The suicide test episode.

A project log for Sweet Dreams CNC (multimachine)

There are solutions and there are elegant solutions.

Crum3leCrum3le 09/26/2015 at 19:530 Comments

The suicide test;

So the machine has progressed past turning it on without any magic smoke pouring forth, all the bells are ringing and the whistles whistling.. (why don't bells belling?) spindle has been powered up and a couple of small tests run. But I'm still air cutting at this point. ...The commissioning continues.

Time to up the ante.

Bearing in mind that body parts may well be found in the same vicinity as gantries and other moving parts, cue the Suicide Test.

For the benefit of the casual CNC visitor, The suicide test is conducted as follows;
1. Set gantry at one end of the X axis.
2. Set speed in excess of 30 metres per minute.
3. Hold down the button and drive the gantry at the highest speed you can get into the stops at the other end of the X axis!

4. Wait for the very loud bang.

5. Repair as required.

I hit the switch and even though I knew what was coming, there was a whirr and a blur as the gantry shot from one end to the other faster than my eye could follow. There wasn't time to take my finger off the button before the limit switches did their job. I don't have an exact figure of the speed it reached but I can tell you that the gantry was still accelerating when it crashed into the the Omcron microswitches at the far end of the profiled rail.

There's 10mm of switch protruding from the box section upright. These particular switches have 3mm of movement, .1mm between open and closed. The speed of the gantry when it hit the switch can be calculated from the time taken between the switch being activated and the distance traveled before stopping. The speed of light and the price of fish being unequal, the gantry was travelling in excess of a "greased lightning" when it got to where it was going!

Confidence went much higher after that test. The limit switches worked great, even in the worst case scenario of a runaway gantry, the worst that I'd expect to see was a crushed switch as the gantry didn't get as far as hitting the metal frame uprights which would have been an interesting end to the frame, and X axis drivetrain.

If you are building a machine, fit the limits and E-stops before you wire the motors. Watching a gantry move at high speed is really not something you want to see from the wrong place. If your gantry moves as quick as this one did, you'll never make it to the power switch in time.