Limit Switch Overtravel

[kolias]

How far an axis will stop after it hits a limit switch?

I assume that an axis will not stop dead after it hits a limit switch so in locating the limit switch I must consider some over travel.

In doing so I can be conservative and allow, lets say 3” to 4” over travel (thus reducing the cutting area of this axis), or lets say 1/2” over travel and risk a damage to the machine.

Anyone has experience with the above?

[Al_The_Man]

It is totally dependant on the mechanics, inertia etc of the individual machine, 3"-4" is rather a long overtravel past an overtravel switch.
Presumably your are turning the motors off, so the inertial required to back feed the motor is normally very large, the majority of the machines I have set up 1" would normally be the very maximum.
It can be observed by a simple test to see what the travel distance is at high feed rate.
Al.

[kolias]

Thank you Al

I figured that it should depend on the individual motor inertia but I thought that a kind of “average setting” might have developed here from all users. 1” I would say should be the max.

Like you said, best to do a test on my own which is very easy to do

[Drools]

Would the number of starts on the lead screw also play on this?
I'm also at the point of mounting limit home/switches and I'm noticing on my Z axis even with the weight of all the aluminum it does not turn at all. I'm using 1/2-10 single start screws.
I have a 5 start screw that I pulled from an old piece of computer hardware and the threads are almost vertical, if I had used it for my Z axis I'm sure the spindle would drop due to its own weight.

[pminmo]

It will stop a lot faster than you think, but it depends on mechanics and electronics. 1/2-10 tpi single start will stop pretty quick with steppers unless there is a really heavy load with a lot of inertia. Fractions of an inch in most DIY machines.

[kolias]

Originally Posted by Drools Would the number of starts on the lead screw also play on this? I'm also at the point of mounting limit home/switches and I'm noticing on my Z axis even with the weight of all the aluminum it does not turn at all. I'm using 1/2-10 single start screws. I have a 5 start screw that I pulled from an old piece of computer hardware and the threads are almost vertical, if I had used it for my Z axis I'm sure the spindle would drop due to its own weight.

I don't think the # of starts on the lead screws has anything to do with the limit switch.

The # of starts to my opinion, has to do with how smooth / fast your axis will travel per turn of the screw.

The problem with your Z axis may be a cause of your motor. What size motor are you using?

[kolias]

Originally Posted by pminmo It will stop a lot faster than you think, but it depends on mechanics and electronics. 1/2-10 tpi single start will stop pretty quick with steppers unless there is a really heavy load with a lot of inertia. Fractions of an inch in most DIY machines.

I like what I read, more usable travel per axis.

As I said, I will give it a try starting at about 3" from the end of the travel and reduce it to the minimum I can get.

[pminmo]

The thread pitch figures into the mechanical advantage when starting and stopping, not the limit switch per say.

I don't use limit switches, I think there pretty much a waste of time for steppers. Most of the software out there has "soft limits" where you define table travel based on a home location. Home switches make life much easier and in my opinion more important than limits. Some of the software also allows duel usage of a home switch to also be a limit switch.

[kolias]

Originally Posted by pminmo The thread pitch figures into the mechanical advantage when starting and stopping, not the limit switch per say. I don't use limit switches, I think there pretty much a waste of time for steppers. Most of the software out there has "soft limits" where you define table travel based on a home location. Home switches make life much easier and in my opinion more important than limits. Some of the software also allows due usage of a home switch to also be a limit switch.

It makes sense what your are saying pminmo

I have MACH3 and haven’t learnt it yet 100%. If I can set “soft limits” why not? Will avoid extra wiring and interference.

Defining the Home place of the machine I agree that it’s more important.

I will give it a try

[Torsten]

Originally Posted by kolias How far an axis will stop after it hits a limit switch? I assume that an axis will not stop dead after it hits a limit switch so in locating the limit switch I must consider some over travel. In doing so I can be conservative and allow, lets say 3” to 4” over travel (thus reducing the cutting area of this axis), or lets say 1/2” over travel and risk a damage to the machine. Anyone has experience with the above?

The amount of overtravel can be calculated.
It depends on your start/stop pulserate setting, this is the rate at wich a motor can be set to from a death stop or can be stoped at instantly.

And the Accel/Deceleration setting this is the rate of speed increase or decrease per step.

Also the max speed pulse rate from where this will be computed.

When the limit switch gives the signal that it has triggered the controller will imediately start to slow down the motors by the amount set in the decceleration value when the speed has reached the stop speed all further pulses will cease and the motor is stopped.

Now you can calculate how many pulses it will take to get back to a stop from your maximum speed.
Multiply the pulses with your resolution and you have the max stop distance.

Of course there is an easier way, if you download "Turbocnc" (free) and set it up for your system it will tell you in the setting menu the resulting stop distance.

Or just test it on your machine and measure the result yourself.

Note when setting the start/stop and acceleration values there is a bit of
personal preference involved.
most people don't like a too tight system because it can be tough on the hardware, however softening the settings will cause more overrun.
Good Luck

[kolias]

Thank you Torsten, very good info, I will keep it in mind

[Al_The_Man]

Steppers may be a different case, I was talking only in the case of servo's as I use these solely.
But on an over travel, I have Never kept the servo's active, I disable them immediately by drive enable input as well as dropping the drive power.
The only thing to watch is if the Z axis is fairly heavy, it may back feed, but if it is at all a considerable weight, it should either have counter balance or brake, or both.
Al.