Ken Nicholas Photography Astronomy & DIY CNC

Emergency Stops

June 29, 2015

Well as we go along and all seems well with the world another small annoyance pops up.

Decided I was going to do a small job in aluminium, nothing too large just a small bracket to mount a 25 pin D connector for the breakout of port 2, this was to be the first metal cutting job and done with a 1/8th end mill.
Drew up the detail and generated the code and off to the machine, setup and away we go, about 10 mm into the first pass and I get an Emergency stop.
No apparent reason so reset rewind and start again, about another 10 mm further into the cut and another Emergency stop????
This process went on for another dozen times until the job was finally cut but unfortunately with a minor error on a restart from the wrong point in the code. Fortunately just a test.

Went out again today to find a reason for all this as the errors were a mixture of just "Emergency Stop" or "Emergency Stop with Limit switch"
So powered everything up picked up an allen key and gave one of the Limit switches a tap and bingo Emergency stop. Went around the other Limit Switches giving them a small tap and the same result, Emergency stop from a small tap causing a vibration as was most likely the case when cutting the aluminium. This vibration as it turns out is enough to cause the Limit/Micro switches break contact momentarily and this is enough to trigger the watchdog and bang an emergency stop.

So now I have to go to trouble of isolating the switches somehow to stop any machine vibrations from effecting them or change the setup to a "Normally Open" but this is a less desirable way to operate as one would have to do a switch safety check before each use to confirm there correct operation.

Just another annoyance of the problems of designing and building from scratch but as I work in a research and development environment this is what happens when you build something completely new.

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Due to the microswitches being sensitive to vibration, you may need to consider slotted optical switches. I would use lever type switches for the physical hard limits but use optical for working limits. I am designing an electronic tensioner for racquet stringing, where I propose using optical for normal operating ranges, and hard limit switches to cut power for when something goes wrong.

Thanks John,
Yes after a bit of investigation that turns out to be the case so I will be getting some optical switches to get over this problem.