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Author Topic: Single home switch and soft limits enough?  (Read 4537 times)

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Single home switch and soft limits enough?
« on: January 10, 2013, 01:34:50 PM »
Hi!

I am working on my Sieg X2 CNC project. Someone told me that they would suggest using only one home switch and then setting soft limits on each end of the travels on X and Y axis. On Z axis a home switch somewhat up on the Z axis travel and soft limits.

In case this is OK, I can save a lot of wiring and making brackets for swithces. I would need only three swithces instead of six.

Do you think this is a good idea?

Offline Hood

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Re: Single home switch and soft limits enough?
« Reply #1 on: January 10, 2013, 01:58:26 PM »
It all depends on whether your machine would damage itself if it hit an axis end that had no limits to stop it.
Hood
Re: Single home switch and soft limits enough?
« Reply #2 on: January 11, 2013, 03:13:53 AM »
Hi Hood.

With soft limits set up some distance from the ends, wouldn't there be a quite low likelihood that the machine hits an axis end?

Even with switch based end limits (as opposed to soft limits), wouldn't those end limit swithces need to be placed some distance away from the axis ends so that the machine have time to stop before it hits?

How do other people place their end limit swithces on small X2 style CNCs with steppers? What is common practice?

The guys at CNC Fusion said that it is not so probable that the ballscrews or nuts will be damaged. They were more concerned about stall damage to the stepper motors.

Offline Tweakie.CNC

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Re: Single home switch and soft limits enough?
« Reply #3 on: January 11, 2013, 04:51:36 AM »
If it’s any help…

My limit switches are mounted in such a way that any over travel (to the mechanical stop) does not damage the switch. The switches will stop the axes travel during the usual G01 moves but with G00 moves the axes will hit the mechanical stop, however, because the limit has been activated the motors will be stopped.

With a high torque motor operating at a high current this setup may leave a lot to be desired but it suits my way of working and has served me well for many years now.  ;)

Tweakie.
KEEP SAFE !

Offline Hood

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Re: Single home switch and soft limits enough?
« Reply #4 on: January 11, 2013, 06:32:30 AM »
If for some reason you have lost position then Mach may not know where the axis actually is, in that situation  SolftLimits will not protect you.
Limits will stop you instantly, or rather the axis will be told to stop instantly, this may mean a slight over travel depending on the friction etc but on the majority of machines this will be within a very short distance.
If your steppers will be unlikely to do any damage to the machine then maybe they are not needed, that will all depend on your setup.

Hood

Offline rs232

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Re: Single home switch and soft limits enough?
« Reply #5 on: January 11, 2013, 01:16:33 PM »
I am repairing a Bridgeport milling center and it only has 1 limit switch for each axis.
They are located at the center of travel, lets say "X" travels 800mm and the switch is located at 400mm
There is a extra switch in the same enclosure that work as Extreme limit switches.
I'm planing to do the same in the machine I'm building.

Ric
« Last Edit: January 11, 2013, 01:18:44 PM by rs232 »

Offline Hood

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Re: Single home switch and soft limits enough?
« Reply #6 on: January 11, 2013, 01:38:25 PM »
Ric,
 are you saying is there is a switch fixed mid axis and it is triggered at either end of the axis? And additionally there is another switch on each axis that triggers close to the end of axis?

If so then that is the normal configuration for industrial machines. One switch is a home switch and it is used to signal the control to search out the index pulse of the encoder for homing, the other is a limit which is to stop the axis immediately should it over travel.

In Mach you can have a similar setup, ie one switch per axis and have them all tied together in series and using one input. You can configure them as both limits and home switches in Mach. When homing the switches are treated as home switches, all other times they are treated as limits.
 I personally prefer to have separate home switches and I home to my servos encoder Index pulse, for that however you need to have servo drives or a controller that supports the function.
Hood

Offline rs232

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Re: Single home switch and soft limits enough?
« Reply #7 on: January 11, 2013, 03:31:41 PM »
Exactly.
All 3 axis have a twin switch, 1 acts as limit/home and the 2nd one is the extreme limit switch.
I only seen it working on manual mode when over riding the 1st one (limit switch)
This is on a Bridgeport Interact 520V
I can take some pictures on Monday, still waiting for the technician to fix the problem with the hand encoder wheel.
Re: Single home switch and soft limits enough?
« Reply #8 on: January 13, 2013, 06:16:31 AM »
Hi. I think I will start with one switch + soft limits and later goto several hard limit switches. I want to get running asap.

Offline Hood

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Re: Single home switch and soft limits enough?
« Reply #9 on: January 13, 2013, 07:02:23 AM »
Why not have the single switch per axis triggered from both ends of axis and set it up as both a limit and home switch for that axis, you can even link all in series (assuming normally closed switches) and only use one input.
Doing that will mean you only use 3 switches and one input and you will have home and limits.
Hood