Still confused with travel values

Hi all

Apologies for such a basic thread but I am going round the twist trying to get this right.

I am still confused regarding the values Table Travel, Home location limits, soft limits etc etc. I think this is what caused me to abandon EMC years ago and go with Mach3. This time around I don't want to that but before I go any further I need to get a grasp of this.

I just measured my machine and I have the following values:

X Axis: Hard Stops are 2590mm apart. Travel between limit switches is 2490mm.
Y Axis: Hard Stops are 1390mm apart. Travel between limit switches is 1290mm.
Z axis has no limit switches yet, I usually just go to max travel and set the machine coordinates to zero for Z.

For each of these the proxy sensor mounts are 100mm wide hence the difference between stops and limits

When Mach3 homes each of X & Y is backs off the switches 3mm effectively making the usable travel on X to 2487 and on Y 1287. I actually set the soft limits to be 2480 for X and 1270 for Y in Mach3 soft limit screen. Z is set to 0 min, -100mm max

So, translating this to StepConf is not going well, I am still confused o regarding the Home Location and Table Travel settings. I have entered the following for X:

Home Location: 0.00
Table Travel: 0.00 to 2480

Z is set to:
Home Location 0.00
Table Travel: -100 to 0.00

Am I close or am I still doing this wrong based on the values above? I suspect all the issues I am currently having is driven from this step so if I get this right I *should* be able to get going doing other things.

in stepconf:

Home location refers to where the axis will rapid to after the homing process.
It shouldn't be exactly on the soft limit ends (but I'm pretty sure stepconf will add or subtract a small amount to fix this)

table travel defines the length of travel (without hitting the limit switches) and the machine origin position.
The left box should be the low number and right the high number
The origin (zero) should be within the table travel.
I can't think of a reason of to pick a particular origin position most people but it a one end.

home switch location (which you didn't mention) is where the home switch is, offset from the machine origin.
most people use a limit switch as a home switch which would mean that the home switch location would be the same as one of the table travel entries.

So based on what I can figure out from what you have said your entries seem fine.
You didn't mention your actual problem.
Another thing to check is that the axis movement are in the right directions.
moving positive should make the DRO go more positive.
The graphical display should match the actual machine ( remember that AXIS shows movement based on the TOOL moving.)

Hope that helps
Chris M

Might want to look here...
www.linuxcnc.org/docs/html/config/ini_homing.html

Rick G

Thanks guys

Chris, the errors I am seeing are to do with limit errors and trying to sort out machine coords versus table coords. I'll start a thread in the right place on those once I get homing sorted as I think my problems relate to that in some way.

Rick, there was some things on that page which still confuse me, namely soft limits. What decides what the soft limits are and where does one find out what they are? In Mach there is a screen to define those but I can't find anything in LinuxCNC that shows me what they are.

I found this thread on cnczone where this guy uses LinuxCNC on a CNCRP machine: www.cnczone.com/forums/open-source-cnc-m...30-cnc-forum-10.html . He posted his configs so I am going to adjust mine and see what happens.

More testing tomorrow inbetween the Le Mans 24hr coverage

racedirector wrote:

Thanks guys

Chris, the errors I am seeing are to do with limit errors and trying to sort out machine coords versus table coords. I'll start a thread in the right place on those once I get homing sorted as I think my problems relate to that in some way.

Rick, there was some things on that page which still confuse me, namely soft limits. What decides what the soft limits are and where does one find out what they are? In Mach there is a screen to define those but I can't find anything in LinuxCNC that shows me what they are.

I would need the specific error to be sure but I will guess it was an error saying 'movement would be outside axis limits'.
This could mean your machine is configured right but you didn't set the user coordinate system right (touch off)
Need more info about what you are doing and what problems come up.

Soft (software) limits are what you are setting in stepconf with table travel entries.
In linuxcnc after homing it should be VERY difficult to ever hit the limit switches.
Linuxcnc knows where the soft limits are and will de-accelerate when close to them and stop before touching the limit switches.

Chris M

Hi Chris

Thanks for the explanation of soft limits, makes alot more sense now. Will investigate a way of over riding them due to things like missed steps, etc. I saw one screen that had an Override Limits checkbox but for the life of me I can't remember where that was! Old age kicking in....

Cheers

You can not override soft limits. You can override limit switches if you somehow contact one.

Why would you want to override soft limits? if you crashed and have missed steps you should rehome.
If missed steps is common enough that you wish to override limits something is wrong.

There seems to be a fundamental difference with Mach and linuxcnc in this capability.
I'm not sure Mach has the same absolute Machine coordinate system and linuxcnc.

It's just a different way to do things and linuxcnc is a stickler for it.

Chris M

cmorley wrote:

You can not override soft limits. You can override limit switches if you somehow contact one.

Why would you want to override soft limits? if you crashed and have missed steps you should rehome.
If missed steps is common enough that you wish to override limits something is wrong.

I know this is wroing but my Z currently doesn't have a limit switch, thats on my todo list. Occasionally I will crash the Z into its max position which upsets Machs interpretation of where the Z is. In that case I turn off soft limits, manually set where I want the Z and then "home" it by resetting the machine coords.

cmorley wrote:

There seems to be a fundamental difference with Mach and linuxcnc in this capability.
I'm not sure Mach has the same absolute Machine coordinate system and linuxcnc.

It's just a different way to do things and linuxcnc is a stickler for it.

Chris M

It looks like I have alot to learn still about LinuxCNC, I seem to be just scratching the surface so far. I am actually looking forward to learning as much as I can and using the system like it should be used instead of fighting with the software.

If you place your Z axis all the way to the top (maximum travel) in your case 0 and then home the z axis there (again 0) it should never crash at the maximum travel because of the soft limit.
After homing it jog to your Z working location, say the top of the material and "touch off" the Z axis in the coordinate system you are using normally G54.
This sets the top of your work as Z 0 in the G54 coordinate system but your soft limit does not change in the machine coordinate and thus will not effect the soft limit of the Z axis.
As Chris said you could only crash the Z axis moving up if steps were lost, or if you re homed the Z axis in some location rather then the maximum travel.

Rick G

As a side note when finished with the machine issue a...
G53 G0 Z0
This should return the z axis to it's home position (if no steps were missed) so the next time you run the machine check to make sure the z axis is still at it's home position before homing it.

This way you should not be able to crash the z axis by moving up. If you do however crash the z axis by moving down and hitting the material your best bet is to jog the z axis up to it's home position, re home it then jog the z axis to it's work position and "touch off" again.

Rick G