Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: rceebuilder on May 11, 2008, 06:46:34 PM
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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
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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
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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
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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.
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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.
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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.
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.
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
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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.