Adding limit switches

Last Update: 29 November 2019
Documentation in Progress

This part of the resources is still a work in progress, please be patient while I take the time to finish it in the coming days -Chris

Limit switches (also referred to end stops or homing switches) are switches that sit at one end or both ends of each axis to provide a few different functions:

  • Preventing the machine from hitting the ends of each axis
  • Homing the machine

There are many different designs of limit switches, but mechanical and non-mechanical that are used in different CNC machines. As long as they can repeatably detect when the machine has come to the extremity of its travel repeatably and reliably, it should work. Technical details and more can be found in the documentation for each limit switch,

Some users may choose to integrate these switches into their machines. We will discuss the benefit and function of these switches, as well as some thoughts on how to integrate them into your CNC machine.

To preface this discussion, it should be mentioned that your LongMill does NOT need limit switches. You can use your machine completely without issues without them. The LongMill is designed so that even if your machine hits any of the axis, it will not damage or break itself. The LongBoard controller provides inputs that allow users to integrate switches if they choose to.

Using limit switches to prevent the machine from hitting the ends of each axis

Limit switches can be used on each end of the axis to prevent the machine from crashing. When the machine reaches the end of the axis, the switch will trigger and send a signal to the control board to stop the machine from moving any further.

If your LongMill is set up with limit switches, you can tell your controller to set hard limits by sending the command¬† “$21=1″.¬†For more info, visit the GRBL v1.1 Configuration Guide.

It should be noted that it is very important to shield and filter noise along the lines with the limit switches, as interference can cause the limits to trigger erratically. We have found that placing a 0.1uF capacitor across the input and ground for each limit helps to prevent errant triggers.

In larger, more powerful machines such as VMCs and industrial CNC routers, crashing a machine can very well mean costly damage to your machine, and so limit switches are cheap insurance to prevent that. However, on hobby machines like the LongMill, the machine is not powerful enough to cause damage to itself, and so are not really necessary. In fact, in some cases, it can be more of a hassle as accidental triggers can ruin a project.

Using limit switches to home your machine

I should note that switches that are used to home the machine are technically called homing switches but are usually used interchangeably as they can both be used for detecting limits and for homing.

The idea of homing the machine is to move the machine to one end of each axis until the limit is hit so that the machine is able to come back to the same position in relation to itself repeatably. This can be useful in some cases. For example, if you know the distance between the machine’s origin point and the origin point of the workpiece, then if you lose your position, whether it be due to a power outage or for a tool-change, by re-homing the machine, you can find the same position again.

You can find the settings to change in your firmware also on the GRBL v1.1 Configuration Guide.

Adding limit switches to your machine

It should also be noted that we currently do not have any official documentation on where and how to add limit switches to the actual machine itself. However, wiring the limits is fairly straightforward, as you can use the screw terminal or JST 4 pin connectors that are found on the right side of the controller to wire the switches. If you are wiring limits to both ends of the axis, you can put them parallel to each other. All of the switches share the same ground.