Questions from a newby-(sort of)

[TRC1]

I've got some questions for you guy's that have built your machines.
1)Homing the machine. How do you send the machine home? Or where do you set machine "zero"? Do you let it limit out on the limit switch and then reset the zero's in the software? Do you set some kind of pin into the table and indicate it or the edge of the workpiece? If so, how do you "dial" the gantry over to the pin or edge?
Orrrrr, have I been a button pusher on a big machine for so long that I am over engineering a non-existant problem?
2)Speaking of limit switches, once you hit a L/S, doesn't it shut the stepper/servo drive travel down? If so, do you have to manually rotate the motor shaft to move the table away from the L/S to enable movement?
I've got a million more questions and hopefully you guys don't tell me to shut up too soon!
Thanks for shedding some light on this for me! DON

[CNCadmin]

When you home the machine will hit the limit switch, and 0 out all axies.

[Zephrant]

I believe that some people have "home" limit switches, and max-travel limit switches. Hitting the home switches just tickles the software where hitting the travel limit switches actually cuts the power to the motors.

Can someone confirm/deny the above?

Zeph

[TRC1]

That makes the most sense, but I wasn't sure. I haven't seen any hand-wheels on any of the home-built machine pictures. I guess you would jog it off the stop instead of dialing it?

[hardmill]

Nice sig TRC1

PEACE

[balsaman]

I home the machine which sends it to the switches and thats it. It will be at 0,0,0 and you run the gcode from there. Yes you would jog it off the stop with the software.

You could use an indicator of some kind when your work piece is out on the table somewhere and you want to start at one corner. What I have done is moved the cutter to the edge of the workpiece and manually set my zero from there. Generally with a wood working machine a dial indicator is not required because it's wood you are cutting and usually you take material off all sides. +/- 1/32 is plenty close enough. I don't have hand wheels on my machine but I have reached in and turned the leadscrew by hand to tweak my start position against the work piece or whatever.

Ask any questions you like. We are a patient bunch.

Eric

[HomeCNC]

Don,

This is the way I run all my CNC machines. I have a CNC Mill as well as a CNC router. You must use fixed tooling for this to work. I use Home switches because this is one position that is somewhat repeatable. When you home your machine you are setting the machines coordinate system, which is G53. For the record a coordinate system is a spot that the DRO’s read 0,0,0 I always home in the far positive direction.

After I have the machine coordinate system defined, I then move to the material corner (I use Lower Left) and you can find this by edge finder or other methods. What you are doing here is finding the coordinate offset position as measured from the machines coordinate system. In Gcode standard you can have many different coordinate offsets defined. Machine 0 is G53. The first offset is G54, second is G55 and so on. By finding the edge of the material in X and Y you will have a negative number for both. Next I find the Z position of the stock. I do this with no tool in the spindle or router. I move the Z down until I touch the stock top. This will be a negative number as well. You now have three negative offset numbers for X, Y, and Z. If your Gcode controller is good it should have a place to enter the coordinate offset for G54. You enter the three numbers here.

I get my tool length by clamping a tool in the holder and placing it in the spindle. I move the Z down until I touch the top of the part. I read the Z dro and subtract this number from the Z offset number I got above. This will give you the tool length. You enter your tool numbers in the controllers tool offset database.

Now when you have this data you just make sure that your Gcode has a G54 at the start and a G43 for the tool length offset. When you switch your CNC from machine coordinate system and use the offset your DRO will now read 0,0 when your are at the corner of your part. When you turn on the tool offset and place the correct tool in the spindle your Z DRO will read 0 when you touch the top of the material.

The nice thing about doing it this way is: 1) if you machine off the top of the part you will still be able to do tool changes and not loose where the material was. 2) if you shut down the machine for the night you just home it again and you are ready to go.

[HuFlungDung]

TRC1, I think the only thing that hasn't been covered is the logic used to get off the limit switch: Typically, this would allow movement only in the opposite direction when a given limit is hit, since it is known from what direction the machine must have moved to activate a given limit.

For more accurate homing, usually you would use an extra switch rather than the limits, but that is up to you. The advantage would be that different logic can apply to a dedicated input/output from the home switch, whereas a limit switch needs a different logic.

I am not sure how you would incorporate this homing logic in your controller software, but usually a very slow jog is used to trigger the home switch, and when the controller gets the signal that the switch has changed states, then the controller looks for the very next input pulse from the encoder's index position. This is a single pulse output from a special single line on the encoder's encoded disk. I am assuming you have an encoder on your screws.

[TRC1]

I haven't built my machine yet, I'm still working on getting everything together.I will probably start with 48x48 inches in structural aluminum.
I hope to have some new servos in 1 week and a day (wink,wink) if the subscription gods are looking favorably upon me
I think Hu's answer came closest to the question I was asking. I have past experience with cnc machines but none with hobby cnc's and I haven't seen anybody address these questions in the forums. Thanks for all the great answers, I'm taking notes!
Don

On a side note, I found a guy who says he has about 1000 feet of structural alum., (45X45) that he wants to part with. I will post it in the For Sale forum in a couple of days if it works out.

[keithorr]

Don't know what kind of machine you use, but some machines have the solution built in.

On mine, the homing speed is user selectable, both high and low speed.

The head moves toward the home limit switches at the high speed, say 2000pps, all three axis moving at the same time.

As the limit switch on each axis is "made", the axis in question reverses at the low speed of 100pps until the micro switch just opens and the axis stops moving. This becomes the machine reference point.

The repeatable accuracy is determined by the type of switch used.

The driver box I use is by Ah-ha. Unfortunately, I don't think they are "active" in any meaningful way. They have moved to an undisclosed closet, their website hasn't been updated in months, but someone will come out of the woodwork and swear new boards are available. Support is non-existent. Except for the company being dead in the water, it is a good stepper system.

[TRC1]

Thats exactly what I needed to know, thank you! Are most machines set up like this?
Don

[balsaman]

TurboCNC drives the axis' towards the switchs at the machines max start speed. Once it hits the switch it backs the axis away slowly until just off and then back to the switch slowly till just on. It then resets the axis' coordinates to the home position, which is user defineable in the settings. My home position is 0, 12, 2 for example. This allows for the easiest material placement and tool changes.

Eric