No limits CNC

[Vixen]

The discussion regarding end stops, limit switches and 'home' switches has been moved here from the Mercedes W165 build thread.

It's quite an important subject and definitely one that needs it's own topic which members can refer to, read and perhaps contribute too.

Different machinists have conflicting views on the use of limit switches and stops, the discussion has been examining the pros and cons and the possible consequences.


My much modified Emco VMC, has a table movement of 200 x 100. I do not use limit switches, so I impose a soft limit of 190 x 90 instead. I see no sense in tap dancing on the limit switches or end stops. If you run into either, the job cannot be completed and is probably ruined. 190 x 90 is more than enough movement for most of the parts I need to make.

Mike

[kvom]

I do not use limit switches

Same here; soft limits work fine. 

[internal_fire]

I don't "use" limit switches, but I have them installed and functional. Hit one on rare occasions when something goes really wrong. Not sure anything would break, since the motors are not oversized, but it provides added safety.

Gene

[dieselpilot]

My much modified Emco VMC, has a table movement of 200 x 100. I do not use limit switches, so I impose a soft limit of 190 x 90 instead. There is no sense in tap dancing on the limit switches or end stops. If you hit either, the job cannot be completed and is probably ruined.

Do you not use home switches? Or are they not accurate enough to restart a job?

[Vixen]

Hello All,

I do not have, or use, any limit switches or 'Home' switches on my machines. They can be a source of unreliability and EMF interference. Any limit or home switch needs to be hermetically sealed to prevent the ingress of fine metal dust particles, otherwise they have the potential to cause nuisance tripping or failure to trip.  My philosophy has always been to keep it a clean and simple as possible.

So what do I do about 'Homing'?

Well, I do not use 'Home' switches to define a fixed point on the mill's table. Instead I use a digital touch probe (similar to a Renishaw) to find a corner, or a center,  or a defined position, such as a toolmakers button, on the piece of material to be machined. That position on the material becomes my Zero (Home??) point. My CAD CAM uses the same position to calculate the tool paths. It certainly simplifies tool offsets... there are none. 

This may not be how they do it in a professional machine shop: but it has worked well for me for the last twenty years. 

Mike

[steamer]

Thanks Dave, 

As an engineer, I like to be creative with the tools available to me. If they have limitations.... no problem.... I will try to find a work-round.

My much modified Emco VMC, has a table movement of 200 x 100. I do not use limit switches, so I impose a soft limit of 190 x 90 instead. There is no sense in tap dancing on the limit switches or end stops. If you hit either, the job cannot be completed and is probably ruined. 190 x 90 is more than enough movement for most of the parts I need to make.
I
The cam gear case cover is an exception, it measures approx 240 x 160. Being "T"  shaped I will be able to get away with three set-ups.

Cheers   

Mike

Oh Totally understand!   I was just curious.....

Dave

[kvom]

Same as Vixen.  Machine coords are ignored and part zero is determined for each part (usually wiggler).  If I need to leave the part overnight, I manually move to X0Y0 before shutting down.  The control doesn't move the table when referencing the axes.

My machine does have limit proximity switches, but I never found using them to be a benefit.

[Hugh Currin]

I typically set machine zero to be the corner of my vice and all the way up in Z. Then set work coordinates for each part. Setting machine zero this way is consistent enough, for me, that the soft limits mean something. I also haven't gotten around to putting on home or limit switches. I park the machine at machine zero when shutting down. This is consistent within a few thousands, close enough unless continuing to machine the same part.

Mike: Your work is amazing. I want to see the finished engine, but don't what the thread to end. Thanks.

[steamer]

For a critical part I'll fixture the part, and mark the zero from a specific corner, that way if I crash or lose power it's easy to pick back up.

Dave

[Vixen]

For a critical part I'll fixture the part, and mark the zero from a specific corner, that way if I crash or lose power it's easy to pick back up.

Dave

Corners are good.    They are easy to find and easy to return to. They give both the X and Y work datum (Work Zero)

Oh!! Don't machine off that corner until the last moment

Mike

[Jasonb]

Never found them that good on round parts I tend to use the top ctr on most jobs more than a corner but that's possibly just a carry on from what I tend to do on the manual mills. Also suits me on teh CNC as stock is usually a little oversize in both directions so using ctr ensures i'm not too tight on one or two faces.

The KX3 has home and limit switches but I don't have them enabled, just watch the simulation to make sure the machine is not moving too far. As for home it would probably be more useful if I were using quick change tooling as it takes the head up and gives you room to change but as I'm only going to be swapping a cutter in teh ER collet then I'll just raise the head as far as needed to do that and jog down to set the tool height.

[john mills]

with industrial machines in years of using and programming cnc and older nc machines
the  limits are important to protect the machine from over travel accidental movements and errors malfunctions.
soft limits are important too have used machines with chuck barrier and machine barrier protect the machine
if you hit a barrier and it stops from finishing that is what it is for .toprotect the machine i have had the service man go to another job to come back and say they had crashed the machine did not bother to have set chuck barrier took to much time .well the machine would be months before it would be repaired.it had ripped the tool holder off the cross slide.thousands of dollar cause didn't bother to set barriers.i have had the tail stock in the way
barrier set some how .but the machine did not have one.if you hit a stop you stop i had a day shift operator that didn't set no's of parts relying on barrier to stop .even with low power machines stopping when hitting end of travell can do lots of damage .as for where you put x and y 0  is a matter of when you program what suits the job.
you just need to know where it is.
john     

[steamer]

Wot John said......As a machine designer....I have to agree.   The limits are there to protect the machine

On my little mill, the limits are always active, and the software will flag me and prevent me from running if the program runs out of bounds either by capacity, or because of my zero location....   

Jason.   The point I was making was that two edges of the fixture plate are used as a zero, knowing that a set distance from these two edges is the program zero.    That way I can pick it back up very easily with an edge finder...as compared to sweeping in a round part.     By the way a edge finder can do very good job of picking up a round part....but it usually takes two goes from the X and Y to do it...

The reason for this is that zero on my machine is set from a rotary encoder reference to one of the limit switches, and those switches don't necessarily repeat that great...

Anyway....none of this matters, as I want to see how Mike the magnificent Vixen is going to finish this engine!   

[john mills]

the maximum travel limit switches are the same as maximum travel feed knock out stops on manual machines
there are plenty of good manual machines with power feed badly damaged by these stops removed to get a bit more travel .shear pins and slipping clutches are used as well but don't always work and the machine is still damaged.
John

[Vixen]

Thank you, John and Dave,

You make a very good point about the use of limit switches and stops. They are there to protect the machine from damage. You can avoid using them if you must, but the risk is yours alone.

Much like seat belts in a car. Nobody intends to have a car crash, but if they do, they have the protection of the seat belts to avoid serious injury.

Mike