Robert,
Being new to g-code and cnc I've been trying to learn the best way to
> think about the coordinate spaces and offsets etc (for a cnc mill).
>
> I'm thinking I should use a **fixture** plate with alignment pins etc to
> mount to the table. Some kind of pallet plate to hold the work
> (plugs into fixture).
>
This depends on the type of work and the type (and size) of workpieces you
have.
I make mostly small parts (a few inches long with a max. of 10~15")
sometimes in small series (10 pc) which I then mill out of a larger block by
moving the home position using one of the user selectable coordinate
systems. And I am in the process of adding a rotation axis which mostly be
used for N-sided milling of small workpieces. Production time is not
critical to me so it does not matter if mounting a new workpiece takes 10
seconds or 10 minutes and my mill is a gantry type with an XY travel of
78x47" so there is plenty of room to leave the rotational axis on one side
when it is not being used.
If you have a complex mounting system a pallet system comes in handy. I know
of one setup where a complete 3D shaped housing is being created out of a
15x10x4" block of aluminum, steel mushroom type of mounting pins are being
used to clamp the block in place and after milling one side it is being
turned upside down on a special created clamp (pallet) to create the other
side. Creation of the product would be very hard, next to impossible, if you
do not use a pallet system.
For larger series with short machine running times a pallet system will keep
the machine running as long as possible while you mount a series of rough
blocks or partly finished products on the next pallet.
Palleting systems can be very complex. The simplest one have one pallet
position and complex systems may have multiple pallet positions with their
own home coordinates, or an automatic pallet changer with multiple pallet
positions. These can be as hard, or harder, to control as an automatic tool
changer...
My current thinking is that I would home x and y to the lower left of
> the table. For z I'd use a spacer block off the table to the end of the
> spindle.
>
> From there I'd use global offsets use a global offset (G92) to align to
> the fixture. From there I'm guessing one whould use the users-selectable
> coordinate
> systems to align ...
>
For a manual pallet changer manual homing may be satisfactory. A pallet
changer may use the same home position for all pallets (the pallet is
physically being replaced). But if you go for multiple pallets on one
machine your suggestions look like what I would try - but I never used a
pallet system before.
If your machine has a probe tool that is usable with an automatic tool
changer you may even probe for the proper home position - comes in handy if
your mounting system does not have a mounting precision that is needed for
your operation.
Tool tables looked kinda fancy so I was going to defer using them until
> I had a better mental foundation of the system, using some kind of
> programatic offsets instead.
>
Depends on your software. I mostly use Filou or DeskProto (before anyone
flames me - these are windows applications :-( ) and both include their own
tool table so tool offsets are calculated by the software. But I also do
some hand-G-coding sometimes and miscalculating tool offsets cost me a
mill-bit or two ...
So in the end it all depends: what type of product, is machine operating
time critical, are you creating series of the same products, ...
Regards,
Rob
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