Messages

1

General Mach Discussion / Re: Soft limits problem, still confused

« on: May 01, 2009, 05:41:20 AM »

 I'm not an expert by any means. Someone else more familiar than me might like to comment.
 
 I don't think G31 is a good way to home. G28.1 looks like its meant to be used for homing.
 When using plasma, homing at the top of Z can be confusing, as I mentioned in above post. Sheetcam (with THC POST) compiles code that uses all positive Z moves. This means that Z should be homed at the material, best way by using a floating head, or even electrical contact method.

 If you always use Sheetcam with THC POST, you may never need to home Z at top.

Please understand with conventional CNC , Z is homed at top, and alll  Z moves are negative, but not when using Sheetcam with THC POST.
Sheetcam tries to make it easy for us, by putting in code ( G28.1) that homes Z at material. Then all Z moves will be positive. Then all we have to do is tell Sheetcam that pierce and cutting height are POSITIVE moves. It is easier for us to think in positive moves starting at the top of material. Its also easy because we don't even have to tell Sheetcam how thick the material is. It will always home there with G28.1 combined with floating head switch.

For now it is safer if you can to use a separate and single input for Z home on the floating switch. Tell Mach3 to home "Z negative".

You can combine all the other home and limit switches if you need more inputs available for Z Home. The floating head is not a Z limit, it is a Z home switch.

What you are doing with G31 will work, but there is a lot of room for error, and you have to measure exactly how far down the material is from your Z home at the top, and then type that figure into code.

 I can understand how you are thinking, I was confused for a while too.

 To answer you question about how to ignore Z homing at top of travel, is TO JUST FORGET IT FOR NOW AND HOME Z AT FLOATING HEAD ONLY   AND SETUP MACH3 FOR HOME Z NEGATIVE. This only applies when using Sheetcam with a THC POST.

Ignore or disconnect switch at top of Z for now.


 There are 3 things that all need to work together:-

1. Sheetcam with THC POST
2. G28.1 (included in Sheetcam THC output)
3. floating head Z Home, and set Z Home negative

When using these 3 items, try to forget "normal" Z methods.


I hope my understanding and explanation is right for you and I am not too confusing.
regards,
Shane Dwyer

2

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

3

General Mach Discussion / Re: Floating head switchoffset

« on: April 30, 2009, 05:48:54 PM »

mrpeja
 read what I have written to your other post, and what I posted as well.


regards,
 Shane Dwyer
 AU

4

General Mach Discussion / Re: THC float head switch recognition position hysteresis

« on: April 30, 2009, 05:40:06 PM »

I don't know if the G28.1 Z 3.00 is s conventional implementation of G28.1. I haven't found anywhere yet when parameters are used with G28.1.

I cant find anything about semantics and syntax of proper g28.1 implementation.

But what I did find is that the z3.00 tells the homing sequence to slow down at +3.00, so more accurate homing can be achieved. But to slow down at + 3.00, one has to faithfully assume and set up Z do that it is not homed at the top of travel, and  Z is being (and has been previously) homed near the material. Consequently all Z moves are positive.

In fact Z will be homed at the last material height that you last used the machine. Z homing (with floating head and THC codes) will always be different.

 I also discovered that G92 is somewhat awkward to use, and its better to steer away from it when you can. Only use it if you know what yourself and G92 are doing.

5

General Mach Discussion / Re: Soft limits problem, still confused

« on: April 30, 2009, 05:12:06 PM »

Mrpeja,
It might be helpful if you read my post about switch hysteresis.

With normal cnc conventions on a gantry table, Z is homed at top of travel, and all Z moves are in negative direction. This can be awkward as a person has to think "upside down" or "backwards" The trouble is both you and the z axis cannot always know where the material is exactly, especially with plasma, as the material warps. It is easier for a human to think "If I can home my Z at the material height, I can have a good datum and all my calculations will be done in a positive direction, from this height"

Sheetcam with THC  attempts to do this for you. The THC code in sheetcam homes Z at the material height, AND NOT AT THE TOP OF Z TRAVEL. So when using sheetcam you should have a means of determining a home Z at material height. A floating head with a switch will  do this for you, and the code to read it is in sheetcam.

CNC convention tell us to home our Z at the top, but not Sheetcam with THC.

 I would suggest to ignore homing Z at the top of travel for now, and fit a means to touch the material and then home Z.  Use a separate input on Mach3 for this. All Z moves will now be positive after homing at the material height, cutting height will now be +1.5 mm and not -123mm ( or whatever) , pierce height will be +3mm and so on.

hope this helps

dwyersm





With plasma be aware
dwyersm

6

General Mach Discussion / Re: THC float head switch recognition position hysteresis

« on: April 29, 2009, 04:26:07 PM »

Im using the floating head switch as Z home. I also have another switch fitted at the top of the main Z slide as home as well. The two switches are wired series NC. I dont go that far up in normal usage to make the top one get used. But its there if I need it to set the machine up "normal" Z. ie all  Z moves in neg direction.

But when you think about it for example any home switch will have to be referenced when the slide moves off the switch in the pos direction for that axis. Not when its coming home and hits in in the neg direction.

.ie. to home x for example:-

gantry move towards home sw:
gantry hits home switch:
cnc controller recognises swiitch activation, and should begin routine to reverse direction:
switch is now open (in the case of NC);
cnc controller walks off switch in opposite direction for small distance (which amounts to hysteresis);
switch closes again;
movement stops:
parameters are now read, and machine zeroed.

So you see hysteresis is now taken care of.
Its just like winding the dials on any table on a mill or lathe.
you wind back to take care of slack, then wind in again, and zero your dial.

So to accurately set up floating head switch as home, it must be walked off in opposite direction, and then zeroed.
When using sheetcam and a floating head, the Z axis is actually used in a somewhat "opposite manner" to normal Z. Home will always be at material thickness above the table, and all Z moves are positive from home.
 

7

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