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I have been doing research on building a high performance spindle for a medium to large size benchtop CNC mill for a few months. I recently found out that @shooter123456 was also working on similar goals and I wanted to create this post to collect information and share ideas. I'm sure he and I have slightly different end goals in mind, so I will try to keep this generic.
Background: Anyone who has converted or built a benchtop CNC (up to RF45/PM-940 size) is likely aware of the extreme limitations of spindle options out there. In fact, the spindle is almost always the limiting factor in custom built designs. There are a couple of categories that we can group the available spindles into:
-Mike
Background: Anyone who has converted or built a benchtop CNC (up to RF45/PM-940 size) is likely aware of the extreme limitations of spindle options out there. In fact, the spindle is almost always the limiting factor in custom built designs. There are a couple of categories that we can group the available spindles into:
- Stock Spindle: These are the machine running the original spindles that came with the machine. While this doesn't add much to the cost, these spindles are often limited in speed and torque due to a gear head design and brushed or brushless DC motors. Speed control from the CNC controller may be difficult without special interfaces.
- High Frequency Spindle: There are a ton of router type machines running water or air cooled high frequency AC spindle motors coming from China at relatively low prices. For general machining work, the 18-24k rpm spindle is too fast and the extremely limited torque and rigidity limits their application to light cuts with small tools.
- AC Induction - Belt Drive: Many hobbyists opt for an open loop AC induction motor run on a VFD. The cost of this setup is often significantly more expensive than the aforementioned options, however it offers great torque and control over the motor speed. Hobbyists are usually limited by the size and weight of the motor, as well as the cost. Speed is limited without special considerations to 7200rpm (2x motor nameplate) and torque falls off rapidly above 3600 rpm. Rigid tapping is possible with special considerations.
- Servo Spindle - Belt Drive: Typically seen on the high end hobby builds, a servo spindle offers a flat torque profile across the entire speed range and precise control of velocity and position. Rigid tapping is easily accomplished with this spindle configuration. Hobbyists are often limited by the cost of these systems and the maximum speed of the lower cost models (3000rpm). Servos rated for 5000 rpm are available up to 2.5HP but may be difficult to find.
- 8000-12000 rpm: Higher spindle speeds offer cutting performance for tools 3/8" and under.
- 3HP: Roughly the maximum single motor rating before 3 phase power is required.
- Substantial low end torque (>30Nm): Allows use of larger diameter tools, facemills, and improved performance in steel.
- <75 dBA Noise: Comfortable to work around.
- Thermally stable: <130*F at 100% duty cycle.
- Quick Change/ATC Compatible: Pneumatic/Hydraulic tool release
- Low TIR: .0002-.0001 TIR at spindle taper
- Closed Loop: Encoder Feedback of spindle position for speed control and rigid tapping
- Common Taper: R8/BT/CAT taper, minimal tool change height is ideal due to limited travel in hobby machines
- High Spindle Rigidity: Thick spindle walls and large bearings for improved cutting performance
- Tool Compatibility: Tooling system equipped to hand tools up to and including 1/2" shank. Options available for larger tools if needed.
- Cost: Cost should not exceed $1-2000 over the purchase price of the machine.
-Mike