I recently converted a PM833-TV to CNC. I am now working on a spindle motor upgrade. My motivation for the upgrade is twofold. 1. The original motor runs almost 7500 rpm to achieve a spindle speed of 3200 rpm. This means it is very loud. 2. I machine mostly aluminum, so I would like to increase the speed to 5000-6000 rpm. I also wanted to use a one speed pulley system.
Full disclaimer: I have degrees in both mechanical and electrical engineering, however I never professionally practiced either. I am retired and love to learn and tinker with technical stuff. I welcome all feedback.
I watched several videos with folks using 2hp, 3hp and even 5hp motor on similar mills. My first impression was that seems like a lot of weight to put on these benchtop mills. More weight means potentially larger ball screws, larger servos/steppers, and more wear & tear on the gibs. Furthermore, more weight means more inertia and more force/wear on the ball screws and ball nuts every time the z axis accelerates or decelerates.
I also viewed a few videos and read posts with AC servos being used for spindle motors. They claimed the servos provided enough torque to approach the limits of the rigidity of the mills.
Here is what I have learned. Both AC servos and Inverter duty motors share similar constant torque curves. This means they provide their full torque from about zero rpm up to their rated rpm. Above the rated rpm, torque falls off quickly. Torque is defined as twisting force, so these motors provide their full twisting force up to the rated rpm. This is ideal for a milling machine because you need max twisting force at both high and low rpm.
I began looking at 1.8KW and 2.6KW low-cost Chinese AC servo motors. You can find these all over the internet from the mid four hundred dollar up to very low six-hundred-dollar range. The 1.8 KW (about 2.4 hp) options put out 6 Nm of torque and weight just under 15 lbs!
The 2.6KW (about 3.5 hp) option puts out 10 Nm of torque and weighs about 27 lbs! It can produce as much as 25Nm for up to 3 seconds.
By contrast, a 1.5KW, 2HP Black Max Inverter Duty motor is rated at 8.13 Nm of torque. The locked rotor torque is 29.5Nm. I am not sure how long it can be overloaded. This motor weighs a whopping 68 lbs. This motor along with a VFD and resistor cost about $1500 + a bunch of freight.
The OEM motor on this mill weighs about 35 lbs. If you are like me and add a power drawbar, the weight of the power drawbar + 2.6KW AC servo weigh about the same as the original motor. If you use the 1.8KW servo, it will weigh less. Personally, I think the weight really matters.
One challenge with servos is their max rpm. The 1.8KW option has a max rpm of 3000-4000. The 2.6KW option is 3000-3500. For timing pulleys and belts I am working with www.beltingonline.com. For literally a few dollars they will bore out stock pulleys and cut custom keyways. I ordered a 72 tooth 5M HTD pulley for the motor and 40 tooth pulley for the spindle. These were the largest pulleys they sell to fit in the mill and smallest pulley possible with a 40mm bore for the spindle. Belt, pulleys, custom boring and keyways cost $175.00 delivered from the UK. These guys are fantastic. Ted in tech support calculated that this pulley system can deliver 7.5KW.
This will give me a 1.8:1 pulley ratio. This translates to a max spindle speed of 5400 – 6300 rpm. The spindle torque will be about 5.5 Nm from 0 to 5400 rpm. The Black Max motor would have a pulley ratio of 1:1, so it will produce 8.3Nm of torque at it’s synchronous speed of 1800 rpm but will have significantly less at speeds over 1800 rpm.
To put these numbers in perspective, an engineer (Uwe on the Centroid Forum aka Suntravel) with 40+ years of experience working on milling machines in Germany, converted a small mill using a 1.5KW JMC AC servo with a 1.68:1 pulley ratio. He stated “1.5 kw Servo will be ok to get the mill to its limit of stiffness. I have mine with 1:1.68 for higher spindle speed, buts still ok to tap M16x1.5 with 800rpm in tool steel.
I think either of these AC servos are great options for spindle upgrades. I chose the 2.6KW JMC servo from the available Chinese options because the JMC manual was excellent, the frame and shaft are larger than comparable Chinese offerings, and Uwe provided me with a schematic and the programming necessary to interface with my Centroid Acorn controller.
I am a happy to provide any drawings and any information for anyone interested. I purchased the 2.6 KW AC servo motor from TOPCNC (338) on ebay. It was not available anywhere I looked, I messaged them, and they located it for me. They were great to work with. Servo motor and servo controller cost $633 delivered.
I am currently waiting for the pulleys to arrive. Below are images of Dr. D’s modified motor mount. Being a bit nuts for a quiet machine, I took of the cover to the servo controller, sketched it in Fusion 360 and modified it so I could mount a larger 80mm quiet Noctua fan on the exterior and removed the internal fan. Yes… I am a bit nuts… but I like to listen to music while setting up parts on my mill and hate the sound of whirring fans.
When comparing AC servo vs. Inverter Duty motors, I view the AC servo as a sports car engine with a tremendous power to weight ratio. It is light, high powered and will provide tons of torque. It is pefect for intermittent loads that change all the time. Just like a car.
The inverter duty motor on the other hand is like a diesel engine. It is designed to output up to 115% of its rated power all day long and will most likely last for years doing so. It is a continuous duty motor. It is similar to a boat engine with constant load. If I were boring large holes in hardened steel all day long…every day…I would choose the inverter duty motor. I would not however choose the inverter duty motor for high speed milling as this motor at 5600 rpm cannot provide much torque.
If I were using the mill for a small shop with intermittent loads, I would choose the AC servo. It will most likely last for many years.
Servo Driver With Modified Cover:
Servo Motor and Motor Mount: