I am amazed that it is so long since I updated this thread, and equally amazed how little progress the actual machine has made in the last year.
This is partly because I spent a lot of time sailing (crossed the Pacific) and partly because I have been working on the software side of things.
So, an update. I have one axis working. But to get the other axes working with the drivers I have I need to update their firmware, and there is no Linux tool for that, so I am writing one.
I will post about that, with pictures and possibly video later.
The subject of this message is spindle feedback. It is handy for closed loop spindle-speed control (though I don't think spindle speeds have to be _that_ accurate). It is imperative for rigid tapping and gear hobbing though.
The Harrison Mill is a "Universal" mill, with a horizontal spindle and a vertical adapter. The mills came with two options of motor-to-gearbox pulleys and two vertical adapter ratios. With both fast ratios you have 100 to 2500 rpm. Guess which ratios I have? I have 47 rpm in the lowest gear, and 1000rpm in the highest. The VFD lets me boost that a bit.
In the long term I hope to make a pair of the high-speed vertical gears. You simply swap a 51:51 set for a 36:66 set inside the head rotate housing. However, that means that the spindle feedback needs to be in the head.
I decided to use some Allegro ATS667 devices running off the gears in the head:
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These are (as far as I can tell) unique sensors that contain a magnet and a Hall device, and can detect anything magnetic with logic output.
Allegro MicroSystems - ATS667LSG: True Zero-Speed, High Accuracy Gear Tooth Sensor IC
I bought some from eBay as I could not find a UK stockist. Then I mounted them in a bracket on the head cover:
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The one at the bottom is for the index, it picks up off of a screw I put in a handy tapped hole on the spindle.
Because the sensors are running in oil, I needed an oiltight connector. Lemo connectors are lovely things, and vacuum-tight, so I used one of them.
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Hang the expense!
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It works very nicely, and I have spindle speed feedback and an interlock which prevents the pneumatic drawbar activating when the spindle is spinning.