Archive for December 13th, 2017

MPCNC: Tool Length Probe Station

Having a tool length probe station on the Sherline, I had to build one for the MPCNC:

MPCNC Tool Length Probe - plotter pen

MPCNC Tool Length Probe – plotter pen

It’s little more than a flange atop a wide base:

MPCNC Tool Length Probe - Slic3r preview

MPCNC Tool Length Probe – Slic3r preview

The flange offset puts the switch actuator on the midline of the base, not thatĀ that matters, and the base features rounded corners and a suitable legend, because I can.

I clipped the PCB’s through-hold leads nearly flush and stuck it to the flange with 3M permanent foam tape, which seems to work much better than screws & inserts for simple things that need never come apart.

The Protoneer CNC Shield includes a Probe input on the GRBL-compliant A5, although it took me a while to find the legend on the SCL pin in the I2C header. I moved the endstop power jumper to another header, then conjured a quick-and-dirty connector:

Protoneer CNC Shield - Tool Probe Wiring

Protoneer CNC Shield – Tool Probe Wiring

When I embed the endstop switch PCB in epoxy, I’ll add a drop to the connector while engaging in Magical Thinking. The whole Arduino + CNC Shield must go into an enclosure after I finish measuring the motor currents.

To forestall discussions about switch repeatability and accuracy, suffice it to say the MPCNC doesn’t claim to be much more than a woodworking router, so thoseĀ switches seem Good Enough.

The OpenSCAD source code as a GitHub Gist:

The original doodles show a severely over-complexicated solution desperately searching for an actual problem:

MPCNC Tool Length Probe - doodles

MPCNC Tool Length Probe – doodles

Putting a large flat pan at the end of a relatively long lever arm, with the pivot arranged to put the pan level at the switch actuation point, made sense at the time. Give the relatively small tools I expect to use, directly ramming them into the switch lever should work just as well.

Putting all that complexity in harm’s way seemed like a Bad Idea when I sat down and looked at it in cold blood.


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