Quick Tip: IF you have Home/Limit Switches, here is an example and description on setting them up

[cncsnw]

Prior to the Acorn, the "Centroid way" was this:
- Two limit switches per axis, one plus one minus.
- If a limit switch was tripped, then the control would prohibit further movement of that axis in that direction.
- If a limit switch was tripped, the control would permit that axis to move in the other direction (though not in Fast jog mode)
- Any of the limit switches could be used as a home switch as well.
- The software recognizes that tripping a limit switch is not an error, if it is in the process of homing using that switch.
- When possible, the limit switches can be hardware direction inhibits to the drives, as well as logical inputs to the PLC and CNC software.
- Software travel limits are a convenience, to stop jogging moves before you even get to the limit switches, and to preempt running a programmed move that would trip a limit switch.

Since tripping a limit switch did not prevent you from jogging back away from it in Slow, Continuous jog mode, there was never a need for a "limit override" switch.

Even when the limit switches are also hardware directional inhibits to the servo drive(s), since the plus and minus directions are controlled by separate switches, you can still jog off of a tripped limit.

All that is still the case with Allin1DC and Oak controls, and optionally can be the case with Hickory and even Acorn controls.

Acorn introduced the ideas of "HomeAll" and "LimitAll" to get around its shortage of PLC inputs. That is when "you don't need limit switches, because software travel limits work perfectly" became the company line.

As you note, software travel limits work perfectly, as long as everything else works perfectly. They do not prevent the operator from jogging the machine outside its travels prior to homing. They do not prevent the machine from running outside its travels when it has lost position, whether due to mechanical slippage or to electrical interference.

If you are setting up a machine with Acorn, and you want to forego limit switches, then do yourself a favor and make sure you have reasonable stop cushions at each end of each axis. That way nothing gets bent or broken if/when you accidentally overtravel an axis.

[cnckeith]

with the addition of "HomeLimit" inputs in the Acorn Wizard, this allows a single switch to act as both a limit and home switch just like oak/allin1dc

also you can now have a dedicated limit switch inside of a home switch (a limit "protecting" a home switch) with Acorn as well.

also another related recent addition is M297 which allows to move beyond the software travel limits. common use would be during a rack mount tool changed where the rack is protected by soft limits but the m297 is issued to allow the tool change moves to move beyond the soft limits.

[Max]

I assume that with the use of a "HomeLimit" input that a limit wired in series can also be incorporated at the opposite end of travel to axis home while only using that one input.

I am about to complete my final machine assembly and have built my design around limit switches at both ends of axis travels.

Yes I am aware that it is not necessary as the soft limits will do the work for me and be reliable. I work on the addage that if something can fail, then I would like to remove that possibility

[ShawnM]

I assume that with the use of a "HomeLimit" input that a limit wired in series can also be incorporated at the opposite end of travel to axis home while only using that one input.

I am about to complete my final machine assembly and have built my design around limit switches at both ends of axis travels.

Yes I am aware that it is not necessary as the soft limits will do the work for me and be reliable. I work on the addage that if something can fail, then I would like to remove that possibility

If I were a betting man I would put money on your “mechanical” switch failing BEFORE the software.

I don’t know how fast your rapids are but if they are high you might also consider installing a hard stop after your limit switch because if you run at it full speed it can travel past the switch before it comes to a stop. Also make sure you have the switch pass by the flag and not directly at it because you’ll break things. I’m in the router world and with rapids on one of my machines at 1350 IPM (or 34,290 mm/min for you) I prefer the software knowing ahead of time where it has to stop, so it decelerates properly to stop in the exact same spot every time.

I dont use limit switches for travel but I don’t think what you want to do will work. If the switches are wired together the Acorn will think it hit its negative limit when at the far end of travel and you won’t be able to back off that far switch it you trip it.

[cncsnw]

It isn't software failure that causes machines that rely solely on software travel limits to get bent or broken. It is mechanical failure (slipping drive train), electrical failure (lost command pulses), and operator error (fast jogging before finding home).

If you only have two inputs to spare, you could wire the switches at the homing end of each axis all together in series to one input, and designate that input as the home and limit for that end of each axis; then you could wire the switches at the other (non-homing) end of each axis all together in series to a second input, and designate that input as the limit for the other end of each axis.

I do not know if the Acorn Wizard explicitly supports that configuration or not, but you can set it up that way on the Machine Configuration -> Motor Parameters table in the CNC12 software.

[cnckeith]

It isn't software failure that causes machines that rely solely on software travel limits to get bent or broken. It is mechanical failure (slipping drive train), electrical failure (lost command pulses), and operator error (fast jogging before finding home).

If you only have two inputs to spare, you could wire the switches at the homing end of each axis all together in series to one input, and designate that input as the home and limit for that end of each axis; then you could wire the switches at the other (non-homing) end of each axis all together in series to a second input, and designate that input as the limit for the other end of each axis.

I do not know if the Acorn Wizard explicitly supports that configuration or not, but you can set it up that way on the Machine Configuration -> Motor Parameters table in the CNC12 software.

yes you can do that with the Wizard for Acorn/AcornSix

[ShawnM]

It isn't software failure that causes machines that rely solely on software travel limits to get bent or broken. It is mechanical failure (slipping drive train), electrical failure (lost command pulses), and operator error (fast jogging before finding home).

Yes, my bad, I’m assuming the the machine is built properly, setup properly and operated properly.

[Max]

Thanks for all the replies. Yes I do have cushioned physical stops at the both ends of all three axis travels. I plan on not needing them, but if too many holes align in the Swiss cheese they are there.

I understand the possibilies of what can cause an over run of travel limits. For the bench test I have used Limit All and Home All inputs. The appeal of HomeLimit is the bonus of one more available Acorn input. With only eight imputs this is like gold.

The biggest issue that I have with my configuration is that the limit switches for my Y travel will be buried deep in the bowels of the machine and not accessible after assembly without a full machine strip.

This answers my question - yes I can use HomeLimit and have one more input available!

[richardb15]

The biggest issue that I have with my configuration is that the limit switches for my Y travel will be buried deep in the bowels of the machine and not accessible after assembly without a full machine strip.

You should wire the limits and home switches all as normally closed, so if you have a wire, connection, switch failure etc the failsafe shows up rather that being a silent failure. Ideally you want them accessible so during your regular machine checks (we all do them, right.... ) you can run round and manually press all the switches and check the input on Acorn toggles. Burying a NC switch deep inside a machine is just begging Murphys Law to ensure that is the first part that fails once you are running, and therefore putting you in a world of grief. I would really encourage you to move the Y switches to somewhere more sensible. The Y you are thinking of using for homing gets switched every time the machine is homed, and my most common mechanical issue on any of my CNC machines is limit switches sticking open. The plungers always get sticky with old slideway oil, coolant and dirt and a quick spray of WD40 and a few pushes sorts them, but you need access. Just my thoughts and trying to save you a full machine strip every few months!