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Improved Sherline Probe Length Switch Repeatability: Selah!

Re-running that probe length switch test a few weeks later produced these results for three trials over the course of two days.

Probe Repeatability - Dec 2010

Probe Repeatability - Dec 2010

The Z-axis differences are all relative to the first reading on the first day, so this includes whatever Z-axis changes take place without doing anything else on the mill in between the tests. I turned the power off after making the first set of measurements, so the steppers restarted with up to a plus-or-minus one full step offset; that works out to:

(0.050 inch) * (1 rev / 200 steps) = 0.00025 in = 0.0064 mm

Because EMC2 doesn’t actually know where the stepper is, any uncommanded motion will show up as an offset when the probe switch trips, which is exactly what we see here.

Two things of interest:

  • The -0.05 mm offset between the two days could well be part of a single step offset
  • Successive probe positions during a single test don’t change by hardly anything at all

Conclusion: a cheap mechanical switch works just fine and an even cheaper switch was still good enough.

The dataset looks like this…

Trial	16 Dec	16 Dec	17 Dec A	17 Dec A	17 Dec B	17 Dec B
0	25.859616	0.000000	25.806118	-0.053498	25.810032	-0.049584
1	25.860282	0.000666	25.808900	-0.050716	25.810696	-0.048920
2	25.863610	0.003994	25.808214	-0.051402	25.813354	-0.046262
3	25.863610	0.003994	25.809368	-0.050248	25.812028	-0.047588
4	25.864276	0.004660	25.810032	-0.049584	25.812025	-0.047591
5	25.862945	0.003329	25.812162	-0.047454	25.814018	-0.045598
6	25.864941	0.005325	25.812690	-0.046926	25.812719	-0.046897
7	25.864276	0.004660	25.810696	-0.048920	25.813540	-0.046076
8	25.864276	0.004660	25.812690	-0.046926	25.814012	-0.045604
9	25.864276	0.004660	25.813354	-0.046262	25.812690	-0.046926
10	25.864941	0.005325	25.813477	-0.046139	25.814018	-0.045598

The raw data, just for completeness…

Probe test data - Dec 2010

Probe test data - Dec 2010

Selah!

(I’m giving a talk and show-n-telling my Sherline CNC milling machine at Cabin Fever Expo right about now, so having this data readily available seemed prudent. The talk & handouts are there.)

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  1. #1 by david on 2011-01-15 - 12:44

    I bet you meant to say one step is .050 *mm*, not .050 *inches*. :)

  2. #2 by david on 2011-01-15 - 12:45

    ignore me: the view from pocket ie cut off that calculation after the first )…

    • #3 by Ed on 2011-01-17 - 07:20

      There’s a tired joke in there somewhere, with the punchline involving IE, but I just won’t kick that horse in the teeth…

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