Category Archives: Mechanics

Variable-Frequency Drive

I’ve just finished the variable-frequency drive (VFD) for my 1970s Schaublin 102 lathe. Before I dig into details, there’s a youtube video here:

My new VFD drive in action

I bougth this lathe around a year ago and it came with a bulky, two-speed 3-phase motor. My workshop at the time didn’t have a wall outlet for 3-phase power so I decided to run the lathe on regular 1-phase 230 volts using a frequency inverter. I knew that this kind of motors run poorly on inverters but I tried anyway. The result was even worse than expected, it barely ran and lacked any torque.

The old drive

So as expected I had to get a more suitable motor. After a bit of online research I ordered a physically smaller, 080 size Lenze MF series motor. This series of motors is specifically designed for inverter operation and offers constant torque over a wide range of frequencies. I sized it to offer slightly more torque than the original motor but that still resulted in a considerably smaller size and weight.

New motor temporarily mounted

I temporarily mounted the new motor with an adaptor made of 3 sheets of wood and added a cheap RPM meter I ordered on Alibaba. With a pot and two switches quickly mounted on top of the motor I got a working lathe. The new drive offered enough torque in all situations I encountered as well as a wide range of speeds. The main downside was that I lost the 3-speeds the lathe originally offered because the original V-belt pulley with its 24mm bore would not fit the new motor’s 19mm shaft. So I started to design a more permanent arrangement.

Carelessly mounted Alibaba RPM meter worked reliably (but super slow)

Just turning an adapter for the original pulley was not really an option because that would have resulted in a top speed of 8000+ RPM while only going down to about 200 RPM in low gear. So I decided to use the original pulley but with a fixed 2:1 reduction before that, giving me a more useful 100-4200 RPM range. For that purpose, I decided to use a 25mm, HTD-5M toothed belt.

Basic arrangement with shaft, bearings etc.

After playing around with several different designs in Fusion360 I started turning the various parts above. The motor, gearing as well as the inverter and controls were to be held together by an aluminium structure.

The 2:1 gearing at the core of the new drive

With the mechanics in place I started designing the electronics around it. For me, that’s the easy part 😉 Obviously, I need a pot and a 2-way switch to control the speed and direction plus a main switch. But since my lathe doesn’t have a lead-screw and hence can’t cut threads I thought I might one day want to add a CNC control just for that purpose. So I wanted to be able to remote-control the entire drive as well as to provide the necessary signals for a CNC control to monitor the speed.

Basic construction using 4 aluminum plates

While the chinese RPM meter worked reliably, it reacted super-slow to changes in speed and the bulky sensor proved difficult to mount in an elegant way. On top of that, it offered only a resolution of a single impulse per rotation and no information regarding the direction of rotation. I googled a bit and ordered a GTS45 gear tooth sensor from Renishaw. Its output is a standard, 90 degrees out-of-phase quadrature signal. Despite being a quality, industrial grade sensor, it only costs around 20 EUR and they are happy to sell them in single quantities. I also ordered a maching 90mm, 64-tooth tooth weel.

The remaining 64-thooth thooth-wheel with sensor in the background

I turned away most of this tooth wheel until only a rim with an 80mm bore remained. That then fitted nicely besides the pulley on the headstock. I decided to design my own RPM meter and to fully integrate it into the rest of the control. This reduces everything to a single PCB which simplifies wiring quite a bit.

Front panel

I wanted to have nice, large switches on the front panel even though the forward/reverse switch only provides a control signal and doesn’t switch any power. I also turned a know for the pot out of 1.4305 stainless steel. Together with the 4-digit 7-segment display, the physical layout was pretty much given. So I had to design the PCB to somehow fit in.

Back side of the front panel

To simplify things, the control runs right off mains voltage which is converted to 12V and 5V DC right on the board. The 12V rail is used to power the sensor as well as to operate 3 DR12 relais. Everything else runs on 5 volts. The inverter is controlled by two of the relais (one for forward, the other for reverse) while the third relay is used to choose beween two speed sources, one from the front-panel pot, the other one from an external input.

PIC microcontroller and input filtering

At the core of the design is a PIC16F18855 microcontroller. There are numerous inputs and outputs:

  • Forward, Reverse and Speed output to the inverter
  • Inputs for the forward/reverse switch and the pot on the front panel
  • Inputs for the quadrature signals from the sensor (as well as power supply for the sensor)
  • Inputs & outputs for remote operation: Enable, On and Reverse digital inputs, speed analog input (0-10V) and quadrature digital outputs

All digital inputs are RC-filtered and Schmitt-triggered before they are used. The PIC consumes all these inputs and then controls the display and the 3 relays accordingly. The cleaned inputs from the sensor are then also made available for the remote control via ULN2003 darlington pairs.

State of digital inputs is displayed via LEDs

The state of all digital inputs as well as the relays is clearly displayed using LEDs. While these are invisible inside the case, they are valuable while programming and setting things up.

Final assembly

With the motor coated with 2-component paint in RAL7031 blue grey, everything was assembled and wired using shielded wires. All that remained was to install it on the lathe and to try it out.

Finally. The new drive is mounted on the lathe and ready for use

For testing the remote operation functionality, I had to build some sorts of a remote control first. Besides the switches and a pot it also needed an RPM meter capable of dealing with quadrature signals. For the fun of it, I designed my own. I’ll describe that in a future post.

Remote control using a DIY RPM meter

As always, eagle files and code are available on github:

Dividing Head Controller

This post ist about the CNC conversion of a manual dividing head aka indexing head. If you’re not familiar with that kind of equipment, there’s a wiki page here.  One makes use of interchangeable indexing plates and and the internal worm gear to accurately divide the circle. Parts like cogwheels and the like can be machined this way. A video of the finished project can be found here on my youtube cannel.

The downside is that a high level of concentration is required to not mess things up. Often a single distraction is all it takes to ruin a part. Besides the fact that constantly changing indexing plates can get tedious. So I decided to mount a stepper motor to that indexing head and to design a controller to take care of that motor.

There are many affordable and well-designed stepper motor drivers out there so I decided to use one of those rather than building my own. So an external driver takes care of translating the 5 volts logic step / direction signals into the (typically 12, 24 or 48V) power signals required to drive the stepper motor. What this circuit does is to provide a user interface and to generate that step / direction signal.

I decided to use this motor driver from Planet CNC that comes with a 2×5 pins 100-mil header for the logic signals. So I also put such a connector with a corresponding pin-out on my board. Then a single ribbon cable (provided with the motor driver) is all it takes to hook up the driver.

The user interface consists of a 4×20 character LCD display and two rotary encoders with push buttons. The display is a Midas MCCOG42005A6W that I have used in several of my other projects before. It is very compact and comes with an I2C interface which saves quite a few pins on the microcontroller. There is also a buzzer to provide some acoustic feedback on button presses and the like. If it enoys you, you can always turn it off in software.

As in all my designs, the rotary encoder signals are nicely debounced in hardware as described here.

The board runs on a 24V supply that is also used to drive the motor. The microcontroller, a Microchip PIC1826J50, runs on 3.3 volts. Furthermore, the motor controller requires a separate 5V supply to power its logic. So I first designed a switching converter that generates a 5V output from any input voltage in the range of 6 to 30 volts. A linear regulator then produces 3.3 volts out of that 5V rail to power the PIC and other on-board logic. Unfortunately, a bug as creeped into my PCB layout – hence that fix with a piece of wire just below the coil.

With the PIC running on 3.3 volts and the motor driver at 5 volts, I also had to provide some logic-level conversion.  A 74AHCT125 line driver / buffer and a few resistors take care of that.

The PIC also comes with a USB interface so that the board could be controlled remotely from a PC. All the hardware for that is present on the board but I haven’t written any software for that yet. Most of that can be copy-pasted from other projects such as the solar charger but I simply haven’t done any of that yet.

Finally, there is a temperature sensor and a fan output on the board. I’m not currently using it but if there is need for a fan for the motor driver and/or the power supply, you can connect a fan directly to this board and have it temperature controlled. For the fan output, the buzzer and the display backlight are driven by a TPL7407L that already includes the free-wheeling diodes necessary to drive inductive loads such as a fan.

I’ve mounted all the power supply, the motor driver as well as this board in a nice, compact case that I bought at a flea market earlier this year.

Nice, solid ground connections are provided to all relevant components. The USB connector is accessible from the back through an extension cable.

The other USB connector belongs to the motor driver and is used for configuration.

Finally, the two knobs for the rotary encoders were turned out of aluminum at the lathe.

The rest of it is mainly mechanics. This may seem somewhat off-topic on this blog but expect to see more of it in the future 😉

Here are the parts required for the mechanical part of the CNC conversion. With the exception of the two cog wheels for the timing belt, they are all machined out of aluminum on a manual mill.

First, a spacer is mounted using three existing, M5 threaded holes.

The main body is then screwed onto this spacer and the cogwheel is mounted.

The hub of the cogwheel was turned out of steel and then press fitted to the aluminum cogwheel. This provides for a firm, true-running.

Then the motor with the 22 tooth sprocket can then be mounted together with the 15mm HTD-5M timing belt. Together with the 44 teeth cogwheel on the other end, this provides a 2:1 geering. The dividing head’s internal worm drive adds another factor of 90:1.

Since the motor, a Sanyo Denki 103 H7823 1740, has a resolution of 200 steps per revolution, this translates to a very convenient 0.01 degrees per full step.

Now all that is left to do is to fix the cover plate. As usual, all the relevant files are on github: