The Othermill is a great fabrication machine, but its spindle is open loop, meaning there is no feedback about how fast the spindle is really going. Sometimes though it's crucial for good results to know exactly the spindle speed to regulate other parameters (feed rate) accordingly: here comes OtheRPM.
It's a spindle attachment (yes, the Othermill Pro provides such a nice feature) with its companion 4 digit 7 segment display giving you the information you need!
If you are curious about the whole fail & learn process, have a read at the whole story.
The spindle speed is displayed on a 0.56" 4 digits 7 segments display, which are available in two flavours: common-anode and common-cathode. The project supports both types via two separate PCB designs, just pick the one that fits your needs and set the COMMON_ANODE definition in the source code: true means you are using a common anode display, false a common cathode one.
Power is provided via the
J3 connector and you have multiple options:
- a regulated power source, like a USB port
- use pin 1 and 3 of
- it can be either a
- do not populate the voltage regulator
- no need to populate the voltage divider resistors
- use pin 1 and 3 of
- a non regulated power source, like a LiIon battery
- use pin 1 and 2 of
- you need to populate the
- you can populate the voltage divider resistors
R9, so to get a warning (blinking display) on low battery
- set the definitions in the source code accordingly to your setup, in particular:
- set MONITOR_BATTERY to true
- set VIN to your oprating voltage, like 5.0 or 3.3
- define the BATT_LOW, when the display starts blinking, like 3.4
- define the sensing voltage divider resistor values, where R1 is the upper resistor and R2 the lower one
- use pin 1 and 2 of
This Othermill Pro extension (or attachment, if you prefer) requires a little permanent modification to your machine: you will have to create a black mark on the white pulley of the spindle subsystem.
I used a black permanent marker with a fat chisel tip to draw a black square mark long about 10mm going from top to bottom. At first pass, the marker left a dark grey spot so I left it to dry a bit (one minute) and marked it again. At the fourth or fifth pass, I got a pretty black band really standing out from the white background: exactly what I was looking for!
Depending on the circuit working voltage, reflectivity of the marker used on the spindle and performance of the IR sensor assembly you will have to re-calculate the resistor values for the inverting schmitt trigger
Luckily for you and for me, a nice and easy to use on-line calculator is available, you have to measure a couple of voltages though: the high threshold correspond to the voltage output by your IR receiver when the the black mark is in front of the sensor while the low threshold corresponds to no black mark.
To proceed with those measurement you should partially assemble the spindle attachment board leaving out the above mentioned resistors. At this point you can mount the partially assembled board and power it with the voltage you intend to have when operating:
3.3V because that will considerably influence your measurements.
Apply the black mark on the spindle and measure the two voltages with your multimeter (between ground and pin 2 of the
U4 op-amp). Also, use the potentiometer on the board to maximize the voltage gap as much as you can: in my case, they were
2.2V, not bad at all.
On the on-line inverting schmitt trigger calculator I typed
1.33V (slightly above the minimum voltage) and
1.95V (slightly below the maximum voltage), selected E6 as resistor series and those are the values I'm going to use:
While testing I was powering my breadboard from USB at
2.23V respectively as minimum and maximum voltage so I went for
1.95V to get
This project is specifically targetted at the Othermill spindle, but the same concept applies to other CNC machines, including all those cheap Chinese machines known as 3020, 4030, etc... That's the reason for the existence of the project CNC-RPM.
Men at Work
Once properly set up, this contactless tachometer can determine the real speed of the spindle with a good accuracy, allowing for fine tuning and greater machine control, as those videos demonstrate.