I’ve been running into an issue where jogging the machine to the limits results in a limit trip followed by a motor fault. Sometimes reaching the limit without the fault seems to result in lost steps. Running a job that worked at the limits of the machine resulted in intermittent job stops with a limit trip, but backing off the limit sensor runs into the hard stop.
I seem to have a couple of different issues going on here and I’d like some advice.
The frame is a bit out of square (about 1.4mm on a 2’x4’ frame), and a result is that with the default travel limits the Y axis bumper is hit on the one side well before the default travel limit is hit. While the frame is only out about 1.4mm on the Y axis, the travel bumper on the right side of the table seems to be hit about 2.2mm before the bumper. I’ve tried reducing the Y axis travel limit and adjusting the limit sensor, but I don’t think I have this right yet.
I think I am also seeing a similar issue sometimes on the X axis, as I had a job that mislocated on the X axis by about 1mm between after jogging away from a limit fault, but I haven’t been able to trace it.
A few specific questions:
How much room is there for reducing the bumper to a slightly shorter bumper to regain the Y axis travel on the one side? How close to running off the gear rack are things if I get a shorter bumper? It looks like I could source different bumpers from McMaster Carr without much trouble, but I have no idea what hardness bumper is appropriate. Ideally, the homing + travel limit should mean the bumper is never hit of course, but realistically this could still happen.
How much variance is there in the limit switch detection distance? How far should the software limit be backed off the trigger point of the sensor? I’ve tried backing off 0.2mm from where I can see the limit trigger, but I still sometimes see the limit trigger when jogging at full speed to the limit. What is best practice here?
I have found it pretty hard to get things down to that last mm with the hardware. Not sure why, probably length inaccuracies in the 8020 rails. Everything is tight as far as I can tell, and I really don’t want to pull the whole thing apart right now and start indicating / shiming the railing to get things lined up. I figured 1mm was well within what the auto squaring should cope with.
I have a whole mdf frame and box bolted inside the thing right now to add rigidity. I would need a few days to tear it all down, and then I need to redo all the alignment of the spoil board to the machine which is good within 0.001" right now. If that is the only solve I will do it, but that seems like I might be chasing my tail a bit.
Maybe as a mid winter Christmas project I will pull the whole thing apart, but if I am doing that then I want to check the rails for parallelism and other things as well.
Yup. I have the gantry square by this method to within about 0.002", measured with an long inside caliper. Turns out my 2x4 table is just small enough that my longest rod caliper can do the measurement.
This does not mean the underlying frame is as square. That is off as I say by about 1.5mm.
The result is that after the gantry is squared by the hard stop homing the left side is a bit closer to the front stop than the right side, and the right side effectively has a shorter travel distance before hitting the back stop if that makes sense.
I either want to figure out how much to shorten the y limit to be reliable, or to shift the sensor to keep the Y limit at 24" if that is possible and safe.
I would rather not tear apart the table and get the frame square to within a fraction of a mm just to get back 2-3mm of working space.
Can’t you just change the y soft limit through the configuration Wizard? You just want the gantry to travel a little bit less than it is right now, correct?
Soft limits are very reliable in CNC12. I’ve never gone beyond the set soft limit if I’ve homed the machine properly.
Yes, I was trying to figure out how close to the sensor trigger I can put the soft limit. I backed it off .2mm and it does not trigger when jogging slowly, but sometimes if I go all the way across the table it was still triggering. I can play more tonight with the back off, but I was trying to figure out if there was guidance on how much room to give it.
Also if I could cheat and put on a slightly shorter bumper on that side.
There’s a slightly hidden utility you can use in the “UTILS” menu. It’s called “travel limit setter” This will seek to your sensors for you and set the max travel your machine can actually do based on where the sensors are in real life.
Run it and it will walk you through it. You can always go back to “factory” soft limits by re-selecting them in the wizard.
Ran this and it took about .3mm more away, but I can reliably jog without hitting any motor faults or sensor trips now.
Good stuff. Rebuilt my model of the table in fusion for my fixturing and got my new table surfaced.
I am trying a triple layer of MDF this time with 2 layers below the table surface and fitting into the openings between the cross members. I have bolted supports onto the cross members to hold the whole thing on ledges all around as well as the top layer being bolted to the table. All three layers are glued together and I bolted t-track in parallel to the Y axis every 100mm or so.
With the new spoil board and mdf sides and bottom bolted on to the frame I have added a couple of hundred pounds to the frame and increased the damping and rigidity a bit. I can see the difference in the movement, but I don’t have a way to measure it scientifically. I will try some brass or aluminum machining maybe later this week to see if the cuts are cleaner.
