Primer on Homing and the Use of Limit Switches
I know, you are finished building the CNC machine, and now, you need to set up the limit switches and be able to home the machine. Argh, the concept of restricting the machine and making it know where it is standing seems like a serious challenge. Well, not really. I am going to demonstrate a few techniques on connecting switches and how to home the machine, but in a rather unique way. You will learn how to connect limit switches and home switches using two different circuit types, normally open (NO) and normally closed (NC). I will also demonstrate a method of homing that uses the end mill (bit) to find the home position. This is not meant to be a complete comprehensive tutorial on limit switches and homing the machine, but it will get you running.
There are a few fundamental methods that you can use to provide limits to your machine: soft limits, limits using an electro mechanical device (switch), or physical blocks or the machine structure. I would not advise the latter, as this can damage the machine if large motors are used. Soft limits are generally used in combination with limit switches. Soft limits prevent the machine from exceeding set values for the extents of the x, y and z axes. For example, if the machine has a routing area of 24"x48", the x would have a soft limit of 0 for the x-- and 48 for the x++, or if the origin is in the middle, the soft limits would be -24 for the x-- and +24 for the x++. That would be for a single axis. you would apply the same principle to the y and z axes depending on the travel and where you want the origin. The configuration for soft limits is quite flexible.
So, if there are soft limits, why is there a need for limit switches. Safety, is the main reason. Limit switches are attached to industrial CNC machinery because the power of the motors to move the machines are usually quite large and high torque. Usually, there is a person (or people) in the vicinity controlling the machine. If, for some reason, the machine lost track of its position, which can happen easily, the machine can potentially run past its physical limit and injure (possibly fatally) the people around it.
So, what about homing? Machines need to know where their origin is located for all of its axes. Some CNC machines have the origin in the middle of the work piece, and some at one corner. I am in the "one corner" camp. I'll bet you are wondering, what about the z-axis... Should it be in the upper most position, or at the surface of the work piece. That configuration is totally up to the controller and CNC programmer. The top of the z-axis is a known distance from the table, so the math is pretty easy to determine the surface of the work piece. I, personally, like the machine to determine the surface location, so in the video, I demonstrate a technique to do this (almost) automatically, using a three plate method.
Homing can be done with switches, or using any device that permits a complete circuits using electrically conductive material. In the video, I demonstrate the latter approach of using the end mill as part of the switch. Sure, limit switches can be used to determine home, but there is still the possibility that the bit is not in the same position, especially for changing the tool (end mill, bit, etc.). The end mill (or bit) that is in the router (spindle) is the part of the machine that does the cutting, so it is obvious that the bit find its home position (origin).
Also, check out Chris Cockrums's setup and how he uses a plate to zero the z-axis.
Shortly, I will devise a script that can be loaded into Mach3 and executed for homing to automatically home and backing off of the plate using the three plate method.