The Shop > CNC
The basics
Will_D:
Have managed to fit a stepper to the x-axis and am getting into the DDCNC controller and its parameters (There are a lot).
Have set up soft limits, backlash compensation, and am starting to speak "baby g-code"
Question about Z axis tool sensor:
Helps if you have one and know how it works!
Quick google finds the cheapest at about €15 and the most expensive is €150 plus.
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)?
Just checked that Tungsten Carbide conducts but what about exotic ceramics (not that I can afford any)
I will be simulating this and the hard limits/home limits by a simple switch (NO/NC) tomorrow.
awemawson:
The Z height sensor doesn't NEED to be a precisely known height as it is used comparatively. Set your longest tool using it to zero off set, then all your other tools are measured and their 'shortness' is set as an offset in your tool table.
Touch off your workpiece top surface with the longest tool and this becomes Z Zero, and all the other tools will be correct when their offset value is read from the tool table.
Of course if the height of the sensor IS known precisely then it can be used on the workpiece instead of touching off Tool Zero
DMIOM:
--- 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
PK:
Here's the first one I made:
Spent hours getting that sliding fit perfect..
All subsequent units have just been blocks of conductive things. The one that came with our 1325 router was just a 10mm thick disk of brass.
Others have used un-etched copper clad PCB material, and I've seen a commercial unit that did the same.
If you are bringing Z down at 30mm/s and it can accelerate at 3000mm/s/s then it's going to take about 1mS to stop. Average speed (ignoring fancy acceleration curves) will be 15mm/s, so it will overshoot 0.025mm..Now that might be a lot on a big machine, but small machines flex an order of magnitude or two more than that...
PK
Bee:
Will - what size motor have you used? nobody answered your original query about the 425ozin ebay advert.
John gave us
1)180 ozin for a 4in rotab.
so advice please on
2) 6in rotab
3) dividing heads of various sizes
These would always be direct drive but a mill could use 2:1 or 3:1 gearing with effect on speed too so next question is for the basic intro
4) what step rate is sensible for the stepper pulses and
5) what traverse speed in/min is adequate for a hobby mill
6) what speed for Z axis if different from above
having established speed, and assuming direct drive
7) what size stepper for each axis on say X2 and X3 mills.
John again gave us I think 325 ozin for a Z axis on a ?? mill moving the whole head without counterbalance what about
8) quill only and
9) raising knee
as opposite ends of the spectrum.
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