BASIC Machining - RPM Display for your Mill

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BASIC Machining - RPM Display for your Mill

Like most smaller milling machines, the Wabeco CC F-1210 has no RPM display or RPM control from a CNC computer. The machine has a 0-100% dial to set the RPM between 0 and 7500. Knowing the RPM helps duplicate a setup and determine the correct feed rates to get the best finish on aluminum. By taking a few covers off the machine spindle and motor, we identified two solutions for monitoring RPM: (a) a rotating magnet could be placed on top of the spindle shaft's cover cap where speed could be monitored in a non-contact with a hall-effect sensor; or (b) the motor's continuous torque feedback in a wired solution. We chose to use the magnet and hall-effect sensor to get started and the project worked really well. We'll design a circuit for the continuous torque feedback next before settling on one.

BASIC Stamp/Melexis 90217 Hall-Effect Electronic Circuit

The BASIC Stamp software to control the circuit is quite simple. The MLX90217 hall-effect cam sensor outputs a pulse with each detection of change in magnetic field. The sensor has an on-board A/D converter for digital sample and hold. The BASIC Stamp's COUNT command measures frequency and through parallel I/O control displays the data on a Hitachi 44780-compatible 2x16 display.

The following electronic components are required to build the circuit:

BASIC Stamp 2 Source Code
Melexis 90217 Hall-Effect Cam Sensor
Noritake 5X7 Dot Character VFD Module CU16025ECPB-W6J
Resistors (10k Ohm, 4.7k Ohm, 5.6k Ohm)
Capacitors (0.01 uF)

Building your RPM Display Unit Hardware

(a) Remove spindle cover cap. Remove the steel cover from the mill spindle. Loosen the two set screws and remove the stainless steel cover cap. Measure the inside diameter.	(b) Turn the magnet holder on lathe. Based on the diameter of the inside of the cover cap and the size of your magnet, turn an aluminum rod down to size. Bore the middle to press fit a magnet. Part off at about 3/32".

(c) Test fit finished parts. This picture shows the small round magnet, the magnet holder and the spindle covering cap. Test fit the parts.	(d) Put magnet in holder and holder in cap. Press the magnet into the magnet holder disc. Press this assembly into the covering cap so the magnet is visible on top. The set screws should still be able to seat without interfering with the magnet holder.

(e) Replace covering cap back on mill. Put the covering cap assembly back onto the top of the spindle. You can now replace the protective cover and install the two set screws that held it in place before you removed it.	(f) Prepare the BASIC Stamp circuit. In order to fit the electronics into a project box slightly larger than the LCD, I cut a BASIC Stamp 2 Carrier Board in half. Shown below is the power switch, power jack, BASIC Stamp 2 Carrier Board, Noritake VFD and the MLX90217 sensor.

(g) Mount RPM Display in Place. Put your electronics and VFD into a small project box. The MLX90217 hall-effect sensor is mounted on a standoff on the bottom of the box so it can be placed within 1/8" of the magnet.	(h) Test the RPM Display on your mill. Before you permanently mount the enclosure, try it for a while. Ours will be mounted by gluing two magnets inside the black project box to hold it firmly to the top of the spindle cover.

(i) VFD shows RPM, percent and bargraph. The Noritake VFDs provide a high-contrast, visually interesting display.	(j) Bargraph fills display as RPM increases. The bargraph will fill the VFD's second line at 100% of the Wabeco's CC-F1210's top speed of 7500 RPM.

(k) Testing the different speeds. The example BASIC Stamp 2 code updates every second but you can change the constant in the COUNT command.	(l) Project completed . . . for now. Of course, this simple project helped us identify a series of improvements. The project needs a dedicated PCB design, a mechanical system to adjust the hall-effect sensor's height and perhaps a more sensitive hall-effect sensor so the distance from the magnet could be increased. Using at least two magnets could increase resolution.