Wednesday, August 9, 2017

Lathe: TouchDRO

I've wanted a DRO on my lathe for a while to deal with the back lash and confusing dial markings. TouchDRO is the best way to go (for many reasons), but first I needed to attach digital scales to the carriage and cross slide.

When I do threading on the lathe I'll just back out my bit and run the lathe in reverse to rest the carriage, so I have no need for a threading dial and had removed it a while ago. That left a great threaded hole to mount the scale bracket to. I suppose if you still have the threading deal there you could always just sandwich the bracket between the apron and dial. The bracket was easily made from 1/8" 1" aluminum angle. It's a bit overkill and I'll probably trim it down a little more later, but it's not really in the way as is.The screws holding the read head in place use crazy glue as a thread locker, since Loctite will attack plastics.

The design I ended up with places the scale below the lead screw, so it's fairly well protected from swarf. The mounting for it is quite stiff, so I only have the scale attached at one end. That was particularly helpful since I didn't want to try drilling into the lathe' body at the head since it houses the motor right there.The end of the scale was wrapped in electrical tape to electrically isolate the scale from the lathe.

Carriage scale in place with stock readout connected.
Carriage travel at the extreme right of the bed is limited slightly.
 The cross slide digital scale sits to the right of the cross slide. The read head is screwed directly to the carriage and the scale's bracket is connected  slide itself. The bracket was made from a non-conducting composite to electrically isolate the scale. The read head needed 1.5mm machined off the cover's mating surface to lower below the height of the cross slide. The read head is secured by two screws to insure it can't rotate.

New iGaging scale mounted to cross slide

The tachometer, like the mill's, uses a Hall effect sensor since they're much easier to set up than an optical sensor and are just as accurate in this application. The lathe previously had a spindle extension installed, and for the tach's magnet I drilled a hole on the extension and used JB Weld to mount a small neodymium magnet in it. 
Spindle extension with magnet mounted.
 The tach's sensor was mounted to the outside of the lathe's gear cover. I considered mounting on the inside but space would have been an issue and it works perfectly well on the outside. I covered the top of the sensor with epoxy putty to protect it and keep any swarf from shorting it. If you look closely you can see I've bent the sensor itself up and away from the spindle to provide a better orientation to the magnet. The sensor's USB cable is run down the back of the lathe to the Arduino's case.
Hall effect sensor mounted on gear cover.

Unlike the mill's Arduino, I constructed this one using a prototype board. It's much cleaner and easier and I highly recommend it, even though it added $8 to the build. I used standard USB A connectors for the scales' interface since both connectors and cables are much easier to find. This forced me to change the cables on both scales, but that didn't cost much. The tachometer's plug is also USB to avoid the issue I had using a 3.5mm headphone jack for the tach on the mill. Everything was mounted in an old Dell laptop power supply brick's case I had on hand. Neodymium magnets were glued to the case's top for mounting on the back of the lathe.

I'm using a Motorola RAZR phone as the Android device running the TouchDRO application. Since the lathe only has four readouts (X,Z, diameter, and tach) the phone is adequate. It's currently mounted with magnets to the top of the headstock using a bracket I fabricated.
All done.
Like with the mill, setting up a TouchDRO system has made the lathe a lot easier and nicer to use, and I would hate to ever be without it.