Limit switches while operating?


(Mike) #1

This is probably another newbie question, but I’m somewhat confused by the limit switch functionality. Do the limit switches only function when you first power up the machine, press job and it homes to the back right corner? I mean, once you zero it to start cutting something, if for some reason your design causes the machine to reach the limits, shouldn’t the switches stop it from going beyond what’s possible and slipping on the belt? Or since it’s been homed, shouldn’t it always know where it’s at and never allow you to exceed the table dimensions in any direction? I’ve made some mistakes recently where I loaded something that went outside of the limits and had to rush to get it stopped as the motors kept turning (of course it was right after getting the steps / mm perfectly calibrated earlier that day). One other thing I noticed recently; I thought that after homing it wouldn’t allow you to manually jog outside of the limits / force the belts to skip; however, it did yesterday as I was trying to find the limit on the upper left of the machine for surfacing the waste-board. Are my machine settings wrong?


(mikep) #2

They aren’t really limit switches, they are homing switches, and only used when homing the machine to provide a known location reference. You can set up “soft” limits, in which the machine keeps track of it’s location vs. the size you tell it the machine is, but this doesn’t seem to work well for everyone.

The machine isn’t powerful enough to bend anything (mostly) and will just run into the stops and make a lot of noise (skipping steps) if it runs into the physical limits of the machine. That said, it’s possible to run an endmill into the front plate of the machine depending on how your wasteboard is set up, which might or might not break the endmill…and might or might not cut a slot in it. You’re way more likely to run into your clamps.

Running into the physical limits of the machine generally won’t hurt your belt calibration (it could I suppose if you just left it that way, and maybe eventually break a belt).

Regarding jog outside area - your settings -might- be wrong.


(Neil Ferreri) #3

In addition to what Mike said, in your grbl settings $20 enables / disables “soft limits”. So, $20 = 1 will enable soft limits that won’t allow you to jog past the limits of your machine. This is assuming your machine is homed, your max travel settings ($130, $131, $132) and steps/mm ($100, $101, $102) are accurate, and there are no mechanical issues. No harm in enabling the soft limits if the other settings are accurate. The controller will enter a feed hold state and then alarm as soon as it receives a command that would take it beyond the limits.
$21 = 1 will enable hard limits. Basically that will turn your homing switches into limit switches. The problems with hard limits are:

  1. You lose position, and the machine must be reset and re-homed.
  2. Physical switches are prone to EMI, especially whn the spindle and dust collection are going. You may receive false detections.

(Mike) #4

Great information. Thank you both. I wouldn’t have even thought about getting into this hobby if it wasn’t for such a great forum.

Thanks!


(Tim Foreman) #5

That’s actually not true. They are hard limit switches if you have $21 = 1. If the machine runs into them while machining it will stop and error.

However, if you are going fast enough, the machine will run into the end plates even if it trips the limit switches.


(mikep) #6

Fair, however

  1. Neil mentioned that.
  2. Doing this makes machine work inconsistently - limits only work at the home position. Not at the other extents. I think that’s actually more dangerous than no limits at all. Inconsistency has shown to be a huge source of “people” mistakes.
  3. As has been mentioned elsewhere (often, and Neil above), people have had problems doing this (noise) causing intermittent stoppages. Soft limits are limited by skips, which are really common in the situation where you run into something, like a mechanical limit, or a crash, which ends up being exactly when you want them to work every time, and they won’t.
  4. They aren’t intended as limit switches, they are intended as homing switches only. Given how many people have had their switches fail, I wouldn’t really rely on them as a safety of any kind.
  5. Limit switches, in my opinion, though not a bad idea, really shouldn’t be relied on - you should know what the machine is going to do. Mistakes do get made, they always will, but given that, again, you’re really not going to hurt the machine by running into the stops. The belts don’t slip - the steppers skip steps first (unless your belts are too loose, in which case, you’re still not hurting anything)

(Tim Foreman) #7
  1. I have limit switches on both ends of travel on the X and Y. So they are indeed, limit switches.

  2. Intermittent stoppages due to limit switches are due to electrical noise and should be addressed. I used shielded wire for my limit switch wiring and have no issues with stoppages.

  3. I don’t know how you get “intention”. They are switches at the limits of travel. They are “limit switches”. I’ve not had any switches fail, but I’m not using the Carbide 3d switches.

  4. The majority of the times that I trip my switches are when I’m jogging and hit the button too many times or accidentally have the distance wrong. I’d rather trip the switch than have the motor start cogging.

But however you want to run your machine is up to you. You can do what you like. Keep in mind that these machines are kits, and some people have different upgrades installed. We can all learn from each other but lets be careful about speaking in absolutes.