Archive for November 9th, 2017
Being a big fan of having a CNC machine know where it is, adding endstops (pronounded “home switches” in CNC parlance) to the Mostly Printed CNC axes seemed like a good idea:
All the mounts I could find fit bare microswitches of various sizes or seemed overly complex & bulky for what they accomplished. Rather than fiddle with screws and nut traps / inserts, a simple cable tie works just fine and makes the whole affair much smaller. Should you think cable ties aren’t secure enough, a strip of double stick tape will assuage your doubts.
A snippet of aluminum sheet moves the switch trip point out beyond the roller’s ball bearing:
I’m not convinced homing the Z axis at the bottom of its travel is the right thing to do, but it’s a start:
Unlike the stationary X and Y axes, the MPCNC’s Z axis rails move vertically in the middle block assembly; the switch moves downward on the rail until the actuator hits the block.
Perforce, the tooling mounted on the Z axis must stick out below the bottom of the tool carrier, which means the tool will hit the table before the switch hits the block. There should also be a probe input to support tool height setting.
The first mount fit perfectly, so I printed four more in one pass:
All three endstops plug into the RAMPS board, leaving the maximum endstop connections vacant:
Obviously, bare PCBs attached to the rails in mid-air aren’t compatible with milling metal, which I won’t be doing for quite a while. The electronic parts long to be inside enclosures with ventilation and maybe dust filtering, but …
The switches operate in normally open mode, closing when tripped. That’s backwards, of course, and defined to be completely irrelevant in the current context.
Seen from a high level, these switches set the absolute “machine coordinate system” origin, so the firmware travel limits can take effect. Marlin knows nothing about coordinate systems, but GRBL does: it can touch off to a fixture origin and generally do the right thing.
The OpenSCAD source code as a GitHub Gist: