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Offline Kristin D

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Home and/or limit switch
« on: September 06, 2008, 02:43:28 PM »
I am trying to figure out how and where to put home and/or limit switches on my Taig mill. The Z axis has a tee slot on one side where I can easily mount a home switch to home to just short of the mechanical top stop which is where I think that may belong. I can't really see the need for a lower limit as depending on the tool having a hard limit there is not going to keep me from crashing into the table, is that where the soft limits come in? Are soft limits set in the program coordinates or machine coordinates? For the Y axis I was thinking a switch mounted to the motor/leadscrew mounting plate will work and contact the movable slide, setting home as the furthest out position of the table. There is a pin fixed to the Y axis carriage that I can use to contact a switch mounted on the Table for my X home which I suppose should be table full right hand side for XY=0,0.

Does this make sense in the grand scheme of things, home would be table full right, full out toward the front and Z axis full up position? I am still figuring if I should use slotted opto sensors or microswitches so I really need to clear up my confusion with "Home" position before I can make the mountings and actuators or I might end up with a mirror image of what I need.

Once all this is done, if I have set fixture('s) say for engraving a plate I should be able to home the machine and either just load the program code or use the MDI to move to my X,Y zero position and run, that is my limited understanding.

Thanks,

Kristin

Offline Hood

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Re: Home and/or limit switch
« Reply #1 on: September 06, 2008, 03:32:41 PM »
I am trying to figure out how and where to put home and/or limit switches on my Taig mill. The Z axis has a tee slot on one side where I can easily mount a home switch to home to just short of the mechanical top stop which is where I think that may belong. I can't really see the need for a lower limit as depending on the tool having a hard limit there is not going to keep me from crashing into the table,

Personally I would also have a lower limit as it might save damage to the Z axis if for instance you never had a tool in and for some reason it went the wrong way and banged off the lower part of the ballscrew, maybe your motors would stall before any damage was done?


Quote
is that where the soft limits come in? Are soft limits set in the program coordinates or machine coordinates?
Softlimits are in machine coords, X and Y min and Z Max are 0  and X and Y Max and ZMin are the travel of these axis. (remembering Z Min is always a negative number)


Quote
For the Y axis I was thinking a switch mounted to the motor/leadscrew mounting plate will work and contact the movable slide, setting home as the furthest out position of the table. There is a pin fixed to the Y axis carriage that I can use to contact a switch mounted on the Table for my X home which I suppose should be table full right hand side for XY=0,0.
You can put the home switches wherever you want. Its easiest to think of the tool in relation to the work rather than the table movement. You really want to have your tool lower left and fully up in the home position, that would be table fully right and away from you and z fully up.  If you chose to have the limits at the other extremes of X and Y you can still do it but you will need to set a “Home Off” value for these axis


Quote
Does this make sense in the grand scheme of things, home would be table full right, full out toward the front and Z axis full up position? I am still figuring if I should use slotted opto sensors or microswitches so I really need to clear up my confusion with "Home" position before I can make the mountings and actuators or I might end up with a mirror image of what I need.

 As said above think of the tool but fully right and away from you for table  is easiest to set up as you don’t have to worry about setting a Home Off value. If you use Optos and shield them in some way from much and swarf then you can mount the switch in the middle of the axis travel and have a flag either end that will break the light beam. Best if you can have the flag adjustable as it will allow you to fine tune. Z can be done in a similar fashion. You could also have a bar that runs the full length of an axis that blocks the opto and have slots cut in either end but that is not so adjustable.

Quote
Once all this is done, if I have set fixture('s) say for engraving a plate I should be able to home the machine and either just load the program code or use the MDI to move to my X,Y zero position and run, that is my limited understanding.

Thanks,

Kristin

If you keep the same work offset then yes

Hood
Re: Home and/or limit switch
« Reply #2 on: September 06, 2008, 03:49:41 PM »
Kristin,

    I think you've got it pretty well figured out, and pretty much the way my knee mill is setup.  I agree there seems little use to a Z "down" limit.  I used microswitches, and feel that was a good choice.  I seem to get pefect repeatability, to within well under 0.001", and they'll be less susceptible to damage from coolant/oil.  If they do ever fail, replacements are as close as the nearest Radio Shack, for about $2 each.

Regards,
Ray L.
Regards,
Ray L.

Offline Kristin D

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Re: Home and/or limit switch
« Reply #3 on: September 06, 2008, 04:05:30 PM »
Hood,

Thanks I knew one of my axis was the wrong way! I just was looking at table forward as mechanically it looked like the easy way out for switch mounting. If I put a limit on Z can that be wired in series with the home switches or would I need another input for limit switches (which I suppose I can wire all in series should I put X & Y limits too)? I am trying to wrap my head around the idea of a central switch and two flags, I can see it with using mechanical switches but the opto switches I have would lend themselves better to having the bar block them 99% of the time since they have a wide slot I could use .125" thick aluminum bar with adjustable apertures at the ends. Now the quandary is, how to set up the inputs active high or low so a failure will stop the machine. I know I saw something along those lines in one of the videos as to the preferred active state so it was "failsafe"

Kristin

Offline Kristin D

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Re: Home and/or limit switch
« Reply #4 on: September 06, 2008, 04:08:26 PM »
Kristin,

 I think you've got it pretty well figured out, and pretty much the way my knee mill is setup. I agree there seems little use to a Z "down" limit. I used microswitches, and feel that was a good choice. I seem to get pefect repeatability, to within well under 0.001", and they'll be less susceptible to damage from coolant/oil. If they do ever fail, replacements are as close as the nearest Radio Shack, for about $2 each.

Regards,
Ray L.

Ray,

So at home position where is your table? Full right and out or Full right and in?

Kristin
Re: Home and/or limit switch
« Reply #5 on: September 06, 2008, 04:17:15 PM »
Kristin,

    Home position is with the table full right and out, which I believe is the most common place.  But, you can put it anywhere you like, even in the middle of the table if you like (but then you can't use the same switches for home and limits).  It really makes no difference where home is, as long as *you* know where it is.

Regards,
Ray L.
Regards,
Ray L.

Offline Kristin D

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Re: Home and/or limit switch
« Reply #6 on: September 06, 2008, 04:23:16 PM »
Well that works for me as I mentioned it is easier mechanically to mount the switch that way and also gets the table clear of the spindle for setting parts. Then all I need (haha ALL!) to do is set up the work offset, I can feel the brain hemorrhage coming on already. ::)

Kristin

Offline Hood

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Re: Home and/or limit switch
« Reply #7 on: September 06, 2008, 04:28:40 PM »
If you have Home in a place other than lower left fully up (tool in relation to table) then you will need to set up a Home Off distance for that axis so that the Machine Coordinates are zeroed  to lower left fully up.

As Ray says if you use home positions other than axis extremes then you will need seperate Hom and Limit switches for each axis. If you have the switches at the extremes you can use the same switch for each axis.
 Depends on your switches whether you can connect all in series, I have 3 inputs to Mach on the machine I use optos on, not tried connecting in series.
On my other machines I have good quality HoneyWell plunger switches  and my homing looks for the switch then looks for the index from my motors encoder then backs off a preset amount of encoder counts. This is only possible with either drives that can do this or Eds BOB (No longer made I believe :(  ) but may also be a feature in the SmoothStepper in the future.

Hood
Re: Home and/or limit switch
« Reply #8 on: September 06, 2008, 04:32:52 PM »
If you have Home in a place other than lower left fully up (tool in relation to table) then you will need to set up a Home Off distance for that axis so that the Machine Coordinates are zeroed to lower left fully up.

As Ray says if you use home positions other than axis extremes then you will need seperate Hom and Limit switches for each axis. If you have the switches at the extremes you can use the same switch for each axis.
 Depends on your switches whether you can connect all in series, I have 3 inputs to Mach on the machine I use optos on, not tried connecting in series.
On my other machines I have good quality HoneyWell plunger switches and my homing looks for the switch then looks for the index from my motors encoder then backs off a preset amount of encoder counts. This is only possible with either drives that can do this or Eds BOB (No longer made I believe :( ) but may also be a feature in the SmoothStepper in the future.

Hood

Many stepper drivers also have a "Phase 0" output that can be used the same way, switch resolution permitting.

Regards,
Ray L.
Regards,
Ray L.

Offline Kristin D

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Re: Home and/or limit switch
« Reply #9 on: September 06, 2008, 04:54:14 PM »
OK,

I think I have it, going to set up the home switches, wire them is series N.C. and get moving. I have 5 inputs on the controler card 10,11,12,13 &15 and a second port if I run out of inputs/outputs so that is not a big problem using seperate I/O for everything. Really not planning on making this too complex (Famous last words) I do want to add spindle motor control and perhaps a coolant output but I am sure I'll find another one or two things like the spindle speed if I swap to a variable speed drive.

Kristin