Boxford VMC190 - Limit switches advice

[swood1]

Hi all,

I need to add limit switches to my CNC machine. Does anyone have experience of adding these to a Boxford VMC or similar machine?




Regards

Steven

[JohnHaine]

Why do you need to add them? Yes they are nice to have but not essential. Were they not fitted originally?

[swood1]

I'm worried about hitting the end stops again. I damaged a ball screw on the x slide which cost me.

There is a proximity switch on the z axis which works well. I'm using mach4, it maybe that I can do it in the software but I haven't worked it out yet.

Regards

Steve

[JohnHaine]

OK, good reason! If Mach 4 is like Mach 3 then it will have soft limits - look up in the manual. I'm surprised that the VMC wouldn't have limit switches originally, so are they actually fitted but not connected, or have they been removed? If the former then find them and use them, if the latter fit new ones, it should be obvious where they were fitted?

[depronman]

I have a vmc190 from the mid 90’s and it as home switches on all three axis
I use linuxcnc but mach3 and 4 support soft limits as does linuxcnc

You home the machine so in G53 it knows physically where it is then the soft limits are applied from that known ‘home’ position
I personally set the soft limits 1mm short of the physical limit of movement on each end of each axis
As the home switch is already located at one end of the physical movement of that axis the soft limit is 1 at one end and 250mm at the other assuming that there is 252mm of axis travel before it hits something or runs off the ball screw

There is a tick box to ‘ ignore soft limits ‘ just in case you do get beyond the soft limit area so you can carefully jog back into the soft limit area. I’ve never needed to use this but noted that it was there.
In mach3 as you approach the soft limit boundary the speed is gradually reduced especially if you are jogging or moving at a fast speed
In linuxcnc it just stops at the boundary
I’ve found soft limits work perfectly BUT if your machine was to skip steps then the soft limit may not protect you from damage but if your machine is skipping so many steps then you have much bigger issues to resolve

I’m not sure about mach3 as I don’t use it much these days but linuxcnc (my go to machine control software) will detect that the gcode would exceed the soft limit area and warn / prevent you starting the gcode which is a real life saver on a small mill like the vmc190

Cheers. Paul


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[swood1]

Unfortunately my machine doesnt look as though it ever had limit/home switches. The only one I have is for the Z+ axis.

Paul, is there any chance of seeing how your switches are fixed to the machine? It maybe that I have the same holes in my table to do the same.

I spent some time today learning about soft limits and homing etc. This should atleast keep me out of trouble for now.



Regards

Steven

[depronman]

The one on the X axis is easy to see when the table is moved full left it is at the right hand end of the table at the front of the table. It is covered by the tin work which covers the x axis stepper. Cable comes from the control cabinet in flexi trunking to the proximity switch

I would be amazed if boxford ever made the vmc series of machines without home switches

Soft limits are not any good without the limit switches as the machine must have a reference point for the soft limits to be effective
Paul


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[swood1]

There was no limit switches on my mill and it came straight from a school. I dont believe there ever has been any on it as would expect to see an addtional hole through the box for the wiring. But I have now added my own switches in discrete positions, just now need to work out how to program them in Mach4.

Thanks all for any direction.


Regards

Steven

[depronman]

Steve. I assume that it was fitted with homing switches (as was my own vmc190)
If so I just set up soft limits which effectively provides ‘virtual’ limit switches
This is a perfectly good solution and is safe unless the machine looses steps, but if that is happening then you have bigger problems that must be solved first

My Cyclone lathe has both home and limit switches fitted so I wires the limit switches into the EStop circuit and again set up soft limits. To date it as never triggered a limit exceeded switch (and hopefully never will)
I manually (with the stepper disabled) wound the x and z just past the soft limits onto the real limit switches to test out the circuit and indeed the EStop circuit is enacted and stops the machine. I then realised that I needed an emergency override switch mounted in the control cabinet to allow the axis to be jogged clear of the limit switches.
Having a rather limit number of input pins this was the best I could do with limit switches but does work well

Paul


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[swood1]

Hi Paul,

My machine had no switches within the cabinet apart from 2 on the opening doors. I have been thinking about adding a couple more for homing switches but currently need to understand how I get the ones I have added to work correctly in mach4. I was hoping once the switch was made the carriage would stop dead but that doesnt seem the case... I probably need to do some reading up on how to set them up again... I struggle with Mach4 as there seems to be a duplication of information within the software, I dont understand why they had to make it so complicated :(


Regards

Steve