About homing and switches...

[JW1977]

*Kitwn*
I actually thought the accuracy where that good. Ive got a Prusa Mk3 3D printer and if im not mistaken it works that way without switches.
I have to look into this some more but i guess it cant hurt using the switches anyway.
Your machine looks nice on the profile picture there!

[Robin Hewitt]

Bouncing off home switches is for setups where the material position is known and invariable.

For everything else you set the material then position the tool so it agrees with the G code.

[JW1977]

Are you meaning that bouncing of home switches is for open loop systems?

Would be happy for a brief explanantion of how you set the tool so it agrees with the G-code, real basic...

[cropwell]

Are you meaning that bouncing of home switches is for open loop systems?

Would be happy for a brief explanantion of how you set the tool so it agrees with the G-code, real basic...

First of all the Prusa Homes by detecting motor stall current, so it has a reference point for a start.

You need a reference point, otherwise your machine does not know where it is. So Home switches provide that. and set machine 0,0,0. If you have a fixture (jig) for the workpiece, you can then use work offsets to relativise your Gcode to the work. This gives you repeatability.

BUT you don't have to do it that way, especially for one-off machining. You can set x and Y zeroes manually above the workpiece and then touch-off against the material top (manually or with a touch plate and macro!). So your Gcode then operates as is relative to the 0,0,0 you have set on the machine.

Whether your system is open or closed loop is irrelevant, as you still need your start reference.

[JW1977]

First of all the Prusa Homes by detecting motor stall current, so it has a reference point for a start.

You need a reference point, otherwise your machine does not know where it is. So Home switches provide that. and set machine 0,0,0. If you have a fixture (jig) for the workpiece, you can then use work offsets to relativise your Gcode to the work. This gives you repeatability.

BUT you don't have to do it that way, especially for one-off machining. You can set x and Y zeroes manually above the workpiece and then touch-off against the material top (manually or with a touch plate and macro!). So your Gcode then operates as is relative to the 0,0,0 you have set on the machine.

Whether your system is open or closed loop is irrelevant, as you still need your start reference.

I see, and i begin to get it... Thanks!

[Kitwn]

*Kitwn*
Your machine looks nice on the profile picture there!

The trick is not to look to closely, the shiny blue paint hides a multitude of sins. The Z axis is still the 18mm plywood mock-up. I really will get round to buying some 160mm wide aluminium bar to build the proper version soon, honest.
Kit

[paulus.v]

Hi JW,

Let me try to clarify what the homing on index function on a servo motor is.

The incremental encoder on a servo motor generates pulses when the motor moves. The motor driver is capable of counting the pulses and store them on power off but if, for example, you move the motor with the power off or by a power failure where the axis moves from inertia, that axis position is lost. So the absolute position of a cnc machine cannot be determined reliable from the servo incremental encoders data and homing sensors are needed.

Most incremental encoders have an additional "index" signal that provides a reference position of the rotor. It is output at each motor revolution. This signal can be used to increase the accuracy of the homing to an almost perfect and repeatable position. The homing on index routine moves the axis until the home sensor is triggered then rotates the motor to the index position. In this case the sensors accuracy need only to be grater than the axis movement of half a motor revolution. And the sensor be mounted so that when triggers the motor shaft is at about 180 degrees from the index position. Depending on the actual setup, the amount of axis movement per motor revolution, switches could have enough accuracy for homing on index.

Why is homing important?

First of all because if you have homing and set each axis maximum movement, the machine knows its limits! And will never hit a limit switch. If you load a g-code that exceeds the machine working area you get a warning. Otherwise you will only find out that you made a mistake when the axis hits the limit and ruins your work because...

Secondly, if something bad happens and you have to cut power to the machine you loose the workpiece position. If you have accurate homing, the workpiece zero position is referenced to the machine zero (home). If those numbers are stored you can easily resume the work after a power failure or if you want to pause and continue the next morning... just home the machine and go to the stored workpiece coordinates. And continue cutting where you left off.

Paul.

[JAZZCNC]

Paulus.v Explanation of Homing on an index is spot on, however, I think you'll find that most Closed-loop Hybrid servo encoders don't provide an index Pulse so can't use that feature.

Regards Homing and Zeroing etc then you need to get into the habit of thinking in terms of Machine coordinates and Work coordinates. Home switches define the Absolute ZERO machine coordinate position. This positon NEVER changes and is used to reference WORK coordinate ZERO.
So when you send the machine to HOME your sending it Machine ZERO. From this position, you can also define machine limits.

WORK zero can be anywhere inside the work envelope of the machine. You'll often see reference to WORK offsets, these are just areas inside of the machine work area which can have separate WORK zero positions.
This means you can assign each area to a Fixture G-code ie: G54 G55 G56 etc and use these codes in your cutting G-code file so it moves to the next fixture and starts cutting from new Zero point.
This is useful for instance when have several different parts or materials setups on the bed. For instance could have G54 reference to point on the Bed while G55 references to corner of a Vice and so on.
Each would have there own Zero WORK coordinate and all are referenced from the MACHINE Zero.(HOME). From MACHINE ZERO any location inside any Fixture Offset can be located accurately.

[routercnc]

Picking up on question E) in post one - you could use a Hall effect sensor but they need a magnet 🧲 on the moving part and are likely to attract steel chips and filings and become unreliable. Proximity sensors detect steel or aluminium so are a better option. Your other options include cheap small micro switches or chunky industrial ones.
Ideally have the detected object on the axis run past parallel to the face not up to it so a fault means the axis crashes into the hard end stop not the sensor.
Oh and welcome back Jazz 🍻!

[JW1977]

Thanks!

Proximity sensors seem like the option i will choose.
Is there anything special i need to now about using them?
As i understand i will use 6 of them and one of each axis will double as home switch right?