There are bunches of viewpoints. I have used both methods through the years. Mind you, I am from a metal cutting world and from a manufacturing world. I have worked in 8 different companies for between 3-10 years in each one, programming Lathes, Machining centers both Vertical and horizontal. I even worked in one place on a twin spindle CR Onsrud router. Done a lot of interesting things.
Even on a lathe we could debate where to put "Z" zero and why. I'm not going to go there
Years ago when I programmed machining centers, no CAM or computers to write code, I programmed "Z" zero to top of part. The philosophy was simple. Minus "Z" numbers were cutting, and plus numbers were not. You can make the same argument on a lathe to locate "Z" on face of part. MOST swiss lathes, or bar machines are face of part.
(not a repeat)
Years ago when I programmed machining centers, no CAM or computers to write code, I programmed "Z" zero to fixture location point - (essentially, bottom of part). The philosophy was simple. All programmed numbers were from a common program zero, regardless of how the part was placed in the fixture. You can make the same argument on a lathe to locate "Z" on the chuck jaws.
Then there is tool setting, and radius compensation, and other stuff too.
Every company I worked at had strong opinion as to which way was best. They all told me to do what I felt best, but if I did something that was not to local convention - whoooeeeee - that was a disaster. Soo, I learned to follow local convention. Now, to me, it really doesn't matter where "X" - "Y" - "Z" zero is, I just roll with it.
Soooo - who is right and who is wrong. I'm not going to answer that either way. They are both right. Vectric knows this, that is why you have choices.
Here in this forum, there is no defined local convention.
What you will hear over and over is the same thing.
Top is good, because---------
Bottom is good because -------
And they are right.
It can get confusing because there "SEEMS" to be nobody making a strong case for what to do. That's cause any way is going to work well, and you have 10 choices.
Again - it's all about personal preference. My preference is Top - Center I have all my reasons and if I had employees they would do it MY way. I DO deviate sometimes depending on the situation. Top Center does not work for tileing.
Lots of people do it lower left bottom. That philosophy is - all numbers are positive and any negative numbers "could" be a potential problem.
Notice - in my philosophies, most of the reasoning is in the sign of the numbers in the program. Let me ask, do we read the g-code any more? If not, does the sign of the numbers matter? For me, at home and with Vectric I don't really care about the numbers, and I am a g-code guy. I am still actively programming long have professionally. At work I program mostly lathes and I program "Z" zero to face of part.
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