Reply with QuoteProbably be destroyed in a run away.
Can I run my limit switches this way OR will they be destroyed.
Hey guys,
I've been running my table since June but I just neared completion of finally getting the limits all hooked up. I had someone over tonight not even into CNC but he question if the motors would stop fast enough to not wreck the limits. By the time the limit is switched, there is only about a 1/16" of play left over. He thought I should have mounted them the other way. Can you please tell me based on the picture if they are ok this way.
Thanks.
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Probably be destroyed in a run away.
I personally wouldn't do it that way, but it may be OK. In a runaway it probably would kill them, but in normal operation it would depend on a few things. The max speed of your machine, machine drag, screw lead, etc. will dictate how far the axis will travel after tripping the switch. If all of these things combined together are sufficient or deficient, depending on how you look at it, then tripping the switch could happen early enough to shut it down without damage. Also, in normal operation, at least how I have my machine set up, the axis limit will only be tripped during the homing sequence. For example a machine set up with homing and limits will home, and then the software limits will not even allow the machine to drive past the software limits which should be inside of the limit switches. The only time the machine could go past would be a true runaway, or a case of lost steps where the computer doesn't know anymore where the machine is at. For homing, you should be able to set the homing speed such that the machineisn't travelling fast enough to cause damage.
Hey, thanks for the replies. I would always hate to walk away from the machine and wonder if it's safe to blink. Since my post I was thinking of ways to slightly modify what I already have. I'm thinking of making some sort of L shaped bracket that mounts to the existing bracket. You're right with Mach and machine limits, I would 'probably' be ok but I would like to have piece of mind. It's only 4 brackets that would have to be changed. My Z axis is the correct way.
Thanks again.
It has always been my way of thinking that if your limit switch is bad, you would want to hit the hard stop asap anyway. So.... if you set your hard stop a few thousands away from your switch what harm is done? Chances are you're not going to be there to hit the e-stop within a few thousandths of a second any way. I"m thinking about building my limit switches IN my hardstops.
[FONT="Times New Roman"]Darryl[/FONT]
[URL="http://www.cnczone.com/forums/open_source_cnc_machine_designs/144485-you_might_redneck_if.html"]Redneck CNC[/URL]
I would mount the switch in such a way that the gantry would role over the switch to trip it, not a head type switch, that's why it is a rocker switch. But not seeing more of the machine its hard to suggest a good mounting area for the switch.
Thanks,
Joe
www.joescnc.com
joecnc2006 at yahoo
Well I just answered my own question today. Can't run them the way I have them in the pic. There is just too much momentum for the gantry to slow down and it bottoms the limit switch out. For homing, it's just fine. That's slow enough to stop with a little space to spare. I'm gonna use the existing brackes that I have and just mount something to the top of it and reposition the 4 limits.
Do you have the soft limits set up. Not sure about Mach3, but LinuxCNC will only go about 0.010" past a soft limit at rapid on my machine in jog. If a program has a command that would exceed the soft limits, it won't even start the g-code program.
No soft limits at this time. I guess my goal back during the build was to use my home & limits as dual purpose and then not use soft limits. I want to say that you either use machine limits OR soft limits, but you cannot do both at the same time.. Correct?
First off, you never want a hard bind if you can avoid it. That's hard on mechanical parts. I don't know about hard and soft limits on Mach, but I will know next week. If I was using steppers, and didn't have a choice, I would certainly only use hard limits. If you miss step, the control won't know and you can bind up an axis. I'm no stepper guy, everything I do is servo, but I do know they can loose steps, probably not as much as they used to though.
IMO, you can and should use soft and switch type limits together. Wouldn't you want to know in advance that your g-code is going to exceed your machine's limits before you execute that 10,000 line program that has a looooong run time? Soft limits allow the software to know this before you even start the run. Maybe you would only have to re-position the part slightly to bring it within the limits
Then the hard limits can come into play if you have a runaway or if you miss steps and the machine runs up to the limit switch
