Upgrading a Bridgeport Spindle Motor - Did I Buy The Wrong Servo?

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Thread: Upgrading a Bridgeport Spindle Motor - Did I Buy The Wrong Servo?

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    Default Upgrading a Bridgeport Spindle Motor - Did I Buy The Wrong Servo?

    I recently moved my shop from an industrial space to my home garage and in order to fit my Bridgeport Boss II R2E4 mill into the garage I had to take the head off. With the head off, I thought it would be a great time to upgrade the spindle motor. I converted the mill to LinuxCNC 3 years ago, see build thread here, and have always been dissatisfied with the factory spindle motor. I've been toying with the idea of replacing the 3 phase, 1.5HP factory motor with a servo motor for a while, and I finally pulled the trigger on a Chinese servo last week. However, I think I may have purchased an underpowered servo motor as a replacement.

    There are a few reasons why I wanted a servo motor for the spindle:
    • Automatically set spindle speed in G-code. I can't tell you the amount of times I've done a tool change and broken a tool because I forgot to adjust the spindle speed for the new tool.
    • Absolute positioning of the spindle. For probing I would like to account for tip misalignment by rotating the probe for each probing event. I would also like to make a small, three to four, tool rack on the side of the table for tool changes.
    • Get rid of the vari-drive unit. It's loud and takes forever to change speeds. I'm lazy and anything I can do to streamline a process will make me less likely to take shortcuts.
    • Lower total mill height. I have lowish ceilings in my home garage and can't fit the factory motor/vari-drive assembly on top of the mill unless I cut into the ceiling and re-do some of the rafters.
    • Increase the mill's power. The factory motor is only 1.5HP and I've bogged the cutter quite a few times with HSM tool paths in steel.


    I purchased a 130ST-M10025-BZ servo motor and drive combo on E-bay from China last week. The motor is rated for 2500RPM max speed, 2.6kw (3.48HP) and 10Nm. I initially thought this would be fine, but I forgot to account for the gearing necessary to get the max desired spindle speed of 6500RPM, which requires a ratio of 2.6:1. The factory Bridgeport motor is 1.5HP and 1425RPM.

    With the vari-drive and back gear of the original setup, torque at the tool is decreased when spinning faster than the base motor speed and increased when spinning slower, correct? With the servo motor, the torque at the tool will always be the same, regardless of spindle speed, right? With the 2.6:1 reduction in power, the new servo motor has an effective 1.33HP at the tool. This means that the new motor will have the same equivalent power as the factory motor spinning the tool at 1,900RPM. Considering I do most of my milling above this speed that seems fine, but they also sell a 3.8kw (5.1HP) motor. Should I try and return this motor and upgrade to the more powerful one?

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  2. #2
    Community Moderator Jim Dawson's Avatar
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    Default Re: Upgrading a Bridgeport Spindle Motor - Did I Buy The Wrong Servo?

    I don't know about that particular servo motor, but most of them will operate up to 2x the rated speed with little torque loss until approaching the absolute max speed. And for the most part the torque rating is for continuous, with peak torque at up to 4x intermittent. The spec sheet for that motor is not really complete based on most of the spec sheets.

    The original BP motor has a torque rating of about 6 Nm

    Yes, the higher the speed on the vari-drive, the less the torque at the tool. Switching into back gear gives you approximately 8 times the torque at the tool, and of course about 1/8 the speed.

    Servo motors have a more or less flat torque curve so torque is pretty constant over the entire speed range.

    Worst case you could reduce your step over a bit to reduce the load on the tool, or just get a bigger motor. There is always the question of just how much power your machine frame will take, it was designed for the 1.5 HP motor, but I'm sure it will take more than that.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Upgrading a Bridgeport Spindle Motor - Did I Buy The Wrong Servo?

    Quote Originally Posted by tightmopedman9 View Post
    I recently moved my shop from an industrial space to my home garage and in order to fit my Bridgeport Boss II R2E4 mill into the garage I had to take the head off. With the head off, I thought it would be a great time to upgrade the spindle motor. I converted the mill to LinuxCNC 3 years ago, see build thread here, and have always been dissatisfied with the factory spindle motor. I've been toying with the idea of replacing the 3 phase, 1.5HP factory motor with a servo motor for a while, and I finally pulled the trigger on a Chinese servo last week. However, I think I may have purchased an underpowered servo motor as a replacement.

    There are a few reasons why I wanted a servo motor for the spindle:
    • Automatically set spindle speed in G-code. I can't tell you the amount of times I've done a tool change and broken a tool because I forgot to adjust the spindle speed for the new tool.
    • Absolute positioning of the spindle. For probing I would like to account for tip misalignment by rotating the probe for each probing event. I would also like to make a small, three to four, tool rack on the side of the table for tool changes.
    • Get rid of the vari-drive unit. It's loud and takes forever to change speeds. I'm lazy and anything I can do to streamline a process will make me less likely to take shortcuts.
    • Lower total mill height. I have lowish ceilings in my home garage and can't fit the factory motor/vari-drive assembly on top of the mill unless I cut into the ceiling and re-do some of the rafters.
    • Increase the mill's power. The factory motor is only 1.5HP and I've bogged the cutter quite a few times with HSM tool paths in steel.


    I purchased a 130ST-M10025-BZ servo motor and drive combo on E-bay from China last week. The motor is rated for 2500RPM max speed, 2.6kw (3.48HP) and 10Nm. I initially thought this would be fine, but I forgot to account for the gearing necessary to get the max desired spindle speed of 6500RPM, which requires a ratio of 2.6:1. The factory Bridgeport motor is 1.5HP and 1425RPM.

    With the vari-drive and back gear of the original setup, torque at the tool is decreased when spinning faster than the base motor speed and increased when spinning slower, correct? With the servo motor, the torque at the tool will always be the same, regardless of spindle speed, right? With the 2.6:1 reduction in power, the new servo motor has an effective 1.33HP at the tool. This means that the new motor will have the same equivalent power as the factory motor spinning the tool at 1,900RPM. Considering I do most of my milling above this speed that seems fine, but they also sell a 3.8kw (5.1HP) motor. Should I try and return this motor and upgrade to the more powerful one?
    I have never seen a R2E4 with a 1.5Hp motor standard, they came with 2Hp with an option of 3 Hp, so if yours had a 1.5Hp at a guess someone had changed it, 3hp to 5Hp is fine on these machines

    Mactec54


  4. #4

    Default Re: Upgrading a Bridgeport Spindle Motor - Did I Buy The Wrong Servo?

    Quote Originally Posted by Jim Dawson View Post
    I don't know about that particular servo motor, but most of them will operate up to 2x the rated speed with little torque loss until approaching the absolute max speed. And for the most part the torque rating is for continuous, with peak torque at up to 4x intermittent. The spec sheet for that motor is not really complete based on most of the spec sheets.
    If I could run the servo at 5,000RPM then I would only need a 1.3 gear reduction. Without any official recommendation about this though I'd be concerned about killing the motor. Would the issue just be overheating? If so, I could easily install a cooling fan for the motor.

    Quote Originally Posted by mactec54 View Post
    I have never seen a R2E4 with a 1.5Hp motor standard, they came with 2Hp with an option of 3 Hp, so if yours had a 1.5Hp at a guess someone had changed it, 3hp to 5Hp is fine on these machines
    You're right, I just double checked the name plate and saw it is a 2HP motor. I'm not sure why I thought it was 1.5HP. I guess the servo motor I got is even more underpowered than I originally than I thought.



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    Default Re: Upgrading a Bridgeport Spindle Motor - Did I Buy The Wrong Servo?

    ...just to add one more question to this Linux thread

    Is Ridge Tap with Linux/Mesa/Encoder setup possible with a normal 3Phase Induction motors?

    Last edited by machinehop5; 08-17-2021 at 08:15 PM. Reason: linux


  6. #6

    Default Re: Upgrading a Bridgeport Spindle Motor - Did I Buy The Wrong Servo?

    Quote Originally Posted by machinehop5 View Post
    ...just to add one more question to this thread

    Is Ridge Tap with Linux/Mesa/Encoder setup possible with a normal 3Phase Induction motors?
    Yes it is, and it is quite easy to set up. I used a VFD to power my motor and had a high wattage brake resistor that could stop the spindle in about 1.5 seconds. With the added resistance of tapping the spindle would usually stop in ~.5s while rigid tapping. Synchronized Z axis motion will follow the tap until it stops, so I would set the maximum Z depth a little above the bottom of the hole, in the case of tapping blind holes.



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    Default Re: Upgrading a Bridgeport Spindle Motor - Did I Buy The Wrong Servo?

    Quote Originally Posted by machinehop5 View Post
    ...just to add one more question to this Linux thread

    Is Ridge Tap with Linux/Mesa/Encoder setup possible with a normal 3Phase Induction motors?
    If the control software / Hardware has spindle Encoder support / feed back, it can do Ridge Tapping

    Mactec54


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    Community Moderator Jim Dawson's Avatar
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    Default Re: Upgrading a Bridgeport Spindle Motor - Did I Buy The Wrong Servo?

    Quote Originally Posted by tightmopedman9 View Post
    If I could run the servo at 5,000RPM then I would only need a 1.3 gear reduction. Without any official recommendation about this though I'd be concerned about killing the motor. Would the issue just be overheating? If so, I could easily install a cooling fan for the motor.
    .
    I don't know if that motor will even turn 5000 RPM, the 2500 RPM spec in the ad could be the max RPM. There is just not enough information in the ad to form an opinion. Speed is basically a function of voltage and frequency input to the motor, the torque is a function of the input current. Power output is a function of voltage x current. There is a bit more to it than that, but that will suffice as a simple explanation. Over driving the motor too much will generate waste heat.

    Take a look at these spec sheets for SureServo2 (Delta) servo motors from Automation Direct. This will give you an idea of the proper specs and normal operating range of servo motors, and a bit of a tutorial on applying servo systems. https://cdn.automationdirect.com/sta.../ss2motors.pdf

    Going to a larger motor and gearing it higher to get the RPM you want might make sense. You can buy high speed servo spindle motors, but for most of us it would require taking out a second mortgage to buy one.

    Jim Dawson
    Sandy, Oregon, USA


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Upgrading a Bridgeport Spindle Motor - Did I Buy The Wrong Servo?

Upgrading a Bridgeport Spindle Motor - Did I Buy The Wrong Servo?

Upgrading a Bridgeport Spindle Motor - Did I Buy The Wrong Servo?