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### Author Topic: Plus or Minus ? What is the norm ?  (Read 8134 times)

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#### MJR

• 35
##### Plus or Minus ? What is the norm ?
« on: May 23, 2006, 06:36:47 AM »
New to CNC.
If I use the bottom left corner (as seen on a CAD drawing or the Mach3 screen) would the DRO & the g-code show a positive number when the tool moves away from this Home position & then a negative to move back or is it a negative to move away from Home then a positive to move back ?
Also, on the Z axis is it a positive to move the tool downward toward the table/project to be cut, then a negative to move back up to home ?
Thanks all.

#### sshneider

• 401
##### Re: Plus or Minus ? What is the norm ?
« Reply #1 on: May 23, 2006, 10:02:35 AM »
I suppose it's really a matter of preference.  But, I think the Classical CNC traditionalists would tell you that if Lower left =home, moving away on the X & Y axis would be a positive value.  As far as Z is Concerned I think most dude have Zero when the cutter is all the way up and then go Negative to cut into the part. HTH.

Regards,
Sid

• 361
• When the going gets weird, the weird turn pro.
##### Re: Plus or Minus ? What is the norm ?
« Reply #2 on: May 23, 2006, 05:49:22 PM »
Hi ,

I think I can see some of the confusion.

Ok usually the REF ALL AXIS HOME for x and y is 0,0 and in the lower left corner (machine coordinate with home switches). When you go y4 it will move to y +4. If you then go y2 it will move y2, all of these numbers are positive. you would have to to go y-4 to get in to - numbers, but if your home is 0,0 then you couldn't physically move past your home position witch should be the physical limits of your machine. Now all of this is assuming you have home limit switches installed and properly configured.

else { you don't have any home switches and you are using the zero buttons to set a home position, then depending on how you generate your gcode will dictate where the 0,0,0 position is relative to the 0,0,0 on your part. in this case things could go to - based on how your part g code was made.

OR you do have home switches installed and you are setting a offset relative to the machine coordinate.

This is kind of a confusing aspect of cnc work. The best thing to do is go to the WWW.mcahsupport.com site and look at the coordinate systems video, then the homing and offsets video. It took me a couple of times through the videos and a lot of experimenting to get my head around it.

Oh yea and usually the Z 0 at the up most position, and negative into the work.

Just thought of something else in case you aren't thoroughly confused yet. There are settings for Incremental and absolute positioning. I would recommend looking this up in the mach manual. It will probably do a better job of explaining it than I would.

« Last Edit: May 23, 2006, 05:57:44 PM by chad »

#### MJR

• 35
##### Re: Plus or Minus ? What is the norm ?
« Reply #3 on: May 23, 2006, 07:42:32 PM »
Thanks all.  Starting to make better sense.
If I start a project in the middle of my axis travel and the center of my project, then I could easily have some negative values. Correct?

I seem to still have problems setting my Z info.  I run a test DXF & my Z does weird things.  Still missing something in the Z stuff.

I have just played with Mach3 for only a very short time.  I have been looking for someone in the Calgary, Alberta, Canada area that could maybe spend some time with me in the training side.
Thanks again.

#### sshneider

• 401
##### Re: Plus or Minus ? What is the norm ?
« Reply #4 on: May 24, 2006, 02:39:00 AM »

If I start a project in the middle of my axis travel and the center of my project, then I could easily have some negative values. Correct?

The answer is ... "That depends".  If you are using Machine Coordinates the answer is No.  Machine Coordinates assume that your Home position is Zero and since you normally never move beyond the home position in the negative, any movement you made to cut your part would HAVE to be in the positive.

Now, if you are using a fixture offset, then the answer could be yes.  Example.... from the Home position you move to the center of your table by moving both you X & Y axis in the positive.  Then yonce you are in the middle, you reset the X & Y axis to Zero.  You have now created a new starting point to cut that particular part.  So, after having reset your DRO's to Zero, if you now moved X & Y toward the Home position, your DRO's would show negative values.

I hope that makes sense.

Regards,
Sid