Never crash my end stops again!

My CNC mill converted to a 3D printer does include end stops for all three axes. I use the Z end stop as a means of shutting down the system when a print completes. Because of this, I need to temporarily bypass the end stop to run the machine off of Z max. I accomplish this by using an RS232 style connector for the end stop and e-stop inputs. Unfortunately, on too many occasions, I started using the machine again without reconnecting this plug.

My anti backlash lead screw nut for the Y axis broke previously, as described in this previous post. I laser cut a Plexiglas part to fix it, but that part was pretty fragile. Under normal circumstances for 3D printing, it held up fine. One crash on the Y and stop and it snapped like a twig!

I happened to find a replacement anti backlash nut that was in fact identical to the original for a reasonable price. It was NOS (New Old Stock if you have not heard the term) on eBay, which was a lucky break. Purchasing this thing new would cost an arm and a leg. I once again disassembled the machine and did the job, which took a few hours. Later on I once again backed the machine off of Z max but did not reconnect the interlock plug. I had one short crash on Y and resolved to NEVER let it happen again! Replacement parts are tough to find and tough to make, and flipping the machine to repair it is a serious pain in addition to risking breakage of other parts in the process.

My solution was to implement a push button, temporary Z end stop bypass so that I can bring the machine off of Zmax after a print job completes, but that allows me to screw together the connector for the end stops and e-stop. This push button switch uses the NC (Normally Closed) connection to bypass the Z interlock while the button is pressed. This way if I hit an X or Y end stop and trip the interlock – something that had better be very rare – I can unscrew the connector, back off of Xmax or Ymax, and screw it back together. Basically, this makes is inconvenient enough to keep me from carelessly leaving it unattached and allowing a future crash. All I need to do now is hold the bypass button while resetting the e-stop in Mach 3, back the machine off of Z max and then let go of the bypass button. I do not need to detach or reattach anything. Where I used to work we would call manual steps like disconnecting and reconnecting something an “administrative interlock” which means it involves a person doing something. In principle, this is pretty much always a bad idea for a high reliability system because it is just too easy for a person to slip up. A real interlock triggers automatically when something bad happens, and does not let you operate unless everything is back in order. Now my system is more like the latter case.

I’m sharing this because I bet lots of other people face this problem. If you are using small enough stepper motors, an end stop crash will probably not hurt anything, but with Nema 23 motors, you can do some damage. We have a Bridgeport at work that uses huge servo motors, and there is no fooling around with that thing. It looks like it could rip your arms off! I suggest to everyone that A: you should use hardware end stops and B: you should implement a system to make sure they are always on line. Your machine will thank you with less repair requirements.


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