Setting up soft limits in UCCNC .. "Logic"

[John11668]

Hi folks .
Trying to get used to UCCNC
Am trying to get my head around Soft limits and having some difficulty.. Not really intuitive could we say?
So taking things one step at a time lets look at a single axis, for which of course we will choose X

So I have entered the settings for the home switches and they work, home and display resets to zero. so I am fine so far .
I select to use soft limits in general config
My thinking is that I want "soft limits" to define a work area in x positve which is bounded by the lower limit ( say 10) and an upper of say 350. I figured that the software would then stop any motion at 10 and prevent the machine hitting the home limit switch.
So I set lower limit to 10, upper to 350 but now when I home , the table goes all the way to the switch and DRO resets to 10 ??? says soft limit is reached (when actually the home switch has been reached) and a "write offset on homing" appears as 10. So what is the logic in this ??
I have read the manual , which isn't giving me clarity other than i need to set numbers for the soft limits.
I have sought for video tutorials but so far haven't found any which clearly explain what is going on here

I am not getting my head around it . I need to get this clear in my mind before I move on to Z axis and begin to regale you all with the issues setting the soft limits on there.

[ngwagwa]

Just set lower limits to 0

[John11668]

Just set lower limits to 0

OKay ngwagwa and thanks for response , but that doesnt really clarify in my mind what soft limits are trying to achieve . it sounds like a fudge to overcome the problem

[Doddy]

I'm not at my machine at the moment, and barely remember this - but can you not set the axis to, after homing, traverse to a fixed offset (within the working envelope, and within the desired soft limit). E.g. home to zero, move to 12, with a soft-limit of 10?

EDIT: I'm thinking of what I do with my Lathe which is LinuxCNC, rather than UCCNC, so the above might be a red-herring of the grandest order. Take with a pinch of salt.

[John11668]

And Doddy


I presume I can do that by calling up an offset in the code , but am wanting to understand the process too.
Having declared soft limits , am I then homing to the soft limit and writing off 10 mm of my table

[ngwagwa]

[QUOTE=Doddy;119103]I'm not at my machine at the moment, and barely remember this - but can you not set the axis to, after homing, traverse to a fixed offset (within the working envelope, and within the desired soft limit). E.g. home to zero, move to 12, with a soft-limit of 10?

Just tried that Doddy and when the machine backs off after going home it only goes to the softlimit value - so would work if back off and softlimit was set the same.

John. Softlimits prevent machine going past the limits to prevent a crash and (on my set up at least) home switches don't limit movement when jogging.

If you look in configuration - Axis Setup for each axis there is a back off units box - set this value and when homing the axis will move back by te set value - make sure you set it positive or negative s required.

Softlimits will reduce the size of the working area - I have X backoff as -3 and softlimits at 0 so I don't lose anything.
Hope this explains it ?

[John11668]

So are we saying that the ONLY reason for softlimits is to prevent the machine traversing beyond the home switches and crashing into endstops ( and I guess killing the traverse if we try to go beyond the positive limit, regardless of whether switches are fitted there) . in which case it seems to make sense to have a soft limit of zero and maybe an arbitrary back-off of say 5 , (and I guess that wound be +5 to bring it back within the working area)

So then I guess that I choose an offset in my G code routine to bring the tool to a new start position on my workpiece , Is that so ?

[ngwagwa]

So are we saying that the ONLY reason for softlimits is to prevent the machine traversing beyond the home switches and crashing into endstops ( and I guess killing the traverse if we try to go beyond the positive limit, regardless of whether switches are fitted there) . in which case it seems to make sense to have a soft limit of zero and maybe an arbitrary back-off of say 5 , (and I guess that wound be +5 to bring it back within the working area)

So then I guess that I choose an offset in my G code routine to bring the tool to a new start position on my workpiece , Is that so ?

Correct re purpose of softlimits ie prevents crashes at end of travel.

Whatever back off you set you don't need to alter the gcode as Zero is still zero. So to clarify on my machine with a back off of minus 5mm after machine has been homed the DRO will display -5mm when displaying machine co-ordinates.

[John11668]

Correct re purpose of softlimits ie prevents crashes at end of travel.

Whatever back off you set you don't need to alter the gcode as Zero is still zero. So to clarify on my machine with a back off of minus 5mm after machine has been homed the DRO will display -5mm when displaying machine co-ordinates.

But doesnt a backoff of -5 take you beyond the home limit presuming you are homing in negative direction from an x positive position on the table .??

[JAZZCNC]

So to clarify on my machine with a back off of minus 5mm after machine has been homed the DRO will display -5mm when displaying machine co-ordinates.

I'm not sure that it works quite like that. It's been a few months since I set up a UCCNC machine but I think it Backoff works like most other controllers.? Which is that it looks for the Home switch then after hitting the switch Zero's the DRO, then backs off the set amount and Re-Zero's DRO again. When finished the MACHINE coordinates will show Zero.

It does this so you can have your HOME position or MACHINE ZERO at any point on the machine ie: Centre of travel, away from where the switch is physically located.

SOFT LIMITS are ALWAYS taken from MACHINE ZERO and limit the travel in the + & - directions. So in my Centre of travel example above with 1000mm of total travel, you would have Min soft limit = -500 and Max soft limit =+500.

If MACHINE ZERO was at the switch then soft MIN =0 Softmax =1000

Hope that helps clear a little of the confusion.!
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