accelerate or decelerate the spindle to the correct speed. The GCode will also issue other commands, which will activate relays on the BOB that are hooked up to the VFD to:
    • M3 to run the spindle Forwards
    • M4 to run the spindle in Reverse
    • M5 to Stop the spindle
  • Step and Direction (or sometimes Clock Wise / Counter Clock Wise [CW / CCW] or sometimes Quadrature) control. Some BOBs will take a Step and Direction signal, instead of a PWM signal, to generate the 0 V to 10 V analog output control voltage, just like in the previous bullet point to control a VFD. Sometimes a Servo Motor will be used as a spindle (instead of a spindle VFD pair), and the Servo Motor will typically want a Step and Direction input signal. The SmoothStepper can handle any of those setups.
  • Modbus control. A USB or Ethernet modbus module can be used to control some VFDs.  The main advantage of this is that you can just command the desired speed directly, and you may be able to monitor parameters inside the VFD.

 

Setting up a VFD

VFDs have a lot of parameters (some times hundreds) in them, that need to be set up correctly in order to get them to work the way you want them to.

 

Spindle Cooling

Spindles can produce a fair amount of heat, and that needs to be dissipated.

Air cooled spindles have a fan on their shaft so that they can blow air over the motor to cool it down. Since you need to keep the air flowing, there is a lower RPM limit you should not run your spindle below, or the air flow may not be sufficient to keep it cool. This fan will make the air cooled spindle louder than a comparable water cooled spindle, but either should be quieter than a router.

Water cooled spindles need tubing and a pump to circulate water from a bucket or a chiller. For short periods of run time a bucket may be sufficient to keep the water cool, but a chiller is needed for longer run times or hotter environments. Since the water is always running to keep the spindle cool, you can run it at lower RPMs than a comparable air cooled spindle. In a garage or other area that may freeze, anti freeze may need to be added to the water. In any situation, you may want to add anti corrosion treatments to the water.  You need to periodically check the connectors and tubing on your water cooling to make sure that they don't start leaking on your equipment.

 

Spindle Collets

Spindles have a much wider range of collet families than routers do, and each family has individually sized collets that can each clamp or hold tools or bits with a small range of shaft sizes. Some common collet families are:

  • ER-11 has about 7 different collet sizes that can hold bits 1/32" to 1/4" OR 0.020" to 0.276" OR 0.5 mm to 7.0 mm
  • ER-16 has about 10 different collet sizes that can hold bits 1/32" to 3/8" OR 0.020" to 0.394" OR 0.5 mm to 10.0 mm
  • ER-20 has about 12 different collet sizes that can hold bits 1/16" to 1/2" OR 0.039" to 0.512" OR 1.0 mm to 13 mm
  • ER-25 has about 15 different collet sizes that can hold bits 1/16" to 5/8" OR 0.039" to 0.630" OR 1.0 mm to 16 mm
  • ER-32 has 20 to 25 different collet sizes that can hold bits 1/16" to 13/16" OR 0.039" to 0.827" OR 1.0 mm to 21 mm
  • ER-40 has about 23 different collet sizes that can hold bits 1/8 " to 1" OR 0.118" to 1.024" OR 3.0 mm to 26 mm
  • Please note that different vendors will support slightly different tool sizes and number of sizes in each collet family. There are other collet families out there too.

If you are running jobs where you need to switch bits (or tools) on a regular basis, an ATC or Automatic Tool Changer can quickly become indispensable.

 

Concluding Thoughts

From my observations I would say:

  • Half to slightly more than half of the hobbyists choose to run a router instead of a spindle.
  • Most people who are trying to earn money or run a business will run a spindle instead of a router.
  • The vast majority of spindles are water cooled, not air cooled.
  • Routers have more runout than spindles. The more precise the work you need to do, the more incentive to go with a better quality spindle.

If you are new to setting up a VFD, please research the one you are going to buy and make sure that you can find a setup guide that works with the same products you are going to use. Spending an extra $20 or $50 to get a VFD that others have set up a guide for can save you hours or days worth of frustration - especially when some of the very cheap no name Chinese VFDs have no documentation and no setup guides at all.

 

 

 

 

 

 

 

 

 

 

 

 

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