Electronic Leadscrew for the New Lathe

[awemawson]

Are you incorporating backlash compensation Steve?

[vtsteam]

I've thought about backlash as a topic,Andrew, but concluded in this hybrid system (an electronic leadscrew) vs full CNC automation, it is not a concern. The reason is the same as it is with a dial on a screw handle. I will manually be backing to a start position that is past the point that backlash will be taken up when the stepper starts to drive the leadscrew in the cut direction again.

Similarly in an ordinary screw cutting change gear lathe, there will be backlash in gears, leadscrew and halfnut, and even the threading dial drive. But because you start the cut in a position prior to contact, backlash is taken up by the time the cut begins.

I've taken an incremental rather than an absolute positional approach to the software, so far at least. When you are dealing with positional coordinate system, backlash becomes important. Conventional CNC is positional.

[awemawson]

My CNC mill, wire eroder and lathe have pitch error correction tables that map the ball screws along their length. I can't believe there's much error, certainly not enough to bother with for my sorts of projects.

My first CNC conversion (literally more than two decades ago !) to a Taiwanese Mill /Drill was practically unusable for profiling until I embodied backlash compensation. My first motivation to CNC it was the pain of filing out the D shaped 25 way connector mounting holes for various projects ! Programmed in assembler 

[vtsteam]

Andrew, cool! 

I was just thinkin, people must be getting tired here of just talk and no pics, but so far it's kind of uninspiring to look at. Nevertheless, somethin is usually better'n  nothin so ........here's my current mess:



Clockwise from upper left, encoder, stepper motor, step driver, power supply, breadboard, Arduino.

[vtsteam]

Setting up an array for the various likely pitches so I can choose them with switches. Since there are 6 switches, that gives me 64 possible pitches.

I've been looking at what the common ones are, to populate the array and have come up with:
Inch: 80 72 64 56 48 40 36 32 28 27 24 20 18 16 14 13 12 11 11.5 10 9 8 7 6 5 4 tpi
Metric: .3 .4 .45 .5 .6 .7 .8 1.0 1.25 1.5 1.75 2.0 2.5 3.0 3.5 4.0
Feed Speeds (tpi equiv): 100 150 200

This is 45 total entries, so will fit in the array size available, with room for additions.

I should say that the Metric pitches and the 11.5 Inch pitch are for future work, as everything else will program now with integer math throughout. I have a good idea how to do the others also with integer math, using something like the DDA algorithm but it's more involved.

Integer math keeps the speed of processing up in FORTH and on the Arduino.

[vtsteam]

I connected an 8 pin DIP switch to the Arduino. The switches now correctly select any of the programmed inch pitches above, and the encoder turns the stepper motor at the selected pitch ratio. Encoder direction is mirrored by the stepper. It's basically ready to test on the lathe.