Where is home?

[Scruffynerfherd]

Where should the table be in home position for a taig mill? All the way to the right for x and all the way to the back for y?

[toddferrante]

Are you talking about a taig mill with CNC done by the factory? or by an aftermarket conversion company? or a do-it-yourself CNC conversion? The answer could be different for each of them.

The "home position" (the position the mill goes to after the switches are tripped) doesn't really matter. I set mine so the spindle is clear and the table is in a position where doing setups is easy.

The best position of the switches themselves depends on if you have position feedback on your axes. For steppers with no feedback, you want the switches near the end of travel of an axis such that the initial position when the home command is given is always on the same side of the switch. Which way the axis moves (positive or negative) doesn't really matter as long as the home switch is tripped before you hit a mechanical stop. For the Z axis, it makes sense to put the switch at the upper limit of spindle travel, but for the X and Y axes, the end of travel where the limit switch trips doesn't really matter.

You might also consider installing home switches on both sides of the travel and switching between them every few months. This would tend to distribute wear on the screw more evenly over time, and might possibly minimize excessive backlash from developing in one area of the screw travel. This is also a good reason to do setups for jobs across the table, not just setting up on the same end over and over again.

Todd F.

[Fixittt]

the term "MACHINE HOME" and its variants refer to a known location typically at the ends of each axis travel where the machine zeros the machine coordinates. This gives it a reference point to other locations in the working envelope to where things might be set to.

For example, if you have home switches, and you home the machine. It goes out and triggers each switch one at a time. When it finds these it zeros the machine coordinates. Now lets say that you have a vice setup and you are doing a production run thru out a few weeks. You can program a fixture offset to your part and each time you turn the machine on and home it, you can now use the fixture offset to find that exact location in "SPACE" to work from again. The trick here is that it needs to know "Where" it needs to start looking from. Which is where the switches come in.

Here, watch this.......
http://www.youtube.com/watch?v=Fvfh5Ou0FRM&feature=g-upl]understanding home switches in mach 3 - YouTube

[Jeff-Birt]

The 'home' in machine coordinates, that is based on the physical position of each axis is when the Z axis is all the way up the X axis is all the way to the right, and the Y axis is all the way back. This places the bit above the left front corner of the table. From this position moving the bit to the right (table moving left) causes the X DRO to get more positive, moving the bit toward the rear (table toward front) causes the Y DRO to get more positive and moving the bit down causes the Z axis to get more negative. Always judge axis movement as though the bit were moving. The process of 'homing' lets your CNC control program (Mach3) and you know where the axis is at physically.

Now lets say you clamp down a piece of stock on the table. You GCode knows nothing about where the stock is at in relation to your machine coordinates. This is where local coordinates come in, you switch to local coordinates mode and then 'zero' to the edges of your stock. Now when your GCode runs it moves relative to this local 'zero'. There are exceptions to this as your Gcode can explicitly ask to move in Machine coordinates but that is another matter. The big picture though is that the machine coordinates tell you where each axis is physically at and local coordinates tell you where the bit is at w.r.t. the stocks 'zero' point.

[Robin Hewitt]

I can only think of 2 circumstances where you might need a home position...

1: You want to come back tomorrow and start machining the same billet

2: You have a work holding fixture bolted to the bed and more pieces to make

Either way it helps if "home" is relocatable so you don't have to wind to the ends of all 3 axes.

[Jeff-Birt]

I can only think of 2 circumstances where you might need a home position...

1: You want to come back tomorrow and start machining the same billet

2: You have a work holding fixture bolted to the bed and more pieces to make

Either way it helps if "home" is relocatable so you don't have to wind to the ends of all 3 axes.

What you are describing is not a HOME but is a local zero. HOME is (typically) the (0, 0, 0) physical position of the machine. (Some really large machines will have the home sensor mid travel so they do not have to move all the way down a 40 foot long table to home.) The HOME is to let the controller know the physical position of each axis when you first start the machine.

You may indeed have some fixture bolted to the bed, but then you need to know the offset from HOME to the fixture to use it without zeroing to the fixture each time.

[UUU]

It's probably easier to have the sensors/switches physically close to the axis motors, so that you can organise the wiring more neatly. That's assuming the shielding is good enough!

[109jb]

What you are describing is not a HOME but is a local zero. HOME is (typically) the (0, 0, 0) physical position of the machine. (Some really large machines will have the home sensor mid travel so they do not have to move all the way down a 40 foot long table to home.) The HOME is to let the controller know the physical position of each axis when you first start the machine.

You may indeed have some fixture bolted to the bed, but then you need to know the offset from HOME to the fixture to use it without zeroing to the fixture each time.

That is exactly what he is talking about. If you have accurate home switches then both Mach3 and EMC2 save the fixture position in relation to the machine's home position. Simply start up the machine, home it and then you are ready to run.

For example, if the part zero location is +3.72, +2.21, -1.12 inches from the machine home position in x,y,z respectively, then after starting up and homing, the DRO on the machine controller will read -3.72, -2.21, +1.12.

You need accurate home switches to do this, but there are many types that can achieve it. Optical and proximity switches come to mind.

Another reason to have a home position is if you are running a machine without limit switches. Then you can set up software limits. You manually move the machine to its home position and let the controller know it is there. Then the machine references this position for the software limits using the defined travels of the axes. For example, if your x axis mechanical travel limit is 12 inches you might define home to be 0.1 inches from one end and thn tell the controller software that the machine travel is 11.8 inches giving a buffer of 0.1 inches on each end. Not sure about Mach3, but EMC2 won't drive the machine past the software limits.As long as you haven't lost a ton of steps, you won't ever get to the mechanical limit of the machine.

[UUU]

Yes, Mach3 has a software limit system that will prevent travel beyond the parameter-set limits.