homing when work-pieces exceed the size of the worktable

[pascalken]

Hi,

we are converting a fully encapsulated CNC machine whereby the work-pieces often exceed the size of the worktable.
Thereby homing with the work-piece attached to the table becomes a problem because the work-piece hits the housing before the end-stop is triggered.

Can you advise on how to deal with this problem?
This is in particular important in the case of a power cycle.

Thank you

[tblough]

You could change your home location to the middle of the travels.

[pascalken]

Thank you for your reply.

Yes, I though of that, however when the table is left of the middle home location, the machine should move to the right and vice versa, when the table is right of the machine the machine should move to the left to find the home position.
Also, the point of hitting the home switch is different from left to right, or from right to left.
I guess that in that case only manual homing is the only solution.
Furthermore, there is no protection because there are no limit-switches for the end of travel.

I was thinking of some sort of (light) barrier close to the wall of the enclosure, on either side, which can detect the work-piece before it hits the wall.
When hitting the barrier the homing procedure knows how to move to the middle limit-switch.

Are there machines equipped with such a device? If yes what model, type or manufacturer?

[tblough]

You would have to manually jog to the correct position before starting the homing routine. Soft limits will take care of protection after homing.

With modifications to the cncm.hom file, and possible PLC modifications, I'm sure you could make the homing process as complicated as you want.

[Allin1Chris]

Think you could accomplish this a few ways. I would first setup both your plus and minus home switch inputs in the motor configuration screen if not already.

I would customize the home file to ask for homing direction for each axis (Choose between M91 or M92). I would use M26 with the L command to account for the different position of the switches in home file. (It may take a bit of fine tuning and adjustment to get machine zero location consistent between the two switches).

Could give something like that a try.

[cncsnw]

Another solution would be to provide a switch-trip rail that keeps the home switch tripped through one half of the axis travel, and clear through the other half.

Then the homing macro could know which way to move, to find the edge of the switch, by looking to see whether the switch is currently tripped or not.

For example, supposing you have INP2 wired as the X axis homing switch, and it is tripped (open) throughout the plus half of axis travel and clear (closed) throughout the minus half of travel, then you could home just to the minus side of the switch-trip point using code like this:

Code: Select all

IF [#50002==1] THEN GOTO 20 ; if X- switch is already closed (clear), skip clearing move
  M105/X P-50002 F50   ; switch was open (tripped): move X- until INP2 closes
N20
M92/X   ; home X+ to designated home switch
M26/X   ; set axis home there