Author Topic: Soft limits problem, still confused  (Read 8396 times)

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Offline mrpeja

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Soft limits problem, still confused
« on: April 27, 2009, 05:54:57 AM »
Hi Hood,
I must ask for help again.
I use plasma table with THC300, and SheetCam postprocessor Plasma THC300 - G31 - corner slowdown.post.
I home my Z axis torch carriage at the top position. Z axis travel is 100 mm
When I start the cut, Z axis goes up, touch the limit switch and this is the end!
I see in generated G code the first instruction is to go UP 50 mm (my safe Z position).
I've been advised to insert snippet code: G31 Z -100 F500 and to set Soft max to 60 mm, Soft min to -50 mm and Home offset to 60 mm and Auto zero checked. Pretty confusing for beginner like me!
Sometimes it works, but sometimes, when I load another G code file,  I get Soft limits warning when I start cutting, and question to proceed or not.

Actually, don't knew where to look to find problem source.

I have idea install another limit switch and to home down to find zero position. And without code snippet.
Please advise.
BTW, What exactly is mentioned with those three lines of code: (I selected mm for my units, NOT inch)
N0040 G53 G90 G40
N0050 F1
N0060 S500

mrpeja

Offline mrpeja

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Re: Soft limits problem, still confused
« Reply #1 on: April 27, 2009, 06:13:07 AM »
Ho Hood,
THIS IS VERY INTERESTING: THE SMALL WINDOW WITH THE MESSAGE "SOFTLIMIT WARNING...CONTINUE?"
HAS THE NAME MACH4 !!!!
mrpeja

Offline Hood

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Re: Soft limits problem, still confused
« Reply #2 on: April 27, 2009, 06:36:07 AM »
It seems that you will either have to alter the post processor or the code it produces or as you say swap the limit so that home is at the lowest position.
 I am not sure how the normal procedure is for Plasma as I have never had or used one but it does seem from what you are saying all Z moves are positive. You could probably keep your home switch where it is  and set a Home Off value for that axis as the travel your axis has. Example is if you home fully up and the distance down to your plate is 150mm then you would set the Home Off value to 150. When you homed the  Z DRO (in machine coords) would now read 150 instead of zero and as you jogged down to the plate it would  go towards zero.

The message regarding Mach 4 is nothing to worry about, there are a few things that have that as at one time there was going to be a Mach4 but the features were then integrated into Mach3 instead.

Your snippets of code mean
G53 = machine coords
G90 = Absolute distance mode
G40 = Cancel cutter radius compensation

F1 = feed of 1 unit per minute (in your case anyway)

S500 = means spindle speed of 500, although you will not have a spindle the plasma may be using the spindle function for torch height or some other feature.



Hood
« Last Edit: April 27, 2009, 06:46:42 AM by Hood »

Offline mrpeja

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Re: Soft limits problem, still confused
« Reply #3 on: April 27, 2009, 10:55:55 AM »
Hi Hood,
One must carefully look at Error mesage line! All the time I suspected at Z axis, but problems were with X or Y axis! I have set material bottom left coordinates too big!
mrpeja

Offline Hood

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Re: Soft limits problem, still confused
« Reply #4 on: April 27, 2009, 10:59:26 AM »
You wont be the first to have missed that and you wont be the last :) I suspect it will just be a matter of days before something similar will have me thinking for a while ;D

Hood

Offline mrpeja

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Re: Soft limits problem, still confused
« Reply #5 on: April 27, 2009, 05:12:00 PM »
Hi Hood,
I'm wondering why Machine DRO and Z axis DRO sometime have the same number after homing (60 mm, that is offset)
and sometime Z axis DRO is different (zero or whatever). This happens during testing G code step by step, when I do homing in the middle of "cutting"
mrpeja

Offline Hood

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Re: Soft limits problem, still confused
« Reply #6 on: April 28, 2009, 02:05:07 AM »
Not really sure what you are meaning, if it is that the DRO start the same and then after a while they differ then the only explanation I can think of is the code is shifting the offset.

Hood

Offline dwyersm

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Re: Soft limits problem, still confused
« Reply #7 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

Offline mrpeja

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Re: Soft limits problem, still confused
« Reply #8 on: May 01, 2009, 02:11:58 AM »
Hi dwyersm,
Is solved my problem as follows. Please advise if this is what you mentioned.
First I home Z axis at top, to  switch connected in series with other axes.
On the beginning I inserted code snippet to find material surface and to zero Z axis work coordinate:
G31 Z-100 F500
G92 Z0.0
Then goes code for safe Z
G0 Z50
and etc.
Is this a way to ignore homing Z at the top of travel ?
Should I map the same input (probe) as input to home Z instead of common input for all axes?
mrpeja

Offline dwyersm

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Re: Soft limits problem, still confused
« Reply #9 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