What is homing on a CNC machine?
When I first started using my CNC machine I didn’t use homing, I didn’t use it because I didn’t really understand it and probably didn’t really need to in those early days however when you start to really use you CNC machine you’re going to want to know about homing and limit switches because they make using your machine a lot easier.
Homing a CNC machine requires that you have homing switches or limit switches. You can have different homing switches and different limit switches however most systems use limit switches as homing switches.
Limit switches stop the machine from moving too far in a direction that can damage your machine. You can move your CNC machine as far to the right as it will go and without a limit switch installed you can still move it further so it will push against the frame possibly damaging the frame, pulling screws or bolts out, maybe damaging the linear guides or motors with the excessive force.
Not something you want happening. A limit switch is positioned at such a place so when your machine has moved as far as it can the switch is operated by something like the side of the machine to prevent it going any further to cause damage.
In my set up I only have three limit switches that also function as homing switches for the machine.
I have an article more specifically about limit switches here if you need more information.
The three limit switches I have on my CNC machine are, one on the Z axis so it goes as high as it can then switches, one on the Y axis so it goes as far forward as it can then switches and one on the X axis so it goes as far left as it can then switches.
What are soft limits on a CNC machine?
A soft limit is like a limit switch but works in the software, hence the term soft limit. Once you have your three limit switches also known as hard limits installed. You enable them in GRBL and enable the soft limits. I have an article on GRBL here if you need more information about it.
You then set the values of the soft limits in numerical values to do the same job as the hard switches but at the opposite ends that the hard switches are installed.
If your installed your X axis switch so your CNC machine moves as far left as it can, zero the left setting then manually jog the X axis as far right as you want it to go and if that reads 400mm on your CNC machine software you enter 400 as the numerical value for the soft limit for X.
You do this for all three Axis and then you have a machine that will move as far up, left and forward as the hardware switches will allow and as for down, right and back as the soft limits will allow due to the values you programmed into the X,Y and Z soft limits in GRBL.
You now have a machine that you can’t damage by moving it too far and it can’t damage itself if you run a G code program in that attempts to move it too far.
In the arrangement I have with three hardware limit switches enabled and soft limits enabled I also have homing enabled in GRBL.
How do you home your CNC machine?
When my CNC machine is powered on and the CNC software OpenBuilds CONTROL starts I have to home it before it will do anything else. You just click on the homing button in the software and the Z axis moves as far up as it will go then stops. The X axis moves as far left as it will go then stops and the Y axis moves as far forward as it will go then stops.
I then zero the machine so X, Y and Z all display zero on screen. This is now the home position and regardless of where ever I’ve moved the machine in the past this will always be a known position. So if you’ve got a G code program that you’ve used before and you home the machine the G code program will always start cutting in exactly the same position.
This is really useful if you make the same items at different times especially if you have a clamping system that is the same. For instance I make parts out of an acrylic plastic sheet and I have a wood edge along the front and the same to the left making a square “L” shape. The acrylic sheet clamps by being pushed against this fixed “L” shape so by homing the machine I know exactly where it will cut on the sheet with the G code programs. So I can put a sheet in and run a G code program take out the cut sheet and put another in as simple as that.
Advantages of homing a CNC machine
That’s one of the huge advantages of homing a machine.
You don’t have to have your CNC machine home to the up, left and front positions either. I know some CNC machine operators like to have their machine home to the back so ever things out of the way when they start to set a job up. I just like mine to the front left but you don’t have to do it that way. The way a machine homes can be set up in the GRBL settings which you can read about here if you need more information.
Incidentally I think this set up with three limit switches and soft limits is the best way to do things as it saves you having to wire up another three hardware limit switches. I’m always puzzled as to where you would position the Z axis down switch anyway, as how far the machine goes down changes depending on how long your cutting or drilling bit is and as long as it can’t go too far up, the depth is something you set before you run a job usually using the probe which takes care of the Z low settings.
If you need to know about CNC probes I have an article here.
I have three different articles, including this one on homing a CNC machine and one on limit switches and one on GRBL settings. These three subjects are quite interrelated and you may want to read them all to get everything set up properly.