Homing & Hard Limit Switches

[leinadltd]

My name is Daniel. I need assistance in setting up my homing and hard limits on my metal lathe.

Currently, the homing switch works as long as I have the limits disable. When the limits are enabled, I get a limit Z-axis limit trip (see photo).

I have attached photos to illustrate my metal lathe setup and my Acorn CNC setup. As you can see from the photos, I use micro-switches to indicate the homing and limit positions. I have provided BOTH the homing switch and limit switch diagrams that are utilized for this set up. Please review my diagrams to make sure that I set it up correctly. I used the Normally Closed (NC) contacts on the micro-switches for my setup.

The homing switch is ran in series with the Z-axis and the X-axis using a Red (+24V) wire from Acorn IN1, and then terminates on the common side (White) wire of the 24V terminal block.

The Z-axis physical hard limit is ran in series with the tail-stock micro-switch using a Black (+24V) wire from Acorn IN2, and then terminating on the common side (Green) wire of the 24V terminal block; thus, if the machine “hits” either the spindle or tail-stock limit the unit physically stops.

The X-axis physical hard limit is ran using a Black (+24V) wire from Acorn IN3, and then terminating on the common side (Green) wire of the 24V terminal block; thus, if the machine “hits” the cross-carriage limit the unit physically stops.

I have included the Acorn CNC photos indicating my input selection control. If I disable the limit switch then the machine will home properly. But then I run the chance of breaking my insert or damaging my tool as the machine will continue past the physical stops.

I have watched several videos in trying to get this set up correctly; however, each person’s situation is unique. I have added photos to show my homing and software travel limits. I could use some help in setting up my homing, hard limit switches, and software travel limits. This way I can home the machine correctly, and while utilizing the metal lathe if it encounters a hard limit, it will shut-down and/or abort the job process. I have also attached a zip report showing the failures on the limit switches to help aid in a resolution.

[ShawnM]

Please excuse my ignorance but is a lathe that different than a router or mill? I'm confused as to why there are two switches in each location? Isn't all that's needed similar to a router or mill, just one N/C switch at one end of travel and setting software limits for the other end. I'm really confused by this setup and hopefully I'll learn something here.

[Gary Campbell]

Shawn...
That's a holdover from the DIY world with weak open loop steppers and hobby controllers like Mach or ShopBot.

[leinadltd]

One switch is for the homing process, and the other switch is for the limit. I have already broken a tool-holder due to the lathe running past the mechanical stop that was created. I need to implement a hard limit via a switch to shut down the lathe to prevent breakage. Hope this helps explain the photos and setup.

[cnckeith]

how about simplifying the setup and use one switch per axis and have that switch act as both the limit and home switch.

[Richards]

I read this thread this morning and I'm a little baffled. Let's see if I understand what the desired goal is.

You want to use homing switches to zero each axis.
You want to use limit switches to keep the machine from crashing.

Mechanically, I can't see how the current setup will work. I use a homing sensor to zero an axis. The proximity sensor (or roller ball on a mechanical switch) is perpendicular to the target so that the sensor or switch does not act as a hard stop when zeroing an axis. When I home an axis, I move the axis until the target is detected, then I reverse the axis a fixed amount until the axis is at its zero point. The homing sensor or switch only detects its target during the homing routine. When I use limit switches/sensors, I also mount them perpendicular to their targets. The target is positioned so that the switch/sensor does not act as a hard stop. I used rubber bumpers on my Shopbot router to stop an axis if a limit sensor failed. The rubber bumpers acted as a hard stop with just a little "give". That router jogged at speeds up to 30" per second. A crash at that speed could cause mechanical damage.

If both the homing switch and the limit switch are mounted together, then chances are that the limit switch will be sensed at the same time as the homing switch. The limit switch will put the machine into a fault condition, requiring a reset. So, mounting those switches at separate locations seems necessary to me.

The tail-stock, because it is moveable, limits the travel of the Z-axis. Having software limits will not stop a crash if the tail-stock is moved towards the spindle. Having the limit switch sense the position of the tail-stock before a collision occurs seems like a good idea. If the target is perpendicular to the switch or sensor, and if there is a safety zone allowing for sufficient over-travel for the axis to stop without crashing, then a limit switch near the tail-stock seems like a good idea.

To summarize:
Use homing switches/sensors with their target perpendicular to the switch/sensor. Allow over-travel. Do not let the homing switch/sensor become a hard stop.

Use limit switches/sensors with their targets perpendicular to the switch/sensor to show that a physical limit is close. Allow sufficient over-travel for the axis to stop before crashing. Do not allow the switch/sensor to become a hard stop. Install a bumper with a little give to act as a hard stop.

[leinadltd]

how about simplifying the setup and use one switch per axis and have that switch act as both the limit and home switch.

Yes, that would be a great idea! Allows me to have spare micro-switches as well.
How would I setup that up in the Acorn INPUT section? How would you wire it up? Would it be in series or parallel? Would you sketch a rough diagram and take a picture of it so I can "see" what your though process is for the setup? Do we need to setup one as NC and the other as NO?

Thanks, Dan

[ShawnM]

Dan, there’s a wiring diagram in the Acorn schematics that shows you exactly how to wire up a single, n/c limit switch per axis in series. Then you setup software limits for max travel. Works perfect every time. Save your self the aggravation and remove all those unnecessary switches and wiring.

Here's a link for the schematics.
https://www.centroidcnc.com/centroid_di ... matics.zip

[ShawnM]

Shawn...
That's a holdover from the DIY world with weak open loop steppers and hobby controllers like Mach or ShopBot.

Thanks Gary, my head was spinning looking at the pics and reading his post. That just seems crazy to do it like that.

[leinadltd]

Dan, there’s a wiring diagram in the Acorn schematics that shows you exactly how to wire up a single, n/c limit switch per axis in series. Then you setup software limits for max travel. Works perfect every time. Save your self the aggravation and remove all those unnecessary switches and wiring.

Here's a link for the schematics.
https://www.centroidcnc.com/centroid_di ... matics.zip

Thank you for the information. Yes, I used that schematic for my homing switch coming out of IN1, and it worked fine (see diagram above). The next step was to add the "soft" limits in the software as shown in the picture. The lathe is at home position in the photo above. The travel limit for Z-axis is +11.05" and the travel limit for X-axis is -3.95". I am not sure if I have them in the right spot or not is the problem. Next, I moved the Z-axis +5.00". Set Z0, then moved it -6.00, thinking the micro-switch would have stopped the machine.....and ran over the micro-switch and broke an insert. This led me to add the second micro-switch to avoid running the machine into the tool-holder, spindle, or tail-stock. I added the second micro-switch using the same pattern as the homing schematic in series coming out of IN2, and when the machine moves and "trips" the limit it works fine....the problem is when I added the second micro-switch and "home" the machine I get the error message "Z-axis limit" (see photo above) and the machine shut down without finishing the homing process. Right now, in order to "home" the machine I have to disable the limits. What did I do wrong? What setting in Acorn did I mess up? I need someone to help me figure it out....I do not have much hair left.

Thanks,
Daniel