[neomagik]

Hi all,

I am about to wire in my limit/home switches but I'm not sure if what I am proposing will work.

My BOB has 4 inputs so I was planning to use one as an E Stop, one for X Y & Z limits wired in series (NC) with the X & Y doubling up as home switches and one normally open input for my Z home switch but using my cutting tool/mill and a piece of aluminium of a known thickness on the table to close the circuit.

Can Mach 3 be configured to work in such a way?

[Fastest1]

I dont think you can use the same switch for a home and a limit. When the home switch was touched, Mach wouldnt know and would stop. Use the 3 inputs (1 for each axis) and 1 input for the E stop. Wire each pair of switches of an axis in series. When homing let it hit the switch and back off, then proceed to where ever your "home" would be (this will be a physical amount of distance or location from the switch set by you thru software). It works very well. If you use the the input pins as I have described you can have 3 axis simultaneous homing.

[neomagik]

Fastest1

What I meant was to have a home switch at the X-- and Y-- end that also acts as a limit switch if needed wired in series with limit switches at the X++, Y++ and Z++ end.

I wanted to keep an input free to home the Z axis as it needs to be configured as a normally open contact if I am to home it using the method I descibed earlier.

I thought Mach could be configured this way??

[M250cnc]

neomagik

All limits and homing can go to one input. When homing the switch is used as a home switch. When running it is used as a limit.

I also suggest you set soft limits to come into play before you hit an actual limit. This has the advantage of Mach not running the code if you are trying to go beyond the physical machine limits.

One input to be used for Auto Tool Zero

I suggest the E STOP be wired in such a way that if you had a computer malfunction that the E STOP would still work IE it turns everything off including the spindle DO NOT RELY ON SOFTWARE FOR THIS FUNCTION

I use the E STOP input as follows as i am using a VFD to control my motor there are times when the VFD will switch off due to excessive braking or acceleration it also has an inbuilt relay so the 5v goes through the relay so should the VFD switch off then i get an automatic E STOP in Mach

Phil

[Fastest1]

neomagik

All limits and homing can go to one input. When homing the switch is used as a home switch. When running it is used as a limit.

I also suggest you set soft limits to come into play before you hit an actual limit. This has the advantage of Mach not running the code if you are trying to go beyond the physical machine limits.

One input to be used for Auto Tool Zero

I suggest the E STOP be wired in such a way that if you had a computer malfunction that the E STOP would still work IE it turns everything off including the spindle DO NOT RELY ON SOFTWARE FOR THIS FUNCTION

I use the E STOP input as follows as i am using a VFD to control my motor there are times when the VFD will switch off due to excessive braking or acceleration it also has an inbuilt relay so the 5v goes through the relay so should the VFD switch off then i get an automatic E STOP in Mach

Phil

Though I agree that you can acheive a software home this way, I dont believe this is actually a home switch. From what I have come to understand, the machine can not pass a limit but can pass a home switch. they can not be configured to do both. All limits can be on 1 pin and all homes can be on another pin. but all cant go to the same pin. Also you can only home 1 axis at a time unless all axis' use their own pin.

[brtech]

Nope, that's not true with Mach. Read the documentation. Many routers have combined limit and home switches.

There is a consequence, which the manual mentions. You can't start a home operation with the router either at the limit/home (tripped) or on the "other side" of the limit/home. It's going to move the tool in the direction of home assuming the tool is presently somewhere in the middle, and it better be able to do that. It can be just off of the home switch, which is where the tool usually is placed when you home.

It is true that if you have more than one home switch on the same pin that it can only move one axis at a time, but at least in the default configuration, it always does that. Mine homes Z, then Y, then X. I have 4 inputs, combined home and limit, because I have a double drive on X. If you have dual drives, you slave them, but they home independently. This squares the gantry to the table if you align them correctly.

[hemsworthlad]

Personally I wouldn't use limit switch's.?
Unless your machine is very small or very fast you'll find that you never get any where near the limit's.! Save your self the hassle of installing and the potential noise issue's related with limit switch's, soft limit's work fine.
The combined home & limit on 1 input is ok but does have issue's if the home switch is far away from the limit and it's a pain remembering to drive it back the right side of the home switch.
Really if you must use limit's and it's done properly thru hardware/relay's etc then you also need a way to over ride the system so you can drive them off the switch's, a mometary button type override something that cant be permently left switched off bypassing the limit's.

DONT RELY ON SOFTWARE TO ESTOP. You want everything to cut dead and shut down in a controlled way when the O-**** button is pressed and the only way to do this properly is thru relay's etc. . . relying on software for e-stop is dangerous.

Cheers
Dean.

[neomagik]

Basically, I want to set the X-- and Y-- limits as 0, 0 (home) and the table top as 0 for Z--

I can then use offsets as needed.

Can I use a separate input for the Z axis homing?

[neomagik]

Surely the easiest way to do an E stop is to plug the router and power supply to the controller to a common switched supply. Then you just hit the switch and cut all power in an emergency.

[brtech]

Yes, you can, but usually you set home for Z at the top.

Among the reasons for that are that if you have a random tool in the spindle, and you drive it to the limit switch, you can ram the tool into the table.

Usually, you home to the top, and then you use a touch off tool to work surface top.
There are some folks who have a surface zero routine with conductive button in the table that zeros the tool to the table. I haven't spent the time to figure out why they do that.

[brtech]

Most controllers have E-Stop inputs. It's good to use them, rather than just cut power.

[M250cnc]

Personally I wouldn't use limit switch's.?

Dean i take it from this statement that you yourself do not use limits ?

Unless your machine is very small or very fast you'll find that you never get any where near the limit's.! Save your self the hassle of installing and the potential noise issue's related with limit switch's, soft limit's work fine.
The combined home & limit on 1 input is ok but does have issue's if the home switch is far away from the limit and it's a pain remembering to drive it back the right side of the home switch.

Lots of mistakes with the above as the home switch and limit are the same, then you will not be far away from it if you hit it. The other thing is if you lose position then soft limits are useless and you will hit something hard. I already mentioned to use softlimits as well, potential noise issues can be overcome but i have never suffered from it.

Really if you must use limit's and it's done properly thru hardware/relay's etc then you also need a way to over ride the system so you can drive them off the switch's, a mometary button type override something that cant be permently left switched off bypassing the limit's.

Not true, there is a setting that enables you to drive off a limit switch after you press the reset button (ON SCREEN)

DONT RELY ON SOFTWARE TO ESTOP. You want everything to cut dead and shut down in a controlled way when the O-**** button is pressed and the only way to do this properly is thru relay's etc. . . relying on software for e-stop is dangerous.

Yeah i already mentioned that.

neomagik

There are lots of ways of doing things and everybody does it differently, but i agree with brtech in using the top of the material as Z zero when you take a cut this way you are going below the surface which is a Z minus number. Why this way, well say i have a vice on the table with some stock and i want to cut a 3mm deep slot, so i zero to the top surface set Z to -3 easy, now try it your way zeroing from the table surface, lots of potential for error.

Now it's true with combined home and limit switches that you can only home one axis at a time but it can be quite an accurate way to continue from where you left off or if you have to reset after a collision. This is a very fast way to get very close to where you were at and is not possible without homing switches.

I use separate home switches now (Optical) on my lathe and mill and they are accurate to 0.0001"

HTH Phil

[hemsworthlad]

Surely the easiest way to do an E stop is to plug the router and power supply to the controller to a common switched supply. Then you just hit the switch and cut all power in an emergency.

Yes but not a good way and by the time you have managed to get to the switch the damage is well and truely done.
Better way is to design the control box to use relays to control power to router vac etc that all run thru the estop system then when estop is pressed power shut's down instantly in a controlled way and only to the things you want stopped, leaving power to things like bob which then can send a signal to Mach that an estop as accured.
This way you know your safe and also if done right the e-stop safety wont let you start backup again unless all is well with the system.

Is sounds complicated but it's not really and this way things are stopped instantly and properly with less risk of damage to both the job/tools and more importantly to you.!! . . . . . How would you get to the switch if the router grabbed you. ?? . . . Think about it.!

[hemsworthlad]

Dean i take it from this statement that you yourself do not use limits ?



Lots of mistakes with the above as the home switch and limit are the same, then you will not be far away from it if you hit it. The other thing is if you lose position then soft limits are useless and you will hit something hard. I already mentioned to use softlimits as well, potential noise issues can be overcome but i have never suffered from it.

Think you mis-under stood.? I dont use Limit's but do use Home switch's.

If the machine is built properly and running within it's motor capabilty then losing position while in normal use shouldn't happen, if position is lost due to me hitting estop or stopping machine mid cut then i just home and position is regained.
Also I did say depends on size and speed of machine, my machine has a cutting area of 1100x1400 so i never get near the limit's.

Not true, there is a setting that enables you to drive off a limit switch after you press the reset button (ON SCREEN)

From this statement I take it your running under software control.?
The on screen button wont wont reset a properly designed control box that use's relay's etc to shut things down when a limit triggers estop.
A properly designed system wont and shouldn't allow a restart untill the problem as been corrected or found.

Like you say people have their own way of doing things, me i like to do it safe but at the same time sensible and without hassle's. When the machine conditions are right and inputs are limited then from my experience limit's are not a big priority on a larger stepper based machine.

Regards
Dean.

[hemsworthlad]

Neomagik:

Bottom line is, If all you want is to home then just use home switch's, if you want all the extra safety of hardware limit's and your limited on input's then go the combined switch route and live with the mentioned short comings.

The Z home position again is a personal preferance, me I do it the same as the others and home to the top and cut in the negative.!

If you want any help with a control box/schematic that use's relays etc rather than using software then drop me a PM.

Cheers
Dean.

[neomagik]

The reason I use the table as Z position 0 is because I'm cutting two halves of a scale helicopter mould out of 75mm thick foam insulation. I use the factory formed flat surface of the insulation to sit on the table and I need it to be exactly the right depth after the cut so that I can put the two halves together to form the finished shell.

I create the toolpath from a 3D model of the helicopter and I set the 0 position in the model as table level, this way I know that nothing in the toolpath that I create is anything below 0 so there is no chance of cutting into the table as long as I've homed it accurately.

If someone can think of an accurate way of homing to the top of the Z and cutting in the Z-- leaving the factory formed bottom surface of the insulation that's sitting on the table as the finished surface (the one that butts together with the other half) then please let me know!!

[brtech]

First of all, you need to read about the work offset stuff. The video tutorials on the Mach3 site explain this pretty well. Home makes sure the tool knows where it is. You want to make sure you know where the work is relative to the tool.

On rigid work, especially things that are already pretty parallel to the table, you can use a "touch off plate" to determine the top of the work surface. The way this works is to clip one side of a circuit to the tool, and use a conductive touch off plate of known thickness. The circuit is an input to your break out board. There is a routine available which lowers the tool until it contacts the touch off plate. The top of the work surface is then the tool location plus (minus if zero is top) the (known) thickness of the touch off plate.

In your case, you want to use the bottom of the work surface as the reference. One way to do that is use a touchoff plate thicker than the material, put it on the table, and set the touch-off offset (what would normally be the thickness of the touch plate) to the touch off plate thickness - the height of the material. That will tell Mach that the work top surface is where it nominally is, but will actually use the bottom surface as the reference. There may be a way to have the touch off routine just use the table as the reference, but I don't recall seeing that.

The beta version of the new screen set for Mach has the touch off mechanism built in. I haven't had a chance to play with it yet myself.

Of course if the foam has two parallel surfaces originally, you can just use the basic touch off plate with the top as the reference.

[wendtmk]

Personally, I don't understand why folks wouldn't want to use both home and limit switches on their machines, unless space to mount them is a consideration. Home and limit switches are probably the most inexpensive items we put on our machines, and for that little money spent, they provide enormous peace of mind that something won't break just because of a mis-typed G Code, or a soft limit not catching the over run. And having separate home switches allows you to set your machine home exactly where you want it, and not be stuck with using offsets from your combined home/limit switches.

Why compromise on your machine setup, when for a really nominal cost you can set up your machine exactly the way it should be set up?

Mark

[brtech]

Personally, I don't understand why folks wouldn't want to use both home and limit switches on their machines, unless space to mount them is a consideration. Home and limit switches are probably the most inexpensive items we put on our machines, and for that little money spent, they provide enormous peace of mind that something won't break just because of a mis-typed G Code, or a soft limit not catching the over run. And having separate home switches allows you to set your machine home exactly where you want it, and not be stuck with using offsets from your combined home/limit switches.

Why compromise on your machine setup, when for a really nominal cost you can set up your machine exactly the way it should be set up?

Mark

One problem is not enough inputs. On my double drive X axis router, to get separate home and limit switches, I would need 5 inputs. My Gecko only has 4 + ESTOP. I really want another for the touch off plate.

Also, noise in the limit switches is sometimes a problem. Mechanical problems plague any mechanical switch. Any false trigger can ruin a work piece.

At least in my case, there doesn't seem to be any reason not to have home be the same as a limit. I do get the issue that you have to make sure the tool isn't on the home limit switch when you start a home operation. That is the real advantage, in my opinion, of separate switches. Other than that, since you can define the offset from the home switch to the home position, why would you want to have home <> limit? The manual describes some huge machine tool where it made more sense to put home on one axis in the middle of the range, but that was a pretty extreme example.

I'm not sure there is any G-code sequence, mistyped or not, that would cause the tool to exceed the softlimits. You would have to be able to zero the machine coordinate DROs from Gcode or something like that. That leaves the possibility of a software glitch in Mach that drove the tool past the soft limits. I definitely believe that is possible, albeit very unlikely. My hard stop will deal with it. The limit switches have LESS reliability than Mach in my estimation.

[wendtmk]

One problem is not enough inputs. On my double drive X axis router, to get separate home and limit switches, I would need 5 inputs. My Gecko only has 4 + ESTOP. I really want another for the touch off plate.

Also, noise in the limit switches is sometimes a problem. Mechanical problems plague any mechanical switch. Any false trigger can ruin a work piece.

At least in my case, there doesn't seem to be any reason not to have home be the same as a limit. I do get the issue that you have to make sure the tool isn't on the home limit switch when you start a home operation. That is the real advantage, in my opinion, of separate switches. Other than that, since you can define the offset from the home switch to the home position, why would you want to have home <> limit? The manual describes some huge machine tool where it made more sense to put home on one axis in the middle of the range, but that was a pretty extreme example.

I'm not sure there is any G-code sequence, mistyped or not, that would cause the tool to exceed the softlimits. You would have to be able to zero the machine coordinate DROs from Gcode or something like that. That leaves the possibility of a software glitch in Mach that drove the tool past the soft limits. I definitely believe that is possible, albeit very unlikely. My hard stop will deal with it. The limit switches have LESS reliability than Mach in my estimation.

Put all your limits on one line, and keep your home switches separate. X home, Y home, Z home, and limits. Four lines.

Noise affecting limit switches is going to affect them regardless of whether they are separate from the home switches or not, so that's not really an argument against them.

As to why you'd want them separate? For a number of reasons..

You misunderstood about the G code causing the machine to exceed the limits. I was trying to say a typo might be out of the machine limits and if there was a controller or software glitch that disabled the soft limits, that typo would cause a machine crash. If it's possible, it can happen. A limit switch is cheap insurance, especially if the soft limit glitch happens on a rapid. Why would you want to send your machine crashing into your hard stop? That gets a wee bit hard on the equipment.

Setting your machine homes inside the limit switches gives you just that little extra bit of insurance. And the insurance is cheap. I'd much rather ruin the workpiece than the machine.

Mark