Topics

1

General Mach Discussion / How can one use a PLC in CNC?

« on: April 30, 2009, 05:59:13 PM »

I an an electrician and have some PLCs laying around. Anyone know how these could be used in CNC apps, mainly X-Y tables? which I have two of.
I'm thinking PLCs could be used for additional inputs and outputs, motor speed etc.

 I saw this post and PLCs used here:-

http://www.machsupport.com/forum/index.php/topic,5536.0.html

regards,
Shane Dwyer, AU

2

General Mach Discussion / THC float head switch recognition position hysteresis

« on: April 28, 2009, 07:23:17 AM »

New to plasma, cnc etc.

While setting up floating head sw for THC 300 I lowered cradle to gently touch a sheet of paper on top of 6mm material, zeroed Z, and kept lowering at a very slow rate in 0.5mm increments. At Z-7.2mm switch was triggered on input on diagnostic screen in mach3.
 I did this several times, repeatability was good, and arrived at a value of 7.2mm every time.

 I then used this value in sheetcam as POST variable switchoffset=7.2. (post THC300)
 However when running the code in mach the torch appeared a little higher than programmed height of 1.5 mm. I then manually edited code and changed the line to 6mm. Cutting height was now perfect.

Heres a snippet generated from sheetcam and re-edited (on line 150) to 6mm by me:-

N0110 G00 Z50.0000
N0120 X79.7916 Y53.5415
N0130 G28.1 Z3.00
N0140 G92 Z0.0
N0150 G00 Z6.0000
N0160 G92 Z0.0
N0170 G00 Z2.5000
N0180 M03
N0190 G04 P0.5
N0200 G02 X80.2516 Y52.6608 Z1.5000 I-1.9960 J-1.6030 F500.0


I cant figure out the discrepency of 1.2 mm. Only thing I can think of is that mach actually takes the value of the triggered input after switch goes from "on" back to "off" again. This accounts for an approximate hysteresis of 1.2 mm in the Z  home switch.
 When running the code, does mach read and assign the switch change on touching it, or backing away from the switch?

Should the actual travel of the floating head be read after the floating head has backed away from the switch? and not from when you would assume it has touched the switch. There is a hysteresis difference of distance here.

 As another matter of interest, what is the code on line N130 for? G28.1 Z3.00
why 3mm?
 what would happen if if was:- G28.1 Z0.0 ?

regards,
Shane,
Brisbane, Au