|Forum topic by JAAune||posted 10-23-2014 04:03 AM||2140 views||0 times favorited||10 replies|
10-23-2014 04:03 AM
This topic might have fit better in the blog section but I thought the CNC people would probably fail to see it and they are the folks who’d be interested in reading this.
I’m not the only one around here using a shop-built CNC router and sooner or later, everyone will encounter the situation none of us wants to face: malfunctions.
Last week I had the router running with a new program. I failed to spot a mis-aligned cut in the simulation and the result was a 1/2” bit trying to hog 7/8” of hard maple in a single pass. With the IPM the machine was running, the steppers stalled and the router almost did the same.
A hasty reset prevented excessive damage to the parts being cut but the router itself kept running which wasn’t right as it’s supposed to shut down on a reset. I soon discovered that the router would always run and at full RPM anytime I had the CNC turned on. No shut-off or speed control was possible from the PC.
Circuit boards are not my skill set but I did take a class on building the things and we had an evening of diagnostics lessons. So out came the multimeter and schematic documents.
The router has a 120AC line and a speed sensor wire going to a Super PID board in the control box. A laptop running Mach3 is visible at the edge and it connects via an ethernet cable. Basic electrical knowledge indicated that the router was getting full power when it shouldn’t have which pointed to a problem with the SuperPID device that controlled the flow of electricity to the router.
The control box is wired in an orderly fashion and all wires are numbered which helps a lot with diagnostics.
Here’s the SuperPID board. No visible switches so I looked up the instructions to see the schematics.
Since the machine runs with Mach3, the important terminals are “run” and “PWM”. Without power to “run”, the router will default to always on. PWM is the controlling signal from the computer. Both terminals are on the 5-volt system.
That’s the 5-volt supply.
As I learned in class, diagnostics consists mostly of using the multimeter to trace where power is going and where it isn’t until something is discovered that’s out of spec. The black lead gets touched to the negative on the 5-volt supply and the red lead is touched to various terminals on the 5-volt system until a reading that fails to match the schematics is discovered.
I was mostly interested in “run” and “pwm”. “Run” tested out fine at just over 5-volts. “PWM” also tested out properly since it showed 5-volts when I gave the spindle on command from Mach3 and showed 0 volts when I gave the spindle off command.
So the SuperPID gave every indication of functioning properly from the control side. The problem had to be on the 120v side that sent juice to the router. Without electrical knowledge, I had no idea where the switch was and went to Google to run searches on variations of “SuperPID” and “router always running”. An hour of searching unearthed a forum discussion that brought up the TRIAC.
One more search and a YouTube video on TRIACS was all the information I needed to make the diagnosis. TRIACs are devices used to control the amount of current passed on to an electrical device like a motor. The excessive cut I’d subjected the router to likely caused the SuperPID to send more power through the TRIAC which created a permanent short.
That is the TRIAC. It has the part number printed on the top so another Google search brought me to Mouser Electronics where the part was sold. $8 for a pair of TRIACS and $15 for a tube of thermal paste plus shipping was the total cost of the repair.
The TRIAC was swapped out this afternoon and the CNC is fully functional once again. Hopefully other owners of home-brew CNC equipment find this little experience of mine helpful should they ever encounter that moment when their machine malfunctions.