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I have little preference to where the machine homes, and don't know why home position matters.Originally Posted by Machineit
If I was building a new machine, and got to chose home position, I would probably chose the top left though.
I use the left side of a part as Zero much more often than the right hand side, so if home was on the left, I would not have to traverse as far to get to Part Zero.
Look at it this way...
If you look at the panel, and push the "-X" key that is pointing to the right, the table moves to the right. The table/part are moving in the cartesian coordinate system in a positive direction. The tool, IN RELATION TO THE PART, is moving in a negative direction.
There is no hidden agenda here. The TOOL moves negative when you press the "-" direction key. Obviously, the reverse is true when you push the "+" direction key.
Mike
I get it guys.
I just very vehemently disagree with it.
With all due respect to your training, experience and opinions I think you are being inconsistent here.
CNC machines use the Cartesian coordinate system and conventionally work in the lower left quadrant. This means that,in machine coordinates, all axes move move in a negative direction from the origin or home position, which is in the top right, rear corner of the machine.
If you choose a different location for the home position then you are working in a different quadrant. You are upset because the jog buttons on Haas machines 'move the table' not the tool, and say this is confusing. But your machine would move the tool (or table) the 'wrong' way which, I think, would be even more confusing.
P.S. I checked the Haas gantry machine button layout and it is the same as all the VMCs.
An open mind is a virtue...so long as all the common sense has not leaked out.
Well now you see, this I did not know.
I spent 20 years as a conventional guy, CNC is new to me.
Why do we care where home position is?
Or for that matter which quadrant we are in, in relation to home?
I do care where I am in relation to my part zero, but home is just a place to park, isn't it? And pick up the machines position of course, via proximity or limit switches, or whatever they use to figure out where the machine is.
For that matter, if there is a reason that we want to work in the -X -Y quadrant, home could still be the top left, or the middle for that matter, you would just have a parameter in the machine that tells it when it is home, it is not at machine 0.,0.,0. but -30.,0.,0. or whatever...
I am sure I am missing something here, as I really do not care where my machine parks, or picks up its references from, as long as it does it the same every time.
The reason I would choose the top left if I was making a machine from scratch is simply due to wanting less motion to get to my most common starting point.
Nothing more, and I don't have any strong opinion about that.
These are not wise A$$ remarks I am writing here either.
I really had no clue that there was a standard to machine home positions and that we always want to be in the negative quadrant.
Why is this?
What are the advantages?
How do we justify the button position on the gantry mills?
In this instance, the bulk of you are telling me that the table/part is moving in the direction of the buttons we press, so it makes sense.
This is not the case on a gantry, the tool IS moving then, and the part/table is sitting still.
Do other manufacturers use this same convention?
The only other CNC I ever ran, other than my Bridgeport, was a Fadal, many years ago, and I seem to remember there being a dial to chose the axis, and then just a knob to move, I could be wrong though.
You can do this with Haas machines as well. You select the axis you want to move by pushing "+X", "-X", "+Y", etc., then use the hand wheel to move the axis around. Here is where you might get frustrated again. If you choose the "X" axis, and then turn the wheel to the right, or "+" according to the hand wheel's sign, the table moves to the left meaning the tool moves to the right, or positive.
All of these movements make sense in the Cartesian system, to me. The one that really messes with my head is when you throw in the 4th axis. I can never remember which way + and - are until I go out and crank the dial and watch.
Mike
The answer to your first questions and the thing you are missing is the history of CNC. Computer Numerical Control (CNC), has only been around for about 30 years plus or minus a few years. Prior to that it was Numerical Control (NC), using punched paper tapes and legacies of the paper tape days still exist. The one I find most amusing is the definition for the M30 command which still shows up in the Haas manual; "Program End and Rewind (Tape)".
On NC controlled machines memory was minimal or non-existent and each line (Block) of code was executed as the tape incremented forward through the tape reader. This meant that the code had to be as compact as possible and there was no such thing as Work Offsets; all the part coordinates had to be taken from machine zero.
Obviously milling machines even prior to NC had the Cartesian Coordinate system embodied into their three axes so for NC the only decision to be made was which octant of the three dimensional Cartesian system was going to be used; i.e. where was a logical position for the Cartesian Origin or machine Home?
Choosing any of the upper octants would have put machine Home at the surface of the table. Choosing either of the lower right octants would put Home at the limit of Z travel above the table, but then motion would be negative for Z, positive for X and positive or negative for Y depending on whether the front or rear octant was used. Only the lower left front octant puts machine Home above the table and provides for all axis motion to be negative; it also means the work (table) is in front of the spindle from the viewpoint of the operator. And as far as I have been able to determine reading about the history of NC this is why that octant was chosen.
Nowadays with modern CNC machines with the capability of numerous Work Offset locations, it is more or less irrelevant where machine Home is located. However it is convenient to have consistency between machines and there is no reason to change from the original choice.
Regarding the button location and function on mills or gantrys I think it is probably for simplicity in manufacturing and I have not really noticed any awkwardness when I have switched between the different machines.
Incidentally CNC is newish to me. Except for two weeks in 1974 I spent 39 years on manual machines before switching CNC in 1999. The two weeks was when I started to learn programming on a tape controlled Warney Swasey lathe. It was only two weeks because the company went backrupt and my career plans suddenly had to be changed.
An open mind is a virtue...so long as all the common sense has not leaked out.
Good post Geof. I'll add two things.
First: I remember a thread here where someone mentioned a machine he had that zeroed X and Y in the center of the table. He also mentioned the problems it caused by running into some fixtures during homing. We all know those jobs where that could happen. So, homing back in the corner seems more practical.
Second: When it come to pushing a button and something moves, what is important is what is actually moving. The brain can easily be fooled into seeing something other than what is actually happening. So, when we push that right button, our brain expects something to move right. That could be the tables of most of the mills or it could be the gantry on that type of machine. We expect to see the movable object move, not some abstract reference point.
As an example of this, take your car into a car wash that has the mechanism that move front and back to wash and scrub your vehicle. Who has not looked up and suddenly put your foot on the brake because it appeared that your car was moving forward and not the gantry moving back? It is just one way to trick the brain into a false reality.
No system is perfect, but I certainly see no reason to mess with this one.
Mike
Haas VF-2, HA5C, Hardinge CHNC 1, BobCAD V23
Excellent post Geof, thanks!
Using that logic, and the little I remember from my 2 weeks punching NC tape during my apprenticeship, I would think we would want to have home be the bottom left corner, upper octant, so that we are always working in quadrant 1, instead of 3.
All coordinates would then be X+ Y+ and Z- requiring less typing, since we wouldn't need a negative sign for each and every coordinate.
Fewer keystrokes means fewer chances of making an error.
I remember very little of the NC stuff though, we did 2 projects at an NTMA training center that took up 2 weeks of class, after work.
Well then, shouldn't we home in the upper left corner so the toolchanger and everything is clear of the table?
Far less danger of interference there.
But we just established that Haas disagrees with this.
If what is important is what is actually moving, not an abstract concept, the gantry mills would not use the same key positioning as we are forced into.
And this is the crux of this discussion summed up very well.
I actually feel the abstract is more important, everyone else feels otherwise.
I AM the cutter, where do I want to go?
That is probably why I do not see any difference between the mill and the gantry; the motion I see matches my finger location.
I will say when I am writing a program in MDI I often have to hold my finger pretending it is a tool to figure out what sign to put on a particular coordinate. If I simply look at the machine I can get myself crossed up. When writing programs on the machine I prepare handwritten pages away from the machine and think in terms of tool movement while doing them. Often when I edit on the machine during proving a program I get crossed up because I am looking at the part and my brain switches to 'table movement' mode.
An open mind is a virtue...so long as all the common sense has not leaked out.
And there you have it.
You have to think about it.
You have to remember.
If you are always the cutter, and never the part, it becomes instinctual.
You have to think in 2 different ways here, and that seems silly to me.
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