What happens with limit switches???

[itsme]

Hi there,

I'm just in the process of converting a mill to CNC and have decided to use TurboCNC to run it. I've had a good look at TurboCNC v4.01 and how it is all setup, but need some help with limit switches.

I'm clear on how to wire it all up and then set it up in TCNC to make them work, but I have absolutely no idea what the software is going to do once the machine hits a limit. Does it just stop the axis that hit the limit? Does it stop all axes? Does it have to sorted out by going into jog mode and 'reversing' out? How does it work????

Thanks
Warren

[Bubba]

itsme,
When you have the program PROPERLY set up, the axis will move to the limit switch at the Homing speed set in the ini. When the switch "trips", the axis will reverse until the switch "un-trips". Depending on the type of switch, this could be several .010s" or it might be nil in the case of an optical.
If you do a G28, ALL axis will move to home and each one will stop. Then each will reverse direction until home. You can (when the program is working right) home a single axis in the jog screen and it will act as indicated above.
IF it just goes to the home switch and stops (no reversal) then you do not have the paramaters properly set up (as me how I know
If you want a version of 4.01 that will do single axis homing under G28, let me know as I have compiled a version that works for me.

[Monte]

Below is a snip directly from the manual. This is for Limit Switches, not Home Switches. Of course proper wiring and program configuration is assumed.

Monte


"If you hit ANY limit switch, the machine won't move again until you either disable the switch, or use Jog mode to back away. These are for preventing "going off the rails" only - use the home switches for calibrating the machine."

[itsme]

Hi there,

Thanks for the replies - they've cleared up a few {a lot} of the issues and created a few more...

Bubba, it sounds like you've set up your limit switches to act as home switches as well. Am I correct in thinking this (ie. is it possible to use the same switch for home and limit?)? If so, how exactly does the software work for the home switches? Do you tell it to 'home' and then it goes in a certain direction (depending on setup) until it hits the home (limit) switch and then reverse out a bit like you explained? Would this position, just off the limit switch, be home or can the machine move back further to make home a different point?

I have always avoided the subject of home switches, mainly because I never knew what they were exactly or how they worked, so I may as well learn about them sometime (like now)... I'll also go and play around with the settings in TCNC to see what I can find.

Thanks again
Warren

[CNCgr]

I haven't set my home switches up as limit switches yet but I intent to. It is supposed to work like this: you use 3 home switches and 3 limit switces (wired in series). If you use a home command it does as Bubba said. If it trips a home switch while NOT on a homing move it works as a limit switch.

Originally Posted by itsme Would this position, just off the limit switch, be home or can the machine move back further to make home a different point?

No, you can't go past the limit switches unless you use a manual bypass switch/button. You can, however, set the home position to be something other than 0, e.g.30mm. This way when it trips the switch it sets itself to 30 and it's like the home position is 30mm futher away, you just can't go there.

[itsme]

Hi there,

Thanks for the info.

What I am now thinking of doing, is using 1 limit and 1 home switch per axis that are wired in series (like you said, CNCgr). The 3 axes will be individual (wired in parallel???).

Is it possible to make the home position something like -5mm (negative five)? That way, when the switch is tripped, it will make home -5mm, so that the code can go to 0 and still have some clearance before it hits the switch???

I hope this makes sense.

Warren

[Bubba]

itsme,
IF I am reading you correctly, YES. A lot will also depend on the type of switch used. For instance, if a microswitch is used, there is hysterious (sp?, I spell so bad even the spell checker can't figure it out As a result, it will trip out as some point and then move back. This will give you exactly what you want, to be able to move to 0 and not trip the limit. Now, if you have say an optical limit, you will have less hysterious and therefor less movement room. I have an optical for setting my Z height and you can breathe on it and make it change state! The home point can be set to any refrence, but I still use 0 as mine and then use fixturing (G54 etc) to set the origin of my work piece. This way, the home switch provides a REPEATABLE refrence position for machine coordinates and all my parts to be machined have a seperate origin for them. This makes it easy to cad/cam the part up as I have the ability to control its refrence (origin) point.
To answer CNCgr's question, Yes I use a combination home/limit switch and have done so since V3.x of Turbocnc. On the X and Y axis this is accompolished by using ONE switch that is actuated by stops on the table (switch mounted in the center of travel). This way, I can use more of the table travel (and on a mill/drill) this is limited in the first place. For instance, on the Y axis, my limit to limt travel is a tad over 7" and I quite often use the entire 7" to machine a part! If I had seperate home/limit switches, there is a problem of where to mount them and depending on how I set it up, it would reduce the travel. For the X and Y, I use AA Mini Din lever operated switches which are also coolant resistant. Trying to mount more than one on an axis would be an exercise in making blivits (10 lbs of stuff in a 5 lb bag For the Z axis, I used regular micro switches as these are up out of the way of coolant and swarf. Originally, I used micro switches for the X and Y and quickly found out what coolant will do for the operation of these. All switches are setup for NC operation.
Hope this helps, but let me know if you need more info.

[CNCgr]

Originally Posted by itsme What I am now thinking of doing, is using 1 limit and 1 home switch per axis that are wired in series (like you said, CNCgr). The 3 axes will be individual (wired in parallel???).

The home switches need to be individual i.e. one pin for each. The three limit switches can be connected all to one pin in series. Total 4 pins. N.C. switces are better so that if a connection fails it will be interpreted as a switch press event and the machine will stop.

Originally Posted by itsme Is it possible to make the home position something like -5mm (negative five)? That way, when the switch is tripped, it will make home -5mm, so that the code can go to 0 and still have some clearance before it hits the switch???

Yes, you can setup any position you want. As Bubba said, though, it's better to use fixture settings and set 0 whereever you want (e.g. workpiece corner etc).

Bubba, I think you mean hysterisis?

Nikolas

[Bubba]

CNCgr,
Thanks, thats exactly the word I was trying to spell. I need more coffee and maybe my brain will engage!

[Oldmanandhistoy]

Originally Posted by Bubba When you have the program PROPERLY set up, the axis will move to the limit switch at the Homing speed set in the ini. When the switch "trips", the axis will reverse until the switch "un-trips"

Firstly I have to say I find home switches and the way the machine homes with Turbocnc perplexing. When setting up my first machine and using the home command the Z axis would home first on its own then Y and X would home together. I actually like this method of homing as it made sure the cutter did not hit anything while homing.
On my second machine I thought I set up the software as the first machine but all axis would home at the same time but not reverse off of the switches.
I am now setting up my 3rd machine and would like to use the home switches as limit also.
Could some one please explain why my axes do not reverse from the home switches after homing.

John

[Bubba]

John,
It sounds like something is wrong in the configuration! The best advice I can give is to have ONLY one axis configured to work and test it. IF it works correctly, then disable that axis and move on to the next one. Repeat as necessary.
Back when I was setting mine up, I found that if one axis was not exactly correct, then all will fail as you have noted. Also, when checking out an axis, I would trip it in the middle of the return by hand. That way, I could make a change as necessary without having to go through the jog screen etc to back it off the limit. To check each switch before trying, I use the "Active Port" feature "F2" "<ALT>+F2" and then manually move the switches to confirm proper action.

Then to further confuse the issue, I have modified Turbocnc to allow single axis homing in MDI. Then I have an aux. Z axis homing switch and under g-code, home to it with a tool in the quill and it will automatically handle tool length compensation:})

Bubba

[Oldmanandhistoy]

Thanks Bubba for your time and suggestions I will try as you have described.

John