Milling Machine Tachometer Mark II
Image by Adam N. Ward
An incandescently boring two minute video displaying the high accuracy tachometer I produced for my low accuracy milling machine.
Tedious particulars for nerds:
Makes use of a Hall impact sensor (latching type) to generate a square wave signal from a magnet attached to the milling machine’s draw bar. The sensor is taped to the dust cap that covers the drawbar and therefore does not interfere with changing the tool holders or chuck.
The brains of the unit is a Parallax P8X32A multi-core microcontroller (aka "Propeller"). Electronics enthusiastss may believe that this is overkill for a tacho and I’d agree, but I do still program to add a lot more characteristics to this, and utilizing a Propeller chip will permit these to be implemented with no compromising on pin count or timing precision.
There are a couple of minor glitches in the firmware for this device. You might notice that after the RPM drops belo a particular worth, the final identified speed tends to remain in memory and get displayed later on momentarily as soon as the speed picks up once more.
At present the accuracy of the RPM measurement is by no means much more than .five RPM away from the accurate worth. This is correct more than the variety of 60 RPM up to approximately 1,000,000 RPM as far as I can tell.
Certainly the four-digit show is only helpful up to 9999 RPM. My milling machine will not go any more rapidly than 1975 RPM when it is free of charge-operating with no tool in the taper. I am employing the worth "9999" to indicate that the unit is unable to establish the true speed.
It is *By no means* secure for the LED show to say that the machine is running at 0000 RPM. This is a security measure.
The RPM readout is never ever much more than 1 revolution out of date – this is due to the algorithm used – it counts the number of CPU clock cycles in between constructive going edges on the HE sensor. There is a 1 second timeout on the half-cycle of the square wave. SO if in doubt, the readout often defaults to 9999 – which is the failsafe error worth.
There will be much more specifics about this build in the subsequent handful of weeks. I program to add X and Y axis position indication as the subsequent phase of the project.
Hope you identified this somewhat fascinating. 🙂
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