Cal,

> unidirectional approach feature.
>
> Is this specialization supported by G-code and EMC?
> OR is backlash compensation the avenue to achieve the required results?
>   
Yes and No on both your questions :-)

Yes, unidirectional is supported - by your CAM software (but this does 
not work for milling circles or other complex shapes).
No, unidirectional is not supported - by EMC. EMC does what it is 
commanded to do, milling left to right or right to left, by the G-codes 
fed to it.

and

Yes, backlash is the way to achieve good results. Is is a very cheap way 
to build a CNC system.
No, backlash is not the way to do this. If you want real good results 
then you should have a servo driven system with (glass) rulers on the 
axes to measure the actual position.
Or use a system that has no backlash - note that these do not exist. 
This is just a system with a backlash that is OK for your application or 
a backlash that is so small you can't measure it.

I used ball screws on my machine and I adjusted the whole thing such 
that I cannot measure the backlash on the nut on my large mill.
My small mill uses some linear drive units I was given - there is some 
backlash on these units so I'll go for backlash compensation on that one.

In the end it all comes down on what you have to work with.
do remember that backlash compensation has its limitations - as with 
hand operated mills: If you mill an edge of a block using climbing 
(conventional) milling, the cutter may climb up against the material in 
the milling direction which may result in an improper workpiece.

Regards,

    Rob

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