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SEM DC Servo Motor Encoder Retrofit Upgrade, preparing the servo for the new CNC control.
Step by step pictorial guide to installing a new Rotary Encoder and direct wire power and encoder cable on a 20 year old DC servo motor for use with a new Centroid CNC control.

SEM DC Servo Motor Rotary Encoder Installation Retrofit Upgrade
Before upgrade...   After new encoder, cable and strain relief has been installed, motor is ready to mount back on the machine and run.

With thousands of DC servo motor based CNC machine tools worldwide, when it comes time to upgrade the original CNC control with a modern Centroid CNC, significant savings can be realized if the existing DC servo motors can be reused. This can often be accomplished by installing a new rotary encoder on the existing servo motors.
Over the last 30 years there have been a wide variety of makes and models of DC servo motors used on a wide variety of CNC machine tools. Many of these motors can return many more years of reliable service on the machine tool with an encoder upgrade. While each make and model of DC servo motor is a little different in design, the process of installing a modern high resolution encoder is often very similar from motor to motor. It is also common at the same time as the new encoder upgrade to install new servo motor power and encoder cables.

On this page we'll show you how we updated a typical 20 year old SEM DC servo motor for use with a new Centroid CNC control. This motor was originally equipped with a low resolution encoder and a tachometer for feedback to the original DC Servo Drive. We will be installing a new 10,000 line (40,000 counts per rev) encoder and new power and encoder cables with strain relief. (Note: No tachometer feedback is required with a Centroid DC Servo Drive so the tach will not be used.)
With a Centroid retrofit it is very uncommon to reuse the original DC Servo Drives as Centroid offers new modern digital DC Servo Drives that are made in USA, reliable, affordable and provide better performance than old DC Servo Drive technology. So, while it is economical to reuse the old DC servo motors , reusing old DC Servo Drives is not. Centroid's popular "All-in-1-DC" CNC controller, as the name implies, is a complete CNC control built on one printed circuit board and contains a built-in 3 axis DC Servo Motor Drive (up to 15 amps per axis!) along with a built-in preprogrammed PLC with inputs and outputs for common machine tool functions, analog spindle control and more. The SEM DC Servos on this web page are approximately 26 in-lb motors, we will be using the Centroid "All-in-1-DC" CNC controller. The "All-in-1-DC" can control DC servo motors up to 15 amp per axis with correlates to a little over a 40 in-lb rated motor. DC servo motor sizes that are commonly found on CNC machine tools are: 15, 26, 29 and 40 in-lb. While you can use the "All-in-1-DC" on DC Servos slightly larger than 40 in-lb's we recommend going to the Centroid "AC/DC Servo Drive" which comes in a 30 amp and a 60 amp (per axis) version for motors larger than 40 in-lbs.
The "All-in-1-DC" CNC controller popularity is due partly to the large number of older CNC controls that used DC servo motors in the 15 to 40 in-lb size range that can be reused, such as Dynapath, Anilam, Fanuc, AllenBradley, Hurco and Bridgeport and more. . Many of these brands used SEM, Baldor, Glentek and other DC servo motor makes. Reusing the stock DC Servo motor with an encoder upgrade and the "All-in-1-DC" greatly simplifies cnc control installation and configuration.

Photo Albums

- Link to SEM DC servo motor encoder retrofit step by step photo album

- Link to typical Bridgeport Boss servo motor encoder upgrade procedure photo album


SEM Cover Plate DXF drawing download  

Download DXF drawing of SEM DC Servo Motor electrical box cover plate.

Make your own cover plate out of .25" thick aluminum. This plate replaces the cover plate that the stock mil (canon) connectors bolted to and accepts the Centroid Motor/Encoder cable seal and strain relief for easy direct wiring of the servo motor.

Download DXF SEM DC Motor cover adapter plate

View larger JPEG of the DXF

SEM Cover Plate Intercon program download  

Use this Centroid "Intercon" conversational program to make SEM DC Servo Motor cover plates using a Centroid equipped milling machine.

Download SEM_coverplate.ICN


Centroid Servo Motor Cable "Direct Wire" Seal and Strain Relief Cap.

For use with Centroid motor cables, this direct wire cable strain relief makes a water/oil tight seal against both the motor power and encoder cables to any flat surface. Very useful when converting both old and new servo motors to a direct wire configuration.

Centroid part# 5031 (aluminum cap, gasket, 6-32 screws) $19 ea.
click here to purchase.

finished sem dc servo motors  

Encoder Installation time for these SEM DC servo motors including the new power and encoder cable was 1.5 hours per motor.

Machining the Aluminum cover plates took about 6 minutes per plate once the Intercon program was finished and the tools were set up in the machine. Total time about an hour with deburing and cleanup.

An afternoon well spent.


3D printed DC servo motor encoder caps
We came up with these 3D printed DC servo motor encoder caps for the case when upgrading a DC servo to a modern encoder the existing motor end cap is too small. both the .stl 3d print ready file and solid model Rhino file is contained in the link below (solid model is included in case you want to modify the design to suit your particular needs).
here is the download link to zip file containing both the .stl and the .3dm files (special thanks to Mike W on the is project)
3d print of DC motor end cap   3d print of Dc servo encoder cover
3d print of DC encoder cap 3d print DC servo encoder cap
After a prefit, the blue end cap is ready to Glue/epoxy onto the blue main body of the 3D printed encoder cap to support and seal it up.
The 3D print cap design accepts the Centroid DC motor cable strain relief kit found on the Allin1DC webpage. 
Encoder motor cable strain relief

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