The Essential Guide to Limit Switches for Superior Machine Control

A limit switch is a device used to detect the presence or absence, passing, positioning, and end of travel of an object. It is an electromechanical switch, which means it operates when a mechanical force is applied to it by an object. When the object comes into contact with the actuator of the switch, it moves the actuator to its limit where the switch operates. This movement either opens or closes an electrical circuit. Limit switches are commonly used in a variety of industrial control applications, such as machinery, to control the motion of parts or to provide a safety interlock. They can be found in various forms, including whisker, roller plunger, and rod actuator types, among others. Common applications include counting objects passing a point, controlling the limit of travel of an object, or as a safety device to prevent machinery from colliding with people or other equipment. Limit switches are known for their reliability, repeatability, and durability, often being able to withstand harsh industrial environments.

How to Choose the Right Limit Switch

Choosing the right limit switch involves considering several critical factors to ensure it meets the specific needs of your application. Here’s a guide to help you select the right limit switch:

1. Operating Environment:
   • Temperature: Ensure the switch can handle the operating temperature range of the environment.
   • Humidity: Check for the switch’s resistance to moisture or for the need for a sealed unit.
   • Vibration and Shock: Some switches are designed to resist mechanical shock and vibration.
   • Corrosives: In chemically aggressive environments, choose a switch with suitable housing and seal materials.
2. Load and Electrical Requirements:
   • Voltage and Current: Match the switch’s electrical rating with the load it needs to control.
   • AC vs. DC Loads: Some switches are better suited for alternating current (AC) while others for direct current (DC).
   • Inrush Current: If the switch will control an inductive load, ensure it can handle the inrush current.
3. Physical Constraints and Actuator Type:
   • Space: Assess the size constraints and fit of the switch in the required space.
   • Actuator Type: Plunger, lever, roller plunger, whisker, etc., need to be chosen based on the kind of physical contact expected.
   • Actuator Length and Positioning: Select an actuator that will engage properly with the moving part.
4. Mechanical Life:
   • Durability: The switch should have a mechanical life that exceeds the expected number of operations in the application.
5. Switching Frequency:
   • The switch should be able to handle the expected cycle rate without premature failure.
6. Precision and Repeatability:
   • The switch should be able to actuate at the specific point needed every time without significant variation.
7. Installation and Maintenance:
   • Accessibility for installation and maintenance should be considered, as it affects operating costs over time.
   • Snap-action switches require little force to activate and can be useful in delicate applications.
8. Certifications and Standards:
   • Ensure the limit switch meets relevant industry standards, such as IP ratings for ingress protection, UL listings, CE markings, and any other regulations pertinent to the application.
9. Cost and Availability:
   • Consider the total cost of ownership, which includes purchase price, lifespan, and any maintenance expenses.
   • Ensure availability for replacement parts and future maintenance needs.

Remember, the correct limit switch will not only perform reliably but also protect equipment, prolong lifecycle, reduce downtime, and preserve safety standards. Often, it can be helpful to discuss your needs with a manufacturer or distributor to get recommendations based on your specific requirements.

My Tormach mill came with microswitch limit switches. They have a plunger with a roller on the end which contacts a ramp and are relatively immune to flood coolant. However, I found them to be less accurate than desired for use as a homing switch so I devised my own optical homing sensors. The OEM had a repeatability of around =/- .. The optical system is =/-.".

The Grizzly G mill uses mechanical switches in conjunction with spring loaded plungers. The spring loaded feature permits overruns without hard crashes. The parts could be ordered as replacement parts. The actual switch is located in a housing and should also be relatively immune to flood coolant.

If reproducible homing isn't required, either of these mechanical systems should be suitable.

My Tormach mill came with microswitch limit switches. They have a plunger with a roller on the end which contacts a ramp and are relatively immune to flood coolant. However, I found them to be less accurate than desired for use as a homing switch so I devised my own optical homing sensors. The OEM had a repeatability of around =/- .. The optical system is =/-.".

The Grizzly G mill uses mechanical switches in conjunction with spring loaded plungers. The spring loaded feature permits overruns without hard crashes. The parts could be ordered as replacement parts. The actual switch is located in a housing and should also be relatively immune to flood coolant.

If reproducible homing isn't required, either of these mechanical systems should be suitable.

The optical homing sensors that you came up with if you could share what you ended up doing, that would be fantastic repeatability on your system versus microswitches is far superior.

Thanks,
Lynn M.

The optical homing sensors that you came up with if you could share what you ended up doing, that would be fantastic repeatability on your system versus microswitches is far superior.

Thanks,
Lynn M.

I had done a write up some time in the distant past. There is a fairly complete discussion of the project on CNC Zone. https://www.cnczone.com/forums/torm...ng-proximity-sensors-tormach-pathpilot-2.html I also have a full set of documentation on Dropbox.

The modification is by no means plug & play. To get the required sensitivity, I used a special optoisolator and designed my own optics and electronics. The system has been in operation for more than ten years and t date there have only been three failures. One was a component failure; another had to do with coolant getting into one of the optointerupter windows after some prolonged heavy usage. That was clear up with a blast of compressed air and a redesign of the shield corrected the problem. The third occurred recently when I was using the mill for some wood work and a lot of fine dist was generated. Again, a blast of compressed air solved the problem.

I left the OEM limit switches intact as a backup. Should the optical system fail to properly home for some reason, it will hit the mechanical switches some .040" in back of them. PathPilot has a feature in that once the system is homed, it knows where the the limits are and will never exceed them, even traversing at rapid speed so it has been literally years since my mechanical switches have been activated.

Would anybody, have a suggestion or information on what switches they’re using for the limit switch inputs something that’s relatively impervious to flood coolant

Thanks,
Lynn M.

$114 at Grainger. I've used many of these in another life and found them to be high quality.

HONEYWELL MICRO SWITCH General Purpose Limit Switch: 1NC/1NO, 15A @ 600V, 2A @ 250V, Top, Surface​

Item 12U161
Mfr. Model BZE6-2RN

Product Details​

Catalog Page262
Limit Switch StyleGeneral Purpose Limit Switch
Basic Actuator Style - Limit SwitchPlunger
Actuator Style - Limit SwitchPlunger
Actuator Location - Limit SwitchTop
AC Contact Rating - Limit Switch15A @ 600V
DC Contact Rating - Limit Switch2A @ 250V
Contact Form- Switches1NC/1NO
Actuator Length - Limit Switch0.97 in
Body Height - Limit Switch1.75 in
Body Width - Limit Switch3.04 in
Body Depth - Limit Switch1 in
Boot Sealed Actuator - Limit SwitchYes
Mounting - Limit SwitchSurface
Body Style - Limit SwitchMiniature
NEMA Rating1; 3; 4
Wiring Entry1/2 in NPT
Electrical ConnectionScrew Terminals
IP Rating66
MovementRelease Force 0.25 lb min
TravelDifferential Travel 0. in to 0. in; Maximum Pretravel 0.078 in; Minimum Over Travel 0.219 in
Operating Temp. Range-25° to 160°F
Mounting Hole Pattern - Limit Switch2 Hole Horizontal
Mounting Hole Spacing - Limit Switch1.00 in
Mounting Hole - Limit Switch(2) 0.22 in dia
Stem Thread - Limit SwitchNo Stem
Body MaterialZinc Die Cast
StandardsCSA; UL
UNSPSC
Country of OriginUSA (subject to change)