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The basics
philf:
--- Quote from: DMIOM on October 23, 2016, 05:06:42 AM ---
--- Quote from: Will_D on October 22, 2016, 06:10:06 PM ---.....Question about Z axis tool sensor ...... Am I right in assuming that its just an insulated (from the table) block of metal with an exact, known height of say 30 mm that when touched by the tool closes an electrical circuit of the controller (+12v to Probe Gnd)?.........
--- End quote ---
Will,
The essence is that when the tool touches the sensor, a circuit is made (or broken) and the simplest form is, as you describe, a conductive plate or block insulated from the rest of the machine. However, a solid block is unforgiving, and you can damage the tool tip or block unless you sneak up via a Rizla paper or similar. Better sensors have a platform with some 'give' - the contact pad can be driven down (against a spring) if you over-travel but once you back off the platform rises back to its datum until it contacts a fixed lip (and, just like referencing a machine's axis, its usually better to take the position when you back off as that eliminates any over-travel). The platform can be just mounted on a plunger, but there is a small danger of binding especially if the contact is off the centre of the platform or button, so even better is the arrangement like the three-legged (but not Manx!) innards of a Renishaw probe.
Dave
--- End quote ---
I think relying on continuity through the cutting tool to a conductive block (even with an over-travel allowance) is flawed from the start.
With a contact system, in the event of e.g. a wire breaking, the tool will continue driving until something breaks or stalls.
A system which breaks a circuit on the tool touching down (but ideally still with over-travel allowance unless you're prepared to run the tool down at a ridiculously slow speed) is inherently safer in that if a wire breaks or a plug falls out the control thinks the tool has already touched down and the tool won't be driven down at all.
Hope this is of help.
Phil.
PK:
--- Quote from: philf on October 23, 2016, 04:13:14 PM ---
A system which breaks a circuit on the tool touching down (but ideally still with over-travel allowance unless you're prepared to run the tool down at a ridiculously slow speed) is inherently safer in that if a wire breaks or a plug falls out the control thinks the tool has already touched down and the tool won't be driven down at all.
--- End quote ---
But you still have wires, if they break then you're diving for the stop. Yes the over travel helps here. But I've done a LOT of tool height setting with both systems; if I broke 3 tools tomorrow because of a dodgy wire, I'd still reckon I was way ahead of a non automatic tool setting machine.
Oh, and:
As a reliability engineer I offer that a single wire to a conductive block has half the failure modes of two wires going to a switch.
:smart:
PK
philf:
--- Quote from: PK on October 23, 2016, 04:42:05 PM ---
--- Quote from: philf on October 23, 2016, 04:13:14 PM ---
A system which breaks a circuit on the tool touching down (but ideally still with over-travel allowance unless you're prepared to run the tool down at a ridiculously slow speed) is inherently safer in that if a wire breaks or a plug falls out the control thinks the tool has already touched down and the tool won't be driven down at all.
--- End quote ---
But you still have wires, if they break then you're diving for the stop. Yes the over travel helps here. But I've done a LOT of tool height setting with both systems; if I broke 3 tools tomorrow because of a dodgy wire, I'd still reckon I was way ahead of a non automatic tool setting machine.
Oh, and:
As a reliability engineer I offer that a single wire to a conductive block has half the failure modes of two wires going to a switch.
:smart:
PK
--- End quote ---
Sorry PK,
My system is fail-safe and there's no diving for an E-Stop button
Yes I still have wires (and 2 at that) which connect to the control via a jack plug. If the plug gets pulled out or one or both wires breaks then Mach3 gives an error message (from memory "Tool Already Grounded") and the Z-axis doesn't move. Try breaking a wire connected to a piece of pcb material used as a sensing pad and the control will keep driving the tool towards the table until .....
My system also works with e.g. a diamond drag engraving tool which is non-conductive.
Phil.
PekkaNF:
--- Quote from: PK on October 23, 2016, 04:42:05 PM --- .....
Oh, and:
As a reliability engineer I offer that a single wire to a conductive block has half the failure modes of two wires going to a switch.
:smart:
PK
--- End quote ---
How many well defined nodes you have on return leg? Your failure analysis is only half done :poke:
Pekka
Will_D:
Thanks for the replys and warnings about the Probe Overshoot! As I am used to manually touching off the tool didn't realise that the Probe function in the controller will drive the tool into the tool height sensor and so some flexability is required. If and when I motorise the Z axis then I am forewarned.
Am also using a simple switch to explore the Home function of the controller and the Hard limits.
Just waiting for the Y axis ball screw to arrive as I think I have found a simple (ie no milling of the machine) way to fix it in place.
Will
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