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limit switch response
« on: May 11, 2008, 06:46:34 PM »
 Hi everybody,

I have built a new  router table running with Mach3 . It works very well so far. Last  night I have added  limit switches and tried if they work well. My gantry can move  up to 20 inches  per second .  I have tried the worst case and  I jog  the gantry with maximum speed and it hit the limit switch . As you  can imagine this big mass  can not stop suddenly .Then  a new  approach come to my mind. Can't we configure to limit switches  as  to give a stop signal to stop the moving axis  with the  preset deccelaration value ?  Or  can't we add  a new  switches front of the  real limit switches to do this job?

Rceebuilder
« Last Edit: May 11, 2008, 06:53:50 PM by rceebuilder »

Offline Hood

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Re: limit switch response
« Reply #1 on: May 11, 2008, 07:43:47 PM »
Set up softlimits, thats what they are for. You can set a slow zone in your softlimits and it will slow down when in that zone. Hardlimits are meant as a last resort to try and save your machine from damage if all else fails.
Hood
Re: limit switch response
« Reply #2 on: May 11, 2008, 08:11:27 PM »
 off the track here but hood has addressed you problem.

 I like the look of your machine.  I am building along similar lines. 100x100 Steel, Rack and Pinion, Cut area 1900x1250x260.

How did you go getting the rack and pinion alignment accurate - Mine works really well but was difficult to get it to run smooth with the limited equipment I had to  work with.

Bruce

Offline jimpinder

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Re: limit switch response
« Reply #3 on: May 12, 2008, 03:00:45 AM »
Moving that at 20 inches per second - phew I can imagine the crash at the end.

Hood gave you the idea of soft limits which can be set inboard of the limit switches. My limit switches are small laser (pen type pointers) shining onto photo-detectors and therefore do not come into contact with the table - they are very reliable and accurate - and on your table would be easiliy fitted. The beauty about them is they do not get wiped out if the table over-runs for any reason.

I would tend to do the soft limits, laser type switches, and then either solid rubber buffers, or a small hydraulic piston (railway buffer stops) right at the physical limit of the run. I wouldn't want that lot dropping on the floor.
Not me driving the engine - I'm better looking.
Re: limit switch response
« Reply #4 on: May 12, 2008, 05:40:25 AM »
 I know  the  soft  limits  issue. But  it  requires   to  homing   all  axes  on  every  start  up .I'm  affraid  sometimes   it  can  be  forgotten then  soft  limits  can  cause  to  stop  the  machine  in  the  middle  of  the  process. Is  there  another  way ?

 Bruce  thanks  for  your  comment ; to  set  up rack  and  pinion I have  milled  the  rail  and  rack  surfaces   with  a  plano  miller. I  would  prefer  to  use  ground  rack  and  pinion but  they are  too  expensive and  hard  to  find. I saw  ground  ones  in a  exhibition . Pinion  was  rolling  on the rack like  a  dream..

Rceebuilder.

Offline Hood

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Re: limit switch response
« Reply #5 on: May 12, 2008, 09:06:34 AM »
You need to home everytime you restart the Mach, this is a standard on all machines, PC or otherwise, that I have seen, you must home when you start the control otherwise the control is never sure where the machine position actually is.
 
Quote
Can't we configure to limit switches  as  to give a stop signal to stop the moving axis  with the  preset deccelaration value ?
If you did that then you would not have hard limits switches you would in effect have physical versions of the softlimits and you would also be reducing the work size of your machine as you would need these limits inboard of your actual limits. If you then had these limits inboard of your actual limits then why would you need this decelleration area as you would now have that built in.
Quote
Or  can't we add  a new  switches front of the  real limit switches to do this job?
Yes this is possible I would think with a macro, you could take the signal from these switches to lower the feedrate override to something you thought to be a safer speed.
   However I do not see what your aversion to homing is, especially when you have such a rapid machine. You would be better writing a macropump or Brain that looked at the Home LEDs so that if the machine was not homed you would not be allowed to start any code, that way you would never forget to home your machine and you would have a much safer environment.

Just my thoughts on the matter :)

Hood

Offline jimpinder

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Re: limit switch response
« Reply #6 on: May 13, 2008, 04:03:46 AM »
I must agree with Hood

Although you don't think you are homing, you must by definition, tell the machine where it is in relation to the workpiece before you start machining. That is homing in the same way as running to the end of the limits and touching a switch.

Because Mach can use limit switches as homing switches, there seems to be a misconception that home switches MUST be in some inconvenient place. Depending on the type of switch, you can have the homing swtich right in the middle of the machine.

As I said above, I use a laser shining on a detector, it is accurate to 2/10 of a thou. I have several detectors on a bar, and can line up to which one I wish, depending on the job in hand.

Even without homing switches, you can set your soft limits up inboard of your physical limit switches. Soft limits run in Machine Co-ordinates, not program co-ordinates, so therefore once set should guard the ends of your machine. Provided you do not then mess about with your machine co-ordinates you should be alright.
« Last Edit: May 13, 2008, 04:09:27 AM by jimpinder »
Not me driving the engine - I'm better looking.