No limits CNC

[dieselpilot]

Hard limit switches are really only needed for DC servo machines where a drive failure can run an axis uncontrolled to the end stops. Stepper machines simply don't need limit switches, but accurate homing is a nice thing to have. Many newer machines don't have limit switches because failure modes of BLDC or AC drives and the types of signals used today mean that an axis simply can't run away.

If you aren't using tool offsets or don't have to shut down the machine before a part is finished, you can do without. I built my first machine with homing, and would never do without.

If using an encoder index pulse with a limit switch to home it will be as accurate as the resolution of the encoder. The accuracy of the switch doesn't come into play. Emco stepper machines have excellent homing with an index signal and a prox.

[Jasonb]

Not sure about stepper equipped machines not needing limit switches. This is the externally visible damage done to my machine by the previous owner when he ran  it past the x height stop, more damage inside



I must have  ago at enabling them again on mine, they were giving some issues when I first got the machine which I may be more able to sort out now having used it some more.

   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...

That's what I normally use on the manual machines, not so much on the CNC as my usual electronic edge finder is not very flexible so don't want to jog too far or in the wrong direction I guess it all comes down to how an individual works, I'd mostly use a chuck or ER collet fixed to the mill table to hold a round part rather than a fixure plate

[Vixen]

......... I built my first machine with homing, and would never do without.

If using an encoder index pulse with a limit switch to home it will be as accurate as the resolution of the encoder. The accuracy of the switch doesn't come into play. Emco stepper machines have excellent homing with an index signal and a prox.

You say you built your machine with 'homing' and would never do without. Well, I have never had a 'homing' capability, therefore I dont know how to use the 'homing' facility or what advantage it would bring. Perhaps you would be kind enough to give me a short 'how and why' primer.

I have just checked my Emco VMC. It still has the magnetic index and proximity sensors and wiring for each of the X, Y and Z axes still in place, but they are not interfaced to my Linux CNC controller at the moment. Perhaps I am missing a trick by not using the 'homing' facility.

Mike

[Dave Otto]

I was going to comment before but didn't want to muck up Mike's thread.
I guess I'm in the minority here, I have working plus and minus limit switches on all three axes of my mill. I can full rapid into any switch and it slows and stops with out any issue; this is due to properly setup soft limits. It is also nice when loading a job file you know if there are boundary issues before you start cutting.
My machine is a converted knee mill so there are some extra variables that come in to play, I.E. knee position or ram position on the turret that may cause you not to be able to cut your entire job without repositioning something.
I'm using industrial quality snap action switches (Omron) with a spring loaded roller actuator. Testing has shown that these switches will repeat to within .001", plenty good considering the rest of the hardware.

Dave

[internal_fire]

You say you built your machine with 'homing' and would never do without. Well, I have never had a 'homing' capability, therefore I dont know how to use the 'homing' facility or what advantage it would bring.

I have active limit and homing switches. As I said previously I try very hard to avoid using the limit switches. 

The homing switch is never used (by me) for referencing the tool to the workpiece. Instead it allows the entire system to remain centered. If I continually reference the workpiece with an edge finder I can accidently make the machine start to work more and more off center. Eventually this could cause a run-in with the soft limit stops.

LinuxCNC requires homing before it will run, so I do not have much choice in any case. (Requirement probably can be defeated, but I have not tried.)

Gene

[Vixen]

You say you built your machine with 'homing' and would never do without. Well, I have never had a 'homing' capability, therefore I dont know how to use the 'homing' facility or what advantage it would bring.

I have active limit and homing switches. As I said previously I try very hard to avoid using the limit switches. 

The homing switch is never used (by me) for referencing the tool to the workpiece. Instead it allows the entire system to remain centered. If I continually reference the workpiece with an edge finder I can accidently make the machine start to work more and more off center. Eventually this could cause a run-in with the soft limit stops.

LinuxCNC requires homing before it will run, so I do not have much choice in any case. (Requirement probably can be defeated, but I have not tried.)

Gene

Thanks Gene, that helps my understanding of the 'home' function.

My LinuxCNC set-up does not try to automatically 'Home' during switch on. Thats probably because the 'home' sensors are not connected. LinuxCNC sees an open circuit and assumes it has reached the 'home' position.

I position the workpiece by eye between the max travel markers on the bed. I then reference the workpiece with an edge finder and press 'Home axis' to Zero each axis to the workpiece reference point. Much like you do with the DRO's on a manual machine.

We all have different way of working and our machines may be set up differently

Mike

[Hugh Currin]

Dave:

It sounds like you're using soft limits (limits imposed by software rather than physical switches). Then you have physical limit switches active but the machine never trips them as long as the soft limits work. I understand this to be correct and safest way to set up a CNC machine. I should put limit switches on my converted knee mill. However, the vast majority of my parts are too small to push the machine size limits. The few times I've pushed it the soft limits have prevented over travel. The soft limits do well as long as LinuxCNC knows where machine zero is. Oh, well yes, I'm lazy!

On my knee mill it's the quill that is controlled. This does limit the Z axis travel. I usually break a program into two parts to get around any Z limits. For my small parts this is usually moving from end mills to drills/reamers.

What you have makes perfect sense.

Thanks.

I guess I'm in the minority here, I have working plus and minus limit switches on all three axes of my mill. I can full rapid into any switch and it slows and stops with out any issue; this is due to properly setup soft limits. It is also nice when loading a job file you know if there are boundary issues before you start cutting.
My machine is a converted knee mill so there are some extra variables that come in to play, I.E. knee position or ram position on the turret that may cause you not to be able to cut your entire job without repositioning something.
I'm using industrial quality snap action switches (Omron) with a spring loaded roller actuator. Testing has shown that these switches will repeat to within .001", plenty good considering the rest of the hardware.

Dave

[Hugh Currin]

Mike:

Let me try to explain homing as I understand it. This is usually where I learn a lot from more knowledgeable members telling me where I'm wrong. I look forward to this learning.

The commercial machines I've seen go through a homing routine upon startup. They then know where machine zero is and are ready to go. They move each axis until they trip the home switch(s) telling the software where that axis is sitting. Zero may not be at the switch but they apply an offset from the switch to know where zero is.

LinuxCNC can likewise be set up to search for home switches. Can even use a limit switch for a home switch. Many options are available such as. 1) Move axis ? in a given direction until it trips the switch and stops with slight overshoot but take it as set, 2) Move axis ? till switch is tripped with slight overshoot then reverse direction and move slowly till switch un-trips and call this set. 3) Move axis ? till switch trips with slight overshoot then reverse direction and slowly look for next rotary pulse and call this set. These are progresivly more accurate with the last requiring a secondary rotary pulse to pin down ball screw rotation. Accuracy, in the first two, will depend on the switch. Dave says he 's within 0.001" which sounds very good to me. For this it's important to have repeatability in the switch. The advantage is that home position is found automatically and consistently.

As I've said I don't have limit or home switches 'cuz I'm lazy. When I start up the machine I use a wiggler to zero in on my vice and call this X/Y machine zero. Then take the highest Z position as Z machine zero. When I turn off the machine I make sure it's parked at X/Y/Z machine zero. If this is done, and I'm extra lazy, I just home the machine on startup at this starting position. On LinuxCNC I just push X&Y&Z home buttons on start up. The wiggler is likely within 0.001" and the second process within the backlash, maybe 0.002".

Why? Well, the soft limits only work when the machine knows how far it can travel +/-. It measures +/- from machine zero and it knows this from homing. Commercial machines are set up to hit soft limits before hard (switch) limits. I depend on soft limits which depend on my homing the machine manually. I'd not give my machine to a student 'cuz I'm not confident they'd home it adequately. But it's my machine and this works well for me.

Why don't I have home switches, besides being lazy? Because I'm also impatient. Home switches are best at the far end of an axis, either + or -. If I set up a home switch at say the far + X axis the machine would search all the way to the end of the table before finding it. It would then know X machine zero and I'd likely ask it to move back there, near the middle of the table. With my slow machine that takes some time. Well, you say, just put the home switch closer to the middle of the table? Because, if the axis is somehow past the home switch it will search for it till it hits the limit switch. I don't have limit switches so that would be a crash. The only sure way to prevent this is to put any limit switches at the end of travel.

The home position, and machine zero, have little effect on parts. That at least the way I use it. After homing the machine to machine zero (G53) I use work coordinate G54 for part programs. I use a wiggler or CoAx indicator to set X/Y part zero. Then use tool length compensation to set tool height Z for the part. I don't know if commercial machines can home accurately enough to depend on. If they are you could set a fixture on the table and depend on homing to set work coordinates on that fixture. Just start the machine up in the morning and cut parts. I don't anticipate ever being pressed for time enough for this to matter. As long as set up is within my patience threashhold I'm OK.

SO it sounds like I do about what you do. Set or more the machine to it's home position and press home buttons. That lets the soft limits work. Then accurately set the work coordinates to each part. I don't think you're missing anything.

At least that's my process from the last time we all went around this track. I may well change once I read all the comments which are sure to come. But if it does I'm ahead of where I was.

P.S. I'm not a CNC programmer, but I once stayed in a Holiday Inn. :-)

You say you built your machine with 'homing' and would never do without. Well, I have never had a 'homing' capability, therefore I dont know how to use the 'homing' facility or what advantage it would bring. Perhaps you would be kind enough to give me a short 'how and why' primer.

I have just checked my Emco VMC. It still has the magnetic index and proximity sensors and wiring for each of the X, Y and Z axes still in place, but they are not interfaced to my Linux CNC controller at the moment. Perhaps I am missing a trick by not using the 'homing' facility.

Mike

[john mills]

I have always worked with industrial machines and have always used home positions
the home position is the position the machine can find and is always the same.the first lines of the program tell it where you have made zero on the part you are doing .when you find your position on the part by what eva means you like to use the machine can find its home position so whenever you turn on the machine you go to the home position then you have told it where you have put it on the part .if the power goes of you don't know any more where it is so you go home and your ready to go .i used to use a vice a lot so i knew where the corner of the vice was so i could position the material from there and be ready to go without having to find that position again .
when doing lots of parts and the machine is not always on you can always turn on home the machine and be ready to go again.it is a constant reference position.the position on the part is where you make it from the home position
and can be found at any time .the home switch is position where the machine can go and find it home position
and is as accurate and repeatable as the machine.
John

[Hugh Currin]

I forgot to mention that I depend on machine zero for tool changes. The post processors I use take the machine to home position for a tool change. They have G53 Z0 , G53 X0 Y0 embedded in the tool changes.

From what John says, commercial machines home accurately enough for part location. That would be an advantage for production.

Thanks.

[Vixen]

Hello Hugh,

Thank you professor, for taking the time and effort to clearly explain the how and why of 'limit' and 'home' switches and how they should be used in an industrial setting. There it is important that all machines and all operators work in the same way, to the same set of rules.

I am well past the age of ever being invited to work in an industrial machine shop and selfish enough not to let anyone else use my CNC machines. So I can devise my own simplified way of working. I have physical markers on each axis to show the extent of travel, so it is easy to position the vice (or vertical chuck for round stuff) well within the available travel limits. I only use (workpiece) part zero, so dont need to bother with offsets (G5xxx etc) . I find it quicker (and safer??) to set Z zero, off the work, after a tool change. Again, this avoids  measuring and setting tool heights into a tool table, and yet more offsets.

Simple and lazy, yes. It works in my shop but I would be a disaster in another workshop.

Mike   

[steamer]

Alright.....Good morning to all.

Mike ....thank you so much for starting this thread and declutering yours.    As it should be    I'm enjoying your thread immensely!

Now....everyone here has a mill...most on this thread run CNC, or have a CNC mill

As you are president and CEO of your mill....feel free to run it however you like!    That would seem quite logical

That said....based on the volume and quality of the outcome,  I think Mike and dare I say myself and many others appear to be doing just fine by whatever unique or esoteric means at our disposal, whether it complies with industrial norms or not....as it would seem...as some of my Midwestern friends would say

"Its not our first rodeo"

So lets strap in and make some engines.....hmmm?

Steamer wants big Vroooom!

[Jasonb]

Hello Hugh,



I am well past the age of ever being invited to work in an industrial machine shop and selfish enough not to let anyone else use my CNC machines. So I can devise my own simplified way of working. I have physical markers on each axis to show the extent of travel, so it is easy to position the vice (or vertical chuck for round stuff) well within the available travel limits. I only use (workpiece) part zero, so dont need to bother with offsets (G5xxx etc) . I find it quicker (and safer??) to set Z zero, off the work, after a tool change. Again, this avoids  measuring and setting tool heights into a tool table, and yet more offsets.

Simple and lazy, yes. It works in my shop but I would be a disaster in another workshop.

Mike   

That's just what I do too and have the Sharpie marks on the machine to prove it:) I decide on the datum point in F360 which is usually top ctr of part be it round, square, rectangular or part machined and locate that position when the stock or part is placed on the machine. Each tool height is set from the part usually with a 20mm block so I just enter Z as +20mm after touching off a new or changed tool.

The only thing I need to do about the code is delete the three instances that F360's post processor for mach3 puts G28 into the list of code and it's ready to run.

I'd like one of these then you could machine whole crankcases in short time and bring back all those long lost designs where castings are no longer available. He uses a novel way to separate the parts from the block in several of his videos - a reverse chamfer tool and then a slitting saw to cut almost through then the part can be snapped off, not sure I'd trust my machine to leave a whisker thin piece of metal as I'd probably launch the part across the workshop!

<a href="https://www.youtube.com/watch?v=I--_63_wysk" target="_blank">http://www.youtube.com/watch?v=I--_63_wysk</a>

[stevehuckss396]

I'd like one of these then you could machine whole crankcases in short time and bring back all those long lost designs where castings are no longer available.

You don't need all that fancy stuff to machine a crankcase. With a little planning and careful setup you can do  a crankcase fairly easy.

This was 6 setups, no homing switches, manual tool changes on a $1300 scratch built milling machine.

[kvom]

On my mill homing to find machine 0 would move the table and spindle at a moderate rate.  As each access sensed its limit switch it would stop and reverse several though.  In theory this was repeatable to +/- .001".

With Mach3 it was never necessary to home, and with soft limits the machine is protected.  With a switch to PathPilot I need to reference each axis (i.e., click a button for each axis), but a configuration option lets this happen with no table motion.  However, referencing changes the values in the DRO.  For that reason, I shutdown the machine at the x0y0 position so that after referencing I can rezero the X and Y DROs.  I rarely leave a tool after shutdown, so the Z axis isn't an issue when referenced.  However, referencing Z sets its limit, so I'll typically jog it near to its upper limit before referencing.

No limit switch is provided for negative Z, but there's a rubber cushion at the bottom of the ball screw.