[Mr.Chips]

Mach3, ESS SmoothStepper, C25S BOB, DIY CNC

After eleven years of running my CNC and not ever touching Machine Coordinates I am in the process of adding homing and limit switches, and theirs the rub. I have watched numerous you tube videos on the Machine Coordinates subject however my problem is where to zero them.

I'st I will square my Gantry to the table. Then adjust the Y axis positive stops so that they are both touching with the Y axis moved all the way forward, which will be used later to auto square the gantry.

Now the gantry is against the front positive stops. The next part is the one I cannot find the answer to.

1. How far away from the positive stops should I move the gantry and set both the Master and Slave homing switches when they are activated?

2. That distance that you mover the gantry away from the positive stops is that Y zero?

3. Is that point also the Zero point of the Y axis Machine Coordinates?

Are these assumptions right? If not please tell me where the Y zero and Machine Coordinates Zero should be set.

Thanks
Hager

[deadlykitten]

hy
... let's say that full axis travel is 1000
... put those limit switches at "a" and "1000-a", where "a" represents the deceleration distance from the top speed * 1.5..2..3 ( safety factor ); "top speed" should be <= servo max capable speed, and represents the top speed that you intend to use on your machine; idea is that if your machine reaches the limit at top speed, it should be able to break without overshooting this means that the limit switch, when touched, should imediatly start to decelerate all axis towards 0 ( there are also other types of limit switches, hart type, that once touched, the machine can not continue it's movement, and the stop is caused because the servo reaches a high torque )
... declare the soft limits at "a+b" and "1000-a-b"; b represents the clearence between the software limit and the hardware limit, that depends on machine construction; for example :
...... mill : x travel is 1500, table is 1550, b is 5, and x travel is centered among table width with <1mm accuracy
...... lathe : x travel is +360-140, b is 2.5, and x center may shift each time the turret is re-aligned; this means that limits may become +360.1-139.9; if you restore the limits to their initial values, then b will become b1 and b2, 2.4 and 2.3, thus both clearances will change with how much the x center has switched

... after hard limits (a) and soft limits (b) had been declared, you may declare 0 anywhere you like; for example :
...... mill, xy axis : 0 is located at the middle of the distance between "a+b" and "1000-a-b", thus at 500
...... mill, z axis : 0 may be declared like above, or right on the surface of the table; like this, if you put a vise, and measure the distance from over the jaws to the table, using a depth caliber, that that point should represent also the z axis coordinate; of course, on a mill, z0 may mean that spindle face is touching the table

i hope you find it usefull / kindly

[ger21]

1) Leave enough room so the axis has room to stop without crashing into the hard stops.
2&3) Generally, the home switch position would be the axis Machine Coordinate zero position. But you have the option of specifying the machine coordinate position relative to the switch position. You can basically place the machine coordinate system anywhere you want, regardless of the home switch position.

[Mr.Chips]

Thanks DK, I'm running a CNC in inches

This is totally new to me so let me rephrase it and let me know where I'm going wrong, which I can easily do.

It sounds like Homing and or Machine Coordinates are best used with Soft Limits. Soft Limits can use braking to slow down the axis before contacting the Limit/Homing switch. Is this correct?

If you don't use Soft Limits you need to find out/measure how far it takes the axis to stop when traveling at top speed/Rapid movement. Is this correct?

So, if I use soft limits I don't need to measure the stopping distance when traveling at top speed/Rapid movement. Is this correct?

In soft Limits you can specify at what distance from the switch you can slow down and approach the switch, correct?

Sorry for so many questions, like I said I'm totally new to Soft Limits, Machine Coordinates, and setting up same, I only know what I have possibly learned in passing but I wasn't interested as for eleven years I didn't use either one.

I have never crashed my CNC into the positive stops until I tried to setup Machine Coordinates and Limit/Homing switches on my own, destroyed one coupling, so I need to understand this.

Thanks
Hager

[deadlykitten]

hello mr chips what i shared is available for a machine that does not need homing ( or zeroing ) each time it is powered up ... thus it was for a machine with absolute encoders

if you need to zero the machine each day, then i need to re-formulate my answer ...

so, do you speak about a machine that needs to be zero'ud at power up ? or it has absolute encoders, thus it is not losing it's zero, even if it is shut down + power on ?

[Mr.Chips]

Thanks DKfor offering to rephrase your reply.
It’s my impression that once you set the homing and limit switches as will as the soft limits and Machine Coordinates that those will be saved and in effect after after a shutdown and restart. All of these settings are stored in Mach. Least that’s my limited understanding of them.

I understand your comment about needing to know how long it takes to stop when moving at a high speed, but does the Soft Limits do away needing to know the stopping distance, since the Soft Limits will automatically slow down to a pre set speed and distance.

OH there’s so much I don’t exactly understand.
Hager

[deadlykitten]

It’s my impression that once you set the homing and limit switches as will as the soft limits and Machine Coordinates that those will be saved and in effect after after a shutdown and restart.

hello mr chips i am sorry, i don't have experience with mach

so far, i know that there are these types of motors/servos/etc :
... absolute encoders : thus you set the limits once, and they will be there forever ( at least until a heavy crash, or something )
... relative encoders, thus they need to be set to zero :
...... each time the machine starts up
...... each time the machine starts up and also between operations
*possibly other types

also, there are these types of limit switches :
... hard type : once they are reached, the axis can no longer continue it's movement, thus they are like a wall
... lever type : once they are reached, there is sent a comand to stop all axis, thus all axis will continue their movement, but only to break
*possibly other types

so, my dear mr chips, there are a few combinations ... but there is common stuff between them, and also particularities

at this moment, i don't understand exactly what you wish to know, and i don't know the arhitecture of that machine, so my replies are general, and maybe i won't be able to help you somehow, we need to narrow it

Soft Limits

about soft limits, there are these cases :
... some machines are capable of auto-setting their soft limits ( by bounching 1st into the hard limits, using a low torque mode, then auto-setting soft limits with clearances )
... some machines require to have the soft limits declared inside the parameters area ( off course, is possible to declare them wrong, thus way beyond the limit switch, and in such a case, only the limit switch will save you )
*possibly other types

so, again, there are combinations

now, this being said, i will try to answer your questions :

but does the Soft Limits do away needing to know the stopping distance, since the Soft Limits will automatically slow down to a pre set speed and distance

soft limit is a cnc-software parameter, and it does not allow the machine to move beyond it

it does not slow down at a pre set speed ... no no no; look, imagine these cases :
1) if you move the machine in manual mode, and you continue jogging, once the soft limit is reached, the axis should stop ( maybe also display an alarm )
2) if inside the program is :
2.1) G01 position_beyond_limit, then the machine should raise an error before executing that line; however, there are machine that will move only until the soft limit, and raise no errors, and you may believe that everything is ok for example, i was drilling deep, and it was beyond the soft limit, but the machine did not raise any errors, it simply drilled as deep as it could, and only after i took the part down from the machine, i saw that the holes were too short ... duh, stuff happens
2.3) G00 position_beyond_limit, the the machine may only travel to the soft limit, and stop there

so again, there are combinations, machine specifics

but in the end, the machine won't slow down to a preset speed and distance ( like you said in your question ), but will actually stop at the soft limit position; it may stop and raise an error, or it may continue the program, but will never go beyond the soft limit

Soft Limits can use braking to slow down the axis before contacting the Limit/Homing switch. Is this correct?

normally, the soft limits are within the hard limits range, thus, there is always clearance between soft limit and hard limit

on a normal cnc, the axis does never travel inside that clearance ... if it does, and if somehow reaches the hard limit, then something went wrong but there are machine, that can be joged manually into the soft limit, and will actually stop and raise an alarm ...

If you don't use Soft Limits you need to find out/measure how far it takes the axis to stop when traveling at top speed/Rapid movement. Is this correct?

so far, i have never seen a cnc machine that does not use soft limits ... but, if there is one, then behaviour will depend on the limit switch type :
... if it is a hard type, machine will hit it like into a wall, then :
...... the machine may stop because of servooverload
...... or the machine may break, or at least take a knock
... if it is a lever type, then all axis will be comanded to stop, thus after the lever type has to be enough room left for the axis to decelerate and stop, without overshooting

also, if the limit switch is broken, the machine will overshoot

So, if I use soft limits I don't need to measure the stopping distance when traveling at top speed/Rapid movement. Is this correct?

In soft Limits you can specify at what distance from the switch you can slow down and approach the switch, correct?

i guess i have allready answered those

i hope you find it usefull / kindly

ps : is a tiny mess, hope you can see the light, or at least a spark
ps2 : i have used the maximum number of smileys alowed : )
ps3 : thx god, is corona time, otherwise i wouldn't have had time to write so much

[Mr.Chips]

Thanks DK, I really appreciate all you have written, it has given me a lot of insight into working through my questions.
Regards
Hager

[deadlykitten]

I tried to setup Machine Coordinates and Limit/Homing switches on my own, destroyed one coupling

i guess that the effect was that the soft limit got beyond the hard limit .... machine tried to go to zero, and it went in full speed across a zone where it should be in low speed ( couplings, or other organs, are designed to break, in order to prevent expensive damage )

i have also messed with soft limits and i did mistakes ... now, if i try to change a limit, i am looking to be sure that there is still clearance to the hard limit; when i did not understood them, i actually took down the machine covers, to see what makes them tick, and i spent hours into the parameters area the case that scares me the most is a faulty hard limit, that may give one a false sense of safety

i have changed limits, for example in order to replace 362.582 with 360 , thus to use round numbers, and identical, on different machines, so to ... whatever ... mr chips, limits are changed in these cases :
... machine got realigned
... machine is ok, but a grid shift is required, in order to :
...... sync different machines between them
...... sync different machines with a unique tool presetting instalation
...... sync with simulation software
...... declare faster the program zero and the tools zero

all those are normal, but are used rarely, because few machines allow shifting from parameters, or they are too old, and no one knows how to do it ... however, if all those are done ok, and understood, they may boost productivity

if i may, why did you changed those limits ? what do you wish to achieve ? also, that coupling needed to be replaced regardless of your actions, so don't worry / kindly