Position Setting Part 1.
X – Y positioning.
I do quite
a lot of label and instrument panel engraving using ‘Traffolite’ (a
multi-layered phenolic plastic) and in most instances the work is cut to size
first and the engraving layout is then centred on the blank. Centre finding of
the work is just a matter of drawing two diagonal lines, corner to corner and
where they cross is the centre. To zero the machine to this centre position is
relatively easy if the tool is an engraving point but becomes much more
difficult when using a ‘slot drill’ or ‘ball nose’ tool.
solution to this problem is to use a cheap and readily available laser ‘cross
hair’ pointer and the following describes how I have implemented this with my
Mach3 setup. Obviously, there is always more than one way to do everything so
please feel free to make changes and / or improvements to this implementation as
you see necessary.
hair pointer is fixed to the Z axis (as close to the spindle centre as is
reasonably practical) then the offset (between the pointer position and the
tool position) is measured using the X and Y axis DRO’s. I have used one of the
screen designers to add two new buttons to the Mach screen – one for turning
the pointer on / off and the second to home the spindle (tool) to the current
pointer position on the surface of the work.
To use, I
mark the surface of the work with the datum point (usually the centre), turn on
the pointer and jog the X and Y axis until the cross hairs align with this
datum mark then click the ‘Home to Sight’ button – the X and Y axis then move
so the spindle is now aligned with the datum.
of positioning is extremely quick and easy to use and it has now become a tool
which I would not be without.
should be noted that the accuracy I am able to achieve with this device is
better than 0.5 mm and whilst this is just fine for a router (where alignment
is often to a pencil line on the surface of the work or the edge of an
engraving blank) it would not really be a suitable addition for a mill which
may require precise alignment to an edge where the conventional edge finders
can achieve a much greater degree of accuracy.
number of problems with laser cross hair pointers which have to be overcome.
(1) I found, with my version, that the metal
casing of the pointer is electrically connected to it’s positive (+) supply and
as my machine is of all metal construction (with the frame electrically
connected to GND) and with the pointer mechanically and electrically connected
to the frame it needs a negative (-) supply.
There are a
number of different ways to resolve this issue but I chose to use a small,
isolated, dc power source (ex-mobile phone charger) which has it’s positive (+)
output connected to GND (machine frame) and it’s negative (-) output connected
to the pointer with the on / off switching accomplished with a solid state
(2) The optical output from the cross hair pointer
is extremely bright and because the reflectivity of various materials and their surface
textures vary it is desirable to be able to adjust the apparent brightness of
the cross pattern. In addition, it is a factor of our eyesight that makes
coherent laser light appear to ‘sparkle’ therefore the narrower the perceived
beam width the better it is to view.
the photon beam from diode lasers is plane polarized I have fitted a rotatable
disc incorporating a piece of polarizing film and when this disc is rotated
between 0 degrees and 90 degrees the apparent brightness varies from full on to
completely off thus enabling an optimum brightness to be selected.
Chinese seem to be the masters of the ‘sloppy fit thread’ and the focus ring
(incorporating the cross pattern) will move during use unless it is suitably
locked in position.
clamping the pointer to the machine frame I have also clamped the focus ring.
It is a bit tricky to initially get it set correctly, when done this way, but
it only has to be done once and it is worth taking the time to get it right.
The focal distance has to be set to an optimum for the travel of the Z axis and
the pointer has to be mounted perpendicular in both planes so that the position
of the cross pattern does not move (on the surface of the work) when the Z axis
is raised and lowered. The orientation of the cross pattern is not that
critical as it is only the crossing point that is used but I have aligned mine
with the X and Y axis (at least, as near as I can).
the cross hair pointer, as mentioned earlier, I have added two new buttons to
the Mach3 screen. These are VB buttons and can be easily added using one of the
free screen designer programs available from the Artsoft website.
ON/OFF’ button will toggle the pointer on/off (turn it on if it is off and turn
it off if it is on) and within Mach contains the following script ;
If IsOutputActive(OUTPUT5) Then
DeActivateSignal(OUTPUT5) 'turn off pointer
ActivateSignal(OUTPUT5) 'turn on pointer
Here I am
using Output #5 which is mapped, within the Mach3 setup, to the LPT output pin
connected to the solid state relay shown in the previous schematic.
The ‘HOME TO
SIGHT’ button will move the X and Y axis to the position of the cross hair,
reset the X and Y axis DRO’s to zero and turn the pointer off.
button contains the following script ;
Xs=GetOemDRO(59) 'Xscale DRO
Ys=GetOemDRO(60) 'Yscale DRO
Xmove = -73.8344 * 1/Xs 'move
distance adjusted for Xscale factor
Ymove = 10.5625 * 1/Ys 'move
distance adjusted for Yscale factor
DeActivateSignal(OUTPUT5) 'turn off pointer
Code "G91 G0 X" &Xmove &
"Y" &Ymove ' makes an incremental move
While IsMoving () ' waits while that
Code "G90" 'goes back to
DoOEMButton (1008) 'zero X DRO
DoOEMButton (1009) 'zero Y DRO
The ‘Xmove’ and ‘Ymove’ values, in the above script, will
need to be changed to suit the positioning of the pointer on your own machine
but they can be easily measured by fixing a piece of scrap to the work table
and then by using an engraving point in the spindle making an indentation into
the scrap. The X and Y axis DRO’s are then zeroed, the pointer turned on and
the X and Y axis jogged until the cross pattern is exactly centered on the
previously made indentation. The X and Y axis DRO’s will now read the correct values
to be entered into the script, changing the sign (+ or -) as necessary (In my
case, the spindle is offset from the pointer in the –X and +Y directions so the sign of the values
in my script reflect this offset).
I think I have covered everything here but if I have missed
anything or failed to explain anything clearly please ask.
In Part 2. I will describe my implementation of ‘Auto Tool
Zero’ or tool height setting relating to the surface of the work.