Limit switch overrun

Why don’t we position limit switches so that the machine can safely travel past them as it stops after triggering rather than ramming into them? I mean just as a safety precaution incase something goes all speed racer on ya.

I’ve seen advice elsewhere to allow for overrun of max_velocity^2/(2*max_acceleration).

(I apologize in advance if I’ve conjured up bad memories from phys class)

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I’m not following. What machine?

To travel past them you would need optical switches.

On the CNC’s you should not be ramming into them. The switch lever should trigger and stop the machine far before you make a hard stop.

Not necessarily. If you rotate the switches 90 degrees, so that the switch triggers when the stop block rolls past the arm, you can roll past the location without a problem.

This is the plan that I have for my laser, so that I can home to a 0,0 position, but still allow it to go to about -5mm or so in X and Y. I was originally going to use optical stops, but the 2 pin connectors are easier to deal with mechanical switches.

I see what you are saying. You most certainly could trigger on a different surface. I have always positioned my switches at the absolute edge of my build area, I waste less than 1mm for sure. If I do any adjustments, I go positive instead of negative. So mine are more of a limit switch and a homing position at the same time.

What is the advantage of allowing negative travel?

Not that I have… but for me it would be a convenience thing of allowing just enough to not have to adjust the home position for CAM. Allow just enough negative travel to have 0,0 be where the lower corner of the CAD is, and give the machine enough to be able to go 1 tool width negative, so maybe 6-7mm for 1/4" mills. This makes getting to 0,0 just a homing step, and not home, jog, then set coordinates.

Of course, on my Primo, I have workspace coordinates, which remain persistent after homing again, so it works fine for me as designed.

I think the original question though was maybe to allow more rapid movement on the first homing pass, or at least that’s what the equation posted would do.

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Just to illustrate, here’s my plasma torch mount that I’m currently modifying.

The switch will be fixed to the base plate. As the torch holder part rises it will slide by the switch rather than slam into it like I had it before.

If I remember correctly this is the way @uthayne designed his, which is what I based my mount off of

I suppose our gantries are just light weight enough and slow enough that it doesn’t really matter?


I honestly never really thought about it on the Primo, cuz mine re at the end, but I will keep this in mind for any future projects.

Doesn’t that make you pretty coupled to a specific switch type?

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Is that not a standard type of switch?

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Good catch. I have the rollers here and the omrons, they have a nearly 5mm difference in trigger contact distance.

That is a clever idea for the Plasma torch but I don’t think the design change is necessary for the other machines. For several of my previous machines I have achieved the same affect without hardware changes. Instead I adjusted the firmware.

When I wanted an axes to have 0 be offset from the limit switch I always adjusted the firmware to do this. I don’t remember the variable name in marlin but I think it was in the advanced Config file. So the machine would home, tap the switch, bounce back, approach the switch again slower, tap the switch, then move to the offset distance I set so the machine would start at my defined 0. Then the machine could go negative from what it thought was 0.

At one point I made the offset the center of my cutting area. So 0,0 was in the center. That worked but it wasn’t as convenient as I thought it would be.

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My question isn’t about homing zero so much as it is about building with the oh shit factor in mind.

When the switch goes high and the firmware steps on the breaks it always takes time and distance to come to a stop. Homing sequences will then back off until the switch goes low, then repeat really slowly to maximize accuracy. The question is is it better to let the machine glide to a gentle stop that first time or let it slam in to something?

(Not much of a question though, v1ers have shown it doesn’t really matter for V1 machines. More just something to talk about)

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There is a small benefit to being able to choose between hard stops and homing, rather than having to always use one or the other. (Although technically it is still possible by having hard stops at X-max and Y-max, it is annoying to jog all the way over there.)

I have endstops installed on my machine in such a way that the gantry can move substantially (maybe 10mm) past the point where the endstops trigger, so I can use hard stops if I want. For my double-decker this especially matters because I crash into the hard stops to square the Z axis, and then I home to square X and Y.

Most people would be able to get away with either hard-stops or dual-endstops, so it’s not a huge deal, but I don’t see any downside to being able to move a bit past the limit switch without breaking.

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