New build – input/comments appreciated

[ngundtoft]

I am confused - apparently I am lacking some basic knowledge about the limit/home switching concept (but I am new to all this, so trying to learn).
Basically I can follow the mail from routercnc, which corresponds with my understanding. But when JAZZ limits the number of switches to 3 , I get confused and have some questions:

• If we agree that the purpose of limit switches is safety and prevents the machine from exceeding its working area - why then is it not required to have limit switches in both ends of each of the axes? (ok Z is a little different).
• I did not really mean that the home position should be in the centre of the bed, it was mainly to distinguish it from the limit switch position. If you always work from the nearest corner (0,0) - as described by routercnc - then the limit switch position and the home position is the same, and what would you need the home position for? Or rather - it only makes sense to use the same switch for both purposes.
• Soft limits are set up in the SW as I understand it. But what are the reference points for these soft limits? Asking because you sometimes get the impression that you can almost manage with these soft limits and omit the mechanical limit switches.
• I can see that there is not much purpose for a normal limit switch on the lower Z-axis, since the tools will vary in length. But how does the router know the cutting depth? I think I saw a video (Obiwan?) where the spindle was moved to a certain (X,Y) position and then lowered (Z), until the tool made contact to a reference point (or activated a switch), to give a reference position. Is this the way to do it - or are there other ways?

[Clive S]

If we agree that the purpose of limit switches is safety and prevents the machine from exceeding its working area - why then is it not required to have limit switches in both ends of each of the axes? (ok Z is a little different).

This is because they use a target at both ends ie the switch will travel with the gantry and so will hit the target at the end of travel in either direction.
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Re point 3 It is the homing that give the ref points for the soft limits.
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re point 4 After you have homed the Z you then move Z down to a ref point (say top of work piece) and then Zero it on the dro so now the machine or should I say the controller knows where zero is and can work out the depth of cut.

[Neale]

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Re point 3 It is the homing that give the ref points for the soft limits.

It is really useful to have reliable, accurate, homing switches so that you can quickly set up reference positions when you switch on each time. Picking up accurate positions from one session to the next is difficult without this, whether you use hard limit switches or not. I don't have home switches on my current machine and it's a pain.

[njhussey]

It is really useful to have reliable, accurate, homing switches so that you can quickly set up reference positions when you switch on each time. Picking up accurate positions from one session to the next is difficult without this, whether you use hard limit switches or not. I don't have home switches on my current machine and it's a pain.

I'll second it being a pain not having homing switches....

[ngundtoft]

Thanks for clarifying this Clive. Obviously (now that I understand it) you can use the same switches for both limit positions. I see now that it is also what JAZZ writes - just wasn't clear to me.
So, since I am not using slaved motors, I should have 3 limit switches.
So even though JAZZ only uses 3 (+1) switches, it sounds like it is a very nice feature to also have the home switches - or can you also use the limit switches for this purpose? I sounds like it is the same position (0,0) + Z?
I can see the advantage in using the top of the work piece as reference, so that you don't have to measure the difference between a fixed reference point and the work piece surface. How do you do it in practice - do you need a manual rotary control or can you do it from the keyboard (without damaging anything)?

[routercnc]

This is often a point of confusion for beginners so don't worry. Keep asking until you get it. Once you have the machine and software in front of you it makes even more sense.

Jazz only has 3+1 (roving) switches but still has the home and limit function. They are just shared from the same switch. In terms of Mach3 (or whatever software) you are either homing an axis (so a switch detection means you are at the home point) or you are manually moving / gcode cutting (when a switch detection trips the e-stop for example). You are never doing both at the same time.

I think from what you are saying in your posts you also need to understand the difference between 'machine coordinates' and 'work coordinates'. This also trips up beginners so make sure that also makes sense. Machine coordinates are where the machine is in absolute space and are set to 0,0,0 by going to (referencing to) the home switches. When you are ready to cut you need to go over and down to the corner edge of the workpiece and set the local work coordinates to 0,0,0. This will be at some machine coordinate absolute location such as 100,100,-50. You can switch between looking at machine and work coords in mach3 on the DROs to see where you are on each axis system. The work coordinate locations can be set using a touch probe, or a piece of paper between flute and workpiece using the jog function (e.g. 0.1mm increment movements) until the paper is just trapped.

Now the machine knows where 0,0,0 home position is (in case the job goes wrong and you need to reset, or you need to stop the job to go home). It also knows where the workpiece 0,0,0 is so all the cutting can take place where the work is on the bed.

Have a read of the manual, ask more questions and eventually this will start to make sense.

[JAZZCNC]

Machine coordinates are where the machine is in absolute space and are set to 0,0,0 by going to (referencing to) the home switches.

Every thing neil says is correct but just want to clear up something that could confuse or be taken wrong by a beginner. Neil says "Absolute" space but this is very different to absolute positioning mode G90 used with-in G-code.
Absolute positioning mode doesn't refer to absolute space but rather it moves an absolute distance in inside the WORK coordinate system. Compared to Incremental Mode G91 which increments by a commanded distance.
ie: G90 X150 would move 150 units away from Work zero. G91 X150 would Increment 150mm from the absolute position so if your at G90 X150 then with G91 X150 you'll end up at X300 absolute work coordinate.

One other slight point I'll make for new folks which I know confuses them is that the Work Zero doesn't have to be on the corner of the Work piece or material. It can be any where you want it to be on the material. Where you located the Zero coordinate in the CAM softare in relation to the part will mostly dictate where on the material you actually place the WORK zero.
ie: For instance if your part was circular like a Wheel for instance then you'll most likely locate Zero at the centre in CAM which means you can't loctate WORK zero on the Corner of material.
In practice often you'll be cutting out of much larger piece of material than the actual parts size so will job to any location that the part will fit within and knowing the Programmed Zero point on the part choose point in that area.


Now to use one switch for Both homes and Limits you need to turn off Home SW safety option in genreal Config. This will tell Mach3 to ignore the Limts while the Machine is Homing.
Your also better to turn on Auto Limit over ride in the setting screen. This will allow you to reset Mach3 and drive off the switch. If you don't turn this on you will need to include a Limit override Switch in your Wiring to knock out the limits while you reverse off the switch.

[Christian Knuell]

Hi,

• If we agree that the purpose of limit switches is safety and prevents the machine from exceeding its working area - why then is it not required to have limit switches in both ends of each of the axes? (ok Z is a little different).

Seeing it from this perspective you'd be right - but my personal opinion is that limit switches are not only unnecessary but even give a deceiving sense of security they cannot provide:

• If a machine can be damaged by chrashing into its axis limits it is just a crap machine...
  (OK - not talking about professional high speed machining centers where you have tons of material accelerated to very high speeds - but even there a limit switch is just the last line of defense - position feedback will stop those machines way before this could happen)
• A machinist not noting the workpiece will require machining outside its limits is just a crap machinist... ;-)
• If the machine looses steps the workpiece will most likely be damaged anyways...
• And limit switches do not detect step losses - a machine may ruin anything including tool changers or sensors within its work area without the limit switches taking any note of it. Which is also why a machine should never ever run unattended.
• So basically: if you really need security you need position feedback.

What sometimes makes sense is reference switches to automatically home the machine. But even though they can make life a bit easier in some situations they are rarely necessary:

• Usually you clamp your workpiece just anywhere on your machine and need to touch it off anyways because your machine has no eyes.
  
  • In this case knowing machine zero is absolutely pointless - it is just a set of numbers without any meaning for the actual machining process.
  • Only in case of step losses it can safe you a little time to get back to the correct workpiece zero - if your workpiece isn't destroyed anyways which will most likely be the case... And a machine losing steps regularly enough to make you want homing switches is again just crap or badly set up ;-)
  • But even in this case you can usually just touch off again (if the reference edges are still available)...
  
  
• Reference switches are only necessary if you have things like tool changers, tool length sensors or fixed clamping devices on your machine.
• And: if the switch isn't precise (or a chip gets in between) machine home will also be imprecise accordingly. Many cheap homing switches are much worse than touching off the workpiece. I program CAM Software and I came across quite some people unnecessarily referencing after every toolchange and wonder why there are visible marks on the finished workpiece that wouldn't be there if they just kept X and Y (or at least used proper limit switches with good repeatability instead of the cheap stuff they bought).

Christian

[JAZZCNC]

Seeing it from this perspective you'd be right - but my personal opinion is that limit switches are not only unnecessary but even give a deceiving sense of security they cannot provide:

I agree upto a point but here are some cases they are required. Servo's for instance will just take off at full speed if there positioning system fails or gets interupted. So in this case they are very much required.

Also with small fast router machines it's very easy to jog from one side to the other which can result in banging into end stops etc often. Esp when first learning. Again I agree a machine that breaks with just a small limit crash is Crap but enough of this banging and eventually damage will occur so best avoided and for just a few sheckels more is it really worth not having.??

What sometimes makes sense is reference switches to automatically home the machine. But even though they can make life a bit easier in some situations they are rarely necessary:

[LIST][*]Usually you clamp your workpiece just anywhere on your machine and need to touch it off anyways because your machine has no eyes.

• In this case knowing machine zero is absolutely pointless - it is just a set of numbers without any meaning for the actual machining process.
• Only in case of step losses it can safe you a little time to get back to the correct workpiece zero - if your workpiece isn't destroyed anyways which will most likely be the case... And a machine losing steps regularly enough to make you want homing switches is again just crap or badly set up ;-)
• But even in this case you can usually just touch off again (if the reference edges are still available)...

[*]Reference switches are only necessary if you have things like tool changers, tool length sensors or fixed clamping devices on your machine. [*]And: if the switch isn't precise (or a chip gets in between) machine home will also be imprecise accordingly. Many cheap homing switches are much worse than touching off the workpiece.

This I mostly Disagree with.!!

Homing switches are THE most useful thing you can put on your machine. I'll give examples to why.

Only yesterday I was cutting parts in large expensive sheet of aluminium when the cutter broke. The part wasn't damaged but it was my last cutter so the Job was stopped. The corner of the material where I choose Work Zero had been cut away. So how without Home switches do I get back into absolute position when my reference point is gone.?
Yes I could go back into Cam and choose some hole to re-reference from etc but this all costs time and effort. Home switches provided resonable quality remove all this hassle.

Example #2 Same large piece of material and Large Job with multiple parts nested into material. So large I don't have time to do in one sitting. Again Zero ref has been cutaway. Home switches allow me to pause the Job or break the Job into smaller jobs and still quickly get back into position.

There are other examples like power cuts or PC crashes etc that back the need for Home swithces but again for the small cost it's silly not having them.
The only thing is that if you need high accurecy homing then you need Good Home Switches. I find even your Cheap Chinese Proximity switches are more than good enough for 99% of Routers or DIY'ers using Milling machines.

So for the Small cost and the fact Limits and Homes can share the same switch it's Stupid not to have them.!!