Hello Hugh and Jason,
I have been looking at the A4988 stepper drive module. They look to be a quick and easy to add and would give me back the big 4.7Amp stepper driver to use with the 4th axis. The only reason I have not done it yet, is the hassle of cutting a hole in the enclosure back panel to fit an additional bulkhead connector,
Looks like there are now three different options to generate the step pulses, 1) N555 pulse generator module 2) modify the HAL software in LinuxCNC 2) Hugh's Arduino.
If you are already using LinuxCNC then the easiest route is to edit the HAL file by adding the six lines of code I showed earlier. No need to understand the intricacies of the HAL programming language. I have done that for you, so just copy and paste into the HAL file.
Thinking again about the mist fluid. I will only get a small amount of lubrication from the 4% imulsion delivered a 15 drops per minute, it will mostly cool. For better lubrication, it will be necessary to increase the strength of synthetic cutting oil imulsion or go for straight 'mist' fluid. Jason and I are both having difficulties in sourcing smallish quantities in the UK, it only appears to be available in industrial quantities of 25 litres or more. So I have a plan....... I am going to take one of my model engines along to one (or more) of the local machine shops and ask them what cutting fluids or mist fluids they use and where do they obtain them from. Hopefully they may offer to give me a small quauntity to try. And, by good fortune, I happen to have an empty 1 litre container in the car.
Mike